Abstract: A method for evaluating a solder joint portion by means of which a part and a substrate are joined to each other includes a preparation process for preparing the substrate including the solder joint portion, a thermal shock process for applying thermal shock to the solder joint portion multiple times, and an evaluation process for obtaining a change of a crystal grain size in the solder joint portion caused by the application of the thermal shock so as to evaluate a lifespan of the solder joint portion based on the change of the crystal grain size obtained.

Abstract: A method for evaluating a solder joint portion by means of which a part and a substrate are joined to each other includes a preparation process for preparing the substrate including the solder joint portion, a thermal shock process for applying thermal shock to the solder joint portion multiple times, and an evaluation process for obtaining a change of a crystal grain size in the solder joint portion caused by the application of the thermal shock so as to evaluate a lifespan of the solder joint portion based on the change of the crystal grain size obtained.

Abstract: A method for the heat treatment of steel, comprising carburization hardening said steel, to form carburized steel, followed by a second hardening, reduces the grain size of steel while precipitating carbonitrides inside the grains of steel, thereby increasing the strength of the heat-treated steel. The carburization hardening comprises heating the steel to a first austenitic temperature range in an atmosphere comprising a carburizing gas and optionally ammonia, cooling the steel to a martensitic temperature range, and quenching the steel, and the second hardening comprises heating the carburized steel to a second austenitic temperature range in an atmosphere comprising a carburizing gas and ammonia, and quenching the steel.