introduction to metalurgy

AN INTRODUCTION

TO METALLURGYComplete and Unexpurgated

Revised and Enlarged(Banned In Boston)

4th Edition

.....Being a Compendium of Knowledge, Folklore, Whimsy, and MetaphysicalRuminations; Garnered from Hitherto Lost, Mystic Treatises of the Ancients andSubstantially Plagiarized from Modern Day Texts; Designed to Guide the InnocentTraveler Unscathed Through the Labyrinth of Pitfalls, Misconceptions, Archaic Ideas,Abstruse Theories, and Pseudo-Experts that Inhabit the Great Void and Contrive toWaylay the Unsuspecting Pilgrim at Every Chance Encounter Whilst on His JourneyTowards Metallurgical Enlightenment.

Concocted by J.D. DufourManager, MetallurgyCameron DivisionCooper Cameron Corporation

Superbly compiled by Cecilia A. HobbsCameron DivisionCooper Cameron Corporation

"In that direction," the Cat said, waving its right paw round, "lives a Hatter: and inthat direction," waving the other paw, "lives a March Hare. Visit either you like; they'reboth mad."

"But I don't want to go among mad people," said Alice.

"Oh, you can't help that," said the Cat: "we're all mad here. I'm mad. You'remad."

"How do you know I'm mad?," said Alice.

"You must be," said the Cat, "or you wouldn't have come here."

Lewis Carroll

Alice's Adventures in Wonderland (1865)

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RAVE REVIEWS FORThe First Edition Of

AN INTRODUCTION TO METALLURGY!

.....To gain a clear understanding of the complexities of metallurgy, read Elements ofMaterial Science by L.H. Van Vlack. Don't read this book!

CHICAGO TIMES

.....A crime against humanity!THE VATICAN PRESS

.....It is difficult to select the most appropriate adjective for this book - vulgar,contemptible, vicious, pusillanimous, obscene, crass, blasphemous, noxious. Allimmediately come to mind and all apply equally well.

BOSTON GLOBE

.....Detente has been set back 5 years.TASS

.....Seldom has there been so prurient and pernicious a volume.CHRISTIAN HERALD

.....Humbug!LONDON EXAMINER

.....In making this contribution to world peace, the author has joined the ranks of Ivanthe Terrible, Stalin, the Marquis de Sade, Genghis Khan, Hitler, Idi Amin, and Attila theHun.

WASHINGTON REPORTER

.....America has finally avenged Pearl Harbor.TOKYO STAR

page - ii

CRITICAL ACCLAIM FORThe Second Edition Of


.....We previously stated that the first edition was the nadir of scientific thought in thiscentury. After reading the second edition, we apologize for having slandered the first.

SCIENTIFIC DEVELOPMENT

.....Great literature withstands the test of time. I don't look for this book being aroundtomorrow.

PARIS COURIER

.....Fertilizer!AMERICAN HORTICULTURALIST

.....Highly recommended for insomnia due to its soporific properties, however, too greata dose results in patient despondency and general blathering. Do not administer toanyone having previous scientific or technical training! Studies show that thesepatients develop homicidal tendencies towards the author that, if left untreated, developinto psychoses typical of the criminally insane.

JOURNAL OF AMERICAN PHYSICIANS

.....Morally depraved! Mentally disturbed!MANCHESTER DISPATCH

.....Bombed again!NAGASAKI NEWS

.....One questions the author's motive - Maliciousness? Trying to kill time? Out to makea fast dime? Some budding psychiatric student will find this fertile ground for a futurethesis on deviate behavior.

TAMPA TIMES

.....Takes metallurgy out of the Dark Ages and into the Stone Age.JOURNAL OF METALLURGICAL ENGINEERING

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AND NOW BOUQUETS OF ACCOLADES FORThe Third Edition Of


.....One can only wonder at a society that can put a man on the moon and yet producethe author of this book.

THE PHYSICS SOCIETY OF AMERICA

.....Three strikes...you're out!U.S. SPORTS MAGAZINE

.....Whoever said that the third time's the charm has not yet read this book.LITERARY MAGAZINE

.....The author has done to metallurgy what Alferd Packer did to American cuisine.CULINARY ARTS

.....Male bovine fecal matter.AMERICAN STOCKMAN

.....The author has declared total war! No punches are pulled. No mercy is shown. Noprisoners are taken. There will be few survivors of this devastating course.

WEEKEND WARRIOR MAGAZINE

.....The great abyss between the covers of this book is matched only by the onebetween the author's ears.

JOURNAL OF UNEXPLAINED PHENOMENA

.....Along with the Titanic, the Hindenberg, federal income tax, and API Spec 6A, one ofthe great disasters of modern times.

GLOBAL CURRENT EVENTS

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PAEANS OF PRAISE FOR The Fourth Edition of


...One of the foundations of Darwins theory of evolution is that man has evolved fromlower life forms. The author of this book is living proof that evolution has now gone fullcircle.

NEW BIOLOGY MAGAZINE

...Dante said it best, All hope abandon, ye who enter here!LITERARY SOCIETY

...While we have always deplored the occasional book burning that has stained ourpast, we would gladly strike the match at an auto-de-fe given in honor of this book andits author.

NATIONAL LIBRARIAN

...Plumbs new depths of depravity.AMERICAN SURVEYOR

...This book rates alongside the Piltdown man, water witching, ether, and perpetualmotion as one of the great hoaxes of modern science.

...The whirling sounds you hear when you open the covers of this book are the greatscientists of ages past turning over in their graves.

SCIENCE QUARTERLY

...What to read when you have five minutes of free time, but dont want to be overlydistracted from more pressing matters? This book is the ticket. A highly entertainingvolume that deserves a place alongside everyones commode. Look for it in the pulpfiction section of your local bookstore.

SLEEZE MAGAZINE

...The author has boldly gone where no man has gone before - at least no one in theirright mind.

WORLD NEWS

...The malignant machinations of a myopic, moronic, monstrous, machiavellian,metallurgist.

METALS MONTHLY

TABLE OF CONTENTS

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FORWARD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix

CHAPTER I - THE BASICS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1CRYSTALS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1GRAIN SIZE, SHAPE, AND ORIENTATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2COLD AND HOT WORK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4PHASES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5DISLOCATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9STRENGTHENING MECHANISMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

CHAPTER II - HEAT TREATMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15ANNEALING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15NORMALIZING, AUSTENITIZING, QUENCHING & TEMPERING OF STEELS . . . . . . . . . . . 16AGE HARDENING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18T-T-T DIAGRAMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20CONTINUOUS COOLING TRANSFORMATION DIAGRAMS . . . . . . . . . . . . . . . . . . . . . . . . . 29ODDS & ENDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

CHAPTER III - ALLOYING ELEMENTS OF STEEL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

CHAPTER IV - MECHANICAL PROPERTIES AND HOW THEY ARE DETERMINED . . . . . . . . . 47TENSILE TESTING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48IMPACT TESTING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52HARDNESS TESTING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55FRACTURE MECHANICS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59FRACTURE TOUGHNESS TESTING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63FACTORS AFFECTING FRACTURE TOUGHNESS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69FATIGUE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73THE FRACTURE MECHANICS APPROACH TO FATIGUE . . . . . . . . . . . . . . . . . . . . . . . . . . 75

CHAPTER V - MAKING METALS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81VIRGINS AND ORES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81STEEL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83NICKEL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

CHAPTER VI - SURVEY OF METALS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99THE UNIFIED NUMBERING SYSTEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100CARBON AND LOW ALLOY STEELS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101STAINLESS STEELS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106NICKEL BASE ALLOYS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112ALLOYING ELEMENTS OF NICKEL BASE ALLOYS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113HEAT TREATING NICKEL BASE ALLOYS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115SOME COMMON NICKEL BASE ALLOYS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116BITS AND PIECES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

CHAPTER VII - FORGING, CASTING, & POWDER METALLURGY . . . . . . . . . . . . . . . . . . . . . 129FORGING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129FORGING DEFECTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137ADVANTAGES AND LIMITATIONS OF FORGING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138CASTING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139CASTING DEFECTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145ADVANTAGES AND LIMITATIONS OF CASTING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147POWDER METALLURGY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148

