Properties of weldments in high-strength ferritic heat-resistant steels

Abstract

In recent years, the preservation of natural resources and the reduction of the pressure on the environment have become important tasks. In fossil-fuelled power plants, efforts have been made to improve the efficiency in electricity generation by raising the temperature and pressure of steam. The main conditions for the steam used with the ultra-super critical pressure (USC) boiler of the most recent design in Japan have reached 600°C and 31 MPa. To realise these steam conditions, it is essential to use materials with excellent high-temperature creep strength in the areas subjected to the highest temperature, such as steam and heat conduction pipes. It is not an exaggeration to say that these conditions had not been achievable until high-strength ferritic steels and austenitic stainless steels were developed.1 Incidentally, in pressure-resistant sections of a boiler like the above-mentioned pipes are welded structures of a large scale, and hence, not only base materials but also the properties of weldments affect the life and reliability of pressure-resistant sections. This article deals with the properties required for welded joints of high-strength ferritic heat-resistant steels which have been used for the most recently designed boilers for fossil power plants. It also describes the creep strength properties at the most important joints and the issues of welding consumables for high-strength materials, together with future tasks.