Microstructures - Low Alloy Steels - Lesson-2

Microstructures
by George Langford, Sc.D., Massachusetts Institute of Technology, Cambridge, MA, 1966
Copyright©2005 by George Langford
Low Alloy Steels - Lesson 2 - Seventh specimen

	Here we have an early composite steel. It is a jail bar which consists of alloy steel reinforcing rods embedded in a low carbon steel matrix. In the image at left, shown at 100X, the alloy rod is at left and the plain carbon matrix is at the right. There is no gap between the two, which were bonded together metallurgically during the hot working process by which the original assembly was reduced to the final diameter of about 0.5 inch. Notice the duffusion zone - the alloy rod is locally depleted in carbon and the plain carbon matrix has become carburized. A layer of ferrite lies between.
	The rod has a hardness of Rckwell C39; and the matrix (away from the diffusion zone) has a hardness of Rockwell B51 (i.e., very much softer). The image at left shows the alloy rod at 500X. The frame below shows the plain carbon matrix at the same magnification; both were eteched at the same time with Nital.
	How is the martensite obtained only in the reinforcing rods and not in the plain carbon steel matrix ? Pause to formulate your response, then proceed to the answer below.

Answer: The alloy steel rods transform from austenite to ferrite plus pearlite only very slowly. Hence, the composite jail bar could be air cooled to harden the rods (by avoiding the "nose" of the continuous transformation cooling curve) while the plain carbon matrix was normalized to ferrite plus pearlite. Note that the diffusion of carbon is away from the highly alloyed rods and towards the plain carbon matrix. The alloying elements in the reinforcing rod have raised the chemical activity of carbon in the austenite. Silicon is very potent in that respect.

Go on to the last specimen in this lesson.