Assessment of hydrogen cracking risk in multipass weld metal of 2.25Cr-1Mo-0.25V-TiB(T24) boiler steel

Welding modern high-strength steel with low carbon and impurity contents, preheating may be dictated by cracking sensitivity of the weld metal instead of the HAZ. Standardised methods are mostly developed for HAZ cracking cases; the applied experiments are therefore usually single-pass cracking tests. For multiple-pass welding, and provided cracking sensitivity of weld metal dominates the required preheat, standard EN 1011 does not provide the user with any unified, scientifically validated methodology for the calculation of safe preheat. For welds having chemical compositions different from structural steels, such as low-alloy creep resisting steels, the suitability of a given standard procedure, or calculation method, for multipass welds has to be investigated on case by case basis. A study concerned with the assessment of required T_o/T_i for 7CrMoVTiB10-10 type weld metal in 2.25 %Cr-1 %Mo-0.25 %V boiler steel is presented, using EN 1011–2 Method, the Crack — No Crack Diagram Method developed by VTT, and deep-notch Implant test based analytical method developed by Osaka University and VTT. Necessary precautions for safe welding are given. Recent advances in numerical evaluation of hydrogen diffusion in multipass welding using transient finite element analysis are utilised for a thin to thick plate case and the implications discussed.