IE 121 MetalIE 121 Metal

Asst. Prof. Dr. Oratai Jongprateep

Phases in iron-carbon alloy

Iron-iron carbide phase diagramIron-iron carbide phase diagram

(BCC) ↔ (FCC) ↔ (BCC) ↔ liquid

Review: Reaction in phase diagramReview: Reaction in phase diagram

Phases in Fe–Fe3C phase diagram

-ferrite - BCC

• Stable form of iron at room T (≤912°C)

• Max. solubility of C is 0.022 wt%

• Soft and relatively easy to deform

-austenite - FCC

• Stable at 727 ≤ T ≤ 1394°C)

• Max. solubility of C is 2.14 wt% at 1147°C

• Large interstitial lattice positions

• Not stable below the eutectic T unless

cooled rapidly

• Tough and ductile, suitable for hot working

Phases in Fe–Fe3C phase diagram

–ferrite - BCC

• Stable only at high T, above 1394°C

• Also has low solubility for C

• Tough and ductile

• Not too important technically

Fe3C (iron carbide or cementite)

• Metastable intermetallic compound: at room T- Fe3C

at 650 - 700°C slowly decomposes into -Fe and C (graphite)

• Hard and brittle

Properties versus amounts of phases

Development of microstructure in iron-

carbon alloys

Eutectoid composition

Pearlite occurs at GB of austenite

Hypoeutectoid composition

Proeutectoid ferrite network surrounding the pearlite colonies

White proeutectoid cementite network surrounding the pearlite colonies

Hypereutectoid composition

Isothermal transformation

diagram

T-T-T plot

Isothermal transformation

diagram (T-T-T plot) is generated

from percent transformation

versus logarithm of time

measurements

Pearlite

Ttransf just below TE

Larger T: high diffusion rate

Pearlite is coarser

Ttransf well below TE

Smaller T: diffusion is slower

Pearlite is finer

Mech. properties are different:

fine pearlite is stronger

Adherence /reinforcement effect

Dislocation barrier

Bainite Consist of ferrite and cementite

A structure intermediate between

pearlite and martensite

Nonlamellar eutectoid structure, occur

in form of needles or plate

Has very fine microstructure: TEM

No proeutectoid phase formed

Formation of banite is competitive

with pearlite, can’t transform from one

to another w/o reheating to austenite strips with long rods of Fe3C

Spheroidite Occurs when pearlite or bainite is

heated at T < TE for a long time (at

700°C for 18 hr)

Fe3C phase appears as sphere-like

particles embedded in a ferrite

phase

Driving force: reduction in Fe3C

phase boundary area

Lower strength/hardness compared

to pearlite due to adherence,

dislocation barrier effect

Mech. prop. of pearlite & spheroidite

Martensite Occurs from diffusionless

transformation of austenite

Very rapid quenching prevent diffusion of C

FCC austenite transforms to a body centered tetragonal (BCT) martensite

Has platelike or needlelike appearance

Time -independent transformation: represented by a horizontal line in TTT diagram

it is a function of Tquenched:

athermal transformation

Martentite needlesAustenite

60

m

Mech. Prop. of martensite Hardest, most brittle, abrasive

resistance

Carbon: interstitial solid

solution impede movement

of dislocation

More percent of carbon

more hardness

low toughness and

ductility

Eutectoid iron-carbon alloy and isothermal heat treatments

Tempered martensite

• Martensite heat below eutectoid T (250-650C )

• Fe3C particle in a matrix of ferrite

• Softer and more ductile

Dept of Mat Eng 22

• Effect of quenching medium:

Mediumairoil

water

Severity of Quenchsmall

moderatelarge

Hardnesssmall

moderatelarge

• Effect of geometry: When surface-to-volume ratio increases: --cooling rate increases --hardness increases

Positioncentersurface

Cooling ratesmalllarge

Hardnesssmalllarge

Quenching Medium & Geometry

Dept of Mat Eng 23

Cooling Ex.Cooling Ex.