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ADVANTAGES AND DISADVANTAGES OF CONVENTIONAL POWDERMETALLURGY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150

HOT ISOSTATIC PRESSING (HIPing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151HIPing AS A FORMING PROCESS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151HIPing AS A CLADDING PROCESS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153ADVANTAGES OF FORMING BY HIPing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155ADVANTAGES OF CLADDING BY HIPing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156LIMITATIONS OF HIPing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157

CHAPTER VIII - NONDESTRUCTIVE EXAMINATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161RADIOGRAPHY (RT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162

RT - General Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162RT - Radiation Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164X-ray Film . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168RT - Imaging and Image Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171RT - Advantages and Disadvantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177

ULTRASONIC EXAMINATION (UT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178UT - General Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178UT - Physics of Wave Motion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179UT - Transducers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186UT - Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194UT - Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198UT - Calibration and Standard Reference Blocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201UT - Advantages and Disadvantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202

MAGNETIC PARTICLE EXAMINATION (MT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203MT - General Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203MT - Materials That Can Be Examined . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206MT - Types of Magnetic Particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206MT - Magnetizing Currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208MT - Magnetizing Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211MT - Odds and Ends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215MT - Advantages and Disadvantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216

LIQUID PENETRANT EXAMINATION (PT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217PT - General Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217PT - Types of Penetrants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218PT - Developers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220PT - Liquid Penetrant Examination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221

PROCESSING FLOW DIAGRAMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221PT - Advantages and Disadvantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221

CHAPTER IX - SPECIAL PROCESSES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229ELECTROPLATING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229ELECTROLESS NICKEL PLATING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233THERMAL SPRAY COATINGS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234PHOSPHATE COATING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239FLAME HARDENING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240INDUCTION HARDENING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241NITRIDING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241CARBURIZING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243BORIDING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .244

CHAPTER X - WELDING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249WELDING PROCESSES - GENERAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249

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ARC WELDING PROCESSES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250OXYFUEL GAS WELDING (OFW) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257ELECTRON BEAM WELDING (EBW) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257WELDMENT TERMINOLOGY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259WELD DEFECTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262WELDING METALLURGY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264CORROSION RESISTANT WELD CLADDING PROCESSES . . . . . . . . . . . . . . . . . . . . . . . 267HIP CLADDING VS WELD CLADDING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269BRAZING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270BRAZING METALLURGY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271

CHAPTER XI - CORROSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275THE BASICS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276ELECTROCHEMISTRY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 280TYPES OF GALVANIC CELLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287POLARIZATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292PASSIVITY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292TYPES OF CORROSION DAMAGE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294SULFIDE STRESS CRACKING (SSC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 298BACTERIA AND CORROSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 298CO CORROSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2992CORROSION PREVENTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300CHEMICAL ADDITIVES USED IN PETROLEUM PRODUCTION . . . . . . . . . . . . . . . . . . . . . 301MARINE CORROSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302THE MARINE ENVIRONMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303SEAWATER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305FORMS OF MARINE CORROSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307PROTECTIVE MEASURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311CATHODIC PROTECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312ISOLATING THE EQUIPMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321INHIBITION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337SELECTING INHERENTLY CORROSION RESISTANT MATERIALS . . . . . . . . . . . . . . . . . 339ADDING A CORROSION ALLOWANCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 339ALTERING THE ENVIRONMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 340

APPENDIX A - GLOSSARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345APPENDIX B - ABBREVIATIONS & SYMBOLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 389APPENDIX C - REGISTERED PRODUCTS REFERENCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393APPENDIX D - BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 395APPENDIX E - A CHECKLIST OF FACTORS TO BE CONSIDERED WHEN SELECTING A

MATERIAL FOR MARINE ENVIROMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 397APPENDIX F - FACTORS TO CONSIDER WHEN SELECTING A

COATING SYSTEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 399APPENDIX G - TRIM SELECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 401INDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 411

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AN INTRODUCTION TO METALLURGY

FORWARD

Aaaaaaaaaaaaarrrrrrrrgggggghhh!!!!

How often have we heard that plaintive cry just as some production controlplanner got the word from one of the heat treat foreman that his 410 stainless steelbody cracked in half during heat treat and, although it can't be used to build a valveanymore, the halves would make great matching bookends? How many times has themonth's business been bushwhacked by parts made out of 4130 material that punchedOK at heat treat only to crater at final hardness testing after machining? What hascaused the cancerous growth of our material specifications that once averaged onlyone page long, but are now pushing 15? Fracture toughness, VAR, K , tensileIcstrength, hardness, stringers, J-integral, cracks, laps, elongation, banding, calciumtreating, reduction of area, ESR, macro etch, micro etch, heat testing, heat per heattreat lot testing, equivalent heat per heat treat lot testing, lateral expansion, QTC, PSL 6there seems to be no end to this parade of potential alligators that are stomping out ofthe swamp in our direction with a mean and hungry look in their eye!

All of us in Cameron are well aware of the tremendous impact that materialshave on our business. We've all seen customer requirements become increasinglycomplex. We've all seen what we thought were well planned projects get stopped deadin their tracks after having been unexpectedly snake bit by a material problem at themost inopportune moment. Are you confused by this metallurgical madness? Don't feelbad, I'm confused too and I'm a metallurgical engineer by trade. How is the averageperson in Cameron who is deficient in metallurgical acumen (but is otherwise a welleducated, respectable, and personable individual) to cope with these metal relatedproblems and avoid becoming part of the gators' cuisine?

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A. Make like a tree and leaf!B. Ignore them and maybe they'll go away.C. Accept the inevitable and hope that they at least get indigestion.D. Hope they just ate and are too full for more than a nibble or two.E. Read An Introduction to Metallurgy by your humble servant.

The correct answer, of course, is E (for those of you who picked A, B, C, or D, Ican only wish those rascally reptiles bon appetit!). Perusal of this slender volume willnot make you a metallurgist. After reading it, you probably won't be able to select anappropriate material for all the different services that our equipment may see. Youprobably won't be able to specify a specific heat treatment or perform a failure analysis.But you will have a good, basic understanding of the fundamentals of metallurgy. Youmay not have all the answers, but you will be able to ask the right questions and beable to raise a flag when you come across a potential problem.

You have within the covers of this book a potent weapon, a hefty club, that whenexpertly wielded and judiciously applied, is a powerful persuader for getting thosegators to do a 180 and slink back to the swamp where they belong.

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A NOTE ON THE TEXT

Every effort has been made to ensure that the text of AN INTRODUCTION TOMETALLURGY

is complete and as accurate as possible. If you would like to see someother area of metallurgy covered in a future edition that is not addressed in the presentone or if you have any comments regarding the text please let me know so that the nextedition can better serve your needs. I can personally vouch for the absolute perfectionof the original draft of this book. Cecilia's word processing was flawless. The text, likeCaesar's wife, is above reproach. Any typographical errors or technical inaccuraciesthat may have crept into the finished book could only have done so through theslovenliness of the printer. If you find any discrepancies, please let me know so thatthey can be corrected. The printer will be drawn and quartered and our future businesstaken elsewhere.

Jim DufourCameronP. O. Box 1212Houston, Texas 77251-1212Phone: (713) 939-2141

CHAPTER I THE BASICS

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Simply Amazing!

A metallurgist is a practitioner of the ancient and honorable profession ofmetallurgy. He is concerned with the various methods of manipulating a metal'smicrostructure in order to obtain the most desirable properties. Metallurgy is as muchan art as it is a science: at times it may seem to border on the supernatural. Metallurgyis an extremely powerful tool that should not be given into the hands of theirresponsible. It should be regarded with a mixture of reverence and awe, admiration aswell as trepidation. Let us embark upon our journey into the realm of metallurgy byexamining the metallurgical magnificence of steel. The lessons we learn will beapplicable to all metals.

