Materials Engineering

1
Materials Engineering Heat Treatment Professo
r Yinong Liu School of Mechanical
Engineering The University of Western Australia
2
Todays lecture Heat Treatment of Metals
Diffusive and nondiffusive transformations Nucleat
ion and grain growth Supercooling and
superheating Transformation rate Isothermal
transformation diagrams (TTT) continuous cooling
diagrams (CCT) Bainitic and Martensitic
transformations Speroidisation Normalising Quenchi
ng and tempering
3
Diffusive and non-diffusive transformations
Solid-solid phase transformations are common
phenomena of metals. The knowledge of these
transformations is the basis of a range of
processing techniques, most notably heat
treatment. Solid phase transformations can be
divided into two main categories diffusive and
non-diffusive transformations.
Non-diffusive, or displacive, solid
transformation atoms displaced by less than one
interatomic distance lattice distortion is
retained and manifested in global shape of
crystal. Non-diffusive transformations are also
known as Martensitic Transformations
Diffusive transformation atoms displaced by more
than one interatomic distance.
4
Rate of Transformation
Solid phase transformations occur by nucleation
of new grains and by grain growth. The rate of
transformation is thus determined by the rate of
nucleation and the rate of growth. The process is
driven by thermodynamic driving force, which is
dependent on temperature, i.e., a supercooling is
required for a solid transformation
that occurs on cooling or a superheating for a
transformation on heating. Supercooling is the
temperature difference between the equilibrium
temperature of the transformation and the actual
temperature. Greater supercooling leads to higher
nucleation rate. On the other hand, grain growth
is basically a diffusion process, and the rate of
growth decreases with decreasing temperature. At
a constant temperature, the rate of
transformation and the fraction of transformation
are dependent on time.
5
Nucleation T controlled, thermodynamics Y
1-exp(-ktn) (Avrami equation) Preferential
nucleation sites grain boundaries,
imperfections Grain growth diffusion controlled,
time dependent, Kinetics RAexp(-Q/RT)
6
Isothermal Transformation Diagrams (TTT diagrams)
This is the TTT diagram for 0.77C eutectoid
steel. At temperatures just below the eutectoid
temperature, supercooling is small and driving
force is weak therefore, the transformation rate
is low. Holding at lower temperatures leads to
more rapid transformation. When the temperature
is too low, the transformatino has a tendency to
slow down due to difficulty of diffusion.
7
At higher temperatures below the eutectoid
temperature, diffusion is easier and the pearlite
formed has thicker laminar plates, known as
coarse pearlite. Lowering temperature refines the
laminar spacing of pearlite, producing fine
pearlite.
8
Fine pearlite is a more homogeneous structure and
offers better strength and toughness than coarse
pearlite.
9
TTT diagram of eutectoid steel - Bainite
Further cooling the isothermal transformation
temperature slows down the transformation rate.
At the same time it makes diffusion extremely
difficult, restricting the precipitation of C
from the austenite matrix. This leads to the
formation of another phase, known as Bainite.
The Bainite formed in the higher temperature
regime is called upper Bainite and that in the
lower temperature regime the lower Bainite.
Bainite is harder and stronger (less ductile)
than Pearlite.
10
Bainite
Bainite is formed as a result of insufficient
diffusion. As a result, the ferrite matrix is
actually over-saturated with C and the cementite
is extremely small and exists in strings of
elongated particles.
11
TTT diagram of eutectoid steel - Martensite
Cooling to even lower temperature will suppress C
diffusion completely. This causes the ferrite
formed from the austenite to be so overly
saturated that it distorts the BCC structure into
a BCT structure. This phase is known as the
Martensite. The martensitic transformation is
time-independent (no diffusion) thus the
transformation envelops appear as horizontal
lines. Martensite is formed by fast quenching
from high T to bypass the nose temperature.
12
Martensite
The c/a ratio of the BCT structure, caused by
trapped C atoms, is a measure of the degree of
lattice distortion of martensite. Higher C
contents produce martensites of higher c/a
ratios and higher c/a ratios make martensite
harder and more brittle. High-C martensite is
extremely hard and brittle.
13
(No Transcript)
14
TTT diagram of hypereutectoid steel (1.13C)
In this TTT diagram an envelop for the start of
the formation of proeutectoid cementite is added.
15
TTT diagram of an alloy steel
16
Heat Treatment Procedures

• Annealing
• Annealing is the most frequently used heat
  treatment procedure. It involves heating the
  alloy to elevated temperatures and then cooling
  down slowly (often in furnace). Annealing may be
  carried out for a variety of purposes
• Process anneal to negate the effect of cold
  working so further cold work can be carried out.
  This anneal often involves recovery and
  recrystallisation, usually done at below
  eutectoid temperature.
• Stress relief anneal to remove residual internal
  stresses, e.g., thermal stresses resulting from
  casting, usually done at below eutectoid
  temperature.
• Composition homogenisation anneal to homogenise
  cored structure
• Solution treatment to dissolve precipitates
• Spheroidisation to convert laminar P to
  spheroidite for improved ductility

17
Spheroidite of a carbon steel
18
Normalising To heat the steel to the Austenite
region and then air-cool, to encourage the
formation of fine pearlite or Bainite. This
treatment improves strength and toughness of
coarse-pearlite structured steels and is commonly
used for medium carbon steels for structural
applications.
19
Quenching This treatment involves heating the
steel to Austenite region and then rapidly cool
in some coolant, typically water, oil, or molten
salt. This treatment produces Martensite or
lower Bainite and is applied to high carbon
steels for high hardness.
Hardness of M increases with increasing
C. Hardenability is the ability of a steel to
be converted to martensite upon quenching.
Hardenability increases with C and some alloying
elements, such as Cr, W, and Mo. Hardenability is
determined in Jominy test.
20
(No Transcript)
21
Tempering Reheating of quench-hardened steel to
elevated temperatures to encourage decomposition
of M to FFe3C in order to toughen the
structure. Low-T tempering lt350C, hardness,
cutting tools Medium-T tempering 350450C, good
combination of hardness and toughness,
springs High-T tempering high toughness and
strength, structural applications
22
Tempered martensite
23
Austenite (g, A)
slow cooling
moderate cooling
fast cooling
Coarse pearlite proeutectoid phase
rapid quenching
Fine pearlite proeutectic phase
Bainite
Matensite (BCT)
reheating
Castings
Cold worked
Tempered martensite
24
Ageing or precipitation treatment Solution
treatment at high T to dissolve much alloying
element and then quench to room temperature to
create over-saturated solution. Reheating to
moderate temperature to encourage precipitation
of overly saturated elements out of solid
solution. The microstructure formed is small
precipitate particle dispersed in the solid
solution matrix. Precipitate particle hinder
dislocation movement, causing strengthening and
hardening of the matrix.
25
Essential conditions Over-saturated solid
solution Decreasing solubility Optimum hardening
effect fine, dispersive, thermally stable,
mechanically strong precipitates