CHAPTER 10
PHASE TRANSFORMATIONS IN METALS
LEARNING OBJECTIVES
1. Cite the two distinct steps that are involved
in the formation of particles of a new phase.
2. Make a schematic fraction transformation-versus-logarithm
of time plot for a typical solid-solid transformation, and then note nucleation
and growth regions on the curve.
3. For some solid-solid reaction, given values
of the constants k and n, compute the fraction transformation
after a specified time.
4. Given a fraction transformation-versus-logarithm
of time curve at some temperature, be able to determine the overall rate of
the transformation.
5. Define the terms supercooling and
superheating.
6. Explain how an isothermal transformation
diagram for some alloy is generated from a series of isothermal fraction transformation-versus-logarithm
of time curves.
7. Describe the difference in microstructure
for fine and coarse pearlites, and then explain this difference in terms of
the isothermal temperature range over which each transforms.
8. Briefly describe the microstructures of
upper and lower bainites and of spheroidite.
9. Briefly describe martensite in terms of
its crystal structure and its microstructures.
10. Describe the difference between thermally
activated and athermal transformations, and then cite one example of each transformation.
11. Describe the heat treatment that is necessary
to produce martensite, and explain why it forms instead of pearlite or bainite.
12. Given the isothermal transformation diagram
for some iron-carbon alloy and also a specific isothermal heat treatment, be
able to describe the microstructure that will result. The microstructure may
consist of austenite, a proeutectoid phase, fine pearlite, coarse pearlite,
spheroidite, bainite, and/or martensite.
13. Given a continuous cooling transformation
diagram for some particular alloy and a specific cooling curve, describe the
resulting microstructure that exists at room temperature.
14. Define what is meant by the critical
cooling rate, and given a continuous cooling transformation diagram, schematically
plot the critical cooling curve.
15. Describe or diagram how alloying elements
other than carbon alter the continuous cooling transformation diagram for a
steel. Now explain, in terms of this alteration, why alloying elements make
a steel more "heat-treatable."
16. Schematically diagram how tensile strength,
hardness, and ductility vary with carbon content for steels having microstructures
consisting of fine and coarse pearlite, and spheroidite. Also, explain why hardness
and strength increase with increasing carbon content.
17. Explain briefly why fine pearlite is
harder than coarse pearlite, which in turn is harder than spheroidite.
18. Qualitatively compare the mechanical
characteristics of bainite and iron-carbon alloys that have other microstructures.
19. Cite two reasons why martensite is so
hard and brittle.
20. Describe the microstructure of tempered
martensite.
21. Describe the heat treatment that is necessary
to produce tempered martensite.
22. Compare the properties of martensite
and tempered martensite, and also explain the properties of tempered martensite
in terms of its microstructure.
23. Schematically plot how hardness depends on tempering time
at constant temperature, and briefly explain this behavior.
24. Schematically plot how yield
strength, tensile strength, and ductility depend on tempering temperature (at
constant tempering time), and then explain this behavior.