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Austenitic Stainless Steel
Description of material
NTR20 is a Cr- Ni –N precipitation hardening austenitic stainless steel designed for elevated temperature applications with high mechanical strength and hot corrosion resistance.
Exhaust valves for gasoline and diesel internal combustion engines where a high temperature strength and corrosion resistance to engine combustion products are needed with operating temperature up to 780°C.
NTR20 has a good resistance to oxidation and sulfidation at elevated temperatures up to 850°C and its scaling temperature is close to 900°C. When solution treated and aged, this grade warrants a stable austenitic structure with high hardness and good creep resistance.
NTR20 has good ductility in the annealed condition but a little lower when solution treated and aged. On the contrary, in this last condition, the machinability slightly improves with a right choice of cutting fluids, tools and machining parameters. Abrasive wheel cutting of an aged hardened structure, at maximum values of hardness , must be carried out with great care and with very good cooling conditions, in order to avoid the overheating able to cause grinding strain cracks in the cross-section of bars. This doesn’t usually occur in condition annealed structure or in small bar dimensions such as precision ground bars for upsetting. 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 and it’s suggested to maintain a small clearance between the edge of upper and lower tool in order to facilitates the fracture avoiding large burrs heights.
In solid state joining such as Friction Welding, NTR20 provides a quality bond line. When friction welded with stems of alloy or martensitic steels, a stress relieving or tempering of valves must be done in order to soften the martensitic structure of HAZ and bond line.
NTR20 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 of the range during the extrusion and forging process, could generate internal bursts of stem of valves. The same care should be applied in the 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.
Valves of NTR20 should be always solution treated and aged to obtain the best creep resistance and elevated temperature corrosion resistance of exhaust valves.