Cast Steel Thermal Inspection Procedure

　　Considering the complex shapes of valve castings, which are prone to deformation and cracking, carbon steel castings are typically used for heat treatment during the annealing process.

　　The focus of the inspection is to supervise the supplier regarding the following heat treatment specifications, as well as to verify the hardness values of the castings.

　　One issue to note during the heat treatment of carbon steel castings

　　1.1 Does the furnace heat slowly when the temperature rises to 650°C–800°C?

　　Because during the heating process, especially for complex-shaped carbon steel castings, when the furnace temperature rises to between 650°C and 800°C, it’s crucial to heat slowly or hold the temperature at this range for a while. This is because, within this temperature interval, carbon steel experiences significant volume changes and phase transformation stresses. If the heating is too rapid, it can easily lead to a large temperature difference between the thin-walled sections of the casting and its surface layer versus the core, thereby increasing thermal stress and significantly raising the risk of cracking in the cast component.

　　1.2 Insulation time is sufficient

　　To ensure that the temperature inside and outside the casting remains uniform, allowing sufficient time for the structure to fully transform, thick-walled castings require longer insulation times compared to thin-walled ones.

　　1.3 Carbon steel castings are typically cooled in the furnace.

　　2 Austenitic Stainless Steel Castings: Heat Treatment Specifications

　　Austenitic stainless steel castings typically undergo solution treatment and stabilization treatment for heat processing, resulting in the best corrosion resistance.

　　The focus of the inspection is on the supplier's adherence to the following heat treatment specifications.

　　1 Austenitic Stainless Steel

　　The purpose of solid melting treatment is to completely melt the carbides in steel, forming a single-phase structure. This method involves heating the casting to temperatures between 950°C and 1175°C—specifically, the heating process should begin with low-temperature preheating, followed by gradual heating and accelerated melting of the solid material. This approach helps minimize temperature differences across the surface and interior of the steel casting during heating. The holding time for the solid solution depends on the wall thickness of the cast steel component; typically, it’s calculated at 2.5 to 3 minutes per millimeter of wall thickness. To ensure thorough thermal penetration throughout the casting cross-section, the component is then quenched in water, oil, or air after solid solution holding—though air cooling is preferred only for thin-walled castings.

　　To prevent sensitization in austenitic stainless steel castings, titanium, niobium, and other alloying elements are added to the steel, followed by solution treatment and stabilization treatment. The castings are then reheated to 850°C to 930°C, held at this temperature, and subsequently cooled. During this process, carbon combines with titanium and niobium to form stable carbides throughout the carbon steel matrix.