Annealing, Stress Releiving, Normalizing, Hardening, and Tempering of Steel

Chapter 10

Heat Treatment

In the process of forming steel into shape and producing the desired microstructure to achieve the required mechanical properties, it may be reheated and cooled several times.

Steps for all HT (anneals):

1. Heating

2. Holding or “soaking”

3. Cooling

Time and temperature are important

at all 3 steps

(Stress-relief)

Full Annealing

heats the steel to a temperature within the austenite (FCC, γ) phase region to dissolve the carbon. (50 deg.F above A3-Acm line)

The temperature is kept at the bottom of this range to minimize growth of the austenitic grains. Then, after cooling ferrite () and cementite structures will be fine as well

Resulting microstructure:

For low-medium carbon steels – coarse pearlite and ferrite

It is easily machined

Why hyperetectoid steels are annealed intercritically?

To prevent formation of brittle cementite network on the grain boundaries

This is undesirable condition if machining is to be done

Annealing is performed at temperatures between the critical lines A3,1-Acm

Spheroidizing – improving machinability

Used on steels with carbon contents above 0.5%

Applied when more softness is needed

Cementite transforms into globes, or spheroids

These spheroids act as chip-breakers – easy machining

Performed by heating to just below A3,1 line, holding there (about 20h.or more) and then slowly cooling

Normalizing

Allows steels to cool more rapidly, in air

Produced structure – fine pearlite

Faster cooling provides higher strength than at full annealing

Process Annealing – 3 stages

Recovery (stress-relief anneals)

Recrystallization (process anneals)

Grain Growth

Stress-relief Annealing

Heats the steel to just below the eutectoid transformation temperature (A1) to remove the effects of prior cold work and grain deformation.

This allows further forging or rolling operations.

Stresses may result from:

Plastic deformation (cold work, machining)

Non-uniform heating (ex. welding)

Phase transformation (quenching)

Stress-relief:

Is held at fairly low temperature

Is held for a fairly short time

So that recrystallization does not occur

Recovery (Stress-relief)

If you only add a small amount of thermal energy (heat it up at little) the dislocations rearrange themselves into networks to relieve residual stresses

Ductility is improved

Strength does not change

TS and elongation

Recrystallization

Add more heat and wait some more time, and new grains start to grow at the grain boundaries.

The new grains have not been strain hardened

The recrystallized metal is ductile and has low strength

How much time to wait?

Incubation period – time needed to accumulate stored energy from the lattice strain and heat energy

Then lattice starts to recrystallize

At first fast (lots of nucleation sites)

Slower at the end

How hot is hot?

Most metals have a recrystallization temperature equal to about 40% of the melting point

K,4.0 mr TT

Higher is the temperature – less amount of CW is needed to start recrystallizationCritical CW – the amount when recrystallization cannot happenHigher is amount of CW- smaller is grain size, no matter what was the temperature

Minor factors for recrystallization

Pure metalIf an alloy – host atom – solvent

foreign atom – soluteSolute atoms inhibit dislocations motion, higher temperature is neededInsoluble impurities (oxides and gases) become nucleation sites and refine grainsSmaller initial grain size will recrystallize easier – at less temperature and time

Grain GrowthIf you keep the metal hot too long, or heat it up too much, the grains become largeUsually not goodLow strength

Size of grains vs. temperature

GRAIN

SIZE

Temperature, deg.C200 600400

Microscope images show:  

Cold rolled steel90% reduction

recrystallized after 2 min.at 830°C

Grain growth after 2min @ 930°C.

Grain-Growth is not recommended mainly because:

Energy consumption

Need of expensive equipment

Large grain metals get surface distortion under tensile forces

Quenching media

Involves the principles of heat transfer

See procedures in ASM Metals Handbook

There are 9 possible choices (air, furnace, tap water, oil, brine etc.)

3 stages of quenching

Vapor blanket

Vapor transport cooling

Liquid cooling

What is important?

Improved cooling rate (dT/dt) to beat the nose of the S-curve

Agitate the quenchant – reduce the time spend at the vapor blanket stage

Chose the best fit of quenching media

Consider S/V ratio

Tempering (drawing)

Heating and holding steel below A1 line and slow cooling to room temperature (1 temper cycle)Done in the range 150-650˚CTemper brittleness should be avoided (loss of toughness at higher tempering temperature). Can be avoided by quenching from the tempering temperature

Martempering (Martquenching)Martempering permits the transformation of Austenite to Martensite to take place at the same time throughout the structure of the metal part. By using interrupted quench, the cooling is stopped at a point above the martensite transformation region to allow sufficient time for the center to cool to the same temperature as the surface. Then cooling is continued through the martensite region, followed by the usual tempering.

Special Tempering

Problem of retained austenite

That gives us untempered martensite

2 or 3 cycle tempering is a solution

That gives us total of tempered martensite

Different tempered martensites will have different hardness

Austempering

The austemper process offers benefits over the more conventional oil quench and temper method of heat treating springs and stampings that requires the uppermost in distortion control.

How to austemper?

Quench the part from the proper austentizing temperature directly into a liquid salt bath at a temperature between 590 to 710 degrees Farenheit.

Hold at this quench temperature for a recommended time to transform the Austenite into Bainite.

Air cool to room temperature.

End product is 100% bainite

Advantages of Austempering:

Less Distortion

Greater Ductility

Parts are plater friendly due to the clean surface from the salt quench

Uniform and consistent Hardness

Tougher and More Wear Resistant

Higher Impact and Fatigue Strengths

Resistance to Hydrogen Embrittlement

You should use the Austempering process if:

Material used: SAE 1050 to 1095, 4130, 4140

Material thickness between 0.008 and 0.150 inches.

Hardness requirements needed in between HRC 38-52

Limitations of Austempering:

Austempering can be applied to parts where the transformation to pearlite can be avoided.

This means that the section must be cooled fast enough to avoid the formation of pearlite. Thin sections can be cooled faster than the bulky sections.