physical metallurgy 21 th lecture ms&e 410 d.ast dast@ccmr.cornell.edu 255 4140

Physical Metallurgy21 th Lecture

MS&E 410

D.Ast

dast@ccmr.cornell.edu

255 4140

Content

Review: Plain Carbon Steel

Alloy steels

Influence of alloying elements

Influence of hydrogen (unwanted alloy element)

HSLA Steel specification

Steel properties

Appendix: Read for 4/17

1. Overview

a) Exceeds minimum alloy contents

b) Reasons for alloying elements

c) Nomenclature (AISI designation system)

d) Virtually always heat treated

2) Alloying Elements and their Effects in Steel

Common classifications

a) High Strength Low Alloy Steels

b) Low Alloy Steels

c) Chromium-Molybdenum Steels (Tool Steels)

Review : Carbon Steels

Only element > 1% is Mn.

P, S (except in electrical steels, Lect 20) are generally unwanted. Sulfur is sometimes added in controlled amounts to enhance machinability (embrittles) and P to increase hardening in particular during deep drawing (a cold working process where it can not move to GBs).

Cu increases the corrosion resistance and has some solid solution hardening

Mn, in plain Carbon steels, works mostly to keep sulfur bound up so the it can not form FeS particles

Manganese increases the field (Hadfield steel is austenitic thanks to 13 % Mn) and is a potent carbide stabilizer. A stabilizer is an element that stabilizes the presence of an other compound.

Role of Si: General impurity in Fe (from sand) used intentionally to deoxidize steel or to increase electrical resistivity (transformer steel)

Review: Classification

Alloy steels

• Alloy steels have compositions that exceed the limitations of C, Mn, Ni, Mo, Cr, Va, Si, and B set for carbon steels.

• Exception: steels containing more than 3.99% chromium are classified differently as stainless and tool steels.

Alloy steels are always killed, but can use unique deoxidization or melting processes for specific applications.

Alloy steels are generally more responsive to heat and mechanical treatments than carbon steels (you may recall that heat treating 1020 is generally not worth the cost).

Killed steels are steel that have been deoxidized by the addition of silicon or aluminium, before casting. (If you do not remember go back and revisit steel making, US and Europe, and continuing casting)

In this case there is no (significant) evolution of gas during solidification.

More generally, have a higher degree of chemical homogeneity and freedom from porosity

If a “B” shows up it means Boron containing.

If “H” shows up means fits particular specs for hardenability

Designation AISI

Alloy elements:

Added to make steel “better”. Can be

• Higher yield stress

• Higher ductility

• Higher hardness

• Better machinability

• Higher service temperature

• Better corrosion resistance

• etc….. application specific

Change in lower yield stress point, low alloy steels

Solid Solution Strengthener

P is effective but when it gets into the GB it embrittles.

See lecture 20.

N is introduced by nitriding steel

Elements that promote austenite, I.e. a larger field

• Ni

• Mn

Ferrite forming elements

• Cr, Si, Mo, W and Al. • Fe-Cr alloys containing more than 13% Cr are ferritic at all temperatures up to incipient melting.

Elements that lower Ms

Practically all ! If fully dissolved in phase

Ms = 561 - 474C - 33Mn - 17Ni - 17Cr - 21Mo

Carbide-forming elements

• Cr, W, Mo, V, Ti, Nb, Ta, Zr. Affinity for Carobon increases from left to right

Some overlap with ferrite promoters (no accident think Fe3C )

• Non Fe containing carbidesCr7C3 W2C, VC, Mo2C.

• Double carbides contain both Fe and Carbide former e.g Fe4W2C.

