Microstructures - Low Alloy Steels - Lesson-3

Microstructures
by George Langford, Sc.D., Massachusetts Institute of Technology, Cambridge, MA, 1966
Copyright©2005 by George Langford
Low Alloy Steels - Lesson 3 - Fourth specimen

	Here is a tension member used for prestressing concrete, made from a 4140 steel, heat treated to Rockwell C45 hardness before machining the screw threads. The photomicrograph at left was made at 200X. The second photomicrograph below this one was made at 500X.
	Note the ferrite network near the oxidized surface. This piece broke at low load during tightening and after the concrete had thoroughly set, ruining the structure. Why did it break ? Is there any remedy ? You can look at the answers once you've worked out your explanations.

Explanation: This is a quench crack. On the actual fracture surface one could see the oxidation which occurred during tempering subsequent to the quenching operation. The fracture surface had a temper color in the portion of the fracture that took place before tempering and before the final fracture. The rest of the fracture surface was a metallic grey. The temper color is due to the formation during tempering of a thin layer of oxide whose thickness is near the wavelength of light, about 0.5 micrometers. Only low temperature oxidation (at 200C to 500C) does this. There is no remedy for the failed piece, but to avoid cracking another one, the quench should be in a warm oil (50C or so) to reduce the severity of the cooling rate during quenching.

Specimen 5 is a tool steel.