jäähdytyskäsittelyn

Jäähdytyskäsittely, often translated as chilling or tempering, is a heat treatment process used in metallurgy to alter the properties of metals. The primary goal is to cool a metal from a high temperature rapidly enough to prevent the formation of undesirable microstructures, such as pearlite or bainite, and instead promote the formation of martensite. Martensite is a hard and brittle phase that can significantly increase the strength and hardness of steel.

The specific cooling rate required to achieve martensite formation depends on the composition of the alloy, particularly its carbon content. Higher carbon content generally leads to a higher hardenability, meaning it can be hardened by slower cooling rates. Alloying elements can also influence hardenability, allowing for effective hardening with less severe cooling methods.

Jäähdytyskäsittely is typically performed by quenching the metal in a cooling medium. Common quenching media include water, oil, and air. Water is the most aggressive quenchant, providing the fastest cooling rate, while oil offers a moderate rate, and air is the slowest, often used for alloys with high hardenability. The choice of quenching medium is critical to achieving the desired microstructure and properties.

Following the quenching process, the metal is usually tempered. Tempering involves reheating the quenched metal to a temperature below its lower critical temperature and holding it there for a specific time before cooling. This process reduces the brittleness of the martensite, improving toughness and ductility while retaining a significant portion of the hardness. Without tempering, martensitic structures can be too brittle for many practical applications.