Steel is essentially an alloy of iron and carbon. An alloy is a metallic substancethat consists of at least two elements, at least one of which is metal. Carbon may bepresent in a steel in amounts of up to 2%, but most commercial steels will not havemore than 0.5%. If the carbon content exceeds 2%, then the alloy is classified as a castiron. Other elements may be present in varying amounts either as intentional additionsused to enhance properties or as impurities.

If you look at a piece of polished and etched steel under a microscope, you'llnotice several things. First is that it has a crystalline structure. The size, shape, andorientation of these crystals, or grains as they are more commonly called, play animportant role in determining a steel's properties. The second thing that you'll notice isthat not all the grains look alike. This is because each grain is composed of one ormore phases. Different phases have different properties, consequently, themacroscopic (overall) properties of steel are strongly dependent upon the type and therelative amounts of phases present. There is a third factor that influences a steel'sproperties, but this one you may not be able to detect under a microscope: the amountof cold work. We're going to examine each of these factors in some detail; how theyaffect a metal's properties and how they can be controlled so that we can obtain theproperties we want in a metal. We must first, however, become familiar with crystals.

CRYSTALS

A crystal is a group of atoms that has a particular arrangement that is repeatedover and over again in three dimensions. The smallest repetitive volume of a crystal isknown as the unit cell. The particular arrangement of atoms in a unit cell is called thecrystal lattice.

Metals are usually polycrystalline, that is, they consist of a multitude of smallcrystals rather than a single large one. The junction between two crystals is called agrain boundary. Grain boundaries are what delineate the size and shape of anindividual crystal.

Pure iron may form one of two crystal structures depending on temperature: body-centered cubic (BCC) or face-centered cubic (FCC). The lattice of each crystalstructure is illustrated in Figure 1.

ABody Centered Cubic

(BCC)B

Face Centered Cubic(FCC)

N2n1


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Figure 1: Crystal Structures of Iron

Iron at room temperature has a BCC structure. If we heat the BCC iron, or alphairon as it is called, to 1674(F the iron atoms will rearrange themselves into a FCCstructure called gamma iron. Gamma iron is the stable form of iron up to 2541(F. At thistemperature the iron atoms revert back to the BCC structure. The BCC iron at thistemperature is referred to as delta iron in order to distinguish it from BCC iron at roomtemperature. Delta iron melts at 2800(F.

Metals, such as iron, that have different crystal structures over differenttemperature ranges are known as allotropic. As might be surmised, the properties of anallotropic metal are directly related to the particular crystal structure that happens to bestable at the temperature of interest. Vastly different properties can be obtained inthese types of metals merely by changing the temperature a few degrees so that themetal goes into a temperature range where a new crystal structure appears.

GRAIN SIZE, SHAPE, AND ORIENTATION

Grain size is often denoted by an ASTM grain size number. The ASTM(American Society for Testing and Materials) grain size number, n, is obtained from theformula:

Where,N = the number of grains observed per square inch when the metal is viewed

under a microscope with a linear magnification of 100Xn = ASTM grain size number

Note that the larger the grain size number, the more grains there are per squareinch, consequently the smaller the grains. A "fine" grain steel has an average grain sizenumber of five or higher. A grain size number of five corresponds to 160,000 grains persquare inch.

As a general rule of thumb, virtually all the mechanical properties of a metal atroom temperature increase as grain size decreases. We can control grain size in a


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metal by regulating the heat treatment and the amount of working it receives, and byspecial alloy additions.

One way we can minimize grain size is to cool the molten metal as rapidly aspossible. This causes a greater number of nucleation sites (places where crystals startto grow) to appear in the molten metal thus increasing the number of crystals anddecreasing their size. Grain size will increase when a metal is held above a certaintemperature, and we can prevent grain growth simply by avoiding temperatures greaterthan this. Another way of inhibiting grain growth is by adding alloying elements such asaluminum or vanadium. These elements form hard compounds that will "pin" the grainboundaries in place or act as innoculants that form additional nucleation sites.

Once you have a certain average grain size in a metal, there are only two waysto refine the structure (decrease the grain size): through a phase transformation or byrecrystallization.

We'll discuss grain refinement by phase transformations a little later, but rightnow let's talk about recrystallization. If we take a sledge hammer and pound on a chunkof steel for an hour or so, two things will happen: 1) the steel will be internally strained,and 2) we won't be able to move our arms again for a week. We, in effect, mashed thecrystals so that they've become distorted and broken. Note that we have not made thegrains any smaller. We greatly increased the internal energy of the structure.

If we now heat the material up to an elevated temperature and hold it for a periodof time, new, strain-free grains will start to form at many different sites along the oldgrain boundaries. These new grains will be smaller than the old ones as long as wedon't hold the metal too long at temperature so that excessive grain growth occurs.These new, strain-free grains formed because the structure was in an unstable state atthat temperature because of its high internal energy. It lowered its internal energy whenthe atoms rearranged themselves into new grains, thus returning to a more stablecondition. This phenomenon is called recrystallization. The temperature that recrystal-lization occurs at, after a specified holding time, is the recrystallization temperature.This temperature is generally around one-third to one-half that absolute meltingtemperature in degrees Rankine or Kelvin ((Rankine = (F + 460( and (Kelvin = (C +273() of a metal, but varies with the amount of internal energy of the material.

Grain size, shape, and orientation are all closely related because obviously allthe grains in a metal must interlock so that all space is completely filled. Of the threefactors, grain size has the greatest influence on a metal's mechanical properties. Theshape of a grain can affect some of a metal's properties such as its formability. Grainsmay be spherical, columnar, plate like, dendritic (shaped like a tree), or any number ofother forms. The orientation of grains in most metals is random (orientation here refersto how an individual grain is situated in space, not the grainflow that occurs in formingoperations). It is sometimes desirable to orient the crystal lattices of the grains in ametal's microstructure in a common direction, or a preferred orientation. This orientationcan be brought about by mechanically working the metal in a certain manner and byspecial heat treatments. Special kinds of steels for electrical applications are oftenmade with preferred grain orientations because the electromagnetic properties of grainsare very directional.


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Figure 2: Cold Work

COLD AND HOT WORK

Cold working is deforming a metal below its recrystallization temperature whilehot working is deforming it at a higher temperature. By cold working a material, youintroduce a considerable amount of strain into its structure. The more strain you put intothe crystal lattices, the harder it will be for the atoms to relocate in the deformationprocess. As a consequence the material will become harder and stronger, but also lessductile. Let's do an experiment.

Crush an empty beer can in the middle (see Figure 2). We can bend it in halflengthwise and then straighten it out again without too much difficulty. If we keep onbending it and then straightening it, in time we'll notice that it gets harder and harder tobend the can: the portion of the can that has been bent has gotten stronger. If we keepon bending and straightening the can, it will eventually crack and then completelybreak, indicating a loss of ductility. NOTE: Do not try this experiment with bottled beer!

Hot working takes place above the recrystallization temperature, consequently,any strain induced in the crystal structure will be removed as the distorted grainsinstantly recrystallize. Strain does not accumulate in the metal as with cold working andthe metal will not be strengthened. This is why most forming operations such as rolling,extruding, and forging are done above the recrystallization temperature. Here the metalcan be shaped with a minimum amount of energy and without worry of cracking it.

Metals are often hot worked first and then cold worked. The hot work is done tomove the bulk of the metal into roughly the desired form. It is then cold worked into thefinal form, thus strengthening the metal.

PHASES

A phase is a portion of a pure metal or an alloy that is chemically and physicallyhomogeneous and has a distinct boundary. An understanding of phases is essential toan understanding of the behavior of metals. By regulating the type and relative amountsof phases present on a microscopic level, we can tailor the macroscopic properties of ametal to a specific usage. Two different phases may be chemically identical and differ


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only in crystal structure (e.g. alpha and gamma iron) or, conversely, may have identicalcrystal structure and differ only in chemistry. This is getting complicated. Better go grabanother beer for a backup before reading on. And cheer up, you're already familiar withphases! An example should make this clear.

