Investigation into fusion boundary carbon diffusion in 1/2Cr1/2Mo/1/4V weldments

The steam lines in the majority of power stations operating in the UK were fabricated from kCrkMokV steam pipe material joined using 2CrMo weld metal. Throughout the life of these systems, which operate typically at 565°C and 165bar, weldment cracking problems have been encountered. Many of these problems are well understood and are managed effectively. In recent years a creep cracking mechanism in the heat affected zone adjacent to the fusion boundary has been identified, referred to as Type IIIa cracking. This project has identified significant carbon migration across the fusion boundary of these weldments, from the lower alloyed kCrkMokV to the more highly alloyed 2CrMo weld metal. Carbon diffusion has been identified as occurring during the welding cycle, however the majority of carbon diffusion occurs during post weld heat treatment and particularly during service exposure. Carbon migration results in marked reduction in carbon content in the heat affected zone adjacent to the fusion boundary with a corresponding band of enhanced carbon content in the weld metal adjacent to the fusion boundary. The width of these areas increase with time at temperature. Micro hardness measurements have confirmed a reduction in the hardness of the heat affected zone region as a result of carbon diffusion. Hardness measurements have also pointed to a step change in hardness across the fusion boundary. This may result in a corresponding step change in creep properties. It is suggested that this mismatch in creep properties, coupled with a predominantly fine grained microstructure at the fusion boundary, can result in the fusion boundary heat affected zone region being the weakest region of the weldment.