Difference between Nitrate quenching and oil quenching

Quenching is a critical heat treatment process to enhance the hardness and toughness of metal parts, and nitrate quenching and oil quenching are two commonly used methods with distinct characteristics.
The most obvious difference lies in the quenching medium. Nitrate quenching uses molten nitrate salt (usually a mixture of potassium nitrate and sodium nitrite) as the cooling medium, while oil quenching adopts specialized quenching oil. This difference leads to distinct cooling performances: nitrate salt has stable chemical properties and strong cooling capacity, with almost no vapor film stage during the quenching process, enabling rapid and uniform cooling. In contrast, oil quenching goes through three stages (vapor, boiling, and convection), with a relatively slow cooling rate due to the initial vapor blanket, which may cause uneven cooling in some cases.
Regarding workpiece performance, nitrate quenching results in a gentle hardness gradient, deeper hardened layer, and smaller deformation, which can increase the crush load of parts by about 30% compared with oil quenching. It also leaves the workpiece surface with compressive stress, improving overall performance. Oil quenching, however, tends to produce a steeper hardness gradient and surface tensile stress, and the deformation of parts is usually larger than that of nitrate quenching.
In terms of application scenarios, nitrate quenching is more suitable for complex and thick-walled parts that require high dimensional stability and toughness, such as bearing components. Its cooling capacity can be adjusted by changing the water content of the nitrate salt, making it flexible for different needs. Oil quenching is widely used for general steel parts that do not require extremely high precision, as it is easy to operate, low-cost, and suitable for mass production.