The purpose of heat treatment is to change the internal structure of steel, so as to improve the process performance and service performance of workpieces, tap the potential of steel, prolong the service life of parts and improve product quality. Save materials and energy.
Ordinary heat treatment process of 1 carbon steel
Annealing of 1 steel
The annealing of steel is usually to heat the steel to above the critical temperature Ac 1 or Ac3, keep the temperature for a period of time, and then slowly cool it with the furnace. At this time, austenite decomposes in the high temperature zone, thus obtaining a nearly balanced structure. Generally, medium carbon steel (such as 40 and 45 steels) is annealed to eliminate residual stress, with stable structure and low hardness (HB 180 ~ 220), which is beneficial to the next cutting.
2) normalizing of steel
Normalization of steel is usually to heat the steel above the critical temperature Ac3 or Accm line, keep the temperature for a period of time, and then air-cool. Because of the faster cooling rate, compared with the annealed structure, the pearlite content in the structure is relatively more and the lamellae are finer, so the properties are improved, the grains are refined, the structure is improved and the residual stress is eliminated. For low carbon steel, increasing hardness after normalizing can improve the machinability and surface smoothness of parts; For high carbon steel, normalizing can eliminate the network cementite and prepare for the next spheroidizing annealing and quenching.
3) Quenching of steel
The quenching of steel is usually to heat the steel above the critical temperature Ac 1 or Ac3, keep the temperature for a period of time, and then put it into various cooling media for rapid cooling (V cooling > V near) to obtain martensite structure with high hardness and high wear resistance.
4) Tempering of steel
Tempering of steel is usually a heat treatment process in which the quenched steel is reheated to a certain temperature below Ac 1 line, kept for a proper time and then cooled to room temperature. Because the martensite structure obtained from quenched steel is hard and brittle, there is great internal stress in the workpiece. If grinding is carried out directly, cracks will often occur, and some precision parts will cause dimensional changes during use, thus losing accuracy and even cracking. Therefore, quenched steel must be tempered. Different tempering processes can make steel obtain different properties.
2. Ordinary heat treatment process of carbon steel
1) heating temperature
In principle, the heating temperature for ordinary heat treatment of carbon steel is selected as the critical temperature of Ac 1 or 30 ~ 50℃ above the Ac3 line. However, in production, appropriate adjustments should be made according to the actual situation of the workpiece. The heating temperature of heat treatment should not be too high, otherwise the tendency of coarse grains, oxidation, decarbonization, deformation and cracking of the workpiece will increase. But the heating temperature is too low to meet the requirements.
Table 2- 1 Heating temperature for ordinary heat treatment of carbon steel
Scope of application of square heating temperature (℃)
Complete Annealing of Quenched AC3+(20 ~ 60) Sub-steel
* * * Ac 1+(20~40) spheroidizing annealing after precipitation.
Normalized precipitation of Ac3+(50~ 100) steel.
Accm+(30~50) precipitates through * * * steel.
Precipitation of quenched Ac3+(30~70) steel
Ac 1+(30~70) is precipitated by * * * steel.
Tempering tempering at low temperature 150~250 cutting tools, molds, measuring tools and high hardness parts.
Temper 350~500 springs and parts with medium hardness at medium temperature.
Temper 500~650 gears, shafts, connecting rods and other parts with comprehensive mechanical properties at high temperature.
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