Development of Softening Resistant Steel for Long-term PWHT

by Satoshi Shimoyama, Manabu Izumi, Hiroki Imamura, Kengo Abe, Tokutaka Tani, Yoshinobu Omiya

Publisher - Kobe Steel, Ltd, Japan

Category - General Novel

The most important performance required for LPG tanks is safety, so that it is essential to remove the residual stress generated in the welded zone during the process for manufacturing the tanks by the heat treatment, called PWHT (Post-Weld Heat Treatment). Generally, the softening becomes large as the PWHT time is extended, so that steel plates that can maintain a high strength even after long-term PWHT are required. The most critical factor that affects the softening by the PWHT is the coarsening of carbide. It is noted that Ostwald growth of cementite results in the coarsening, which causes the cementite not contributing to the strengthening any more. For minimizing the softening, therefore, it is important to keep the cementite finely dispersed after long-term PWHT so that it can contribute to the strengthening. To solving this matter, we investigated the effects of alloy element which suppresses softening. With this study, new concept of alloying technology, called KHS treatment (Kobe High-resistance to Softening) which is to retard the cementite coarsening by adding Cr and Mn with good balance, was found. By the KHS treatment, we developed a new steel plate that achieved high strength after 615℃×23hrs PWHT with high strength and toughness of the welded joint.

Please login to borrow the book.