Our discussion has dealt with the cause of retained austenite (parts 1 and 2). We began our discussion of retained austenite as a problem last week, and we will conclude that discussion here. The control and elimination of retained austenite will be our final topic.
Problems of Retained Austenite
An interesting phenomenon occurs as a result of the surface carbon content in relation to where the Ms line will be on the vertical temperature axis of the temperature/time graph.
As the surface carbon content increases, the Ms temperature will decrease with carbon content. Therefore, if one requires consistent surface metallurgy, it is necessary to accurately control the surface carbon content of the steel. Ms will also be affected by the alloying elements of the steel. It is a known fact that the Mf (martensite finish) will be approximately 420°F below that of the Ms temperature. The steel composition (including the surface carbon potential) can reduce the Mf line into negative temperature values.
Retained austenite is a relatively soft metallurgical phase. It can be identified microscopically or the surface hardness value would INDICATE the potential presence of R/A. This means that “mixed metallurgical phases” (martensite and austenite) are present in the steel surface.
The austenite is known to be unstable below the upper critical line of the ICE diagram, and will begin its transformation to untempered/fresh martensite progressively and over time. This means that the surface hardness will increase if the R/A is not addressed.
Further, another change will be occurring. The R/A is a face-centered cubic lattice, which has a different volume from the body-centered tetragonal lattice of martensite. Therefore, the size will change progressively as the hardness increases. This is the inherent danger of having mixed residual phases in the steel surface.
Part 5 will deal with the control and elimination of retained austenite.