91Ó°ÊÓ

Make a copy of the isothermal transformation diagram for an iron-carbon alloy of eutectoid composition (Figure 10.22 ) and then sketch and label time- temperature paths on this diagram to produce the following microstructures: a) \(100 \%\) coarse pearlite b) \(50 \%\) martensite and \(50 \%\) austenite c) \(50 \%\) coarse pearlite, \(25 \%\) bainite, and \(25 \%\) martensite

Make a copy of the isothermal transformation diagram for a 1.13 wt\% \(C\) iron- carbon alloy (Figure 10.39 ), and then on this diagram sketch and label time- temperature paths to produce the following microstructures: (a) \(6.2 \%\) proeutectoid cementite and \(93.8 \%\) coarse pearlite (b) \(50 \%\) fine pearlite and \(50 \%\) bainite (c) \(100 \%\) martensite (d) \(100 \%\) tempered martensite

Briefly explain why there is no bainite transformation region on the continuous cooling

Name the microstructural products of 4340 alloy steel specimens that are first completely transformed to austenite, then cooled to room temperature at the following rates: (a) \(0.005^{\circ} \mathrm{C} / \mathrm{s}\) (b) \(0.05^{\circ} \mathrm{C} / \mathrm{s}\) (c) \(0.5^{\circ} \mathrm{C} / \mathrm{s},\) and (d) \(5^{\circ} \mathrm{C} / \mathrm{s}\)

Briefly describe the simplest continuous cooling heat treatment procedure that would be used in converting a 4340 stecl from one microstructure to another. (a) (Martensite \(+\) ferrite \(+\) bainite) to (martensite + ferrite + pearlite + bainite) (b) (Martensite \(+\) ferrite \(+\) bainite) to spheroidite (c) (Martensite \(+\) bainite \(+\) ferrite) to tempered martensite

On the basis of diffusion considerations, explain why fine pearlite forms for the moderate cooling of austenite through the eutectoid temperature, whereas coarse pearlite is the product for relatively slow cooling rates.

Briefly explain why fine pearlite is harder and stronger than coarse pearlite, which in turn is harder and stronger than spheroidite.

Cite two reasons why martensite is so hard and brittle.

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) \(0.25 \mathrm{wt} \% \mathrm{C}\) with spheroidite, and (d) 0.80 wt\% C with fine pearlite. Justify this ranking.

(a) Briefly describe the microstructural difference between spheroidite and tempered martensite. (b) Explain why tempered martensite is much harder and stronger.