Typically the steel is heated to a suitable temperature, such as sub-eutectoid steel is heated to AC3 (expressed steel heating ferrite all into critical point austenite) above 30--60 ℃, keep after a certain time, so that rapid cooling process which is called quenching. Quenching can be water or oil as a cooling medium. Quenching can improve the hardness and wear resistance of the metal workpiece, which is widely used in various engineering, mold, measuring and require surface wear parts (such as gears, rollers, carburizing parts, etc.). By quenching and tempering with different temperatures, can greatly improve the strength of the metal, toughness and fatigue strength, and get fit (mechanical properties) between the performance of these to meet different requirements. Further hardening can also make some special properties of steel to obtain certain physical and chemical properties, such as hardened steel to enhance its permanent magnet magnetic iron, stainless steel to improve its corrosion resistance. Quenching process is mainly used for steel. 

After quenching of steel, the hardness and strength can be greatly improved, and can improve certain physical and chemical properties. However, due to rapid cooling, there will be stress, ductility and toughness are decreased. To improve the overall performance of steel, usually after quenching by the process known as quenching tempering process. Hardened steel refers to the metal after quenching, martensite, hardness greater than HRC50 steel. 

Characteristics of quenching when cutting 
1, high hardness, high strength and almost no plastic: This is the main characteristic of the cutting of hardened steel. When the hardness of hardened steel reaches HRC50 ~ 60, its strength up σb = 2100 ~ 2600MPa, workability of the material to be processed for grading, hardness and strength are 9a grade hardened steel, are the most difficult to cut materials according to the. 
2, cutting power, cutting temperatures are high: the cut chips from the high hardness and strength of the workpiece, the unit cutting force of up to 4500MPa. In order to improve the cutting conditions, increasing the cooling area, tool selection smaller angle and vice declination. At this time will cause vibration, required to have a better process system rigidity. 
3, easy to BUE: hardened steel with high hardness, brittleness, difficult to produce BUE cutting, surface to be machined can get a lower surface roughness. 
4, the blade is easy to chipping, abrasion: Due to the brittleness of hardened steel, in contact with the blade when cutting short chips, cutting force and cutting heat concentrated near the tool edge, easy to make cutting edge chipping and wear. 
5, low thermal conductivity: thermal conductivity is generally hardened steel is 7.12W / (m K?), About 45 steel seventh. The machinability of the material 9a grade level is below the cutting material is difficult. Due to the low thermal conductivity of hardened steel, cutting heat is difficult to chip away by high cutting temperatures accelerate the tool wear.