The influence of alloying elements on the precipitation behavior of sigma (σ) phase was investigated for conventional SAF2205 and SAF2507 super duplex stainless steel. Time-Temperature-Precipitation (T-T-P) diagram of sigma phase of SAF2507 were shifted toward to shorter times compared to SAF2205. The precipitation of sigma phase was accelerated with increasing Cr and Mo concentration. According to the microstructure observation, the sigma phase began to precipitate at ferrite (α) ⁄ austenite (γ) phase boundaries and grew into ferrite for SAF2507 and SAF2205 steel. In the as-received condition, Cr and Mo concentration in ferrite was clearly higher than that in austenite. Especially, it was found that Mo concentration in ferrite of SAF2507 was higher than that in ferrite of SAF2205. The result of EPMA-measurement showed that sigma phase was mainly Fe-Cr-Mo intermetallic compound and Mo was significantly enriched into sigma phase. The difference of Mo concentration in ferrite significantly affected to the sigma phase precipitation. The secondary austenite formation was also induced by sigma phase precipitation. Cr and Mo were ejected to the remained ferrite ⁄ austenite phase boundaries by secondary austenite formation. Consequently, sigma phase precipitation was more accelerated by the reheating.
Recently, owing to the high demand and high dependence on electric power, the reliability of the distribution voltage of electric power systems has become more important. 22 kV high voltage cutout fuses have been used widely, but some breaking accidents occurred because of pollution ⁄ contamination of bushings. With the increase in contaminants on bushings, the electric fields on the fuse element surface rise to a critical level and trigger corona discharge. The fuse element material which is more resistant to corona discharge compared to Ag element is desired. A new type current-limiting fuse made of stainless steel wire of low cost and simple design has been developed. Some characteristic tests including : breaking test with a high surge current and noise intensity when a voltage was applied. It was confirmed that the new type fuse possesses similar performance characteristics as compared with the Ag element using model fuse.