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In the past twenty years it has been observed by test that there has been an increase in the high temperature strength of the austenitic stainless steel Types 304, 316, 321, and 347 with the result that the ASME Boiler and Pressure Vessel Committee has permitted two increases in allowable stress values since 1952 based on additional data. Creep and stress rupture studies of these materials led to the conclusion that nitrogen in controlled additions had a strengthening effect. Nitrogen grades of Types 304 and 316 referred to as 304N and 316N are permitted for use through ASME Boiler and Pressure Vessel Committee Code Case 1423 originally issued in September 1969 (now Case 1423-2), and efforts are underway to include 304N and 316N in ASTM specifications. Three public utilities have installed 304N for high temperature steam piping, and additional installations are expected.
nitrogen, mechanical properties, austenitic stainless steels, piping, pressure vessels, boilers, tubing
Materials engineer, Detroit Edison Co., Detroit, Mich.