The quenching process, quenching medium and cooling method

Steel quenching process is heated to AC3 or AC1 point above a certain temperature, keep a certain time, then cooling martensite and (with appropriate speed Or) Bainite heat treatment process of the organization. Quenching the purpose is to improve the hardness, strength, abrasion resistance to satisfy the use performance of the parts. Quenching process used most widely, such as tools, measuring tools, dies, bearings, springs and automobile, tractor, diesel engine, cutting machine, pneumatic tools, drilling machinery, agricultural machinery, petroleum machinery, chemical machinery, textile machinery, aircraft and other parts used in quenching process. ( 1) Quenching heating temperature quenching heating temperature according to different steel composition, microstructure and performance requirements to determine. Hypoeutectoid steel is AC3 + ( 30～50℃） ; Eutectoid steel and hypereutectoid steel is AC1 + ( 30～50℃） 。 Hypoeutectoid steel quenching heating temperature, if choose below the temperature of the AC3, then this ShiGang austenitizing not fully exist some transformation of ferrite, ferrite after quenching in quenching organization is retained. Ferrite hardness is low, so that the hardness after quenching can not meet the requirements, it will also affect other mechanical properties. If hypoeutectoid steel is heated to the far higher than the AC3 temperature quenching, austenitic grain back significantly bulky, and damage performance after quenching. So hypoeutectoid steel quenching heating temperature choose AC3 + ( 30～50℃） , so not only ensure sufficient austenitizing, and keep the austenitic grain size small. Hypereutectoid steel quenching heating temperature generally recommended as AC1 + ( 30～50℃） 。 In the actual production according to the circumstance around 20 ℃. In the heating temperature range, the organization for the small grain size of austenite and small parts evenly distributed not dissolve carbide. In addition to a handful of residual austenite after quenching in vitro, its organization for lamellar martensite uniform distribution of fine carbide particles on substrates. Such high hardness, wear resistance, and brittle is relatively small. Hypereutectoid steel quenching heating temperature cannot under AC1, because the steel has not yet been austenitizing at this time. If heated to just above the AC1 temperature, the pearlite transformation austenite, completely and a small amount of cementite into austenite. The austenitic grain size small, and the mass fraction of the carbon has a little higher and eutectoid composition. If continue to raise the temperature, the secondary cementite into austenite, the austenitic grain growing, its rising carbon concentration, can lead to increasing of quenching deformation tendency, quenching organization microcrack and the brittleness increases. Due to the austenite high carbon content at the same time, the residual austenite after quenching, reduce the workpiece hardness and abrasion resistance. Therefore hypereutectoid steel quenching heating temperature is higher than AC1 too much is not appropriate, ACm or heated to complete austenitizing temperature above are not appropriate. Choose quenching heating temperature of workpiece in production practice, in addition to comply with the above general principles, consider the chemical composition, the technical requirements of workpiece shape, size, the original organization and heating equipment, cooling medium, and many other factors, the heating temperature be adjusted appropriately. Such as alloy steel parts, usually take limit, take the lower limit for complex shape parts. Strong new technology selection of quenching heating temperature and the commonly used different quenching temperature. Such as the temperature quenching is hypoeutectoid steel in slightly lower than the temperature of the AC3 austenitizing after quenching, which can improve the toughness, reduce the brittle transition temperature, and can eliminate the temper brittleness. Such as 40 cr, 45, 60 si2 materials and temperature quenching heating temperature of workpiece is AC3 - ( 5～10℃） 。 Using high temperature quenching can obtain more plate strip martensite or make full lath martensite improve strength and toughness. Such as 16 mn steel at 940 ℃ quenching, 5 crmnmo steel at 890 ℃ quenching, 20 crmnmo steel at 920 ℃ quenching, the effect is better. High carbon steel at low temperature, fast, short-time heating quenching, decrease the high carbon steel quenching heating temperature, or the use of rapid heating and shorten the time of heat preservation, can decrease the carbon content of austenite, improve the toughness of the steel. ( 2) Holding time in order to make the workpiece inside and outside the organization transformation, carbide dissolution and the parts are completed austenitic composition homogenization, quenching heating temperature so as to have heat preservation time, both the heat preservation time. ( 3) Workpiece quenching medium used in the quenching cooling medium called quenching cooling medium ( Or quenching medium) 。 Ideal condition of quenching medium should have is to make the workpiece can quench into martensite, and does not cause much quenching stress. This will require more than the 'nose' of the C curve slow cooling temperature, to reduce the thermal stress generated by rapid cooling; In the 'nose' cooling speed is greater than the critical cooling rate, to ensure the supercooled austenite does not occur the martensitic transformation; At the bottom of the 'nose', especially to Ms point temperature, cooling speed should be small, as far as possible to reduce the stress of the organizational transformation.