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Annealing, Stress Releiving, Normalizing, Hardening, and Tempering of Steel
Chapter 10
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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.
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Steps for all HT (anneals):
1. Heating
2. Holding or “soaking”
3. Cooling
Time and temperature are important
at all 3 steps
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(Stress-relief)
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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
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Resulting microstructure:
For low-medium carbon steels – coarse pearlite and ferrite
It is easily machined
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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
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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
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Normalizing
Allows steels to cool more rapidly, in air
Produced structure – fine pearlite
Faster cooling provides higher strength than at full annealing
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Process Annealing – 3 stages
Recovery (stress-relief anneals)
Recrystallization (process anneals)
Grain Growth
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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.
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Stresses may result from:
Plastic deformation (cold work, machining)
Non-uniform heating (ex. welding)
Phase transformation (quenching)
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Stress-relief:
Is held at fairly low temperature
Is held for a fairly short time
So that recrystallization does not occur
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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
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TS and elongation
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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
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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
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How hot is hot?
Most metals have a recrystallization temperature equal to about 40% of the melting point
K,4.0 mr TT
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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
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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
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Grain GrowthIf you keep the metal hot too long, or heat it up too much, the grains become largeUsually not goodLow strength
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Size of grains vs. temperature
GRAIN
SIZE
Temperature, deg.C200 600400
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Microscope images show:
Cold rolled steel90% reduction
recrystallized after 2 min.at 830°C
Grain growth after 2min @ 930°C.
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Grain-Growth is not recommended mainly because:
Energy consumption
Need of expensive equipment
Large grain metals get surface distortion under tensile forces
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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.)
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3 stages of quenching
Vapor blanket
Vapor transport cooling
Liquid cooling
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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
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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
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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.
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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
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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.
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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.
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End product is 100% bainite
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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
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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
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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.