New developments in steels for power generation boilers

Steam conditions in power plants are gradually being raised with a view to energy savings and environmental protection. The steam temperature of 600 degrees C for modern power plant has already been realised, and a goal for the future is the 625 degrees C class with ferritic steels. T91/P91 was developed in the early 1980s and was subsequently widely used for the high-temperature components of power boilers. Its strength, however, was not sufficient for temperatures of 600 degrees C and above. Since 1980 numerous studies on heat resistant steels have been actively conducted, and, among the various steels developed for advanced steam cycles, great progress has been seen in 9-12Cr steels and austenitic steels. In particular, recent ferritic 9-12Cr steels have become stronger than conventional austenitic stainless steels. The most recent 9-12Cr steels have a creep rupture strength of 140 MPa at 600 degrees C for 100 000 hours. Such enhancement in creep strength is achieved by the alloying of W and reduction of Mo. in the near future a strength of 180 MPa is anticipated for ferritic steels through further studies on the role of alloying elements such as Co. This paper presents a historical view of developments over the last several decades, as well as a survey of the current status of steel development for power generation boilers, mainly with regard to creep strengthening and alloy design.