Sensitization of Powder Metallurgy Type 430L Stainless Steel Sintered in Nitrogen-Hydrogen Atmosphere

The degree of sensitization of ferritic AISI 430L (UNS S43000) stainless steel, prepared by powder metallurgy (PM), was examined by exposure methods (ASTM A763-93, Practice W and Practice Z) and electrochemical reactivation methods such as electrochemical potentiokinetic reactivation (EPR) and electrochemical potentiokinetic reactivation double-loop (EPRDL) tests. Two sintering atmospheres, vacuum and nitrogen-hydrogen, were chosen. The latter was the focus of the work and the former was used for comparison. It was seen that nitrogen played an important role in the degree of sensitization, either when dissolved or in intermetallic precipitates. Because heat treatment was necessary to control the microstructural features, a set of heat treatments was established: annealing and aging at 100, 200, 300, 375, 675, and 875°C for different times (90 min and 4, 6, 8, and 48 h). The effect of nitrogen and temperature-time aging on corrosion resistance was investigated. Experimental results showed that there was an optimal aging heat treatment for nitrogen-sintered stainless steel in comparison to vacuum-sintered samples. Furthermore, the results indicated that the electrochemical reactivation methods used for PM ferritic stainless steels were suitable to evaluate the degree of sensitization. Effects of the heat treatments were discussed as the consequence of two competing mechanisms: dissolution and phase precipitation of nitrogen.