Using the supplied isothermal transformation diagram for an iron–carbon alloy of
ID: 1058296 • Letter: U
Question
Using the supplied isothermal transformation diagram for an iron–carbon alloy of eutectoid composition, 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.
Cool rapidly to 670°C, hold for 100 s, then observe at that instant and that temperature using a high temperature microscope or similar device
(a) 100% austenite
(b) 100% martensite
(c) 50% pearlite/50% austenite
(d) 50% pearlite/50% martensite
(e) bainite
Cool rapidly to 670°C, hold for 100 s, then quench to room temperature, and observe at room temperature
(a) 100% austenite
(b) 100% martensite
(c) 50% pearlite/50% austenite
(d) 50% pearlite/50% martensite
(e) bainite
Cool specimen to 700 °C and hold there for 100 hours
(a) Tempered martensite
(b) 100% spheroidite
(c) 100% coarse pearlite
(d) ) 50% pearlite/50% martensite
(e) austenite
Rapidly cool to 640 °C, hold for 10 s, rapidly cool to 350°C, hold for 1000 s, then quench to room temperature.
(a) 25% martensite/25% bainite/50% pearlite
(b) 50% martensite/50% bainite
(c) 100% bainite
(d) 50% pearlite/50% bainite
(e) mostly martensite
Rapidly cool (to room temperature) in less than 1 second.
(a) 25% martensite/25% bainite/50% pearlite
(b) 50% martensite/50% bainite
(c) 100% bainite
(d) 50% pearlite/50% bainite
(e) mostly martensite
Rank the microconstituents in order of increasing hardness
(a) Bainite, coarse pearlite, fine pearlite, martensite, spheroidite
(b) Coarse pearlite, bainite, spheroidite, fine pearlite, martensite
(c) Spheroidite, coarse pearlite, fine pearlite, bainite, martensite
(d) Martensite, coarse pearlite, bainite, fine pearlite,
(e) Spheroidite, martensite, coarse pearlite, bainite, ice cream
Which of the following statements about untempered (ie. freshly produced) martensite is false:
(a) it is very hard and well suited for use in tooling and other high strength applications
(b) it is normally too hard for safe use in any application
(c) it needs to be tempered to a lower hardness as soon as possible
(d) it is full of elastic strain and is very brittle
(e) its hardness increases as carbon content increases
800 n 700 600 500 H 400 300 200 100 10 M(start) M(50%) M(90%) 10 Eutectoid temperature 140o 1200 1000 800 600 50% 400 M A 200 104 10 10 10 Time (s)Explanation / Answer
1) Option C : 50% pearlite and 50% austenite
Explanation: You can see from the graph that if the specimen is held for 100s only half of the transformation takes place and at 6700 C the pearlite formation only takes place.
2)Option D: 50% pearlite and 50% martensite
Explanation : after 100s at 670 0 C 50% pearlite is formed and as it quenched immediately after this to room temperature and s only 50% austenite is avvailable , So only 50% martensite is formed.
3)Option B : 100% Spheroidite
Explanation : if the specimen is held at 7000 C for 100 hours ,theoritically only 100% spheroidite is formed.
4) Option B :50% martensite / 50% Bainite
Explanation: If the pecimen takes the path as described from the graph it can be seen that the above result occurs.
5) Option E : mostly martensite
Explanation: Can be seen from the Graph.
6) Option C : Spheroidite, Coarse pearlite, Fine pearlite,Bainite, Martensite
Explanation: According to experiments and theory.
7) Option A : It is very hard and well suited for use in Tooling and other high strength applications.
Explanation: It contains lot of thermal stresses and strain introduced in it and hence it is not very safe to use freshly produced martensite for high strength applications.
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