Martempering or Marquenching is an interrupted quenching process employed to minimise the risk of cracking and distrotion of the quenched parts. The term Martempering is actually a misnomer and Marquenching is a better term. Let us see how Marquenching helps to reduce the risk of distortion and cracking?
Danger of Cracking and Distortion while hardening:
During quenching there are temperature gradients and the surfcae of the quenched part cools much faster than its core. As a result, the surface of the part reaches Ms temperature first and the martensite transformation takes place first on the surface of the part. By the time the core of the quenched part reaches the Ms temperature, the surface of the quenched part would have already transformed to martensite. As the cooling progresses further, the austenitic core starts to transform to martensite and the volume of the core starts to increase. However, the hard martensitic skin of the part resists this volume growth, giving rise to tremendous internal stresses in the quenched part which can lead to distortion and cracking.
Marquenching of Through Hardened Parts:
Marquenching aims to solve or minimise this problem. The part is quenched in a quenching medium which is held at a temperature just above the Ms temperature. The part is kept immersed for sufficient time, just enough to equalise the temeprature of the surface and the core. The structure of the core as well as the surface is still austenitic at this stage. Now when the part is taken out of the quenching medium and allowed to cool in still air, the core as well as the surface of the part start to transform to martensite at a very slow rate and almost simultaneously. And hence, whatever, volume growth which occurs in the core as a result of the transformation to martensite is accomodated by the soft cushion of austenite which is still available on the surface.
Hence, the internal stresses developed due to volume growth in the core are minimised by Marquenching. This helps to prevent cracking and minimise distortion.
Marquenching of Carburised Parts:
The situation is a bit different in case of carburised parts. The carburised steels are low carbon steels and hence their Ms temperature is fairly high. When these steels are carburised, the carbon content of the surface is increased which results in lowering down of the Ms temperature of the case. Hence, when marquenching carburised parts, the quenching medium is held at a temperature which is higher than the Ms temperature of the case but lower than the Ms temperature of the core. After allowing sufficient time for equalising the temperature of the core and the case, the parts are allowed to cool in still air.
During air cooling, the core which has already partially transformed to martensite completes the transformation along with the case. Thus the residual stresses and the distortion is drastically reduced.
Quenching Mediums for Marquenching:
Nitrate – Nitrite based molten salts and hot quenching oils are the preferred quenching mediums for marquenching.
Limitations of Marquenching:
Due to the high temperature of the quenching medium, the quench severity is seriously hampered. Hence, there is a danger of intercepting the nose of the TTT curve while cooling to the temperature of the marquenching medium. Therefore hardenability of parts to be marquenched should be sufficiently high. Also parts with very high section thickness can not be marquenched satisfactorily. Please remember that even though the residual stresses are minimised because of Marquenching, tempering after marquenching is still required.
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