Application of heat resistant castings

- Sep 26, 2019-

Heat-resistant steel is commonly used in the manufacture of parts for high-temperature industrial sectors such as boilers, steam turbines, power machinery, industrial furnaces, aviation, and petrochemicals. These members require not only high temperature strength and high temperature oxidation resistance, but also sufficient toughness, good processability and weldability, and structural stability determined according to different uses.

Since 1952, China has been producing heat-resistant steel. Some new low-alloy heat-resistant steels have been developed to increase the working temperature of pearlitic heat-resistant steel castings to 600-620 °C, and some new low-chromium-nickel oxidation-resistant steels have been developed. Heat resistant steel castings and stainless acid resistant steels cross each other within the scope of use. Some stainless steels have the characteristics of heat resistant steel. They can be used both as stainless steel acid-resistant steel and as heat-resistant steel. As the ferrite forming element of chromium, aluminum and silicon, the metal surface can be promoted to form a dense oxide film at a high temperature to prevent further oxidation. They are the main elements for improving the oxidation resistance of steel and the corrosiveness of high temperature gases. However, excessive amounts of Al and Si can severely deteriorate room temperature plasticity and thermoplasticity. Chromium can significantly increase the recrystallization temperature of low alloy steels. When the chromium content is 2%, the strengthening effect is the best. Nickel and manganese can form and stabilize austenite. Nickel improves the high temperature strength and carburizing properties of austenitic steels. Manganese can form austenite instead of nickel, but it will destroy the oxidation resistance of heat-resistant steel. Vanadium, titanium and niobium are strong carbide forming elements, which can form fine dispersed carbides and improve the high temperature strength of steel. The combination of titanium, niobium and carbon also prevents intergranular corrosion of austenitic steels at high temperatures or after welding. Carbon and nitrogen expand and stabilize austenite, thereby increasing the high temperature strength of heat resistant steel. When the steel contains more chromium and manganese, the solubility of nitrogen is significantly increased, and nitrogen alloying can be used instead of expensive nickel. Boron and rare earth are trace elements in heat resistant steel. Boron dissolves into a solid solution and distort the crystal lattice. The grain boundary boron prevents elemental diffusion and grain boundary migration, thereby increasing the high temperature strength of the steel. Rare earth elements can significantly improve the oxidation resistance and thermoplasticity of steel. Smelting heat-resistant steel is usually smelted in an electric arc furnace or an induction furnace. Vacuum refining and refining processes are often used for high quality requirements. The processing and deformation of certain high alloy heat resistant steels is very difficult. Casting is not only more economical than rolling, but also has a higher durability strength. Therefore, heat-resistant cast steel occupies a considerable proportion in heat-resistant steel. In addition to sand casting, precision casting technology can also be used to obtain products with smooth surfaces and precise dimensions. Centrifugal casting is commonly used in high temperature furnace tubes for the pyrolysis of ammonia and ethylene. Pearlitic heat-treated steel is usually normalized or tempered, while martensitic heat-resistant steel is tempered to stabilize its structure and obtain good comprehensive mechanical properties and high temperature strength. The ferritic steel in the heat-resistant steel annealing tank production line cannot be strengthened by heat treatment. In order to eliminate the internal stress caused by cold plastic deformation processing and welding, it can be annealed at 650 to 830 ° C, and rapidly cooled after annealing to quickly pass the brittle temperature range of 475 ° C.

Heat-resistant steel castings are mainly used for as-casting, and can also be heat-treated according to the type of heat-resistant steel.