Boron: Type IV Cracking

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Boron: Type IV Cracking

Conference/Talk

Abe, Fujio / Tabuchi, M. / Tsukamoto, S. / Liu, Y.

A lecture given by Fujio Abe, at the Adventures in the Physical Metallurgy of Steels (APMS) conference held in Cambridge University. Some wonderful work on power plant steel containing boron, to prevent the pernicious type IV cracking. The addition of boron without the formation of any boron nitrides during normalizing heat treatment at high temperature minimizes the degradation in creep strength of both base metal and welded joints of 9%Cr ferritic power plant steel at 650oC and long times. The enrichment of soluble boron near prior austenite grain boundaries (PAGBs) by the segregation is essential for the reduction of coarsening rate of M23C6 carbides along boundaries in the vicinity of PAGBs, enhancing the boundary and subboundary hardening in base metal at long times, and also for the production of same microstructure between the base metal and heat-affected-zone (HAZ) in welded joints. The grain boundary segregation of boron retards the diffusive transformation from ferrite to austenite in HAZ during heating of welding, resulting in martensitic transformation. The resultant microstructure of HAZ after post weld heat treatment is substantially the same as that of base metal, indicating no Type IV fracture in HAZ of welded joints. 9Cr-3W-3Co-VNb steel with 120-150 ppm boron and 60-90 ppm nitrogen exhibits not only much higher creep strength of base metal than conventional 9%Cr steel but also substantially no degradation in creep strength due to Type IV fracture in welded joints at 650 C.