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Austenitic Stainless Steel
Description of material
ACNW is a Cr- Ni austenitic stainless steel designed for elevated temperature applications whose higher Tungsten and Nickel contents than AVW warrant a good mechanical strength and hot corrosion resistance.
Exhaust valves for gasoline and diesel internal combustion engines where a high temperature strength and corrosion resistance to combustion products are needed. This grade is normally used in the production of not heavy duty exhaust valves and operating temperature not over 730°C.
ACNW has a good resistance to oxidation and corrosion to combustion products that form inside the engine. When solution treated, this grade offers a stable austenitic structure.
ACNW has a fair ductility in the annealed condition and cold shearing, band or circular sawing and grinding wheel cutting can be used. In the case of turning, its machinability is approximately comparable to the strain hardened austenitic TP 300 series and could be only improved with a right choice of cutting fluids, tools and machining parameters.
In solid state joining such as Friction Welding, ACNW provides a quality bond line. When friction welded with stems of alloy or martensitic steels, a stress relieving or tempering of the valve must be done in order to soften the martensitic structure of HAZ and bond line.
ACNW has a good hot plasticity and 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 temperatures at the top end of the range during the extrusion and forging process, could generate internal bursts of stems of valves. The same care should be carried out for electrical upsetting process in order to maintain the right geometry of the bulb and to avoid hot cracks and internal shortness of the head valve. A low or medium temperature range of hot working is recommended.
Heat treatments don’t increase the mechanical properties of ACNW and only full annealing and stress relieving after friction welded can be done.