Contribution of Electron Microscopy to Investigation of Weld Structures in Cr - Mn Steels

AUSTENITIC CR-MN STEELS ARE UNDER CONSIDERATION AS POTENTIAL REDUCED LONG TERM ACTIVATION MATERIALS FOR THERMONUCLEAR FUSION REACTORS. SINCE WELDING WILL BE EXTENSIVELY USED IN REACTOR CONSTRUCTION, WELD INTEGRITY IS AN ESSENTIAL CONCERN. INFORMATION IS THEREFORE REQUIRED ABOUT WELD STRUCTURES IN THESE MATERIALS AND THEIR BEHAVIOUR DURING PROLONGED EXPOSURE TO REACTOR OPERATING CONDITIONS. ELECTRON MICROSCOPY STUDIES ARE CURRENTLY IN PROGRESS TO CHARACTERIZE WELD STRUCTURES AND ASSESS THEIR THERMAL PHASE STABILITY IN STEELS OF A RANGE OF COMPOSITIONS. SCANNING ELECTRON MICROSCOPY WITH ENERGY DISPERSIVE X-RAY MICROANALYSIS IS USED FOR PRELIMINARY IDENTIFICATION OF THE PHASES, PRECIPITATES AND INCLUSIONS PRESENT IN THE WELD AND IN THE HEAT AFFECTED ZONE. MORE DETAILED INVESTIGATION OF INDIVIDUAL SPECIMENS OF PARTICULAR INTEREST IS THEN CARRIED OUT BY MEANS OF ANALYTICAL TRANSMISSION ELECTRON MICROSCOPY. THE STEELS WERE ALL FULLY AUSTENITIC IN THE ORIGINAL STATE, EXCEPT FOR ONE WHICH HAD A DUPLEX STRUCTURE CONTAINING 8-10 % FERRITE. THE WELDING TECHNIQUES USED WERE SHIELDED METAL ARC, TUNGSTEN GAS ARC AND ELECTRON BEAM. WELD MATERIAL IN STEELS WITH LOWER CR CONTENT WAS VIRTUALLY FERRITE FREE,WHILE IN THOSE OF HIGHER CR CONTENT THE AMOUNT AND MORPHOLOGY OF THE FERRITE WERE DEPENDENT ON COMPOSITION. IN SOME CASES HEXAGONAL EPSILON MARTENSITE WAS FORMED IN BOTH THE WELD REGION AND THE HEAT AFFECTED ZONE. AN ATTEMPT HAS BEEN MADE TO RELATE THESE FINDINGS TO THE PREDICTIONS OF AVAILABLE WELD CONSTRUCTION DIAGRAMS. AS A RESULT OF THERMAL AGING SIGMA PHASE MAY BE NUCLEATED WITHIN THE FERRITE PROVIDED THAT THE CR CONTENT IS SUFFICIENTLY HIGH. WELD STRENGTH, AS EVIDENCED BY EXAMINATION OF FRACTURE SURFACES MAY BE STRONGLY INFLUENCED BY FERRITE/AUSTENITE DENDRITIC CELL BOUNDARIES, GRAIN BOUNDARY PRECIPITATES AND NON-METALLIC INCLUSIONS.

SASAKI Takao; 

1995-03-15

JRC4040

https://publications.jrc.ec.europa.eu/repository/handle/JRC4040,