Consider an ice cube floating in a glass of water. Here we have a two phasesystem: a solid and a liquid, both having the same chemistry, but differing radically intheir mechanical properties. Each of the phases is homogeneous, that is, any givenportion of the liquid is identical to the remainder of the liquid and any part of the solid isidentical to the rest of the ice cube. There is clearly a distinct boundary between the iceand the water. We can control the relative amounts of both phases at a particularinstant in time by either heating or cooling the system.

Iron is somewhat more complicated than our glass of water. Each of the differentcrystal structures represents a different phase with each phase having differentproperties. The rearrangement of atoms at 1674(F (BCCFCC), at 2541(F(FCCBCC), and at 2800(F (BCCliquid) represents a phase transformation.

Let's go back to our glass of water, assume the ice cube has melted, and add ateaspoon full of salt. Here we're adding another dimension to our system, that ofchemistry. If we stir the mixture long enough, all the salt will dissolve (go into solution).How many phases do we have now? Because any given portion of the salt solution isphysically and chemically identical to the remainder, we still have just one phase. If wekeep adding salt to the solution, eventually we will reach a point where no more salt willdissolve. We now have two phases: a liquid phase consisting of a salt solution, and asolid phase consisting of granules of salt. If we heat the water, we can get more salt todissolve. We have thus learned about two ways of controlling the types and relativequantities of phases. We can change the chemistry or the temperature of the system.

Steel, as we have already mentioned, is an alloy of iron and carbon. At veryminute concentrations, carbon atoms can occupy the spaces in between the iron atomsin alpha iron without distorting the crystal lattice. Alpha iron that contains carbon atomsis called ferrite. Ferrite is a very soft and ductile material. The solubility of carbon atomsin gamma iron is much greater than in alpha iron because the interatomic spacing ofthe iron atoms is larger. Gamma iron that contains carbon atoms is referred to asaustenite. Most forging and rolling operations are done while steel is in the austeniticrange because it's easily worked there. But just as salt has a limited solubility in water,carbon has a limited solubility in iron, regardless of the crystal structure or temperature.If the carbon exceeds this limit, a second phase must be formed. The excess carbonatoms will combine with some of the iron atoms to form cementite. Cementite (or ironcarbide) has a stoichiometric formula of Fe C and is a very hard, brittle substance. A3combination of ferrite and cementite has significantly better mechanical properties thaneither phase by itself.

So far we have learned (hopefully!) that the phases present in a carbon steel aredependent on temperature and carbon content. The iron-carbon equilibrium diagram, orphase diagram, indicates which phases are stable for any given combination oftemperature and carbon content (see Figure 3). Equilibrium means that we allowenough time during heating or cooling for any possible reaction to completely finish.


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Let's discuss what happens to a molten steel containing 0.30% carbon as weslowly cool it to room temperature. Referring to Figure 3, we see that the steel remainsa liquid until the temperature reaches approximately 2750(F at point A. Here the liquidstarts to solidify into delta iron. From point A to point B (2723(F) we have a two phasesystem: particles of solid delta iron immersed in a liquid. At point B, all the delta iron(BCC) transforms into austenite (FCC). More and more liquid solidifies as we approachpoint C (about 2650(F), but now it solidifies directly into austenite rather than delta iron.At point C, the liquid has completely disappeared and we have a one phase systemcontaining austenite only.

Decreasing the temperature further, we remain in one phase region until wereach point D, about 1510(F). Here the austenite begins to transform into ferrite (BCC).Once again we have a two phase mixture. At point E (1341(F), the remainder of theaustenite will transform, but because the solubility of carbon is so much less in ferritethan in austenite, the ferrite that forms will not be able to accommodate all the carbonthat the austenite held. As a consequence, the remaining austenite decomposes into amixture of ferrite and cementite (iron carbide).

The horizontal line at 1341(F is called the eutectoid isotherm and point F is theeutectoid point. A eutectoid point is where a single solid phase (in this case austenite)transforms isothermally (at a specific, constant temperature) into two different solidphases (in this case ferrite and cementite). Point G is the eutectic point. A eutectic pointis where a liquid transforms isothermally into two different solid phases (austenite andcementite in this instance). The horizontal line at 2098(F is the eutectic isotherm.

When we cooled our 0.30% carbon steel below point E, we obtained a mixture offerrite and cementite, but if we look at the microstructure under a microscope we will beable to see a difference in the ferrite that formed above 1341(F and that which formedat a lower temperature. The ferrite that formed from austenite above 1341(F is calledproeutectoid ferrite and we can see individual grains of this substance in themicrostructure. The austenite remaining just above 1341(F is very rich in carbon andcorresponds to the carbon content at the eutectoid point. When it transforms, it willproduce a mixture of about 88% ferrite and 12% cementite by weight. This mixture islamellar, that is, it's composed of alternating layers of ferrite and cementite. Theresulting micro constituent is called pearlite.


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Figure 3: Iron-Carbon Equilibrium Diagram

Figure 4: Microstructure of a 0.30% C Steel

Pearlite is not a phase, but a specific mixture of two phases formed bytransforming austenite of eutectoid composition into ferrite and cementite. The lamellarshaped structure of pearlite gives it many unique properties: the hard, brittle cementitereinforces the soft, ductile ferrite thus forming a natural composite. The properties ofpearlite can be changed by changing the spacings between the layers of ferrite andcementite through an appropriate heat treatment. At room temperature our 0.30%carbon steel has a microstructure that looks something like Figure 4.


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In the austenite to ferrite transformation, iron atoms must rearrange themselves.By cooling a metal slowly from an elevated temperature, as we did in the previousexample, we allow the iron atoms sufficient time to move and arrange themselves into adifferent crystal structure. Suppose now, that instead of slowly cooling a metal downfrom the austenite region, we cool it very rapidly or quench it. If we exceed a certaincritical cooling rate, depending on the thickness of the metal as well as its chemistry,the iron atoms will not transform from a FCC structure into a BCC structure as they didduring a slow cool. Instead they will form a body-centered tetragonal structure. Thisstructure is similar to BCC, except that it is not cubic: one dimension of the unit cell islonger than the other two. The iron atoms will instantly try to form a BCC structure. Thecarbon atoms in the austenite will not, however, have time to diffuse out before themetal temperature drops, consequently, they are locked in place. This is becausediffusion (or the movements of an atom) decreases rapidly as temperature decreases.

You'll remember that carbon has a low solubility in BCC iron because of thesmall distances between atoms in the BCC structure. The resulting structure of themetal we quenched will then be essentially a BCC structure that is distorted in order toaccommodate all the carbon atoms. This structure is called martensite. Martensite is ametastable phase. A metastable phase is one that exists in a nonequilibrium condition,but still will not change spontaneously as would most other nonequilibrium conditions.Martensite is a very hard, strong, brittle substance that is one of the chief hardeningagents in steel.

We have one more topic to discuss that involves phases: grain refinementthrough phase transformations. If we heat BCC iron to a temperature higher than1674(F, grains of FCC iron will begin to appear all along the grain boundaries of theBCC iron. The temperature of the metal remains constant while this is going on. Oncethe transformation is complete, we will have an entirely FCC structure, but with a finergrain size. The temperature of the metal can now begin to rise.

If we now cool the metal back down below transformation temperature, theprocess of the grain refinement will repeat itself, only this time it will be smaller BCCgrains that form. This is the basis of grain refinement of a steel by normalizing.Normalizing is one type of heat treatment and we'll discuss it in the next section.

DISLOCATIONS

Football players will more than likely experience at least one knee dislocationduring their professional careers. Trying to pat yourself on the back too often may resultin a severe dislocation of the shoulder. Watching a pretty blonde pass by in a speedingcar may cause a neck dislocation (with complications if you are accompanied by yourwife). Metals are also subject to dislocations, but of a different sort.

