Transcript METALS & MATERIALS

PART 2 : HEAT TREATMENT
ALLOY SYSTEMS
 STEELS
 ALUMINUM ALLOYS
 TITANIUM ALLOYS
 NICKEL BASE SUPERALLOYS
STEELS
 Annealing
 Normalizing
 Stress Relieving
 Hardening and Tempering
 Solution Treatment and Aging
IMPORTANT EQUILIBRIUM
PHASES IN STEELS
Ferrite (α )
…Body Centered Cubic (BCC)
Cementite (Fe3C) … Orthorhombic
Austenite ( γ )
…Face Centered Cubic (FCC)
δ Ferrite
…Body Centered Cubic (BCC)
BCC CRYSTAL MODEL
Packing Density- 68%
FCC CRYSTAL MODEL
Packing Density- 74%
BASIS FOR HEAT TREATMENT:
Fe-C PHASE DIAGRAM
FEATURES
SOLUBILITY OF C IN
a Fe(BCC)-O.O2%
g Fe(FCC)-2.11%
EUTECTIC REACTION
11480C, 4.3%C
EUTECTOID REACTION
7270C, 0.77%C
SIZE OF VOID SPACE
OCTAHEDRAL VOID SPACE
IN BCC
VOID SIZE IS 0.019 nm
SIZE OF C ATOM IS 0.07 nm
OCTAHEDRAL VOID SPACE
IN FCC
VOID SIZE IS 0.052 nm
SIZE OF C ATOM IS 0.07 nm
ISOTHERMAL TRANSFORMATION CURVE
HOW IS HARDENING DONE
1. Heat the steel piece to the specified austenitizing temperature and hold so that
the entire job achieves the specified temperature
2. Quench fast enough so as to avoid the knee to prevent formation of the high
temperature transformation products (pearlite, ferrite and cementite)
EMERGENCE OF MARTENSITE
FROM AUSTENITE
EFFECT OF CARBON
(a)
(b)
H
A
R
D
N
E
S
S
WT% CARBON
Effect of Carbon on
a) Hardness
b) Ms
HARDENESS DISTRIBUTION
(b)
Hardness,Rc
Hardness,Rc
(a)
Diameter
Diameter
Hardness distribution in water quenched steels (a) SAE1045 and (b) SAE6140
EFFECT OF TEMPERING
(MPa)
(1725)
Strength
Depending
upon
requirement, appropriate
tempering temperature is
selected. A typical case is
shown in the graph for 4340
STEEL.
The tempering temperature
depends upon the required
strength and hardness after
tempering. All these are
tabulated and are available
in ASTM literature for every
steel.
(1380)
(1035)
(690)
Temper, C
204
260
315
371
426
482
538
594
Tempering Temperature
650
ANNEALING AND NORMALISING TEMPERATURES
These are softening processes used for
producing steel with high ductility and low
hardness. Though annealing is used in a
very broad sense it has a distinct cycle.
Annealing involves heating the steel to
elevated temperature, holding for a time
dictated by section thickness and cooling in
the furnace. The elevated temperature is in
the range of 0-500c above a3 for hypo
eutectoid steels and 0-500c above a1 (not
acm, to avoid precipitation of pro-eutectoid
cementite along grain boundaries) for
hyper eutectoid steels
Normalising involves heating the hypo
eutectoid and hyper eutectoid steels above
a1 and acm, respectively holding and air
cooling.
HEAT TREATMENT OF ALUMINIUM ALLOYS
 Solid solution strengthened alloys
Soaked in Furnace followed by air cooling
 Precipitation hardened
Solution treated and quenched( quench delay < 15 seconds)
Aged natural (room temperature) or artificial (higher temperature)
PRECIPITATION HARDENABLE
ALUMINIUM ALLOY SYSTEMS
Al-Cu Phase Diagram
Al-Zn Phase Diagram
PRECIPITATION HARDENING PROCESS
1) Solution Treatment- the alloy is heated above the solvus temperature and soaked
there until a homogeneous solid solution (α) is produced.
2) Quenching is the second step where the solid α is rapidly cooled forming a
supersaturated solid solution of αSS .
3) Aging is the third step where the supersaturated α, αSS, is heated below the solvus
temperature to produce a finely dispersed precipitate(θ). The formation of a finely
dispersed precipitate in the alloy is the objective of the precipitation-hardening .
STRENGTHENING PRECIPITATES IN
DIFFERENT ALLOY SYSTEMS
 Al- Cu systems and Al Cu Li systems
Al2 Cu, Al2CuMg, Al2CuLi, Al3Li
 Al –Mg-Si systems
Al5Cu2Mg8Si6

Al-Zn-Mg, Al-Zn-Mg-Cu systems
MgZn2 , Mg(ZnCuAl)2
SOLID SOLUTION STRENGTHENING
Al-Mg Phase Diagram
Al-Mn Phase Diagram
Solid solution strengthening is due to dissolved solute . The solute
atmosphere interacts with moving dislocations impeding their motion.
WHY NO PRECIPITATES IN
Al-Mg AND Al-Mn SYSTEMS?
Despite sloping solvus, Mg coming out of super
saturated solution is extremely sluggish. Therefore
strengthening is only by solid solution .
Very slow cooling such as furnace cooling from
annealing temperature brings out Mg in blocky form as
Al3Mg2, reducing the strength.
In higher Mg containing Al alloys (>4wt%) , these
precipitates appear at grain boundary, reducing ductility
and resistance to stress corrosion cracking.
Post annealing cold work accentuates this problem.
Mn in Al alloys is added below its high temperature
solubility limit. Therefore no question of forming
precipitates.
Both Mg and Mn increase work hardening rate.
Therefore strengthening is done by cold working(Al and
Mg alloys < 3wt% Mg).
OTHER IMPORTANT Al ALLOYS
Al-Si Alloys
 Do not form any precipitates
 Weak solid solution strengthening
 Si improves fluidity
Therefore used as sheets for brazing, Welding rods and castings.
Al-Si-Mg alloys
 Si and Mg in proper proportion produce AlMg2Si
 precipitates. 2 Groups
 1st Group- (Mg +Si ) 0.8-1.2 can be easily extruded and air
cooled.
 2nd Group-(Mg +Si ) >1.4% develops high strength on aging
after ST + Quenching. Cu also added to enhance mechanical
properties.
HEAT TREATMENT OF TITANIUM ALLOYS
 Annealing
 Mill Annealing
 Normal Annealing

Aging Treatment
 Solution treatment and quench
 Ageing at elevated temperature
HEAT TREATMENT
HEAT TREATMENT
DEVELOPMENT OF MICROSTRUCTURES
HEAT TREATMENT OF
Ni BASE SUPERALLOYS
 Solution Treatment

Ageing Treatment
HEAT TREATMENT DETAILS
•HOMOGENIZATION
To make the composition
uniform
•SOLUTION TREATMENT
Heating
to temperature
above γ’ solvus and below
incipient melting to take all
the γ’
into solution,
followed by quenching.
Wrought alloys-1040-1230 C
Cast alloys-1180-1235 C
•AGING
1. Primary aging at 925 C to
precipitate coarse γ’
2. secondary aging at 750 C
to precipitate fine γ’
3. Tertiary aging at 700 C to
precipitate very fine γ’
and to form M23C6
carbides.
- P resen t a t G B s
1 0 0 0 – 10 0 00 n m s
- O b serv a b le o n ly in su b -so lvu s
so ln . trea ted m a teria l
(A b o u t 1 1% after
1 1 0 5 °C so lu tio n isin g )
T o ta l vo lu m e fra c tio n o f g’ ~ 4 3 %