P. Mayr, H. Cerjak, Graz University of Technology, Graz, Austria; J. W. Elmer, Lawrence Livermore National Laboratory, Livermore, CA; T. A. Palmer, The Pennsylvania State University, University Park, PA
Welded components of martensitic 9-12% Chromium steels are widely used for high temperature applications in thermal power generation. Long-term service experience as well as crossweld creep testing data has revealed the fine-grained region of the heat-affected zone (HAZ) to cause premature failures by Type IV cracking. Therefore, severe weld strength reduction factors have to be considered in the design of welded high temperature components.
In this paper, the influence of weld thermal cycles on the base material microstructure of a 9Cr-3W-3Co-V,Nb steel with balanced boron/nitrogen ratio is described. Phase transformations during physical HAZ simulation were monitored by in situ X-ray diffraction using synchrotron radiation. A test weld was produced and crossweld specimens creep tested at 650°C. Extensive metallographic studies using optical as well as most advanced electron microscopic methods gave insight to the formation of an extraordinary HAZ microstructure.
Although the steel did fully transform to austenite on heating and re-transformed to martensite on cooling, the original martensitic base material grain structure was maintained throughout the HAZ. No conventional grain-refined region was formed in the entire HAZ. Creep tests revealed similar creep strength of base material and crossweld specimens.
By elimination of the fine-grained HAZ region, this steel shows high potential to overcome Type IV cracking and exhibit improved crossweld creep strength.
Summary: Phase transformations in the heat-affected zone of a martensitic 9Cr-3W-3Co-V,Nb steel with balanced boron/nitrogen ratio were observed by in situ X-ray diffraction using synchrotron radiation. Although the steel did fully transform to austenite on heating and re-transformed to martensite on cooling, the original martensitic base material grain structure was maintained throughout the HAZ. No conventional grain-refined region was formed in the entire HAZ.
