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)