A dislocation in a metal is a linear defect of the metal's crystal structure.Dislocations are the primary reason why the actual tensile strengths (typically 10,000 -300,000 psi) of common engineering metals are so much lower than the theoreticalstrengths (1,000,000 - 3,000,000 psi) that are calculated from the stress necessary tobreak all the bonds between two adjacent planes of atoms. Ignoring those related to

AScrew Dislocation Movement

BEdge Dislocation Movement


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Figure 5: Dislocation Defects

Figure 6: Dislocation Movement

human anatomy, there are two basic types of dislocations: edge and screw (see Figure5).

An edge dislocation can be thought of as an extra plane of atoms pushed inbetween two existing planes of atoms in a crystal lattice. A screw dislocation is a littleharder to visualize at the atomic level. Figure 5(b) shows that a portion of the upper partof the lattice is displaced relative to the lower. This displacement of one atomic distancecauses the lattice planes of atoms near the dislocation to spiral around the dislocationline much like the threads of a screw hence the name screw dislocation.

Dislocations can move! Apply a large enough shear stress (see Figure 6) andthe dislocation (be it an edge or a screw) will move completely through the lattice. Notein Figure 6 that the top part of each lattice has been displaced by one atomic distancerelative to the bottom after the dislocation has passed through. The lattice is said tohave undergone slip. Dislocations tend to move on certain crystallographic planes inwhich the atoms are most closely packed together. These planes are known as slipplanes.

Do you understand all this? You do? Who are you trying to kid? After reading itthrough the first time? Hey, Einstein died back in 1955 so Einstein you're not. Youobviously didn't read it carefully enough or you're so lost that you don't know you're lost.Go back and read it again.

Dislocation movement is the primary factor why the actual tensile strength ofmetals is just a fraction of the theoretical strength. The shear stress required to move a


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dislocation through a lattice so that the upper part of the lattice is displaced by oneatomic spacing relative to the lower is much, much smaller than breaking all the atomicbonds between two parallel planes of atoms in a perfect crystal and then moving thetop half one atomic space from the bottom. A common analogy for why this is so is theeffort it takes to move a large, heavy rug a short distance across a floor. It will take agreat deal of work to grab an edge and just drag the rug the desired distance. It's mucheasier if you put a ripple in the rug near the edge and then merely push the ripple to theother side. The rug will have been displaced by a small distance. It takes much lesseffort to push the ripple across the rug and cause the displacement than it does to dragthe whole thing because only a small portion of the rug moves at any one time as theripple is pushed. A dislocation is just like that ripple in that only a small part of thecrystal lattice has to move at any one time in order to get a net displacement of theentire lattice.

The amount of deformation that a lattice undergoes when a dislocation passesthrough is only one atomic spacing. This by itself is insignificant. It has been estimated,however, that the dislocation density of a strain-free metal is 10 to 10 dislocations per6 8cm . Thus the cumulative effect of dislocation movements is very significant because of2the vast numbers involved.

There is obviously some distortion in the lattice adjacent to a dislocation. In otherwords, a dislocation has a strain field associated with it. This strain field is veryimportant because it can interact with the strain fields from other dislocations as well aswith grain boundaries, precipitates, foreign atoms, etc. These interactions may make itdifficult or impossible for a dislocation to move. In essence, a dislocation can becomelocked in place. The primary reason why the actual strength of metals is so much lowerthan the theoretical is that metals contain dislocations which are free to move. Impedethe movement of dislocations and you will increase the strength of a metal.

STRENGTHENING MECHANISMS

How can we keep those pesky dislocations from moving around and lowering thestrength of our metal? Through a combination of one or more of the followingmechanisms:

1. reduction of grain size2. cold work3. solid solution strengthening4. dispersion strengthening

We've already discussed the first two mechanisms. The strain field associatedwith a dislocation can interact with a grain boundary and prevent further motion of thedislocation. By reducing the grain size, we increase the total amount of grain boundaryarea thus increasing the interactions with the dislocation strain fields and strengtheningthe metal. Cold work induces a great deal of strain into the crystal lattice which caninteract with the dislocation strain fields and impede the movement of the dislocations.In addition, cold working will increase the number of dislocations. These dislocationswill not all form on the same slip plane. Under an applied stress, some of the

AInterstitial Solid Solution

BSubstitutional Solid Solution


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Figure 7: Solid Solutions

dislocations will move and eventually the strain field of one dislocation will bump intothe strain field of another and both dislocations will be locked in place. The dislocationshave become entangled.

The last two mechanisms are new so we'll look at them in detail. A solid solutionis a homogeneous mixture of two or more kinds of atoms in the solid state. The mostabundant atom is called the solvent and the least abundant the solute. Solute atomscan occupy one of two possible positions within the solvent atoms' crystal structure (seeFigure 7). The solute atoms may take the place of some of the solvent atoms within thecrystal structure. This is known as a substitutional solid solution. The other possibility isthat the solute atoms may occupy a site in between the solvent atoms in the crystal.This is known as an interstitial solid solution.

If the solute atoms are sufficiently large in either a substitutional or interstitialsolid solution, they will cause the solvent crystal structure to become distorted. Thestrain induced by the mismatch in atomic sizes can interact with dislocation strain fieldsthus preventing dislocation movement and strengthening the metal. Note that the soluteatom cannot be too large or else the solvent lattice will not be able to accommodatevery many and strengthening may not be achieved. The large solute atoms are, ineffect, outside the solvent lattice and therefore exert little influence on it.

Dispersion strengthening takes place as a hard, second phase is finelyprecipitated throughout a softer matrix of another phase. The hard precipitates act asbarriers to dislocation movement. They also introduce strain fields into the lattice thatcan interact with dislocation strain fields. Just as with solute atoms, the precipitatesmust not be too large or else strengthening will not be achieved. Dispersion hardeningis the chief strengthening mechanism for precipitation hardenable alloys.

By now you're probably thinking all kinds of nasty thoughts about metallurgy andmetallurgists. Despite what your intuition tells you, metallurgy is not a devil's inventionmeant to confound the innocent. It is, rather, a way of life, an elegant science that isworthy of many hours of quiet meditation and reflection. A firm grasp of its greatprinciples will break the chains that tether your mind to the commonplace and will allowit to soar. The great epochs of man are inextricably linked to the state of their metals'technology hence the Bronze Age, Iron Age, and the Age of Steel. Indeed the progressof our civilization may be said to have always followed the progress of metallurgy. Thisgives you, gentle reader, an awesome responsibility. Significant progress in metallurgy


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can only be achieved when the common man, such as yourself, has been exposed toits basic truths, understands their importance, and has a burning desire to preach themamongst the heathen (mechanical engineers, Texas A&M graduates, plant managers,etc.). Have you scaled Olympus? Have you entered the Elysian Fields? No? Then goback and diligently reread this chapter lest you be accused of shackling the progress ofcivilization. This chapter is the foundation upon which the rest of the course will be built.If, after several rereadings, you still have not attained Nirvana, then grab a beer beforegoing on, it's the next best thing.


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NOTES:


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NOTES:

CHAPTER II HEAT TREATMENT

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Hot Stuff!

If you managed to muddle through the last section, congratulations! You are wellon your way to becoming an honorary metallurgist. Soon you'll be able to amaze yourfriends, your spouse, and yes, even your kids with the breadth and depth of your newknowledge! Soon you'll be able to shovel your way through any snow-job that aCameron metallurgist throws your way! But not yet. First, we have to talk about heattreating.

Hopefully you remember that the properties of metals are determined by:

1. the size, shape, and orientation of grains,2. the types and relative amounts of phases present,3. the amount of cold working.

By controlling the above three items, we can produce the particular properties we wantin a metal. The best way to exert this control is through alloy additions and through heattreatment. In this section, we'll examine the heat treatments that are commonly used inmanufacturing our oil tool equipment.

We use a wide variety of metals in our products and, of course, many differentheat treatments. The specific heat treat procedure used in manufacturing a given partwill be dependent on the alloy involved, the size of the part at the time of heattreatment, the required properties, the available heat treating equipment, and anycustomer requirements. The vast majority of parts in our products are carbon, low alloy,or stainless steels or nickel base alloys. Carbon and low alloy steels are typicallyaustenitized, quenched, and tempered or normalized, austenitized, quenched, andtempered. Stainless steels, depending on the variety, may be annealed, age hardened,or austenitized, quenched, and double tempered. Nickel base alloys are typically usedin the annealed or age hardened condition depending on the specific alloy and therequired strength level. We'll examine each of these heat treatments.

