Microstructures
by George Langford, Sc.D., Massachusetts Institute of Technology, Cambridge, MA, 1966
Copyright©2005 by George Langford
Low Alloy Steels - Lesson 2 - Third specimen
Leather cutting knife at 500X etched
 This is a transverse section of a leather cutting knife shown at 500X with a Nital etch.  The edge became bent because of excessive amounts of soft retained austenite.

The alloy is 1% carbon, 1% manganese, and 0.4% molybdenum, which is known as a "nondeforming" tool steel.  'Nondeforming" is usually meant to mean that it won't change dimensions during heat treatment. 

The microstructure consists of tempered martensite plus about 40% retained austenite, ten times as much as would ordinarily be tolerated.  The retained austenite is not very strong, but when it transforms to martensite during use or during a subsequent heat treatment, the steel can crack or become embrittled.
Ms and Mf temperatures of carbon steels
 The start and finish temperatures of the athermal transformation of austenite to martensite in plain carbon steels are quite sensitive to the carbon content of the austenite.  Austenite with more than about 0.8% carbon will not transform completely to martensite if the austenite is only quenched down to room temperature. 

Can this knife be reclaimed ?
How would future trouble be prevented ?



Pause to formulate your response, and then move on to the explanation.



























Remedies: You could either convert the retained austenite to bainite by a prolonged low temperature tempering treatment, or you could refrigerate the specimen well below the martensite start temperature of the remaining austenite and then retemper.  The preventative treatment is to reduce the austenitization temperature of the tool to lower the carbon content of the austenite.
Why is this possible ?   Think about it, and then proceed.




























Iron - cementite phase diagram Explanation: The iron - cementite phase diagram shown here has been annotated to indicate the carbon contents of the microconstituents of one hypereutectoid steel (i.e., like the present one) or one hypoeutectoid steel (like a so-called dual phase steel). 

Partial austenitization causes partitioning of the carbon (along a tie line) between the two phases of the austenite plus cementite alloy or the austenite plus ferrite alloy.

Note: Next time, refrigerate this knife immediately before tempering, temper, refrigerate again, and retemper.  This is the preferred treatment for eradicating retained austenite.  However, it only works for properly heat treated steels.
Go on to Specimen 4.