Heat Treatments and Mechanical Properties in FeC System

1
Heat Treatments and Mechanical Properties in Fe-C
System

• From the study of kinetics of phase
  transformation we know that a variety of
  microstructures are possible in the Fe-C system
  following a non-equilibrium cooling.
• In this section we will look at some standard
  heat treatments, the resulting microstructures
  and corresponding mechanical properties in the
  Fe-C system.

• Heat Treatments of Steels
• Critical temperatures

Upper critical temperature
Lower critical temperature
Self-study 11.5-11.6 (5th ed), 11.8 (6th ed)
Reading 11.1-11.4 (5th ed), 11.7 (6th ed),
10.7-10.8 (5th 6th ed)
2
Heat Treatments and Mechanical Properties in Fe-C
System

• Normalising
• original microstructure deformed with usually
  large, uneven-sized and irregular-shaped grains
  of pearlite and proeutectoid phase (if present)
• treatment heating to above the upper critical
  temperature, holding for sufficient time to
  obtain 100 austenite (austenitising), and
  cooling in air to room temperature
• resulting microstructure fine grained pearlite
  (with fine lamellae) plus any proeutectoid phase
  that may be present

• Full annealing
• original microstructure anything that is not
  soft enough
• treatment heating to above the upper critical
  temperatures for hypoeutectoid alloys, holding
  for sufficient time for all pearlite to become
  austenite, furnace cooling (thus very slowly) to
  room temperature
• resulting microstructure pearlite with coarse
  lamellae plus any proeutectoid phase that may
  exist - such a microstructure is soft and ductile
  and thus amenable to further forming and machining

Annealing is primarily for sofening and cooling
very slowly is the key. Normalising may harden
or soften steels depending on the history of the
treatment.
3
Heat Treatments and Mechanical Properties in Fe-C
System
4
Heat Treatments and Mechanical Properties in Fe-C
System

• Spheroidising
• original microstructure pearlite that are too
  hard even after annealing
• treatment heating to just under the eutectoid
  temperature and holding for long hours
• resulting microstructure spheroidised Fe3C
  particles (of the order of 5 µm) in a matrix of
  a, a much softer and ductile microstructure

Spheroidised cementite
5
Heat Treatments and Mechanical Properties in Fe-C
System

• Quenching
• heating to the single g region to austenitise
  100 austenite achieved
• quenching in cold medium (e.g. water) to achieve
  rapid cooling
• martensite is the intended product
  (microstructure) although 100 M is often not
  possible as the interior of the sample may not
  undergo the same rapid cooling as the surface
• the material after quenching is very hard but
  brittle (due to M) and thus usually not usable in
  engineering applications

• Tempering of M
• original microstructure M
• Heating to 250-650C, holding for a period of
  time, and cooling to room temperature
• resulting microstructure tempered martensite
  extremely small Fe3C particles (of the order of
  0.1 µm) uniformly distributed in a (in the
  spheroidite, the Fe3C particles are much coarser)
• Tempered M is as strong and hard as M, but much
  tougher and with improved ductility - this
  structure is desirable and consequently, M is
  almost always tempered.

6
Heat Treatments and Mechanical Properties in Fe-C
System

• Microstructure of tempered martensite
• Fe3C particles are about one order of magnitude
  finer compared to those in the spheroidite

7
Heat Treatments and Mechanical Properties in Fe-C
System
Reheat
8
Heat Treatments and Mechanical Properties in Fe-C
System

• Mechanical properties
• Which microstructure to have?
• Depending on mechanical properties required
• Ingredient phases of a microstructure
• a soft and ductile
• Fe3C hard and brittle
• Martensite extremely hard and brittle
• Mechanical properties depend on the amount,
  shape, size and distribution of these phases,
  i.e. microstructures
• Various microstructures can be obtrained through
  different heat treatments

Processing
Microstructure
Property
9
Heat Treatments and Mechanical Properties in Fe-C
System

• Effect of amount of cementite

Eutectoid composition
Strength increases with C
Ductility and toughness decrease with C
10
Heat Treatments and Mechanical Properties in Fe-C
System

• Effect of shape and size of cementite

Strength increases with decreasing thickness of
lamellae in pearlite
ductility is higher in spheroidite
ductility decreases with decreasing thickness of
lamellae in pearlite
Strength is lower in spheroidite
11
Heat Treatments and Mechanical Properties in Fe-C
System

• Effect of martensite and tempered martensite

Hardness M gt Tempered M gt fine P gt coarse P gt
spheroidite
What about ductility and toughness?