MICROSTRUCTURE AND TENSILE BEHAVIOR OF NITROGEN-ALLOYED, DUAL-PHASE STAINLESS-STEELS

Two alloys of-high-nitrogen stainless steel have been heat treated to produce dual-phase microstructures. The first alloy, N10CrNiMo17 1, a Ni-containing stainless steel, was processed conventionally. The secon d alloy, N20CrMo17, a Ni-free stainless steel, was processed to obtain a higher nitrogen content by pressurized electroslag remelting. The m artensite in N10CrNiMo17 1 was homogeneously distributed in the ferrit e and obtained a near-constant volume fraction as a function of interc ritical annealing temperature. Microprobe analysis and microhardness m easurements of the martensite constituent suggested that up to 0.4 pct N was dissolved in the austenite before quenching. Austenite formatio n, martensite transformation, undissolved nitrides, and retained auste nite were evaluated by transmission electron microscopy (TEM). The Ni- containing alloy exhibited classic dual-phase tensile behavior in that continuous yielding was observed together with good combinations of u ltimate tensile strength and total elongation. The martensite constitu ent in alloy N20CrMo17 was concentrated within bands. Comparison of te nsile properties of the two alloys at similar volume fractions and har dness levels of martensite and ferrite showed that the microstructure containing banded martensite had inferior combinations of strength and ductility. The degradation of tensile ductility was accompanied by a fracture mode transition from microvoid coalescence to transgranular c leavage. The deformation and fracture behavior of both alloys were rel ated to the microstructure.