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Annealing_ Stress Releiving_ Normalizing_ Hardening_ and Tempering

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					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

Tr  0.4Tm,      
                     K
Higher is the temperature – less amount of CW is
needed to start recrystallization
Critical CW – the amount when recrystallization
cannot happen
Higher is amount of CW- smaller is grain size, no
matter what was the temperature
Minor factors for recrystallization
 Pure metal
 If an alloy – host atom – solvent
                   foreign atom – solute
 Solute atoms inhibit dislocations motion, higher
 temperature is needed
 Insoluble impurities (oxides and gases) become
 nucleation sites and refine grains
 Smaller initial grain size will recrystallize easier –
 at less temperature and time
Grain Growth
If you keep the metal hot too long, or heat it
up too much, the grains become large
Usually not good
Low strength
Size of grains vs. temperature
 G
 R
 A
 I
 N

 S
 I
 Z
 E
     200      400     600
           Temperature, deg.C
  Microscope images show:




Cold rolled steel   recrystallized after Grain growth after
90% reduction       2 min.at 830°C       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˚C
 Temper 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.

				
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