Transcript Metal Alloys: Their Structure & Strengthening by Heat

Metal Alloys: Their
Structure &
Strengthening by
Heat Treatment
Part 2
Chapter 4
Heat treatment of ferrous
alloys (4.7)
• Heat-treatment techniques: the controlled
heating and cooling of alloys at various
rates
• Phase transformations: greatly influence the
mechanical properties
o Strength
o Hardness
o Ductility
o Toughness
o Wear resistance
Iron-carbon system
microstructural changes
•Pearlite
•Spheroidite
•Bainite
•Martensite
Pearlite
• Fine pearlite
• High rate of cooling
• As in air (fig.4.11)
• Course pearlite
• Slow rate of
cooling
• As in a furnace
Spheroidite
• Pearlite is heated to
just below the
eutectoid
temperature for a
period of time
• Example: a day at
700oC
• Cementite lamellae
transform into
roughly spherical
shapes
• Higher toughness
• Lower hardness
• Can be cold
worked
• Spheroidites less
conductive to stress
concentration
FIGURE 4.14
Microstructure of eutectoid steel. Spheroidite is formed by tempering the steel at 700°C (1292°F).
Magnification: 1000.
Bainite
• Very fine microstructure consisting
of ferrite and cementite
• Bainitic steel is stronger and more
ductile than pearlitic steels at the
same hardness levels
Martensite
• When austenite is cooled at a high
rate such as by quenching in water its
FCC structure is transformed to BCT
(body-centered tetragonal)
• Hard
• Brittle
• Lacks toughness so limited in usefulness
FIGURE 4.15 (a) Hardness of martensite as a function of carbon content. (b) Micrograph of
martensite containing 0.8% carbon. The gray platelike regions are martensite; they have the same
composition as the original austenite (white regions). Magnification: 1000.
Quench cracking
• Internal stresses cause parts to
undergo distortion or even crack
during heat treatment
• Distortion is the irreversible dimensional
change of the part during heat
treatment
FIGURE 4.15 (a) Hardness of martensite as a function of carbon content. (b) Micrograph of
martensite containing 0.8% carbon. The gray platelike regions are martensite; they have the same
composition as the original austenite (white regions). Magnification: 1000.
TTT (time-temperaturetransformation) diagrams
• Allow metallurgists to design heat
treatment schedules to obtain
desirable microstructures
• Fig.4.17a The higher the temperature
or the longer the time, the more
austenite that transforms into pearlite
FIGURE 4.17 (a) Austenite-to-pearlite transformation of iron–carbon alloy as a function of time and
temperature. (b) Isothermal transformation diagram obtained from (a) for a transformation temperature
of 675°C (1247°F). (c) Microstructures obtained for a eutectoid iron–carbon alloy as a function of
cooling rate.
Hardenability
• Hardenability is the capability of an
alloy to be hardened by heat
treatment
• Measures the depth of hardness
obtained by heat
treatment/quenching
• Hardenability is not the same as
hardness
FIGURE 4.19 Mechanical properties of annealed steels as a function of composition and
microstructure. Note in (a) the increase in hardness and strength, and in (b), the decrease in ductility
and toughness, with increasing amounts of pearlite and iron carbide.
End-Quench hardenability
test (Jominy Test)
• Round test bar is
austenized (heated to
the proper
temperature to form
100% austenite)
• Bar then quenched at
one end
• Hardness decreases
away from the
quenched end of the
bar
•
•
•
•
•
•
•
•
•
Quenching media
Water
Brine
Oil
Molten salts
Air
Caustic solutions
Polymer solutions
gases
FIGURE 4.20 (a) End-quench test and cooling rate. (b) Hardenability curves for five different steels,
as obtained from the end-quench test. Small variations in composition can change the shape of these
curves. Each curve is actually a band, and its exact determination is important in the heat treatment of
metals, for better control of properties.
Precipitation hardening
• Small particles of a different phase called
precipitates are uniformly dispersed in the
matrix of the original phase
• Precipitates form because the solid solubility
of one element in the other is exceeded
• The alloy is reheated to an intermediate
temperature and held there for a long time
during which time precipitation takes place
Aging or Age Hardening
• Because the precipitation process is one of time
and temperature, it is also called AGING.
• Age hardening is the property improvement of the
material
• Artificial aging is carried out above room
temperature
• Natural aging: some aluminum alloys harden and
become stronger over time at room temperature
FIGURE 4.22 The effect of aging time and temperature on the yield stress of 2014-T4 aluminum
alloy. Note that, for each temperature, there is an optimal aging time for maximum strength.
Case hardening
• Hardening of the surface
• Improves resistance to surface indentation, fatigue,
wear
• Gear teeth
• Cams
• Shafts
• Bearings
• Fasteners
• Pins
• Automotive clutch plates
• Tools and dies
TABLE 4.1 Outline of Heat-treatment Processes for Surface Hardening
TABLE 4.1 (continued) Outline of Heat-treatment Processes for Surface Hardening
Annealing
• The restoration of a
cold-worked or heattreated alloy to its
original properties
• Increase ductility
• Reduce hardness and
strength
• Modify the
microstructure
• Relieve residual stresses
• Improve machinability
• Steps
1. Heat to a specific
temperature range
in a furnace
2. Hold at that
temperature
(soaking)
3. Cooling in air or in
a furnace
More about annealing
• Normalizing-the cooling cycle is
completed in still air to avoid excessive
softness
• Process annealing
• Stress-relief annealing
• Tempering
• Austempering
• Martempering
• Ausforming