Some Issues on Heat treatment

1
Some Issues on Heat treatment
1. Introduction The key to improve the material
property is to change its structure at atomic
level. This can be achieved through (i) alloying
and controlled heat and cooling basically heat
treatment The basic steps of heat treatment are
Heat -gt Soaking -gt Cooling
2
Some Issues on Heat treatment
Heat -gt Soaking -gt Cooling
Temperature Time of soaking Rate of cooling
Medium of cooling
- Different combinations of the above parameters
- Different compositions of materials and
initial phases of materials
Different heat treatments
3
Some Issues on Heat treatment
2. Purpose of heat treatment

• Soften the metal prior to shaping
• Relieve the effects of strain hardening that
  occurs during cold forming
• Achieve the final strength and hardness required
  in the finished product as one of the end
  manufacturing processes.

4
Some Issues on Heat treatment
Body heat treatment Surface heat treatment
Heat treatment
3. Classification of heat treatment processes
(1) annealing, (2) martensite formation in steel,
(3) precipitation hardening, (4) surface hardening
5
Some Issues on Heat treatment
4. Annealing

• Heat -gt Soaking -gt Cooling
• Reduce hardness and brittleness
• Alter microstructure for desired mechanical
  properties
• Soften metals to improve formability
• Recrystalize cold worked (strain hardened) metals
• Relieve stress from shaping

6
Some Issues on Heat treatment
4. Annealing

• Full annealing
• Normalizing
• Process Anneal
• - Recrystallization anneal
• - Recovery Anneal
• Stress-relief annealing

7
Some Issues on Heat treatment
5. TTT curve principle for Martensite Formation

• eutectoid composition
• preheat or heat up alloy to austenite.
• austenite to various phases cooling rate.
• Pearlite, P Bainite, B
  alternative forms of ferrite-carbide
  mixtures
• Martensite, M

8
Metals Phase diagram for Iron and Carbon
Fe3C
9
5. TTT curve principle for Martensite Formation
10
Some Issues on Heat treatment
5. TTT curve principle for Martensite Formation
Martensite hard and brittle BCT carbon
Tetragonal
Ms the temperature M starts to form. Ms depends
on alloyed element some are lower than room
temperature
11
Some Issues on Heat treatment
5. Heat treatment to form Martensite phase
Austenitizing heat up to a certain temperature
to form ? Quenching cooling media brine the
fastest air the slowest Tempering heat up to
temperature below eutectiod
soak for one hour slow cooling
BCT to BCC
12
5. Heat treatment to form Martensite phase
Austenitizing quenching - tempering
13
Some Issues on Heat treatment
5. Hardenability

• - The relative capacity of a steel to be hardened
  by transformation to martensite hardness
  penetration
• Hardenability Hardness ?
• alloying elements increase the hardenability to
  make TTT curve right to increase the time to
  start the transformation for Austenite to
  Martensite

14
(No Transcript)
15
Heat treatment
6. Precipitation hardening

• A necessary condition for metals that can be heat
  treated to be hard is ? - Martensite formulation,
  which includes the following points
• The composition of metal, in the range 0.1-0.8
  C
• The Ms should be above the room temperature
• The TTT curve should allow the possibility that
  the cooling trajectory passes on the left of the
  nose.

16
- Necessary condition may not be met in practice
for all compositions of steel
- Further, heat treatment for non-ferrous metals,
e.g., aluminum, copper, magnesium, do not follow
the martensite formation. This calls for a new
process.
Precipitation Hardening process

• Formation of fine particles (precipitates) that
  act to block the movement of dislocations
• thus strengthen the metal

17
Necessary condition for the precipitation process

• Composition of the metal has two phases at the
  room temperature, see Figure 1a (next slide).
• When the temperature arises, one phase should be
  be dissolved, i.e., sloping solvus line
• Composition C (next figure) satisfies above
  conditions

18
6. Precipitation hardening
Figure 1a, b
19
6. Precipitation hardening

• 3 steps of precipitation/age hardening
• Solution treatment
• - alloy heated above Ts into alpha-phase and
  held to dissolve the beta phase
• (2) Quenching
• - to the room temperature to create a
  supersaturated solid solution
• - It is noted that the structure of
  martensite is of highly distorted BCC plus
  excessive carbon dissolved Figure 2.
  Supersaturated structure has that same feature.

20
6. Precipitation hardening
(3) Precipitation treatment (aging) - Reheat
the material to the temperature a little bit
above the room temperature, Tp, but below Ts, to
cause precipitation of fine particles of the beta
phase. - high strength and hardness achieved
in this step - Temp. and time for the step
are variables. Higher temp.?
hardness peaks quickly lower
temp? more time to harden, but hardness is more
- Over-aging reduction in hardness
21
Figure 2
22
7. Surface heat treatment

• Thermo chemical treatment
• Composition of part surface altered by addition
  of other elements
• Adding of carbon, nitrogen, or other elements

Nitriding
Carburizing
Carbonitriding
23
Surface heat treatment
Pack carburizing- Pack carbonaceous materials
(charcoal) Very thick hard outer layer
- Carburizing
Gas carburizing- Diffuse Hydrocarbon fuels
(propane in a furnace) thin hard outer layer
Liquid carburizing- Diffuse molten salt bath
containing sodium cyanide, barium chloride, and
other compounds medium sized hard outer layer

• - Low hardness
• - Ductile
• Capable of
• withstanding stress

24
Surface heat treatment
Carburizing -gt HRC 60, Thickness 0.025 4
mm Nitriding -gt HRC 70, Thickness 0.025 0.05
mm Carbonitriding -gt HRC 70, Thickness 0.07-0.5
mm Chromizing and Boronizing -gt HRC 70
25
8. Heat treatment methods and facilities

• Heat treatment furnaces
• 2. Surface hardening methods
• - flame hardening
• - induction heating
• - Laser beam heating