Bearing steel heat treatment

Bearing steel ingot typically conducted in 1200 ~ 1250 ℃ high temperature diffusion annealing for long time, in order to improve carbide segregation. To control atmosphere in furnace, hot working steel billet heating temperature is unfavorable and exorbitant, heat preservation time shoulds not be too long, lest produce serious decarburization. Finishing ( Forging) Temperature is usually between 800 ~ 900 ℃, too high bulky carbide network is seen, too low to rolling ( Forging) Crack. Rolling ( Forging) Wood products should be fast to 650 ℃, in order to prevent the cementite on the grain boundary precipitate net-like, conditional can adopt the control rolling technology. In order to achieve good machinability and quenching organization, before the cold working with spheroidizing annealing of bearing steel to be completely. Annealing temperature is 780 ~ 800 ℃, to prevent the decarburization annealing. If the rolling steel existed coarse mesh cementite, need before annealing for normalizing treatment. Chromium bearing steel is usually between 830 ~ 860 ℃ heating, oil quenching, 150 ~ 180 ℃ tempering. Precision bearing in the organization, should as far as possible to reduce the residual austenite size or make the residual austenite in use process stable, so often - should be carried out after quenching 80℃（ Or lower temperature) Cold treatment and for a long period of time under the 120 ~ 140 ℃ stabilizing treatment. Bearing steel heat treatment process such as normalizing, annealing heat treatment and final heat treatment are two major link in advance. GCr15 steel is an alloy content is less, has a good performance, the most widely used high carbon chromium bearing steel. GCr15 bearing steel after heat treatment with high and uniform high hardness, good abrasion resistance and contact fatigue performance. ( 1) Heat treatment in advance (1) is the fire: chromium bearing steel quenching process, the workpiece through 40 ~ 60 min after thermal insulation, cooling needs faster, immediately after normalizing spheroidizing annealing. (2) the spheroidizing annealing: GCr15 chrome steel bearing, isothermal spheroidizing annealing process is often used to 790 ℃ is considered to be the best spheroidizing heating temperature. Heating to 900 ~ 920 ℃ before annealing, normalizing after insulation 2/3 ~ 1 h. Holding time on the workpiece size, the uniformity of the heating furnace, furnace charging method and the furnace charging amount, annealing the original organization uniformity of the former. Spheroidizing annealing at low temperature is mainly suitable for cold punching, cold extrusion ring of recrystallization annealing. Ordinary spheroidizing annealing, isothermal spheroidizing annealing is mainly suitable for forging ring, thermal shock and horizontal forging ball annealing. Chromium bearing steel ball annealing process. ( 2) The final heat treatment (1) bearing parts: general by quenching and low temperature tempering, its purpose is to improve the strength of steel, hardness, wear resistance and fatigue resistance. GCr15 steel quenching temperature at 820 ~ 860 ℃, oil quenching critical diameter is 25 mm. Commonly used oil cooling quenching. Heat preservation time is longer than alloy tool steel, salt bath heating coefficient values 0. 8 to 1. 5min/mm。 Air furnace heating coefficient of 1. 5 ~ 2分钟/毫米。 160 ℃ + 10 ℃ low temperature tempering, the tempering time is usually 2 ~ 4 h. To stabilize the size precision bearing parts, should be - after quenching 60 ~ 80 ℃ cold treatment, heat preservation time is 2 ~ 4 h, after cold treatment parts return to room temperature, tempering within 4 h, in order to prevent the parts cracking. Low temperature tempering failed to completely eliminate the residual stress will redistribute after grinding. These two kinds of stress will result in changes of parts size, even cracks. Therefore, we should again complement tempering, the tempering temperature is 120 ~ 160 ℃, the heat preservation 5 ~ 10 h or longer. (2) advanced GCr15 heat treatment: because the steel is easy to produce white point defects, large work mold heat treatment is easy craze, the slow heating or 690 ℃ for a long time, Greater than 5 h) Segmented isothermal can reduce the probability of cracking, choose the austenitizing temperature 810 ℃ plus or minus 10 ℃, coefficient of heat preservation a = 1. 6 ~ 0. 9min/mm。 Greater than 60 mm diameter workpiece need water double liquid quenching oil.