According to different heating methods, holding time and cooling conditions, the heat treatment methods of steel castings mainly include annealing, normalizing, quenching, tempering, solution treatment, precipitation hardening, stress relief treatment and hydrogen removal treatment.
Annealing is to heat the steel whose structure deviates from the equilibrium state to a certain temperature predetermined by the process, and then slowly cool it after heat preservation (usually cooling with the furnace or burying in lime) to obtain a heat treatment process close to the equilibrium state of the structure. According to the composition of the steel and the purpose and requirements of annealing, annealing can be divided into complete annealing, isothermal annealing, spheroidizing annealing, recrystallization annealing, stress relief annealing and so on.
The general process of complete annealing is: heating the steel casting to 20 °C-30 °C above Ac3, holding it for a period of time, so that the structure in the steel is completely transformed into austenite, and then slowly cooling (usually cooling with the furnace) at 500 ℃- 600 ℃, and finally cooled down in the air. The so-called complete means that a complete austenite structure is obtained when heated.
The purpose of complete annealing mainly includes: the first is to improve the coarse and uneven structure caused by hot working; the second is to reduce the hardness of carbon steel and alloy steel castings above medium carbon, thereby improving their cutting performance (in general, When the hardness of the workpiece is between 170 HBW-230 HBW, it is easy to cut. When the hardness is higher or lower than this range, it will make cutting difficult); the third is to eliminate the internal stress of the steel casting.
The use range of complete annealing. Full annealing is mainly suitable for carbon steel and alloy steel castings with hypoeutectoid composition with carbon content ranging from 0.25% to 0.77%. Hypereutectoid steel should not be fully annealed, because when the hypereutectoid steel is heated to above Accm and slowly cooled, the secondary cementite will precipitate along the austenite grain boundary in a network shape, which makes the strength, plasticity and impact toughness of the steel significant decline.
Isothermal annealing refers to heating steel castings to 20 °C - 30 °C above Ac3 (or Ac1), after holding for a period of time, quickly cooling to the peak temperature of the subcooled austenite isothermal transformation curve, and then holding for a period of time ( Pearlite transformation zone). After the austenite is transformed into pearlite, it cools down slowly.
Spheroidizing annealing is to heat the steel castings to a temperature slightly higher than Ac1, and then after a long time of heat preservation, the secondary cementite in the steel spontaneously transforms into granular (or spherical) cementite, and then at a slow speed Heat treatment process to cool to room temperature.
The purpose of spheroidizing annealing includes: reducing the hardness; making the metallographic structure uniform; improving the cutting performance and preparing for quenching.
Spheroidizing annealing is mainly applicable to eutectoid steels and hypereutectoid steels (carbon content greater than 0.77%) such as carbon tool steel, alloy spring steel, rolling bearing steel and alloy tool steel.
Stress Relief Annealing and Recrystallization Annealing
Stress relief annealing is also called low temperature annealing. It is a process in which steel castings are heated to below Ac1 temperature (400 °C - 500 °C), then kept for a period of time, and then slowly cooled to room temperature. The purpose of stress relief annealing is to eliminate the internal stress of the casting. The metallographic structure of the steel will not change during the stress relief annealing process. Recrystallization annealing is mainly used to eliminate the distorted structure caused by cold deformation processing and eliminate work hardening. The heating temperature for recrystallization annealing is 150 °C - 250 °C above the recrystallization temperature. Recrystallization annealing can re-form the elongated crystal grains into uniform equiaxed crystals after cold deformation, thereby eliminating the effect of work hardening.