6.2 Hardenability and Heat treatment for Steel

Definition of hardenability

ability of hardening steel→ how easy it is to form marsentite

How to judge hardenability with TTT diagrams

for the same cooling rate, judge TTT diagram of different compositions to which one allows 100% marsentite to form

How does hardenibility changes with alloying additions

addition of such alloy leads to very high hardenability AKA low cooling rate still leads to 100% marsentite formation

(C curve translate down)

What is a CCT diagram

Continous cooling diagram

unlike TTT, which consider isothermal cooling, CCT plots for continuous cooling

C curve shifts down and right

(dotted line-TTT, solid line-CCT)

Reason for difference of CCT and TTT C curve

thermal activation energy decreases as time goes on

→ forces material to transform as temperature drop, causing curve to drop and take a longer time

Effect of annealing

1. remove resdual/manufacturing stress

2. remove work hardening and restore ductility

3. soften steel

3 Steps of annealing

1. Recovery: dislocations entangles and DLCT density decreases

→oppositely oriented DLCT cancels out

2. Recrystallisation: new grains forms to form strain-free microstructure

→ strength decrease and ductility increases

3. grain growth

→ time-induced process that can be controlled to control final property

Condition of Temperature for process annealing to occur

Apply to pre-strained/work hardened auestenite that cools

Temp: around 100 degrees eutectic temperature

(refer to stress relief section of diagram)

Effect of process anneal in terms of property and microstructurely

remove work hardening and restore ductility

→ only ferrite recrystallise while cementite remains its stretched shape

Effect of full anneal

restore equilibirum microstructure of hypoeutectoid steel

microstructure become equiaxed again

Temperature for full annealing

• heated at austenite region and cooled quickly

Process of normalising

heat hypoeutectic steel at austenite region and cooled at a slowly (but faster rate than full aneal)

Effect of normalising

form finer equibilibrium of ferrite and pearlite

Process and effect of spheroidization

Process:

prolonged heating below eutectoid temp to change shape of cementite to spheres

→ temperature (thus driving force) high enough for cementite to from, but not grow lammenarly, therefore forming spheres

Effect: soften steel for easy machining

Process and effect of quenching

cooling quickly to form marsentite

Factors affecting end result of quenching

1. medium used (quenching speed: saltwater>water>oil)→ cracking could occur if quenching too fast

2. composition of steel (quenching rate plotted to different TTT used result in different microstructure)

3. size of component (inside cool slower than outside→ hardening varies in X section)

Reason for tempering after quenching

tempering improve poor property of steel it has after quenching

2 semi-quantitative methods to measure hardenability

1. Critical diameter/ruling section method

2. Jominy end-quench test

Principle of critical diameter method

Quench a solid steel cylinder and measure hardness across diameter

Critical diameter is when the minimum hardness value drop to 50% of the hardness at the end

Principle of Jominy end-quench test

Quench the bottom of one cylinder and measure hardness along the length x

→ could be used to predict size of specimen than hardens successfully

→ critical distance is when hardness is 50% of max hardness

2x_crit=D_crit

Process and effect of tempering

Process:

1. reheat quenched martensite at a not too high temperature ( around 500 degrees)

2. hold

3. quench again

effect: reduce brittleness and retain toughness

4 steps in tempering martensite

1. initial formation of fine ε carbide (Fe5C2) precipitate (<200 degrees)

2. any retained austenite is converted to bainite (<350 degrees)

→ austenite may be present due to CCT crossing the C curve

3. Fe5C2 covert to course Fe3C (cementite) (>500)

4. spheroidisation of Fe3C (over tempering)

Final marsentite phase has no carbon content anymore

Importance of choosing the correct temperature of heat treatment

No matter the carbon content, as temperature increases, all steel hardness reduces and converges to similar value

Principle of case hardening

process: impart hardened tempered maternsite on surface of component while keeping core tough

Method of case hardening of low carbon steel

1. heating in a C/CO2 rich atmosphere (T>1000 degrees)

→ carbon diffuses and absorb slowly into centre of steel

2. form martensite

Method of case hardening for high C steel

apply induction/localised heating

→ quench and temper surface regions to form case