HEAT TREATMENT  Bulk and Surface Treatments Annealing, Normalizing, Hardening, Tempering

Transcription

HEAT TREATMENT  Bulk and Surface Treatments Annealing, Normalizing, Hardening, Tempering
HEAT TREATMENT
With focus on Steels
 Bulk and Surface Treatments
 Annealing, Normalizing, Hardening, Tempering
 Hardenability
Principles of Heat Treatment of Steels
Romesh C Sharma
New Age International (P) Ltd., Publishers, New Delhi, 1993.
Heat Treatment of Steels
 We have noted that how TTT and CCT diagrams can help us design heat treatments to
design the microstructure of steels and hence engineer the properties. In some cases a
gradation in properties may be desired (usually from the surface to the interior- a hard
surface with a ductile/tough interior/bulk).
 The general classes of possibilities of treatment are: (i) Thermal (heat treatment), (ii)
Mechanical (working), (iii) Chemical (alteration of composition). A combination of these
treatments are also possible (e.g. thermo-mechanical treatments, thermo-chemical
treatments).
 The treatment may affect the whole sample or only the bulk.
 A typical industrial treatment cycle may be complicated with many steps (i.e. a
combination of the simple step which are outlined in the chapter).
Thermal (heat treatment)
Treatments
Mechanical
Chemical
Bulk
Surface
Or a combination
(Thermo-mechanical, thermo-chemical)
An overview of important heat treatments
HEAT TREATMENT
BULK
ANNEALING
SURFACE
NORMALIZING
HARDENING
&
THERMAL
TEMPERING
Full Annealing
MARTEMPERING
Recrystallization Annealing
Stress Relief Annealing
Spheroidization Annealing
Flame
Induction
AUSTEMPERING
LASER
Electron Beam
THERMOCHEMICAL
Carburizing
Nitriding
Carbo-nitriding
Ranges of temperature where Annealing, Normalizing and Spheroidization treatment are
carried out for hypo- and hyper-eutectoid steels.
Ful
l
910C
An
ne
A3
alin
g
No
rma
liz
atio
n
N
li
a
orm
on
i
t
za
Acm
Full Annealing
723C
A1

T
Stress Relief Annealing
Spheroidization
Recrystallization Annealing
Wt% C
0.8 %
Full Annealing
 The steel is heated above A3 (for hypo-eutectoid steels) | A1 (for hyper-eutectoid steels) → (hold) → then the
steel is furnace cooled to obtain Coarse Pearlite
 Coarse Pearlite has ↓ Hardness, ↑ Ductility
 Not above Acm → to avoid a continuous network of proeutectoid cementite along grain
boundaries (→ path for crack propagation)
910C
Ful
lA
nne
alin
g
A3
No
rma
liza
tion
N
a
liz
a
orm
n
tio
Acm
Full Annealing
723C
A1

