Is the quality of steel shot related to its dark, shiny surface?

　　After quenching and heating, steel undergoes two cooling methods: isothermal cooling (isothermal quenching) and continuous cooling (ordinary quenching). Isothermal cooling can produce pearlite, sorbite, troostite, bainite, etc., as needed, while ordinary quenching aims to obtain martensite. Various steel parts, mechanical components, tools, and molds must be quenched and tempered before use. The strengthening and hardening that occurs during quenching is due to the formation of martensite.

　　Martensite has two basic forms: lath martensite, which appears as clusters of laths under a crystallography microscope; and plate martensite, which appears as bamboo leaves under a crystallography microscope. In terms of properties, lath martensite not only has high hardness but also good toughness; while plate martensite has high hardness but poor toughness, typically hard and brittle, requiring tempering to sacrifice hardness to improve toughness. Therefore, we hope that the workpiece will have more lath martensite after quenching. In actual production, steel with a carbon content below 0.2% produces entirely lath martensite after quenching; steel with a carbon content between 0.2% and 1.0% produces a mixed structure of lath and plate martensite after quenching; and steel with a carbon content above 1.0% produces almost entirely plate martensite after quenching.

　　Although carbon content determines the morphology of martensite, we can adjust the process to maximize the performance of the workpiece. Low-carbon steel and low-carbon alloy steel have poor hardenability; therefore, by using intense quenching (cold brine), we can obtain almost all lath martensite, achieving a good balance of high strength and toughness. Medium-carbon steel (0.3%-0.6% carbon content) or medium-carbon alloy steel is widely used, with a carbon content between 0.2% and 1.0%. Quenching produces a mixed structure of lath and plate martensite. High-temperature quenching of these steels can yield more lath martensite, significantly improving the toughness of the steel without changing the hardness. For high-carbon steel parts, to obtain more lath martensite, a rapid, short-time quenching process at a lower temperature can be used.

　　Therefore, the same material can achieve more desirable properties through steel shot more appropriate quenching process.