ANNEALING

Annealing consists of heating a material to a suitable temperature, holding for aspecified time, and then cooling (usually at a slow rate). If this seems like somewhat ofa vague definition, that's because there are many different types of anneals. Thepurpose of annealing may be to:

1. soften2. stress relieve3. refine the grain size4. remove gases5. alter mechanical properties6. obtain a desired microstructure

The term full anneal, when applied to steel, involves heating the metal into theaustenitic region and then slowly cooling it (often the metal is left in the furnace and the


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furnace shut off). This produces a very soft, homogeneous microstructure free fromresidual stresses.

A solution anneal involves heating a metal up to a temperature sufficiently highfor a time sufficiently long to dissolve one or more microconstituents into a solid solutionwith the matrix material. The metal is then cooled rapidly enough to keep themicroconstituents in solution.

A process, or subcritical anneal is an in-process heat treatment used to softensteel that has been cold worked during a forming process. The steel is heated up to atemperature just below the start of the ferrite to austenite transformation and then helduntil the desired degree of softening has been attained through recrystallization andgrain growth.

Many alloys that cannot be substantially hardened through heat treatment areoften used in the annealed condition. Examples include Monel 400, Inconel 600,and 316 stainless steel. These alloys, as well as many other nickel base alloys,austenitic and duplex stainless steels, and certain other nonferrous metals, are typicallyannealed to provide a homogeneous microstructure, to stress relieve, to remove theeffects of cold work, and to provide the optimum microstructure for corrosion resistance.Following an anneal, parts may be air cooled down to room temperature or quenched inwater or oil to prevent the precipitation of undesirable phases.

NORMALIZING, AUSTENITIZING, QUENCHING AND TEMPERING OFSTEELS

Normalizing is a special type of anneal that involves heating a steel to atemperature about 100(F into the fully austenitic region, then removing the materialfrom the furnace and allowing it to cool in still air.

Normalizing accomplishes several things. First, it produces a fine grainedpearlitic structure thus enhancing the uniformity of mechanical properties. Second, itcan greatly improve notch toughness. Third, it removes some of the stresses that occurin the material during forging or rolling operations. And finally it homogenizes thematerial making it easier to austenitize in a subsequent heat treat operation.

The austenitizing operation consists of heating the material to a temperatureabout 50(F into the austenitic range and holding the material at temperature until thetransformation to austenite is complete. Once the material is fully austenitic, it is then"quenched." Quenching consists of removing the material from the furnace andimmersing it in an agitated fluid. This causes the material to cool rapidly. Water, oil, or apolymer solution are the fluids used for quenching low alloy steels. 410 stainless steelsare quenched in oil, polymer solution, or air depending on section thickness.

The quenching operation is used to harden the material. It does this through theformation of martensite from the austenite. This transformation will take place only if thecooling rate exceeds a certain limit. Because some of the material may not exceed the


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critical cooling rate (material near the surface of a forging will cool faster than thematerial at mid-wall or the center), some of the austenite may not form martensite. Theaustenite at locations receiving a subcritical cooling rate will transform into ferrite,pearlite, and/or bainite (we'll talk about bainite later on in this section) in varyingamounts, depending on the distance from the quenched surface. This is the reason whyparts made out of the same material, in the same heat treated condition, but withdifferent wall thicknesses may have vastly different properties. It also explains why thesurface hardness of a part may be much higher than the hardness just below thesurface.

The quenching operation will increase the hardness and strength of a steel, butwill drastically decrease ductility and notch toughness. As a consequence, thequenching operation for steels is always followed by another heat treat operation:tempering.

Tempering consists of reheating the quenched steel to a temperature below theaustenitic range, holding the material at temperature for a specified period of time, andthen cooling at a specified rate.

The purpose of tempering is to improve the toughness and ductility of thequenched material. It does this by causing the brittle, hard martensite to dissociate intoferrite and iron carbide. The resulting structure, called tempered martensite, is notlamellar like pearlite, but contains many fine particles of carbide dispersed throughoutthe ferrite. This gives the material the optimum combination of strength, toughness, andductility.

Tempering will reduce the strength of a material from that found in the as-quenched condition, however, it will considerably improve toughness and ductility whilestill maintaining a higher strength level than could be attained in the normalizedcondition. The final properties attained will be dependent on the tempering temperatureand the holding time at temperature.

In some highly hardenable alloy steels, such as 410 stainless steel, there is apossibility that not all of the austenite will transform during the quenching operation.Due to local variations in chemistry, there may be pockets of austenite that are quitehappy with the status quo and have no intention of altering their lifestyle merelybecause some metallurgist pours water or oil on them. This austenite that failed totransform during the quench is referred to as retained austenite and can be quitetroublesome. So, after the quenching operation, we may have a material that consistsof newly formed martensite interspersed with pockets of retained austenite.

In order to improved ductility and toughness, we must temper the material so thatthe brittle martensite will dissociate into ferrite and carbide. During this tempering cycle,some of the retained austenite may decide that being martensite isn't so bad after alland go ahead and transform. This fickleness on the part of the retained austenitecauses complications because now after tempering, our material has a structureconsisting of a strong, but ductile, tempered martensite and pockets of fresh martensite(transformed from the retained austenite) that are hard and brittle. We are going tohave to retemper the material in order to eliminate the pockets of fresh martensite. This


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is why a "double temper" is called out in many of our material specifications. Thesecond temper will always be at a lower temperature than the first so that we don't over-temper the bulk of the metal.

AGE HARDENING

Age hardening, or precipitation hardening as it is sometimes called, is a threestep heat treatment that is used to increase the strength and hardness of an alloy. Notall alloys can be effectively age hardened. We'll explain the reason for this as well asexamine each of the three steps a little later, but first, we have to look at a typical phasediagram of an age hardenable alloy (and you thought you were finished with phasediagrams after the first lesson!).

Figure 1 shows a phase diagram for elements A and B (at least one of which is ametal). Pure A has a stable phase of up to a temperature of T , while pure B is a 1phase up to a temperature of T .2

Remember the salt and glass of water in the first lesson? There we stirred ateaspoonful of salt into a glass of water. The water dissolved all the salt (i.e. the saltwent into solution) and we still had a one phase system. When we continued to add saltwe eventually reached a point where no more salt would dissolve: the solution hadbecome saturated. We then had a two phase system consisting of a liquid (the saltsolution) and granules of salt. By cooling the system, the solution would not be able tohold as much salt and some salt would have to come out of solution. In other words,there is a limit on the solubility of salt in water at a particular temperature and thissolubility decreases with decreasing temperature.

The phase in Figure 1 can only accommodate a certain amount of element Bat a given temperature (just as water can only dissolve so much salt) before a secondphase, , is formed. The amount of B that the phase can contain (the solubility limit!)increases up to a temperature of T . The solvus line in Figure 1 represents the solubility3limit of B in the phase. At temperature T , the phase can hold 1% B atoms. At a4higher temperature T , the phase can hold 2% B atoms in solution. Summing up,5there is a limit on the solubility of B in at any given temperature and this solubility limitdecreases with decreasing temperature. All age hardenable alloys exhibit this behavior,however, not all alloys exhibiting this behavior are age hardenable.

Enough of the preliminaries, let's get down to business. The three steps in agehardening are: 1) solution anneal, 2) quenching, and 3) controlled reheating (aging).We'll examine the heat treatment of an age hardenable alloy consisting of 98% A and2% B. At room temperature we have a two phase system: and .


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Figure 1: Age Hardening

1. Solution Anneal - The first step in age hardening is to heat the alloy upinto a one phase region so that all the will dissolve and the B atomsgo into solution. This requires that we heat our alloy to a minimumtemperature of T . We'll heat it up to T just to be on the safe side and5 6be sure all the B is in solution.

