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Austenitic Stainless Steel
Description of material
21MN is a Cr- Ni – Mn - N precipitation hardening austenitic stainless steel designed for high strength and hot corrosion resistance at elevated temperatures. This grade has lower Nickel, Nitrogen and Manganese contents than 212MN and its Carbon content is higher than all other austenitic valve steels.
Exhaust valves for gasoline and diesel internal combustion engines where a high temperature strength and corrosion resistance to engine combustion products are needed. In addition, 21MN has been used in heavy duty intake valves of certain engines.
21MN has poor ductility in the annealed condition and is difficult to machine due to high Carbon and Mn contents. Only the grinding process should be carried out. In case of turning, a right choice of tool geometry, feed and cutting fluids should be well evaluated to minimize the strain hardening of surface avoiding the possibility of stress delayed cracks. Abrasive wheel cutting is suggested instead of band and circular sawing. However, abrasive wheel cutting should be carried out with great care and with very good cooling conditions in order to avoid the overheating of the cross-section of bars. For extrusion bars, warm or hot shearing are suitable and correct processes. For small bars, such as precision ground bars for upsetting, the cold shearing (cropping) is preferred but the structure of 21MN requires a small clearance between the edge of upper and lower tool in order to facilitates the fracture avoiding large burrs heights and shear cracks. It’s important to point out that this grade, such as similar high Mn - N valve grades, the shift in ductile-brittle transition temperature curve is just 30° - 50° C above room temperature.
In solid state joining such as Friction Welding, 21MN provides a quality bond line. When friction welded with stems of alloy or martensitic steels, a stress relieving or tempering of valves is mandatory and must be done in order to soften the martensitic structure of HAZ and bond line.
21MN is suitable for processing by hot extrusion or by upsetting with electric resistance heating. However, overheating must be always avoided. The choice of hot working temperature and process parameters must always evaluate both the strain rate and the consequent increasing of temperature that is reached after hot deformation. High strain rates and temperature at the top of the range during the extrusion and forging process, could generate internal bursts of stem of valves. The same care should be exercised for electrical upsetting processes in order to maintain the right geometry of bulb and to avoid hot cracks and internal shortness of the head valve. In both processes, a low or medium temperature range of hot working is recommended.
Valves of 21MN should be always solution treated and aged to obtain the best creep resistance and elevated temperature corrosion resistance of exhaust valves.