Effect of sensitization on corrosion fatigue behavior of type 304 stainless steel annealed in nitrogen gas

Effect of sensitization on fatigue behavior of type 304 stainless steel in laboratory air and in 3% NaCl solution was studied using specimens annealed in high temperature nitrogen gas. Annealing in high temperature nitrogen gas was performed at 1100. °C and at 1200. °C for 30. h, resulting in the solid solution of nitrogen and the precipitation of chromium nitride (CrN). The mechanical and fatigue properties were significantly improved by annealing. The improvement was due to the solid solution of nitrogen and the precipitation of CrN. Fatigue strengths of the untreated specimen in laboratory air and in 3% NaCl solution are nearly the same. However, fatigue properties of the annealed specimen in 3% NaCl solution change for the worse, because chromium (Cr) -depleted zones were formed along grain boundaries during the heat treatment, resulting in the remarkable sensitization.In order to prevent the sensitization, the re-solution treatment (RST) which enhanced the dissolution of CrN and the water quenching treatment (QT) which avoided the precipitation of CrN was performed. As a result, the fatigue properties of the RST and QT specimens in 3% NaCl solution were slightly improved, but were still lower than that of the untreated one. Since the oxalic etch tests proved the formation of Cr-depleted zone also in the RST and QT specimens, the influence of sensitization could not be fully eliminated by the both treatments.