• High-speed tool steels usually three types of carbides, which are usually designated M6C, M23C6 and MC. M represents sum of metal atoms. I.e M6C can represent Fe4W2C or Fe4Mo2C; M23C6 represents Cr23C6 etc. For how to stabilize these carbides at high T see Lect. 20

Carbide stabilizers

The stability of the carbides depends on the presence of other elements in the steel. Chemical equilibrium between different carbids is measure by K . K is weight ratio of C in cementite vs C contained in the matrix. K values are

I.e. Mn will promote C to exist as Fe3C rather than being dissolved in the matrix. Thermo language would be that the activity of C is f(xi). Cr even more effective For that reason mallable cast iron (where you want the graphite to come out as globular C) can not contain any Cr.

Nitrides

Nitride former are similar C former

TiN is a well known ultrahard compound. AlN precips distorts lattice and generates high dislocation density

Nitrided surfaces are extremely hard

Nitrogen:

•Nitrogen can form a solid solution with ferrite at nitrogen contents up to about 6%.

• Above ~ 6% N, gamma prime ’ with a composition of Fe4N.

• Above ~ 8%, the equilibrium product is compound, Fe3N.

Nitrogen hardening is a “case hardening” process, producing a very hard thin layer at the surface. Ammonia, nitrogen plasma, or fluidized bed reactors are used

Hydrogen

An unwanted element that generates, in high strength steels, hydrogen embrittlement. Hydrogen diffuses in metals, in trap limited diffusion process.

The effect is due, depending on the situation

a) weakening of the Fe-Fe bonds at the tip of a (growing) crack. The decohesion model can be explained with the effective d electron concentration (reviewed in prelim)

b) reformation of H into H2 molecules generating interior pressure (do the thermo, pressure is tremendous)

c) Hydrogen combining with C to form CH4 again forming high pressure bubbles.

d) hydrogen atmosphere around dislocation

Source of hydrogen

• cathodic protection,

• phosphating, pickling, and electroplating (notorious in high strength fasteners that also need to be corrosion resistant.

• welding with electrodes covered with coating containing moisture

• hydrogen used to cool (electric generators), reducing “air” friction (fly wheels), fuel (hydrogen economy)

Low strength not susceptible

High strength steel very susceptible.

Destination of hydrogen Solubility of hydrogen is enhanced in triaxial tensile fields

Diffusion is trap limited as first discovered by H. Johnson (look into the MS&E lounge)

The classic example is failure of retaining rings in hydrogen cooled electric generators

Why megawatt electric generators use hydrogen cooling

• The specific heat capacity of hydrogen in constant volume is 10.25 J/(kg/°C)

• 14 times that of nitrogen

• 32.5 times more than one of argon.

• Additional Benefit: Low friction loss

The problem is now much better understood, but led to some spectacular failures in the 70’s

Note that crack growth rate increases by an order of magnitude when hydrogen is present

S-N (Woehler) curves

Note that fatigue limit falls to 1/5 of the value in air

Back to our alloy elements….

More than 3 elements.

General approach is to lump all the austenite formers together as “effective Nickel” and all the ferrite formers as “effective Chromium”. Below is UK (Cambridge U)• Ni equivalent = (Ni) + (Co) + 0.5(Mn) + 0.3(Cu) + 25(N) + 30(C)

• Cr equivalent = (Cr) + 2(Si) + 1.5(Mo) + 5(V) + 5.5(Al) + 1.75(Nb) + 1.5(Ti) + 0.75(W)

Different authors have different weighting factors The approach is popular in the welding industry to predict the phase in a weld. Schaeffler diagram.

NOT an EQUILIBRIUM DIAGRAM. But neither is the Fe-C phase diagram… as we discussed

Schaeffler Diagram (US)

HW 21-1

Indicate in the diagram the position occupied by a weld made in

a) 316 Stainless

b) 440 C Stainless

HSLA Steels

Total alloy content < 8 wt%Good combination of strength, formability, and weldability.

From the USS website

I could not find the composition of these steels !

THE END

Next: Auxiliary Material (read for 4/17)

Slides from ku.edu

Reading Material on effect of alloying

Source

They mean embrittlement !!! MS correction took over

Ac and Ar are the eutectoid temperature in the Fe-C phase diagram, as they appear during cooling and reheating.