T
Stress Relief Annealing
Spheroidization
Recrystallization Annealing
Wt% C
0.8 %
Recrystallization Annealing
Heat below A1 → Sufficient time → Recrystallization
Cold worked grains → New stress free grains
Used in between processing steps (e.g. sheet rolling)
910oC
No
rma
Ful
liza
lA
tion
nne
alin
A3
g
on
i
t
a
l iz
Acm
a
m
r
No
Full Annealing
723oC
A1
Spheroidization
Recrystallization Annealing
Stress Relief Annealing
T
0.8 %
Wt% C
Stress Relief Annealing
Annihilation of dislocations,
polygonization
Residual stresses → Heat below A1 → Recovery
Welding
Differential cooling
910oC
Machining and cold working
No
rma
Ful
liza
lA
tion
nne
alin
A3
g
Martensite formation
on
i
t
a
l iz
Acm
a
m
r
No
Full Annealing
723oC
A1
Spheroidization
Recrystallization Annealing
Stress Relief Annealing
T
0.8 %
Wt% C
Spheroidization Annealing
Heat below/above A1 (long time)
Cementite plates → Cementite spheroids → ↑ Ductility
• Used in high carbon steel requiring extensive machining prior to final hardening & tempering
• Driving force is the reduction in interfacial energy
910oC
No
rma
Ful
liza
lA
tion
nne
alin
A3
g
on
i
t
a
l iz
Acm
a
m
r
No
Full Annealing
723oC
A1
Spheroidization
Recrystallization Annealing
Stress Relief Annealing
T
0.8 %
Wt% C
NORMALIZING
Heat above A3 | Acm → Austenization → Air cooling → Fine Pearlite (Higher hardness)
910oC
Ful
l
A3
No
rma
liza
tion
An
nea
ling
ion
zat
i
l
Acm
a
rm
No
Full Annealing
723oC
A1
Spheroidization
Recrystallization Annealing
Stress Relief Annealing
Purposes
T
0.8 %
Wt% C
Refine grain structure prior to hardening
To harden the steel slightly
To reduce segregation in casting or forgings
• In hypo-eutectoid steels normalizing is done 50oC above the annealing temperature
• In hyper-eutectoid steels normalizing done above Acm → due to faster cooling
cementite does not form a continuous film along GB
HARDENING
Heat above A3 | Acm → Austenization → Quench (higher than critical cooling rate)
• Quench produces residual strains
• Transformation to Martensite is usually not complete (will have retained Austenite)
• Martensite is hard and brittle
• Tempering operation usually follows hardening; to give a good combination of
strength and toughness
Typical hardness test survey made along a
diameter of a quenched cylinder
Schematic showing variation in cooling
rate from surface to interior leading to
different microstructures
Severity of quench values of some typical quenching conditions
Process
Variable
H Value
Air
No agitation
0.02
Oil quench
No agitation
0.2
"
Slight agitation
0.35
"
Good agitation
0.5
"
Vigorous agitation
0.7
Water quench
No agitation
1.0
"
Vigorous agitation
1.5
Brine quench
(saturated Salt water)
No agitation
2.0
"
Vigorous agitation
5.0
Ideal quench
Severity of Quench as indicated by the heat
transfer equivalent H
H
f
K
[m 1 ]
f → heat transfer factor
K → Thermal conductivity

Note that apart from the nature of the quenching medium, the vigorousness of the shake
determines the severity of the quench. When a hot solid is put into a liquid medium, gas bubbles
form on the surface of the solid (interface with medium). As gas has a poor conductivity the
quenching rate is reduced. Providing agitation (shaking the solid in the liquid) helps in bringing
the liquid medium in direct contact with the solid; thus improving the heat transfer (and the
cooling rate). The H value/index compares the relative ability of various media (gases and
liquids) to cool a hot solid. Ideal quench is a conceptual idea with a heat transfer factor of  (
H = )
Schematic of Jominy End Quench Test
Jominy hardenability test
Variation of hardness along a Jominy bar
(schematic for eutectoid steel)
Tempering
 ' ( BCT )
Martensite

Temper
 ( BCC ) Fe3C (OR)
Ferrite

Cementite
 Heat below Eutectoid temperature → wait→ slow cooling
 The microstructural changes which take place during tempering are very complex
 Time temperature cycle chosen to optimize strength and toughness
 Tool steel: As quenched (Rc 65) → Tempered (Rc 45-55)
MARTEMPERING
 To avoid residual stresses generated during quenching
 Austenized steel is quenched above Ms for homogenization of temperature across the
sample
 The steel is then quenched and the entire sample transforms simultaneously
 Tempering follows
800
Eutectoid temperature
723
Austenite
Pearlite
600
Martempering
T →
AUSTEMPERING
Pearlite + Bainite
400
300
Austempering
 + Fe3C
500
Bainite
Ms
200
Mf
100
Martensite
0.1
 To avoid residual stresses generated during quenching
 Austenized steel is quenched above Ms
 Held long enough for transformation to Bainite
1
10
102
t (s) →
103
104
105
ALLOY STEELS
 Various elements like Cr, Mn, Ni, W, Mo etc are added to plain carbon steels to create
alloy steels
 The alloys elements move the nose of the TTT diagram to the right
→ this implies that a slower cooling rate can be employed to obtain martensite
→ increased HARDENABILITY
 The ‘C’ curves for pearlite and bainite transformations overlap in the case of plain carbon
steels → in alloy steels pearlite and bainite transformations can be represented by separate
‘C’ curves