Analyze the heat treatment inspection regulations of steel castings

1. Inspection Regulations for Heat Treatment of Carbon Steel Castings

Considering the complicated shape of valve castings and easy deformation and cracking, the heat treatment of carbon steel castings usually adopts annealing. The inspection focuses on supervising whether the supplier is in accordance with the following heat treatment specifications and inspecting the hardness value of the castings.

1. Points to note during heat treatment of carbon steel castings

Usually, the following points should be paid attention to during heat treatment of carbon steel castings:

1.1. When the furnace temperature rises to 650℃~800℃, whether the temperature rises slowly

Because in the heating process, especially for carbon steel castings with complex shapes, when the furnace temperature rises to 650℃~800℃, the temperature should be increased slowly, or kept at this temperature for a period of time. Because the carbon steel undergoes phase transformation in this temperature range, and the volume changes, phase transformation stress is generated. If the temperature rises quickly, it is easy to increase the temperature difference between the thin-walled part and the thick-walled part of the casting, and the surface layer and the center layer Increase the thermal stress of the casting and easily lead to cracking of the casting.

1.2. Is the holding time sufficient?

In order to make the inner and outer temperature of the casting consistent and have enough time to completely transform the structure, the heat preservation time of thick-walled castings is longer than that of thin-walled castings.

The calculation method of holding time is as follows:

a) Calculated according to the maximum wall thickness of the castings in the same furnace, holding 1 hour per 25mm, suitable for castings with a wall thickness of less than 20mm.

b) Calculated according to the maximum wall thickness of castings in the same furnace, the heat preservation is 1 hour per 50mm, but not less than 2 hours.

c) Calculated according to the stacking height (ie the stacking height of the castings). Generally, the heat preservation time of carbon steel castings is calculated at 1m height for 4 hours.

1.3. When carbon steel castings are annealed, they are generally cooled in the furnace.

2. Heat treatment specification for carbon steel castings

2.1. The annealing and heating temperature of carbon steel castings should be adjusted reasonably

2.2. Specification for annealing of carbon steel castings

2. Heat Treatment Inspection Regulations for Austenitic Stainless Steel Castings

The heat treatment of austenitic stainless steel castings usually adopts solid solution treatment and stabilization treatment to make it have the best corrosion resistance. The inspection focuses on supervising whether the supplier is in accordance with the following heat treatment specifications.

1. Solid solution treatment of austenitic stainless steel

The purpose of solution treatment is to completely dissolve the carbides in the steel and obtain a single-phase structure. The method is to heat the casting to 950°C~1175°C. The heating method should be preheated at low temperature, and then accelerated to solidify. The process of melting temperature is to reduce the temperature difference between the surface and the core of the steel casting during the heating process. The solid-melt holding time is determined by the wall thickness of the steel casting. Generally, it is calculated according to the wall thickness for 2.5~3min per mm to ensure that all sections of the casting are all It can be heated through, and then quenched into water, oil or air after solid melting and heat preservation. Water is commonly used. Air cooling is only suitable for thin-walled castings.

2. Stabilization of austenitic stainless steel castings

In order to prevent the sensitization phenomenon of austenitic stainless steel castings, alloy elements such as titanium and niobium are added to the steel, and after solid solution treatment, stabilization treatment is carried out, that is, the castings are reheated to 850℃~930℃, after heat preservation , To make the carbon in the steel fully combine with titanium and niobium to form stable carbides.