Common Heat Treatment Process Of Steel Casting Parts

According to the different heating and cooling conditions, the main heat treatment methods of steel casting are: annealing, regular ignition, homogenization treatment, quenching, tempering, solid solution treatment, precipitation hardening, stress elimination treatment and hydrogen removal treatment.
1. Annealing: Annealing is a heat treatment process for heating the steel casting to Ac 3 above 20~30℃, keeping heat for a certain time and cooling. The purpose of annealing is to eliminate the columnar crystal, coarse isoaxial crystal, Wei tissue and dendritic segregation in order to improve the mechanical properties of cast steel. Carbon steel after annealing tissue: subbatch cast steel is ferrite and pearlessite, cobatch cast steel is pearlite, subbatch cast steel is pearlessite and carbide. For steel casting of all grades.
2. Positive fire: Positive fire is a heat treatment process of heating the steel casting to 30~50℃ above the Ac 3 temperature, making it completely austenitic, and then cooling in static air. The purpose of ignition is to refine the organization of the steel so that it has the required mechanical properties and as a preparatory treatment for later heat treatment. There are two differences between ignition and annealing processes: one is that the heating temperature is higher; the other is the faster cooling. The strength of annealed cast steel is slightly higher than that of annealed cast steel, and its pearl body organization is relatively fine. Carbon steel and some alloy steel castings with large thickness and complex shape are mostly treated with regular ignition. Normal fire can eliminate the mesh carbide in co-cast steel and over-cast steel to facilitate spherization annealing, and can be used as a preparatory treatment before quenching of medium carbon steel and alloy structural steel to refine the grain and uniform organization, thus reducing the defects caused by the casting during quenching.
3. Quenching: Quenching is after heating the steel casting to austenization (Ac. Or Ac & # 8226; above), to cool after a certain time in an appropriate way to obtain a heat treatment process for martensite or bainite tissue. Common water cooling quenching, oil cooling quenching and air cooling quenching. After quenching, the steel casting parts should be tempered in time to eliminate the quenching stress and obtain the required comprehensive mechanical properties
Main parameters of the quenching process of steel casting parts:
(1) Quenching temperature: the quenching temperature depends on the chemical composition of the cast steel and the corresponding critical temperature point. In principle, the quenching temperature of Ac. Above 20~30℃, often referred to as complete quenching. Coanalysis and over-cast steel in Ac. The above 30~50℃ quenching is the so-called subcritical quenching or two-phase region quenching. This quenching can also be used for subcoanalysis steel, the obtained tissue is thinner than the general quenching, suitable for the toughness of low alloy steel casting.
(2) Quenching medium: the purpose of quenching is to obtain a complete martensite organization. For this reason, the casting when quenching the cooling rate must be greater than the critical cooling rate of the cast steel. Otherwise, the martensitic tissue and its corresponding properties cannot be obtained. However, the high cooling rate is easy to cause the deformation or cracking of the casting. In order to meet the above requirements simultaneously, the appropriate quenching medium shall be selected according to the material of the casting, or other cooling methods (e. g., graded cooling). In the range of 650~400℃ steel, so the casting should be cooled at this temperature during quenching. Below the Ms point cooling is expected to slow to prevent quenching deformation or cracking. Quenching medium usually uses fire, aqueous solution, oil, and air. In graded quenched or