Because of the alloying elements and providing that the appropriate austenitizing temperature has been used, there is little chance of any retained austenite forming. Unless they are treated in a vacuum furnace or salt bath, however, these steels will severely oxidize during the cool-down procedure with massive amounts of surface scale being formed. In addition to this, there will most certainly be decarburization.
If decarburization is allowed to occur, there will be a significant loss of surface hardness. If the steel is treated in a furnace atmosphere, great care needs to take place with the atmosphere and temperature control. If the atmosphere is controlled with the carbon potential higher than that of the steel, carburization will occur. This can lead to retained austenite occurring in the surface, which will also lead to a lower hardness and, perhaps, some dimensional instability.
Because of the high austenitizing temperatures of the A-series steels and running with a high furnace carbon potential, the carbon diffusion will be quite rapid in relation to the process temperature.
It must be stressed that the selection of the austenitizing temperature is critical as is the protection of the surface of this group of steels. If the process temperature selected is too low, there is likely to be too high a content of carbon and alloy in the alloy carbides. If the alloying elements are not available in the matrix, the hardness will be reduced because non-martensitic structures can form during air quench.
Conversely, if too high an austenitizing temperature is selected, too much carbon and alloying elements dissolve in the martensite. Hardness values will be low due to the formation of retained austenite as well as the lower volume fraction of carbides in the microstructure.
So, temperature selection is extremely critical to the success of the hardening of any of the A-series tool steels as well as soaking time at the austenitizing temperature. Great care must also be taken with the protection of the surface for both decarburization or carburization.
Because of the alloying elements, the Time-Temperature-Transformation (TTT) curves are far over to the right hand side of the graph, thus allowing time for martensite to form.
If retained austenite does occur, it will be observed by lower-than-normal as-quenched hardness values, which is usually indicative (but not conclusive) of retained austenite. There are two methods of decomposing the retained austenite. These are:
- Multiple high tempering temperatures will contribute to the decomposition of retained austenite.
- Refrigeration/cold treatment in either liquid nitrogen or a solution of dry ice and a suitable freezing liquid followed by tempering.