Inverse hardening a steel of adequate hardenability requires a workpiece of sufficiently large cross section, an appropriate cooling medium, and the right quenching conditions. This article explains the Temperature Gradient Quenching Analysis System (TGQAS), which can measure, record, and evaluate all quenching processes in common use, describing their heat extraction dynamics by corresponding thermodynamic functions. It discusses the metallurgical aspects of steels with an emphasis on two different processes, namely, heat extraction (a thermodynamic process) and microstructural transformation (a metallurgical process) that are initiated at the moment when the austenitized workpiece is immersed in the quenchant. The article describes the uses of polyalkylene glycol copolymer and the effect of hardness and fatigue resistance on AISI 4140 type steel.