PSI - Issue 68

Ema Kukuljan et al. / Procedia Structural Integrity 68 (2025) 822–827 E. Kukuljan et al. / Structural Integrity Procedia 00 (2025) 000–000

823

2

Nomenclature ρ density, kg/m 3 heat conductivity, W/mK T s surface temperature, K quenchant temperature, K specific heat capacity, J/kgK

heat transfer coefficient, W/m 2 K

T f

cooling time from 800°C to 500°C, s

t 8/5

Achieving the desired hardness distribution within a component necessitates proper definition and optimization of the quenching process and its parameters. Numerical simulation of cooling during quenching and estimation of the resulting hardness can help facilitate and accelerate this task Smoljan and Liščić (1999), Liščić et al. (2010). 2. Quenching process and its parameters Quenching is a heat treatment process in which steel is heated and then rapidly cooled to achieve the highest possible proportion of martensite in its microstructure, thereby increasing its hardness. The main part of the quenching process involves rapid cooling of the heated specimen by immersing it in a cooling medium, usually water or oil. The choice of cooling medium and cooling regime depends on the material of specimen and the desired properties. The quenching process can be physically divided into three stages: vapor stage, boiling stage, and convection stage. During the immersion of the metal specimen heated to a high temperature in the cooling medium of significantly lower temperature, a non-stationary temperature field occurs in an isotropic rigid body without additional heat sources, which can be somewhat simplified and described by Fourier's law of heat conduction (conduction): !( cρT ! ) t = div ( λ grad T ) (1) and characteristic boundary condition (convection): - λ δT δn ' s =α ( T s − T f * , (2) Although simplified for practicality, modeling the quenching process is complicated by the fact that the values of parameters describing heat transfer, such as sample material's density ρ , specific heat capacity c , heat conductivity coefficient λ , and heat transfer coefficient α depend on temperature to a greater or lesser extent, as shown in Figure 1.

Fig. 1. Heat transfer coefficient distribution for quenching of steel AISI 4140 in oil without agitation, Smoljan et al. (2015).