On 2018-08-08 14:48:54
Process treatment of cast steel in heat treatment furnace
Heat treatment furnace is easy to produce serious dendrite segregation for steel casting materials and steel castings produced by casting method, resulting in uneven structure and large residual internal stress. Therefore, steel castings, especially alloy steel castings, are usually subject to heat treatment and supplied in heat treatment state.
(1) Precautions for heat treatment of steel castings
① The heat treatment of steel castings is different from that of forged steel or rolled steel with the same chemical composition. The heating temperature of steel castings should be slightly higher than that of forged steel or rolled steel with the same composition (about 20 ℃), and the time of austenitizing and heat preservation should be appropriately prolonged.
② The choice of heat treatment method of steel casting is closely related to its material characteristics.
a. The heat treatment method of carbon steel casting is annealing, normalizing, normalizing and tempering.
b. The heat treatment methods of medium and low alloy steel castings are annealing, normal normalizing, isothermal normalizing, double normalizing, normalizing + tempering, quenching + tempering, etc. Among them, double normalizing can be used for treatment. The first normalizing is to uniformly organize the structure through diffusion, which needs to be carried out at a higher temperature [AC3 + (100-160) ℃]; the second normalizing is to refine the structure, which needs to be carried out at a normal normalizing heating temperature [AC3 + (40-60) ℃].
c. The heat treatment method of high manganese steel casting is water toughening treatment.
d. The heat treatment methods of corrosion resistant stainless steel castings are: annealing, normalizing + tempering, quenching + tempering, homogenization + quenching + tempering, annealing + quenching + tempering, solution treatment + aging treatment, etc.
e. The heat treatment method of heat-resistant steel casting is: normalizing + tempering, etc.
f. The methods of heat treatment for die steel castings are: annealing, quenching, quenching + tempering, normalizing + tempering, annealing + normalizing + tempering, solution treatment, solution treatment + aging treatment, etc.
g. The heat treatment methods of low temperature steel castings are normalizing, quenching, normalizing + tempering, quenching + tempering, etc.
③ The heat treatment of steel castings should be carried out as quickly as possible without deformation and cracking, so as to improve the production capacity. The heating rate depends on the following points.
a. Chemical composition of steel castings. The higher the carbon content of steel casting is, the worse the thermal conductivity is and the lower the allowable heating speed is. The heating rate of alloy steel castings is generally lower than that of carbon steel castings.
b. Section size of steel casting. The larger the section of the steel casting is, the greater the temperature difference between the inner and outer layers and the greater the thermal stress, so the heating speed should be slower.
c. Shape characteristics of steel castings. The more complex the shape of the steel casting and the greater the difference of section thickness, the slower the heating speed is.
(2) The common methods of heat treatment for steel castings and their main purposes and application mainly focus on the differences in process.