Abstract
The increase of chromium and molybdenum content as alloying elements is effective to improve the corrosion resistance of duplex stainless steels. However, the increase of these elements have the tendency to induce the poor hot workability and inferior weldability of the steels. In the weldment the more those elements increase, the more often the embrittlement in HAZ takes place.
The mechanism of the HAZ embrittlement in 28Cr-3.8Mo-Ni-N duplex stainless steels was investigated. It was clarified that there are two reasons for the HAZ embrittlement from the investigation on the effects of nickel and nitrogen contents. One was the sigma phase precipitation in case of the lower ferrite ratio. Another was the nitride precipitation in case of the higher ferrite ratio. In the proper range of nickel and nitrogen content, the HAZ showed the high toughness. The sigma phase precipitation was accelerated by the decrease of the ferrite ratio and the increase of the nickel content. The nitride precipitation was accelerated by the increase of the ferrite ratio and the decrease of the nickel content. The acceleration of the sigma phase precipitation in the increase of nickel content can be explained by as follows. One is that the increase of nickel enriches the ferrite phase in the chromium and molybdenum concentration. The other is that the nickel makes the ferrite phase unstable and increases the transformation driving force.