2. Quench - Once we've gotten all the B into solution at T , we're going to6quench the alloy to some temperature well below T , say T . The B5 4atoms will still remain in solution in the phase even though theamount of B atoms (2%) exceeds the solubility limit at T (1%). By4rapidly cooling the alloy to a low temperature, we have effectively"frozen" the B atoms in place before they had a chance to move. Wenow have a supersaturated solution.

3. Controlled Reheating - The final step in age hardening is the agingprocess itself. This consists of reheating the supersaturated solution toT , a temperature below T , holding for a period of time, and then cool-7 5ing. This causes some of the B atoms to come out of solution(precipitate out). These precipitates are finely dispersed throughout the matrix and introduce a great deal of strain into the crystal lattices.


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The extra strain in the crystal structure makes it difficult for dislocationsto move in response to an applied load, consequently the alloy ishardened and strengthened. Note that although the B atoms do form aprecipitate, this precipitate may not necessarily have the samestructure as the phase.

The aging time and temperature determine how much aging, or hardening,actually takes place. We can overage the alloy by holding it for too long a time at theaging temperature or by using too high an aging temperature. This causes theprecipitate particles to agglomerate and grow, resulting in a decrease in hardness andstrength.

T-T-T DIAGRAMS

In Chapter I we discussed what happened to a carbon steel that was slowly andcontinuously cooled through the austenitic region down to room temperature. You'llrecall that we ended up with a mixture of ferrite and pearlite. Will austenite alwaystransform to ferrite and pearlite? Obviously a loaded question. The answer is no.We've already talked about one case - the formation of martensite - but there are othersas well.

One of the chief means of studying the decomposition of austenite is through themetallographic examination of isothermal transformation products. As an example, if wequickly quench a small test specimen that has been austenitized down to a temperaturewhere austenite is unstable and then hold the sample at that temperature, eventuallythe austenite will transform into something else (isothermal here refers to the fact thatwe hold the austenite at a constant temperature until it transforms). We can thenquench our sample down to room temperature, examine it under a microscope, and seewhat the austenite decomposed into.

Suppose we take a large number of small test specimens of a carbon steelcontaining 0.64% carbon and 1.14% Manganese (AISI 1566) and heat them up into theaustenitic region, quench them to various temperatures, hold them at thosetemperatures for various lengths of time, and then quench them to room temperature.We can then look at each specimen under a microscope and determine whatmicroconstituents are present and in what quantities. This data will allow us to make aplot of the type and quantity of the austenite transformation products as a function oftransformation temperature and holding time. This type of plot is called a T-T-T diagram(the T's stand for time-temperature-transformation). It is sometimes referred to as anisothermal transformation curve. The T-T-T diagram for our AISI 1566 carbon steel isshown in Figure 2.

The first line (from the left) in Figure 2 shows where the austenite first starts todecompose after it has been quenched to a specific temperature. The dotted lineshows where 50% of the austenite has decomposed. The solid line on the right showswhere the austenite transformation is complete. The horizontal line marked Ac is the1temperature above which austenite first starts to form upon heating. The line markedAc is the temperature at which the transformation to austenite is complete. Figure 23


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Figure 2: AISI 1566 T-T-T Diagram

shows that for our carbon steel, austenite may transform into pearlite, martensite, orsomething we haven't talked about yet: bainite. We'll examine each of thesetransformations in some detail.

1. Pearlite - Assume we can instantaneously quench a small piece ofaustenitized 1566 carbon steel down to 1200(F (point 1 in Figure 2).After holding it at that temperature for about ten seconds, ferrite willstart to form along the austenite grain boundaries (point 2). This is theproeutectoid ferrite that we talked about in Chapter I. Proeutectoidferrite precedes the formation of pearlite in hypoeuctectoid steels(steel having less that the eutectoid carbon content of 0.77%). Inhypereutectoid steels (steels having more than the eutectoid carboncontent of 0.77%), however, the iron-carbon phase diagram in ChapterI tells us that some of the austenite will first transform to cementitebefore the remaining austenite transforms to pearlite.

As we continue to hold our specimen at 1200(F, more and more of theaustenite transforms into ferrite until we reach point 3 afterapproximately forty seconds. At this point, all the austenite that isgoing to transform into proeutectoid ferrite has done so. The remainingaustenite has become increasingly rich in carbon because of the


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Figure 3: Pearlite Formation

limited solubility of carbon in ferrite. Point 3 also marks the start ofpearlite formation.

Pearlite forms when iron carbide begins to nucleate out in theremaining austenite along grain boundaries. The subsequent growth ofthe carbides depletes the carbon content of the adjacent areas so thatin these areas the austenite transforms into ferrite. Further carbidenucleation will result in a pearlite "colony" having alternate layers ofcementite and ferrite. This process is illustrated in Figure 3. All theaustenite that did not transform into proeutectoid ferrite will havetransformed to pearlite by the time we have reached point 4. No furtherchanges in the quantities or the types of microconstituents in ourspecimen will occur after point 4. The final structure in our specimenconsists of proeutectoid ferrite and pearlite.

Note in Figure 2 that the lower the temperature at which pearlite forms,the "finer", or more closely spaced, the layers are. Fine pearlite isstronger and tougher than coarse pearlite.

2. Bainite - Assume we can instantaneously quench a small piece ofaustenitized 1566 carbon steel down to 800(F (point 5 in Figure 2).After holding our specimen for about 3 seconds at this temperature(point 6), ferrite will start to nucleate along austenite boundaries. Thelattices of the ferrite grains are coherent with the austenite matrix (thismeans that the lattices match, see Figure 4). As the ferrite forms, theaustenite adjacent to it becomes richer in carbon until it becomessaturated. At this point cementite will begin to precipitate out. Thecarbides form parallel to the longitudinal axis of the bainite "needle." The austenite will have completely transformed to bainite at point 7.No further transformations will occur past this point. The resultingstructure has a feathery appearance under the microscope and isknown as upper bainite. Figure 5 shows the formation of upper bainite.Austenite that isothermally transforms between the knee of the curveand approximately 660(F will form upper bainite.

Coherent Lattices Incoherent Lattices


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Figure 4: Coherency

Figure 5: Upper Bainite Formation

Austenite that isothermally transforms between approximately 660(Fand the line in Figure 2 identified "M " forms lower bainite. LowerSbainite forms when ferrite, supersaturated with carbon, forms alongaustenite grain boundaries. Again the ferrite is coherent with theaustenite matrix. Carbides will precipitate out within the bainiteneedles(see Figure 6). Lower bainite has an acicular (needle shaped)structure that is similar to tempered martensite.

The fact that the ferrite in both upper and lower bainite formscoherently within the austenite matrix distinguishes it from the ferrite inpearlite which forms incoherently. Bainite is considerably stronger thanpearlite because with coherency, perfect matching of the two lattices isseldom achieved. This results in considerable amounts of inducedstrain that will hinder the movement of dislocations under a load thusstrengthening the material. Bainite is hard and brittle in the as-quenched condition and consequently must be tempered.

3. Martensite - Martensite will form if we quench our 1566 carbon steelfast enough so that we miss the knee of curve and go down below the"M " (martensite start) temperature line in Figure 2. As we discussed inSChapter 1, martensite has a distorted body centered tetragonalstructure. The austenite is so unstable at these low temperatures thatit will instantaneously transform. Atomic diffusion is extremely slow atlow temperatures so martensite does not form through a nucleationand growth process like bainite and pearlite do. Instead the austenite


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Figure 6: Lower Bainite Formation

will transform in a shear reaction involving minimal atomic movement.The ferrite matrix that forms is distorted because it must accommodatethe extra carbon atoms that were in solution in the austenite (remem-ber that carbon has a much lower solubility in a BCC structure than aFCC). This distortion is accompanied by a considerable amount ofinduced strain which hardens and strengthens the material. Martensitehas an acicular structure.

