What Is Quenching?

The quenching of steel is to heat the steel to a temperature above the critical temperature Ac ₃ (hypoeutectoid steel) or Ac ₁ (hypereutectoid steel), hold it for a period of time to fully or partially austenite, and then cool it at a rate greater than the critical cooling rate. The rapid cooling speed is lower than Ms (or isothermal near Ms) for the heat treatment process of martensite (or bainite) transformation. Generally, the solid solution treatment of aluminum alloy, copper alloy, titanium alloy, and tempered glass or the heat treatment process with rapid cooling process is also called quenching.

Quenching is to heat the steel above the critical temperature, hold it for a certain period of time, and then cool it at a rate greater than the critical cooling rate to obtain an unbalanced structure mainly composed of martensite (also available to obtain bainite or maintain single-phase austenite as required ) A heat treatment process. Quenching is the most widely used process method in steel heat treatment. ^[1]

The purpose of quenching is to make

Including heating, holding and cooling three phases. The following takes the quenching of steel as an example to introduce the above three stages

Quenching technology is widely used in modern machinery manufacturing industry. Important parts in machinery, especially steel parts used in automobiles, airplanes and rockets, are almost quenched. In order to meet the diverse technical requirements of various parts, various quenching processes have been developed. For example, according to the part to be treated, there are global, local hardening and surface hardening; whether the phase transition is complete when heated, there are complete hardening and incomplete hardening (for hypoeutectoid steel, the method is also called subcritical hardening); The content of time-phase transformation includes graded quenching, isothermal quenching and under-speed quenching.

In addition, due to the characteristics and limitations of the secondary cargo methods, they are all applied under certain conditions. The most common applications are induction heating surface quenching and flame quenching. Laser beam heating and electron beam heating are currently rapidly developing high-energy-density heating and quenching methods. They have attracted attention because they have some characteristics that are not available in other heating methods.

Surface hardening is widely used in machine parts made of medium carbon hardened steel or ductile iron. Because medium carbon quenched and tempered steel is pre-treated (quenched or normalized) and then surface quenched, it can not only maintain high comprehensive mechanical properties of the core, but also make the surface have higher hardness (> HRC 50) And abrasion resistance. For example, machine tool spindles, gears, diesel crankshafts, camshafts, etc. In principle, gray cast iron, ductile cast iron, malleable cast iron, alloy cast iron, etc. with pearlite ferrite based on medium carbon steel can be surface hardened in principle, while ductile iron has the best process performance and has a high Comprehensive mechanical properties, so the most widely used.

After surface hardening of high carbon steel, although the surface hardness and wear resistance are improved, the plasticity and toughness of the core are low. Therefore, the surface hardening of high carbon steel is mainly used for tools that work under small impact and alternating loads, Measuring tools and high cold hard rolls.

Because the strengthening effect of the surface of the low carbon steel after quenching is not significant, it is rarely used. ^[3]