Sheet 5
HEAT TREATMENT OF METALS
1.
"Steel is made hard by quenching." List at least three requirements that must be met to
justify this statement.
2.
Using the Fe-C phase diagram (figure 1) and the isothermal transformation diagram for 1080
steel (figure 2), give the microstructure present in a thin strip of the 1080 steel after each
step of the following heat treatments:
(a) 800˚C for 1 hour
Quench into lead at 700˚C, hold for 15 minutes.
Quench into lead at 500˚C, hold for 1 minute.
Water quench.
(b) 700˚C for 6 hours and water quench.
(c) 800˚C for 1 hour, quench into molten salt at 260˚C, hold for 1 minute, and then air cool.
(d) 800˚C for 1 hour and water quench.
Why would treatment (c) give better mechanical properties than treatment (d)?
3.
Match the suitable heat treatment procedures with the alloys given below and briefly explain
the purposes of each procedure:
Alloys
Heat Treatment
1020
1050
1080
A96061
G3500
homogenisation anneal
normalising
full anneal
quenching and tempering
ageing
4.
Why is it preferable to precipitation-harden alloys by first quenching to a low temperature
and then reheating to the ageing temperature rather than quenching directly to the ageing
temperature?
5.
Can low carbon steels be age-hardened? Can low carbon steels be quench hardened? How
are low carbon steels strengthened?
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Figure 1: Fe - Fe3C Phase Diagram
Figure 2: Isothermal Transformation diagram for
a 1080 steel
6: The kinetics of the austenite-to-pearlite transformation obey the Avrami relationship. Using the
fraction transformed–time data given here, determine the total time required for 95% of the
austenite to transform to pearlite:
7: Using the isothermal transformation diagram for a 1.13 wt% C steel alloy, determine the final
microstructure (in terms of just the microconstituents present) of a small specimen that has been
subjected to the following time–temperature treatments. In each case assume that the specimen
begins at 920 oC and that it has been held at this temperature long enough to have achieved a
complete and homogeneous austenitic structure.
(a) Rapidly cool to 250 oC, hold for 103 s, then quench to room temperature.
(b) Rapidly cool to 775 oC, hold for 500 s, then quench to room temperature.
(c) Rapidly cool to 400 oC, hold for 500 s, then quench to room temperature.
(d) Rapidly cool to 700 oC, hold at this temperature for 105 s, then quench to room temperature.
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8: The Figure shows the continuous
cooling transformation diagram for a
0.35 wt% C iron–carbon alloy. Make a
copy of this figure and then sketch
and label continuous cooling curves to
yield the following microstructures:
(a) Fine pearlite and
proeutectoid ferrite
(b) Martensite
(c) Martensite and proeutectoid
ferrite
(d) Coarse pearlite and
proeutectoid ferrite
(e) Martensite, fine pearlite, and
proeutectoid ferrite
9: Copper-rich copper–beryllium alloys
are precipitation hardenable. After
consulting the portion of the phase
diagram, do the following:
(a) Specify the range of compositions
over which these alloys may be
precipitation hardened.
(b) Briefly describe the heat-treatment
procedures (in terms of temperatures)
that would be used to precipitation
harden an alloy having a composition of
your choosing, yet lying within the range
given for part (a).
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10: Rank the following iron–carbon alloys and
associated microstructures from the hardest to
the softest: (a) 0.25 wt% C with coarse pearlite,
(b) 0.80 wt% C with spheroidite, (c) 0.25 wt% C
with spheroidite, and (d) 0.80 wt% C with fine
pearlite. Justify this ranking.
11: For a eutectoid steel, describe isothermal
heat treatments that would be required to
yield specimens having the following Brinell
hardnesses: (a) 180 HB, (b) 220 HB, and (c)
500 HB.
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