Note in Figure 2 that the amount of austenite that transforms into martensite isdependent solely on the temperature to which we quench our austenite to. The percentof austenite that transforms is independent (unlike bainite or pearlite formation) of thetime that the quenched austenite is held at temperature. Because of this, martensiteformation is referred to as an athermal reaction, as opposed to the isothermal reactionsthat we have been talking about. The amount of austenite that transforms to martensiteincreases as we quench to lower temperatures until we reach the "M " (martensiteFfinish) temperature line in Figure 2. At this point all the austenite has transformed tomartensite.

We can use T-T-T diagrams to show the effect of alloying elements on the heattreat response of a steel. Our T-T-T diagram for AISI 1566 steel can be thought of asthree superimposed curves (see Figure 7A). By alloying with different elements, we canseparate these curves in order to make it easier to obtain the desired transformationproduct (see Figure 7B and Figure 7C).

Increasing the carbon, manganese, nickel, and silicon content of a steel willmove the pearlite and bainite curves further to right, but do not separate them to anyextent. Molybdenum, chromium, and vanadium move the pearlite curve up and furtherto the right while moving the bainite curve to lower temperatures. Figure 8 shows theseeffects for an AISI 4340 steel (nominal composition, 0.40% carbon, 0.80% manganese, 1.80% nickel, 0.25% molybdenum, and 0.80% chromium).


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Figure 7: T-T-T Diagrams

The significance of all this is that by moving the ferrite and pearlite curves to theright we have more time in which to quench our steel from the austenite region to thebainitic or martensitic region without having any of the austenite transform into thesofter proeutectoid ferrite or pearlite. This is desirable because the rate at which we canquench our steel is limited by the quenching medium and the size of the piece we arequenching. The farther the ferrite and pearlite curves are moved to the right, the moretime we have to quench our steel in order to obtain the desired martensite or bainite,the slower our cooling rate can be, and consequently the larger the piece of steel weare quenching can be and still obtain the desired microstructure. We're going toexamine this a little further later on.

T-T-T diagrams are also useful for illustrating the types of heat treatments per-formed on steel. Figure 9 shows an annealing or normalizing heat treat of our 4340steel. The steel is heated up into the austenitic region and then slowly cooled. This slowcooling will result in a microstructure of ferrite and pearlite.

Figure 10 shows an austenitize, quench, and temper heat treatment. Note thattwo cooling curves are shown: one for the surface and one for the center of the partbeing quenched. The differences in cooling rates can be appreciable depending on thesize of the part being quenched and the quenching medium. Ideally the part isquenched so that the bainite and pearlite curves are missed: only martensite is formed.After the part is cooled below M , it is heated up again for the tempering operation.F


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Figure 8: AISI 4340 T-T-T DiagramFigure 11 illustrates a martempering heat treatment. Here the part is rapidly

quenched down to a temperature just above M and held until the temperatures of theScenter and surface of the part become equalized. The part is then quenched into themartensitic range. Typically this "interrupted quench" is done by quenching and holdingthe part in a salt bath or martempering oil until the temperature equalizes throughoutthe part and then is air cooled into the martensitic range. The purpose of this heattreatment is to harden the material by forming martensite, but at the same time,minimize the distortion and residual stresses associated with the martensite trans-formation. We use this heat treatment for heat treating Colmonoy coated parts. It isalways followed by tempering.

Figure 12 shows an austempering heat treatment. Here we again quench oursteel down to a temperature just above M and hold it there until the austenite hasScompletely transformed into bainite. It is then air cooled. Like martempering, it is usuallyperformed in a salt bath. The purpose of austempering is to harden the steel by formingbainite. Although this will not give the same strength as martensite, bainite formationproduces much less distortion and residual stresses than martensite formation. As aconsequence, warping and quench cracking are minimized. Austempering is usuallyfollowed by a tempering operation in order to restore ductility.


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Figure 9:Anneal/Normalize

Figure 10:Austenitize, Quench, & Temper

EGADS! We're finished with T-T-T diagrams. Unfortunately T-T-T diagrams areof limited use in the real world of heat treating because they are based on theassumption that we can instantaneously quench a piece of steel down to a desiredtemperature and also because most heat treatments do not involve isothermal


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Figure 11:Martemper

Figure 12:Austemper

reactions. But let's not be too disparaging about T-T-T diagrams. They have servedtheir purpose well in helping us examine the decomposition of austenite.

To accurately predict the heat treat response of different steels, having differentsizes, and quenched in different mediums, we are going to have to dive into continuous


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Figure 13:AISI 4140 CCT Diagram

cooling transformation diagrams. This, like our T-T-T diagrams, is anothertranscontinental topic that metallurgists like to wax rhapsodic about. So take a deepbreath before we submerge.

CONTINUOUS COOLING TRANSFORMATION DIAGRAMS

Continuous cooling transformation (CCT) diagrams show the changes that steelsundergo when quenched from the austenitic condition. There are two common ways ofpresenting this information. The first is a plot of transformation products as a function oftransformation temperature and cooling time (time after quenching). This is illustratedfor an AISI 4140 steel in Figure 13. Transformation temperature is always plotted on thevertical axis and the cooling time along the horizontal axis. Time is plotted on alogarithmic scale in order to make the diagram compact.

Suppose we want to determine what type of structure we would have in thecenter of 2" diameter bar of 4140 after austenitizing and then oil quenching. If wesuperimpose a plot of temperature versus time corresponding to the cooling rate at thecenter of a 2" diameter bar during an oil quench onto Figure 13 (curve A), we canpredict the final structure. In this case we see that the austenite begins to transform toproeutectoid ferrite at point 1 (after 20 seconds of cooling). By the time we havereached point 2 (after approximately 45 seconds), 5% of the austenite has transformedinto ferrite. At point 2, the remaining austenite begins to transform into bainite. The


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bainite transformation is complete by the time we reach point 3. We now have roughly5% ferrite, 55% bainite, with the remainder being untransformed austenite. Point 3marks the start of martensite formation. Virtually all the remaining austenite willtransform to martensite by the time we reach room temperature so our final structureconsists of roughly 5% proeutectoid ferrite, 55% bainite, and 40% martensite.

A second way of presenting CCT diagrams is illustrated in Figure 14 for an AISI4130 steel. Here the horizontal axis represents the size of bar being quenched. Thereare three scales: one for each type of quenching medium. Again the horizontal scalesare logarithmic to keep the diagram compact. The vertical axis is the transformationtemperature. This type of CCT diagram is a convenient means of determining themicrostructure in the center of a round bar that has been austenitized and then air, oil,or water quenched. Let's look at an example. Assume we take a 6" diameter bar of4130, austenitize it, and then water quench it. We can find out what the resultingmicrostructure will be by drawing a vertical line through the 6" diameter marker on thehorizontal axis for water quenching (line A in Figure 14). Roughly 30% of the austenitehas transformed into proeutectoid ferrite (as indicated by point 1), roughly 45% hastransformed into pearlite (as indicated by point 2 where 75% of the total austenite hastransformed), and the balance of the austenite has transformed into bainite.

CCT diagrams are extremely useful tools. We can use the type illustrated inFigure 13 to accurately predict what the microstructure (and consequently themechanical properties) of a steel part will be at any location within the part as long aswe know what the cooling rate is at that location. The type of CCT diagram illustrated inFigure 14 is useful for determining the maximum size bar in which we can develop thedesired mechanical properties throughout its entire cross section.

So far we have learned that in order for a steel to be strengthened (or hardened)though heat treatment, it must undergo several phase transformations. The desiredtransformation products for most of the types of steels that we deal with are martensite,bainite, or a combination of the two. Martensite is harder and stronger that bainite: bothare significantly harder and stronger that pearlite or ferrite. When we talked about thequenching operation, we stated that austenite will transform into martensite and/orbainite (thus hardening the steel) only if the cooling rate exceeds a certain critical value.This cooling rate is one indication of a steel's hardenability. Hardenability can bethought of as a measure of how easy it is for a particular steel to develop a givenhardness at a given location. A steel with low hardenability requires a faster cooling rate(a more severe quench) to attain t

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