Solidification cracking susceptibility of modified 9Cr1Mo steel weld metal during hot-wire laser welding with a narrow gap groove

Abstract

Hot-wire laser welding with a narrow gap groove was applied to ASTM A213 grade T91 steel in the application of ultra super critical boilers. A filler wire according to AWS A 5.28: ER90S-G was used. The effect of the depth-to-width ratio (D/W) of a weld shape on the susceptibility to solidification cracking was investigated. With a groove width of 3 mm, solidification cracking did not occur with a D/W ratio of less than 0.6 and more than 1.2. The hot-wire laser welding method with a narrow gap groove seemed to be efficient for practical use, when the higher D/W condition was used without solidification cracking. The high temperature strain, which occurred during solidification, was computationally calculated in order to make clear the effect of D/W ratios on the susceptibility of solidification cracking in theory. From in situ observation using a high-speed camera of the hot-wire laser welding phenomena, a three-dimensional Finite Element Method (FEM) was simulated to estimate an equivalent plastic strain in the weld metal during welding. An approach to the moving heat source consistent with such phenomenon was proposed and validated based on experimental measurements. The simulation results presented that a moving heat source by hot-wire laser welding was able to produce a particular weld shape at a higher D/W ratio with a lower strain rate, which corresponded with the relationship between D/W ratios and solidification cracking in experiments. Therefore, such a hybrid process is a very interesting alternative process to reduce the solidification cracking in narrow gap welding, especially in modified 9Cr1Mo steel.