Effect of Bath Temperature on Cooling Performance of Molten Eutectic NaNO 3 -KNO 3 Quench Medium for Martempering of Ste
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CONVENTIONAL quench hardening involves quenching of an austenized steel part into a quench medium. Water, mineral oils, vegetable oils, brine, and aqueous polymer solution are generally used as quench media for conventional quench hardening.[1] The heat extraction during quenching in conventional vaporizable quench media occurs in three stages, namely, vapor blanket stage, nucleate boiling stage, and convective cooling stage.[2] The heat transfer rate is very low during the vapor blanket stage, and then it increases and reaches maximum during the nucleate boiling stage. The heat transfer rate subsequently decreases to a very low value during the convective cooling stage.[3] The heat flux at the metal–quenchant interface during quenching is not only transient in nature but also spatially dependent because of the simultaneous presence of
K.M. PRANESH RAO and K. NARAYAN PRABHU are with the Department of Metallurgical and Materials Engineering, National Institute of Technology Karnataka, Surathkal, Srinivasnagar, Mangalore 575 025, India. Contact e-mails: [email protected], [email protected] Manuscript submitted September 14, 2016.
METALLURGICAL AND MATERIALS TRANSACTIONS A
two or all three stages of cooling at the surface of the probe.[4,5] These nonuniform cooling and high heat extraction rates generally result in microstructural variation, distortion, cracking, and residual stress in the quenched part. These are major defects observed in conventional quench hardened parts.[6] An ideal quench medium is the one that has no vapor blanket stage, very short boiling stage, and cools the part very slowly during the convective cooling stage, when austenite transforms into martensite in the steel.[7] The quench medium should transform the maximum amount of austenite into martensite by abating undesired high temperature diffusion based transformations and minimizing distortion, cracking, and residual stress in the quenched part. These qualities cannot be realized in a single quench medium, and hence, the interrupted quenching process was developed. Unlike the conventional quench hardening process that is a single-step process, the interrupted quenching process is a multistep process. Martempering, a widely practiced industrial heat treatment process, is an example of the interrupted quenching process. Martempering involves quenching of an austenized steel part in a medium maintained at a temperature close to the martensite start temperature of the steel and held for some time. The part is then
removed from the quench bath and subjected to air cooling. Holding the part in a bath maintained at a temperature of martensitic start temperature reduces the thermal gradient in the steel part, thus, reducing thermal stresses. The subsequent step of slow air cooling of the steel part ensures slow transformation of austenite into martensite, thus, reducing distortion, cracking, and residual stress in the quenched part.[8] Martempering the heat treatment process requires a quench medium that can operate in the te
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