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Retained Austenite - Is it Good or Bad?

By Pankaj Deval
4 Aug 2017

Retained Austenite – Is it Good or Bad? And Why?

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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.

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13 Responses

  1. Dr. PapaRao

    Very informative article.. What is the best characterization technique if retained austenite % is 0.5 to 4.0 %. ??

  2. Sainath R

    Sir nice article.
    If RA transforms into martensite, what might the stress required for such transformation??
    Can it be calculated??

  3. Ajay shukla

    I don’t think RA will transform to Martensite during tempering, some other product will be formed.
    Why RA is at surface not at core in carburized component?

    1. Thank you for your comments. My reply is as follows:
      1) There is no doubt that tempering does help to transform at least some of the retained austenite to martensite. How much retained austenite is transformed will depend upon how stable is the retained austenite and the tempering temperature. There are many references available which confirm that tempering does reduce retained austenite. Please click on the follwoung link to check one article on retained austenite http://www.heat-treat-doctor.com/documents/RetainedAustenite.pdf

      2) In a carburised component, the case has higher percentage of carbon content (approx 0.8%). The carbon is an austenite stabilizer. Therefore it is the root cause of retained austenite in the case. However, the carbon content of the core is low (around (0.20%) and hence, retained austenite is not found in the core.

      I hope this answers your query.

  4. Dheeraj Kumar Singh

    If retained austenite is coming more than 30~40% in gears after running of approximate 25000 km on vehicle and even hardness is increased to 64-65 HRC. what does it means?

        1. Such high % of retained austenite for this grade is quite unusual. The only possible reason seems to be the surface carbon might be high. You need to control the Carbon potential during the boost and diffuse periods. What is the carburising temperature and what is the CP during boost and diffuse cycle? What is the hardening temperature?

          1. Dheeraj Kumar Singh

            thanks. I need to check with my heat treatment department for same.
            what will be impact on gear life with this much high % of RA (30~40%)

    1. RA is retained austenite. The aim of hardening process is to convert the austenite to martensite to get maximum hardness and strength. However, for various reasons some of the austenite remains untransformed. Which means that less martensite has formed. So the as quenched hardness is lower if you have high amount of retained austenite. Also the retained austenite is a meta-stable (or unstable) phase and during service, it transforms to martensite which remains un-tempered. Hence the ductility of the component is drastically afftected. Also when the retained austenite transforms to martensite during service, there is a volume growth which causes change in dimensions of the component, which may be undesirable if the part is a bearing race or inspection/calibration gauge. Hope, I was able to answer your question. Please feel free to ask if you need any clarification.

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