The tempering is a heat treatment process that the parts after quenching heated below a certain temperature below Ac1 is heated for a certain time after cooling to room temperature. Quenched steel generally not directly be used, must be together with tempered. The purpose of tempering is to reduce the brittleness of quenched steels, to eliminate or reduce internal stresses, to reduce the tendency of deformation and cracking, to transform unstable quenched Markov and retained austenite into more stable ferrite and cementite or Two-phase mixture of carbides. To ensure that the stability of the workpiece in the course of the organization, shape and size to meet the different parts of the use requirements, then access to the appropriate strength, hardness, ductility and toughness.
Tempering process
The steel tempering process should be based on the chemical composition, the workpiece performance requirements and the hardness and organization after quenching ,then make a appropriate tempering temperature, holding time, and the cooling method after tempering to ensure that the workpiece getting the required performance after tempering. The most important factor that determines the microstructure and properties of a workpiece after tempering is the tempering temperature. Production according to the required mechanical properties and the level of the required tempering temperature of the workpiece, the tempering can be divided into low-temperature tempering, mid-temperature tempering and high-temperature tempering.
Low temperature tempering.
Low tempering temperature range is generally 150 – 250 ℃. Most of low-temperature tempered steel are quenched high-carbon steel and quenched high-alloy steel; after low-temperature tempering it obtain the organization of martensite with fine granular carbide, ie, tempered martensite. After low temperature tempering, sub-eutectoid steel obtains the tempering tempered martensite and eutectoid steel obtains tempering tempered martensite + carbides + retained austenite. The purpose of tempering at low temperature is to appropriately increase the toughness of hardened steel while maintaining high hardness (58 – 64HRC), high strength and wear resistance, while significantly reducing the quenching stress and brittleness of steel. Low temperature tempering in the production of large quantities used in tools, measuring tools, rolling bearings, carburizing parts, surface hardening parts. Precision measuring tools, bearing parts In order to reduce the additional stress formed in the final processing steps, increase the dimensional stability, increase the tempering temperature between 120 – 250 ℃, holding time up to tens of hours of low temperature tempering, sometimes called For artificial aging or stabilization.
Mid-temperature tempering
Mid-temperature tempering range is generally between 350-500 ℃, tempering the body is distributed in the ferrite body with a large number of fine-grained cementite, that is, tempering the body tissue. The tempering of the ferrite in the retained austenite also retains the morphology of the martensite. After tempering temperature in the workpiece internal stress basically eliminated, with high elastic limit and yield limit, high strength and hardness (35 – 45HRC) and good ductility and toughness. Temperature tempering is mainly used for a variety of spring parts and hot forging dies.
Tempering at high temperature. High temperature tempering temperature of 500 – 650 ℃, usually quenching and subsequent high temperature tempering heat treatment process is called quenching. the organization of high-temperature tempering is called tempered sorbite, that is, fine-grained cementite and ferrite. After high-temperature tempering steel get high strength, ductility and ductility which are better overall mechanical properties, hardness of 25 – 35HRC, widely used in carbon structural steel and low-alloy structural steel manufacturing a variety of components of the more complex stress components, such as engine crankshaft, connecting rod, connecting rod bolts, semi-shafts, machine tools and spindles, etc., but also can be used as precision workpieces such as Gauge, mold and other pre-heat solution.
In addition to the above three tempering methods, some high-alloy steels that can not be softened by annealing can also be softened and tempered at 600 – 680 ° C.
Post time: Nov-15-2017