A thin sheet specimen of iron?carbon alloy, with 0.016 weight percent carbon, is heated to 1000 K, where it is allowed to come to equilibrium. Following this, it is slow cooled to 850 K. It is then quenched in iced brine and aged at 313 K for 100 hours. Describe the resulting microstructure.

Question

What will be an ideal response?

Answer 1

The solution to this problem is outlined on the iron?carbon phase diagram below. Note that heating an alloy with 0.016 percent carbon to 1000 K, places the metal in the ? phase region of the phase diagram. If it is equilibrated at this temperature, all the carbon will go into solution in the ? phase. However, on slow cooling when the metal reached 962 K, it becomes saturated with carbon so that below this temperature cementite will begin to form. As may be seen in the phase diagram, the solubility of carbon drops from 0.016 percent at 962 K to 0.00445 percent at 850 K. The cementite that is formed will appear preferentially along the grain boundaries. Thus,at 850 K, the microstructure will consist of ? phase crystals containing 0.00445 carbon with particles of cementite distributed along their grain boundaries.Quenching the iced brine will not significantly affect the microstructure. However, holding at 313 K, where the equilibrium solubility of the carbon is only 3.7x 10-11 percent, causes the ? phase with 0.00445 percent carbon to be supersaturated with carbon. At 313 K, although the carbon diffusion rate is lower than at more elevated temperature, it is still finite. The result is that the carbon now precipitates as small particles in the matrix.