Prof. Tatsuo Inoue passed away on September 23, 2023, at the age of 83. He held a professorship at Kyoto University from 1983 to 2003 and made significant contributions to the theory of heat treatment simulation, which is now widely used. His theory was reported at an international conference in Linkoping, Sweden in 1984. Fundamental equations in his theory cover metallurgical coupling effects caused by changes due to phase transformation, temperature, and inelastic stress/strain as well as carbon diffusion during the carburizing process. Prof. Inoue designated these effects as “metallothermo- mechanical coupling”. Software applying his theory was presented at ASM International’s 1st International Conference on Quenching and the Control of Distortion in 1992, where its advanced nature was recognized. In 1994, Prof. Inoue published a paper on the application of heat treatment simulation to the quenching of Japanese swords, revealing changes in temperature, curving, microstructure, and stress/strain in their model during the traditional quenching process. In 2017, he published “The Science of Japanese Swords” with Sumihira Manabe, a swordsmith, to communicate his specific achievements to the general public.

Heat treatment simulation has progressed to the point where commercial software is widely available, and validations of simulation functions using experimental results have played a big role in getting here. For this reason, the author presents a number of validation cases and explains what relatively simple experiments can reveal about the complex phenomena of heat treating. In the case of validating basic functions, such as heat transfer and phase transformation, the author uses experimental results of the inverse hardening of quenched steel cylinders. When validating software at the stage where stress and strain functions are added, the author uses measurement data corresponding to length and diameter changes and residual stress distributions in normally quenched steel cylinders. Other cases presented include the validation of curving in long specimens cooled unevenly and the validation of distortions and residual stresses in carburized and quenched, induction hardened and nitrided steels.

A variety of test systems have been developed to determine the cooling characteristics of quenchants. Although current test standards specify cylindrical probes for measuring quenchant temperatures and cooling rates, this review concerns the development, implementation, and potential of test systems that use ball probes instead. It assesses the strengths and limitations of different types of ball probes and describes prototype test systems that leverage ball probe capabilities while compensating for inherent weaknesses.

Performances of quenchants have been enhanced and maintained based on their cooling characteristics determined by specific test systems. A rotary-arm type test system with a small ball probe has been developed for this purpose by making prototypes. Its unique concept derived mainly from a circular motion of a small ball probe in quenchants was proposed by Tawara in 1941. The prototypes have been realized by current heating, measuring and mechatronics techniques. Finally the probe material has been changed from nickel alloy to platinum for resolving the discoloration and thermal aging problems on the probe surface. The performance of the prototypes has been verified by systematic tests using specific quenchants under various cooling conditions.

Blade curving due to quenching in the Japanese sword has been recognized by swordsmiths through the ages. In the late 1920s, Hattori noted that the sword curving is induced from not only martensitic transformation expansion in the near-edge region but also non-uniform elastic and plastic strains distributed in the section, based on his experimental results using cylindrical specimens. Our research for an updated explanation on the subject prepared Japanese sword (JS) type specimens made of the same steel and process as the Japanese sword, and model JS (MJS) type specimens with the almost same shape as the JS type specimens, which were machined from commercial carbon steel and austenite stainless steel bars. All specimens quenched by a swordsmith using the traditional way showed a usual curved shape with different curvatures. Curving, temperature, hardness, metallic structure and residual stress measurements for the specimens were performed to prepare their future simulation works.