When steel is carburized in the austenite condition (above the AC3 line and above the ACcm line), the austenite is a very stable metallurgical phase.
After completion of the carburizing cycle, the steel is then cooled down to its appropriate austenitizing temperature. Depending on the rate of cooling, the surface carbon content and the composition of the steel, it is then rapidly cooled. The austenite phase becomes unstable and (hopefully) will decompose to form the hard martensite phase in the surface of the steel after the cooling cycle is complete.
We are assuming that the cooling rate is fixed (water or oil). Martensite is not a high transformation-temperature phase but rather a low temperature beginning around 400°F to 500°F (depending on the steel chemistry).
Typically, the result is an incomplete transformation to full martensite. The retained austenite is then unstable, which means that it will transform progressively and continually if it is not stabilized. If the retained austenite is not addressed and stabilized, the progressive transformation of retained austenite into untempered martensite, or fresh martensite, will occur. The rate of transformation will depend on the operating-environment temperature and the amount of retained austenite. The net result is:
- An increase in hardness (because of the transformation of austenite to martensite)
- A change in dimension (because of the phase change of FCC to TCC)
If, after quenching of the carburized part for surface and case hardening, the part is then cooled in a freezing mixture such as liquid nitrogen or a solvent-based freezing mixture, the retained austenite will decompose into fresh martensite and become stabilized. The part can then be tempered.
There is no “magic” number for the freezing mixture. The decomposition rate is now time/temperature related.
Care must then be taken after the freezing treatment has been concluded. The part should be allowed to warm up to ambient temperature, and this should then be tempered at the appropriate temperature to produce the required hardness and mechanical properties.
How do we see or know that retained austenite may be present?
An indication of the presence of retained austenite is by hardness testing after quenching. If the as-quenched hardness is 3-5 HRC lower than expected, it could be a strong indication that retained austenite is present in the formed case. Although not a guarantee, it is an indication that the austenite may not have completely transformed.
Another method of ascertaining the presence of retained austenite is by microscopy. This will require a careful cutting of the sample to be prepared, followed by the pre-grind, rough polish and final polish. Once this has been accomplished, the sample is etched using a 3-5% Nital etch, soaking for approximately five seconds to reveal the surface and formed case microstructure. (It should be noted that there are other etchants available.) The retained-austenite structure will etch out white with the martensitic needles. One can also see free carbides if the steel is an alloyed carburizing steel such as AISI 8620. The austenite volume can be calculated based on the prior microstructure, prior grain size and the amount of austenite formed in the case.
This article will be continued in part 2.