(a)

(b)

(c)

(a) Rapidly cool to 350°C,

hold for 104s, quench to

Troom

(b) Rapidly cool to 250°C,

hold for 100s, quench

to Troom

(c) Rapidly cool to 650°C,

hold

for 20s, rapidly cool to

400°C, hold for 103s,

quench to Troom

Heat treatment of steel Heat treatment of steel

Dept of Mat Eng 25

Other Heat Treatment Process

Full annealing◦Plain carbon steel with low and moderate percent of

carbon◦Heated to 40oC above critical temperature and then

cooled in furnace◦Coarse pearlite soft and ductile and has small

uniform grainsProcess annealing

◦Stress relief ◦To restore its ductility, so the part can be worked

further into the final desired shape.◦Heated below eutectoid temperature (~ 550o C -650o

C )

Spheroidizing annealing◦Plain carbon steel with high percent of carbon◦Heated below eutectoid temperature and

cooled in furnace◦Spheroidite cementite in ferrite matrix soft

Normalizing◦Heated to the austenite region and then cooled

in air◦Fine pearlite with small uniform grain◦higher strength and toughness, but lower

ductility than full annealing◦Reduce compositional segregation in castings

of forging

Hardening of steel and Hardening of steel and alloyalloy

Dept of Mat Eng 29

• Hardenability : Ability to form martensite• Jominy end quench test : to measure hardenability.

• Hardness versus distance from the quenched end.

24°C water

specimen (heated to phase field)

flat ground

4”

1”

Hard

ness

, H

RC

Distance from quenched end

Hardenability

• Thermochemical treatments to harden surface of

part (carbon, nitrogen)

• Also called case hardening

• May or may not require quenching

• Interior remains tough and strong

Surface Hardening

• Low-carbon steel is heated in a carbon-rich

environment

– Pack carburizing - packing parts in charcoal or coke -makes thick layer (0.025 - 0.150 in)

– Gas carburizing - use of propane or other gas in a closed furnace - makes thin layer (0.005 - 0.030 in)

– Liquid carburizing - molten salt bath containing sodium cyanide, barium chloride - thickness between other two methods

• Followed by quenching, hardness about HRC 60

Carburizing

• Nitrogen diffused into surface of special alloy

steels (aluminum or chromium)

• Nitride compounds precipitate out

– Gas nitriding - heat in ammonia

– Liquid nitriding - dip in molten cyanide bath

• Case thicknesses between 0.001 and 0.020 in.

with

hardness up to HRC 70

Nitriding

• Carbonotriding - use both carbon and

nitrogen

• Chroming - pack or dip in chromium-rich

material - adds heat and wear resistance

• Boronizing - improves abrasion resistance,

coefficient of friction

Other case hardening

Dept of Mat Eng 34

• Particles impede dislocations.• Ex: Al-Cu system• Procedure: --Pt A: solution heat treat (get a solid solution) --Pt B: quench to room temp. --Pt C: reheat to nucleate small q crystals within a crystals.• Other precipitation systems: • Cu-Be • Cu-Sn • Mg-Al

Pt A (solution heat treatment)

Pt B

Pt C (precipitate )

Temp.

Time

300

400

500

600

700

0 10 20 30 40 50wt%Cu(Al)

L+L

+L

T(°C)

A

B

C

composition range needed for precipitation hardening

CuAl2

Precipitation Hardening (I)

Dept of Mat Eng 35

Precipitation Hardening (II)

Metal Alloy and Cast Metal Alloy and Cast IronIron

Dept of Mat Eng 37

Types of Metal AlloysTypes of Metal Alloys

Metal alloys

Ferrous NonferrousNonferrous

Steels Cast iron

Dept of Mat Eng 38

Phase Diagram of Iron-Phase Diagram of Iron-Iron CarbideIron Carbide

Ferrite

Austenite

Delta iron

Dept of Mat Eng 39

SteelsSteels Low Alloy High Alloy

low carbon<025. wt%C

med.

carbon-02506wt%C

high carbon

0. -614. wt%C

Uses auto struc.

sheet

bridges towers

press.vessels

crank shafts

bolts hammers

blades

pistons gears

wearapplic.

wearapplic.

drills saws

dies

high Tapplication

turbines furnaces

corrosion resistant

Example101043101040434010954190 304

Additions none Cr,V Ni, Mo

none Cr, NiMo

none Cr, V, Mo, W Cr, Ni, Mo

plain HSLA plain heattreatable

plain tool austentiticstainless

Name

Hardenability0 + + ++ ++ +++ 0

TS - 0 + ++ + ++ 0

EL + + 0 - - -- ++

increasing strength, cost, decreasing ductility

Dept of Mat Eng 40

+ Cr : to increase strength, to reduce corrosion + Mo : to improve the strength and hardness at high temp. + Ni : to improve toughness, good forming

Deep drawing

Dept of Mat Eng 41

Plain Carbon Steel and Low-Alloy SteelsPlain Carbon Steel and Low-Alloy Steels

Dept of Mat Eng 42

Plain Carbon Steel and Low-Alloy SteelsPlain Carbon Steel and Low-Alloy Steels

Dept of Mat Eng 43

Stainless SteelsStainless Steels

Ferritic S.S. : 14-27%Cr and very low C (<0.12%C)soft, good machinability, magnetic no hardenability

