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An introduction of austenitic stainless steel forging

Posted: 03/05/2019 15:15:47 Hits: 59

Stainless steel is widely used in chemical industry, petroleum industry, power plant, etc., and many products require not only corrosion resistance, but also high strength, so most stainless steels are used after forging. Compared with carbon steel, stainless steel has characteristics such as low thermal conductivity, narrow forging temperature range, strong heat sensitivity, high resistance and low plasticity at high temperature, so on and so forth, which bring many difficulties to forging production. And there are discrepancies in different types of stainless steel forging processes.

Forging of austenitic stainless steel is more difficult than those of ordinary steels, but it rarely produces surface defects. Most austenitic stainless steels can be forged at temperatures above 927 °C. Because austenitic stainless steel has no phase transition in the high temperature range, its forging temperature is higher than that of martensitic stainless steel. But high chromium or low carbon stainless steels do not have the above properties, when forging high chromium or low carbon stainless steels, if the temperature is higher than 1093 °C, different levels of ferrite will be produced depending on the composition, which is harmful to the forgeability.

For austenitic stainless steels, controlling final forging is pretty important. Except for stable and ultra-low carbon stainless steel, the final forging temperature of almost all austenitic stainless steels should be controlled above the temperature sensitive zone and rapidly cooled below 871 °C. This is because thermal cracks and transformation can be easily produced on austenitic stainless steel at low temperatures. Forging of stable and ultra-low carbon steels is performed at a temperature lower than the regular forging temperature. The strain hardening usually occurs at 538 °C to 649 °C. When the hardness requirement is relatively low, solution annealing should be performed after forging.

Sulfur or selenium can play a role in improving the mechanical properties of austenitic stainless steels. The harmful slat-like structure of selenium is less likely to exist. The 321 steel with titanium also has segregated band, so when forging, it is easy to cause surface cracking. 347 stainless steel with cobalt is suitable for forging because it is not easy to produce segregation. When heating austenitic steel, the furnace should be slightly oxidative; the decarburizing and peroxidizing circumstances will produce harmful inclusions or chromium depletion, thereby reducing the corrosion resistance of the steel. The chromium depletion is particularly serious in 309 and 310 steels.

Characteristics of the austenitic stainless steel forging process
18-8 austenitic stainless steel is often used to make long-term-used boiler and steam turbine parts that function below 610 °C, as well as various parts in chemical production. Firstly, 18-8 type austenitic stainless steel is easy to carburize when being heated in the coal stove, so avoid contact with carbon-containing substances during heating, and use oxidizing media to reduce carburization of steel and prevent intergranular corrosion. Secondly, the initial forging temperature of austenitic steel should not be too high, it should be heated slowly at 1150 - 1180 °C. Thirdly, the surface defects of the blank must be removed before heating, so as to prevent further expansion during forging and the forging from being scrapped. Fourthly, the final forging temperature should not be too low, otherwise, precipitation will occur at 700 - 900 °C and lead to cracking. Fifthly, the part must be air-cooled and solution treated after the forging process.

Temperature range
The initial forging temperature of austenitic stainless steel generally does not exceed 1200 °C; the final forging temperature is usually at 825 - 850 °C, which is mainly limited by the sensitive temperature of carbide precipitation. Once the final forging temperature is within this temperature range (480 - 820 °C), the deformation resistance is increased due to the precipitation of carbides, and the plasticity is reduced which leads to cracking.

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