engineering materials 7

1. 1. (5 points) Why are ductile fractures less frequent in practice than brittle fractures?
   2. A fatigue test is made with a maximum stress of 25 ksi (172 MPa) and a minimum stress of -4.00 ksi (-27.6 MPa). Calculate (a) the stress range, (b) the stress amplitude, (c) the mean stress, and (d) the stress ratio.
   3. DS MAR-M 247 alloy (Fig. 7.31 from slides) is used to support a stress of 207 MPa. At what temperature (oC) will the stress rupture lifetime be 210 h?
   4. (20 points) If 500 g of a 40 wt % Ag–60 wt % Cu alloy is slowly cooled from 1000°C to just below 780°C (see Figure below):
   a. How many grams of liquid and proeutectic alpha are present at 850°C?
   b. How many grams of liquid and proeutectic alpha are present at 780°C + ΔT ?5. (5 points) Explain the numbering system used by the AISI and SAE for plain-carbon steels. 6. (5 points) What are some of the limitations of plain-carbon steels for engineering designs?

7. (5 points) What are the principal alloying elements added to plain-carbon steels to make low-alloy steels?
8. (20 points) A 0.25 percent C hypoeutectoid plain-carbon steel is slowly cooled from 950°C to a temperature just slightly below 723°C.

a. Calculate the weight percent proeutectoid ferrite in the steel.
b. Calculate the weight percent eutectoid ferrite and weight percent eutectoid cementite in the steel. 9. An austenitized 80 mm diameter 4340 steel bar is quenched in agitated water. Predict what the Rockwell C hardness of the bar will be at (a) its surface and (b) its center.