Predictive tools for the isothermal hardening of strip steel parts in molten salt; pp. 152–158

The current study focuses on an industrial hardening process where the steel parts are austempered in a molten salt bath. The aim of the study was to compose a predictive tool for more qualified process adjustment and control in a hardening plant. The process approach considers specific 3.0Â to 4.0Â mm thick strip steel products and steel grades with carbon content in the range from 0.27Â to 0.62Â wt%. Austempering in salt produces high hardness and ductility of steel as the final microstructure is bainitic. Salt bath cooling is not so severe as cooling in oil, polymers, or water and has an advantage of a uniform cooling rate. There are several approaches to determining hardenability of steels and interrelating it to the quenching process to predict the final hardness of the parts. The best known are the Jominy hardenability test and the Grossman hardenability number where hardness is presented as the function of the specimen geometry. To determine the quenchant properties the Grossman number, nickel ball, hot wire, cooling curve, and quench factor methods are used. These methods are guiding tools to the engineer for choosing the steel grade and the hardening process. The latest approach is quench factor analysis, which interrelates the cooling curve of the quenchant and material hardenability data with a single number and allows prediction of the properties of the hardened material. The imperfection of the quench factor is limited availability of mathematical constants for different steel grades to make calculations. As the cooling process and phase transformations are nonlinear, the computation accuracy does not seem to satisfy the heat treaters to utilize it in the hardening plant. The types of heat treatment equipment in use have different characteristics, which are related to the technological process used and the design of the machine. The available engineering diagrams for steels and processes are useful to have a general view of the process but still each workshop has to specify the approach to be used in its particular case.