Creep characterization of Boron added P91 steel in the temperature range 600 - 650 degree C

Abstract

Modified 9Cr1MoVNb steel popularly known as Grade P91 is a creep resistant Ferritic steel used in different high temperature components of power plants such as steam pipes, super-heaters, steam headers and in process vessels of chemical industries. However welded P91 grade steel components are reported to show premature creep failure in the temperature range of 600- 650oC. Failures occur in the inter critical heat affected zone (ICHAZ) and grain refined HAZ (GRHAZ) of weld joints, generally known as Type IV cracking of weld. Research attempts have been made to understand and minimize its susceptibility to such cracking. One approach to alleviate this is to modify the parent steel composition with suitable alloy additions. Addition of 60-130 ppm of boron to the existing P91 alloy composition is reported to cause delay in recovery of lath structure resulting in the formation of austenite during weld thermal cycle. This microstructure results in reduction of creep rate across the weld zone and enhances resistance to Type-IV cracking. However a systematic study on the creep behavior of boron added P91 steel and its weld is lacking. It is necessary to understand the creep behavior and to collect creep data on boron added P91 steel for reliable operation and long term life estimation of high temperature components made of this steel.