Spatial Dependence of Heat Flux Transients and Wetting Behavior During Immersion Quenching of Inconel 600 Probe in Brine
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QUENCH hardening for the attainment of superior metallurgical and mechanical properties of steel components is a commonly used method in heat-treating industries. The process involves heating of steel to austenitizing temperature in a controlled atmosphere followed by rapid quenching in a suitable quench medium. Heat transfer and wetting are the two important phenomena that occur during quenching that controls the final metallurgical and mechanical properties of the components. The important parameters which control the metallurgical transformation/heat-transfer condition during quenching are grouped into three categories: (i) workpiece characteristics (composition, mass, geometry, surface roughness, and condition); (ii) quenchant characteristics (density, viscosity, specific heat, thermal conductivity, and boiling temperature); and (iii) quenching facility (bath temperature, agitation rate, and flow direction). Of all these factors listed, only a few can be changed in the heat-treatment shop. The selection of optimum quenchant and quenching conditions both from the technological and economical points G. RAMESH, Senior Research Fellow, and K. NARAYAN PRABHU, Professor & Head, are with the Department of Metallurgical and Materials Engineering, National Institute of Technology Karnataka, Surathkal, Srinivasnagar, Mangalore 575 025, India. Contact e-mail: [email protected], [email protected] Manuscript submitted May 21, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS B
of view is an important consideration.[1] When hot steel components are quenched into a vaporizable liquid medium, the cooling of components occurs by three stages known as vapor blanket, nucleate boiling, and convective cooling stages (Figure 1). Further, the transition from vapor blanket to nucleate boiling leads to the formation of wetting front which is defined as the loci between the vapor film and the occurrence of bubbles. The wetting front moves on the cooling surface with a significant velocity. Based on the movement of the wetting front, wetting behavior during quenching is classified as (i) Non-Newtonian wetting (a wetting process that occurs over a long period of time); and (ii) Newtonian wetting (a wetting process that occurs in a short time period or an explosion-like wetting process). A Newtonian type of wetting usually promotes uniform heat transfer and minimizes the distortion and residual stress development. In extreme cases of nonNewtonian wetting, because of large temperature differences, considerable variations in the microstructure and residual stresses are expected, resulting in distortion and the occurrence of soft spots.[2] The quenching medium has a strong influence on the rewetting and heat-transfer behavior. Water, brine solutions, mineral oils, vegetable oils, and polymer media quenchants are generally used for quench hardening of steel components. Higher cooling rates are observed in all the three stages of quenching for water and brine solutions. For mineral oils, slow cooling rates in vapor- and convective-coo
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