Retained austenite is that fraction of austenite which remains untransformed at the end of the hardening process. Retained austenite is considered detrimental or undesirable in most cases but there are certain applications where some amount of retained austenite is considered as desirable. Let us consider both these cases.
Applications where retained austenite is considered detrimental and why?
Austenite being a softer phase, if RA% is high, you will observe soft spots. The soft spots are detrimental in applications such as cutting tools where wear resistance is one of the most important factors affecting the service performance and life.
Austenite, being a metastable (un-stable) phase has propensity to tranform to martensite. Therefore whatever fraction of austenite is retained (untransformed) at the end of the hardening process is likely to transform to martensite during subsequent tempering or when the hardened part undergoes stress and strain in service. This newly transformed martensite will remain untempered and hence cause brittleness, which can cause premature failure of the component in service. Due to this reason, retained austenite is considered to be detrimental in applications such as tools and dies, where high impact loading is an essential service condition.
The transformation of austenite to martensite is accompanied by increase in volume. Therefore retained austenite, when it transforms to martensite during the service life of the component, would affect the dimensional stability of the component. This is detrimental in applications such as bearings and guages where dimensional stability is extremely important.
Applications where some amount of retained austenite is considered beneficial and why?
Finely dispersed retained austenite resists the propogation of fatigue cracks and improves rolling contact fatigue (RCF) stress. Therefore some amount of retained austenite is considered to be beneficial in certain applications. One such application is bearings working with contaminated lubricants (eg: railway bearings). In some cases, bearing components made of through hardening steels like SAE 52100 are carbonitrided because the carbonitriding process gives higher surface hardness and increases wear resistance and it also promotes retained austenite.
Please stay tuned and subscribe to this blog for more such useful tips on heat treatment. Please also share this post for the benefit of your friends and contacts.
Do you have any questions or challenges regarding retained austenite? Please feel free to post your queries, share your experience or make differing comments in the comment section below.
Or you can take a FREE TRIAL of any of our courses. Please click here.
See you tomorrow.