Determination of Cooling Intensity Using Transient Heat Transfer Technique at Gas Quenching in Vacuum Heat Treating Furnace

Abstract

During vacuum heat treatment, the cooling rate of workpiece at gas quenching decreases in proportion to time. It's difficult to express with cooling rate (ex. K/s) the relation between the metallurgical change and the cooling intensity of workpiece. The purpose of this paper is to examine the cooling behavior of workpiece in vacuum heat treatment using lumped capacitance method. As the temperature difference between center and surface of column of workpiece was small at nitrogen gas quenching, heat transfer coefficients were determined around 40 W/(m²・K) at 0.067MPa and around 75 W/(m²・K) at 0.197MPa. The dimensionless temperature, the cooling time and the time constant had an exponential relation. The time constant consists of physical properties and is also defined as the time at that the dimensionless temperature equals to 1/e. The agreement was confirmed by both of experiment and theoretical calculation. It is concluded that the time constant was suitable for the explanation of cooling intensity at gas quenching under vacuum heat treatment.