THE INTERNAL-NITRIDING BEHAVIOR OF 310 STAINLESS-STEEL WITH AND WITHOUT AL AND TI ADDITIONS

The internal-nitriding behavior in ammonia-hydrogen atmospheres of type-310 stainless steel and 310 to which either 2 wt.% Ti or 3 wt.% Al were added was studied over the range of 550-950-degrees-C. An Fe-24Cr binary alloy was included to assess the role of a BCC crystal structure vs the FCC crystal structure of 310 stainless steel. The BCC alloy exhibited the most rapid kinetics as expected. X-ray diffraction showed only the presence of CrN in all the alloys up to 735-degrees-C. At 850-degrees-C and above, both CrN and Cr2N were detected. The nonformation of TiN and AlN at lower temperatures is attributed to nucleation problems. Precipitates were extremely fine (unresolvable even at 20,000x) at 563-degrees-C and became much coarser with increasing temperature. The precipitate density, size and shape varied across the internal-nitriding zone at the higher temperatures. External scaling was noted at 850-degrees-C and above, however, it was not a continuous film. The activation energy of internal nitriding from 563-735-degrees-C ranged from 3.8 kcal/mol for 310 + 2Ti to 18.2 kcal/mol for 310 + 3Al; from 850-950-degrees-C, the activation energy ranged from 44 (310 + 2Ti) to 56.6 kcal/mol (310 + 3Al). Microhardness profiles show that an intermediate zone exists between the nitride case and the base metal. The origin of this zone is discussed.