Mechanical Properties of Nitrided Austenitic Stainless Steel as Related to Microstructure

摘要

In studies concerned with the deterioration of high temperature materials, four austenitic stainless steels (AISI Types 304, 316, 321, and 347) were nitrided in ammonia for 72 hours at 425C (800F), 540C (1000F), 650C (1200F), 785C (1450F), and 980C (1800F). Tube tensile specimens, nitrided on both the outside and inside diameters, were tested. A nitride case formed on the steels nitrided at 650C (1200F) and higher. Tensile specimens of Type 321 (titanium stabilized) disintegrated during the 540C (1000F) exposure. Ductility, as measured by elongation, was severely reduced at all temperatures of nitriding, whether a case formed or not. Yield strength significantly increased except for the 425C (1800F) treatment where it decreased. Ultimate strength was reduced at all temperatures, but was recovered at 980C (1800F) with all of the steels except Type 321 (titanium stabilized). In many instances, the load-extrusion curves beyond the yield point were very irregular as the nitride case cracked with increasing load. Changes in these mechanical properties were difficult to correlate to the nitrided area. Properties were affected even if no case formed. The nitride case in all four stainless steels was found by X-ray diffraction to consist of (Fe.Cr)N and alpha-iron (martensite). (Author)