UNIVERSITY OF NAMIBIA
FACULTY OF ENGINEERING & INFORMATION TECHNOLOGY
TMEM 3622: ENGINEERING MATERIALS
ASSIGNMENT 1
Topic:
Classification and Characteristics of Steels
Deadline:
11th SEPTEMBER 2020
Note:
ALL assignments must be submitted through the Moodle Learning
Management System.
TOTAL MARK: 50
QUESTION 1
Consider 2.5 kg of austenite containing 0.65 wt% C, cooled to below 727°C.
I. What is the proeutectoid phase?
II. How many kilograms each of total ferrite and cementite form?
[1]
[3]
III. How many kilograms each of pearlite and the proeutectoid phase form? [3]
IV. Schematically sketch and label the resulting microstructure.
[2]
QUESTION 2
(a) A hypereutectoid plain-carbon steel contains 10.7 wt % eutectoid Fe3C. What is its
average carbon content in weight percent??
[5]
(b) In terms of heat treatment and the development of microstructure, what are two major
limitations of the iron–iron carbide phase diagram?
[2]
QUESTION 3
Draw an isothermal transformation diagram for a plain-carbon eutectoid steel and indicate
the various decomposition products on it. How can such a diagram be constructed by a
series of experiments?
[6]
1
QUESTION 4
(a)
Using the isothermal transformation diagram for an iron–carbon alloy of eutectoid
composition given below, specify the nature of the final microstructure (in terms of
microconstituents present and approximate percentages of each) of a small specimen that
has been subjected to the following time–temperature treatments. In each case assume that
the specimen begins at 760°C and that it has been held at this temperature long enough to
have achieved a complete and homogeneous austenitic structure.
I.
II.
III.
Cool rapidly to 700°C, hold for 104 s, then quench to room temperature.
[2]
Reheat the specimen in part (a) to 700°C for 20 h.
[2]
Rapidly cool to 600°C, hold for 4 s, rapidly cool to 450°C, hold for 10 s, then quench
to room temperature.
[3]
IV.
Cool rapidly to 400°C, hold for 2 s, then quench to room temperature.
[2]
V.
Cool rapidly to 400C, hold for 20 s, then quench to room temperature.
[2]
VI.
Cool rapidly to 400C, hold for 200 s, then quench to room temperature.
[2]
VII.
Rapidly cool to 575C, hold for 20 s, rapidly cool to 350C, hold for 100 s, then
quench to room temperature.
VIII.
[2]
Rapidly cool to 250C, hold for 100 s, then quench to room temperature in water.
Reheat to 315C for 1 h and slowly cool to room temperature.
2
[3]
Figure Q4: The complete isothermal transformation diagram for an iron–carbon alloy of eutectoid
composition: A, austenite; B, bainite; M, martensite; P, pearlite.
(b)
What is the normalizing heat treatment for steel? What are some of its purposes?
[4]
QUESTION 5
Draw time-temperature cooling paths for a 1080 steel on an continuous-cooling
transformation diagram that will produce the following microstructures. Start with the steels
in the austenitic condition at time = 0 and 850ºC.
I.
100 percent martensite
II.
50 percent martensite and 50 percent coarse pearlite
III.
100 percent fine pearlite
IV.
50 percent martensite and 50 percent upper bainite
V.
100 percent upper bainite, and
VI.
100 percent lower bainite.
[6]
3