Austenitic S.S. : Cr, Ni addedgood for forming, not magnetic

Martensitic S.S. : 12-18%Cr and high C (up to 1.2%C)good hardenability, magnetic

Dept of Mat Eng 44

Designations,Compositions And Designations,Compositions And Applications for Stainless SteelsApplications for Stainless Steels

45

Cast IronsCast Irons

Gray Iron

Malleable IronWhite Iron

Nodular Iron

Dept of Mat Eng 46

Cast IronsCast Irons Gray cast iron :

◦ C in the form of graphite ◦ used in the engine block

Nodular cast iron : ◦ 3.5%C, 2.5%Si and Mg, Na, Ce, Ca, Li etc. ◦ give the nodular (spherical) graphite◦ good toughness for crankshaft, rocker arm and piston

White cast iron◦ If the cast iron has been quenched, the white cast iron

occurs which has high compressive strength and corrosion resistance

Malleable cast iron◦ because white cast iron is too hard will be annealed

Malleable cast iron

Dept of Mat Eng 47

Nonferrous Alloys

• Cu AlloysBrass: Zn is subst. impurity (costume jewelry, coins, corrosion resistant)Bronze: Sn, Al, Si, Ni are

substitutional impurity (bushings, landing gear)Cu-Be:

precipitation hardened for strength

• Al Alloys-lower : 2.7g/cm3 -Cu, Mg, Si, Mn, Zn additions

-solid solution or precipitation strengthened (structure

aircraft parts & packaging)

-very low :1.7g/cm3 -ignites easily -aircraft, missiles

• Refractory metals-high melting T -Nb, Mo, W, Ta• Noble metals

-Ag, Au, Pt - oxidation/corrosion resistant

• Ti Alloys-lower : 4.5g/cm3

vs 7.9 for steel -reactive at high T -space application

Nonferrous Nonferrous AlloysAlloys

• Mg Alloys

Ni-based SuperalloysNi-based SuperalloysHigh temperature heat-resistance

alloys (760-980oC), which can retain high strengths at elevated temperatures, good corrosion resistance and good oxidation resistance.

There are three types of Ni-base superalloys; nickel base, nickel-iron base and cobalt base.

Applications: ◦Aircrafts, space vehicles, rocket

engines◦Industrial gas turbines, ◦Nuclear reactors, submarines◦Steam power plants, petrochemical

equipment 4

8

Processing of Steel

Iron ore: Hermatite Fe2O3, Magnetite Fe3O4

Blast furnace

Pig Iron

Coke, limestone

Basic Oxygen furnace Cupola Furnace

Steel Cast Iron

Fe2O3 + 3CO 2Fe + 3CO2

4% of carbon along with other impurities

up to 1.2% of carbon 2-4% of carbon

Production of steel from iron ore Production of steel from iron ore

Refining steel from iron ore Refining steel from iron ore

Coke = reducing agent to

produce raw pig iron (contain

4% of C +other impurities)

Basic Oxygen furnace: pig

iron + 30% of steel scrap

Oxidizing the carbon and other

impurities

Cupola furnace: metal, coke

and flux

Steel: up to 1.2% of C

Cast iron: contain 2-4% of C

Processing-Fabrication

Forming operationsForming operations

Rolling Drawing

Forging Introduce plastic deformation by using mech. force

Types of forming

Hot: repeatable

Cold: good surface finishing, mech. prop., expensive

ExtrusionExtrusion

Rolling The ingots are heated and hot-

rolled into slabs, billets, or

blooms

The slabs are subsequently

hot- and cold-rolled into steel

sheet and plate

The billets are hot- and cold-

rolled into bars, rods, and wire

The blooms are hot- and cold-

rolled into shapes such as I

beams and rails

Thick metal sheet

Steel or metal casting

Molten steel is either cast in stationary mold or continuously cast into long slabs from which long sections are periodically cut off (called ingot).

Steel casting

Most popular High production rate

Ingot slab

Casting processes

Continuous casting

Die casting

Sand casting

Investment casting

Casting processes

Casting processes: advantages and limitations

Process Advantages Limitations

Sand Almost any metal is cast; no limit to size, shape or weight; low tooling cost.

Some finishing required; somewhat coarse finish, wide tolerances.

Investment Intricate shapes; close tolerance parts; good surface finish.

Part size limited; expensive patterns, molds, and labor.

Die Excellent dimensional accuracy and surface finish; high production rate.

Die cost is high; part size limited; usually limited to nonferrous metals; long lead time.

Powder met. & joining

Mostly used in metals with high Tmp and low ductility