oilquenched

Oil quenched refers to a metal, most commonly steel, that has undergone oil quenching, a heat treatment in which the material is heated to its austenitizing temperature and then rapidly cooled by immersion in oil. The quench oil is usually mineral oil, though synthetic or vegetable oils may be used. The cooling rate is slower than water quenching, which helps reduce cracking and distortion while still promoting the formation of martensite in many alloy steels.

In practice, the steel is heated to an austenitizing temperature appropriate for the alloy, typically in the range of about 800 to 950 degrees Celsius. After a soak to dissolve alloying elements and homogenize the structure, the part is immersed in oil, often with agitation or in a specially designed quenching bath. The oil may be preheated to influence the cooling rate and mechanical properties. The resulting microstructure is largely martensitic, producing high hardness, though the exact properties depend on the alloy and the quenching conditions.

Oil quenching is commonly used for tool steels and bearing steels, such as 4140, 4340, and 52100, as well as other alloys that respond well to moderate cooling rates. After quenching, many parts are tempered to achieve a balance of hardness and toughness and to relieve residual stresses. Advantages include reduced quench cracking and distortion compared to water quenching, while disadvantages include potentially lower maximum hardness and the need for oil handling and disposal considerations. Safety and environmental concerns, including fire risk and lubrication oil management, are also important.