Reverse transformation behaviour in B containing high Cr heat resistant steel HAZ

Boron addition in the ferritic heat resistant steels significantly improve the creep strength of the welded joint due to prevention of the Type IV failure. It is attributed to improvement of the HAZ microstructure. Coarse grained prior austenite is formed in the B containing steel (B steel) HAZ due to reconstitution of the original austenite observed in the base metal, whereas the prior austenite is significantly refined in the conventional heat resistant steel HAZ. In the present study, ferrite to austenite reverse transformation process has been investigated to understand the suppression mechanism of the grain refinement in the B steel HAZ. 130 ppm B containing 9Cr-3W-3Co-Nb, V steel was used as the B steel. Some other steels with different reverse transformation process were also used to observe the typical feature of each reverse transformation process. HAZ thermal cycle simulation test was carried out for various peak temperatures to observe the microstructure evolution during weld thermal cycle. In addition, ferrite-to-austenite reverse transformation behaviour was directly observed using a laser microscopy with a high temperature stage. When the peak temperature was just above A_C3, coarse prior austenite grains were observed in the B steel, but a few fine grains were also observed around the coarse austenite grain boundary. The volume fraction of the fine grains increased with increasing the peak temperature above A_C3. This phenomenon can be explained if the coarse austenite contains high density of dislocations. Clear surface relief was confirmed during ferrite-to-austenite reverse transformation of the B steel by direct observation of the transformation process. This is a typical feature of martensitic or displacive transformation where a number of dislocations are introduced. It is thus considered that martensitic reverse transformation takes place during the weld thermal cycle in the B steel, and this prevents the grain refinement in the HAZ.