Interior crack and its formation mechanism in overlaying weld of back-up rolls

• Interior crack in overlaying weld of back-up rolls was studied.
• Fracture surfaces without cracks was mainly transgranular fracture.
• Fracture surfaces with cracks was columnar crystals and secondary cracks.
• Optimizing alloy system and reducing impurities were taken to prevent crack.

Through ultrasonic NDT, cracks with various lengths are often found inside the hardfacing metal for re-manufacturing large-scale back-up rolls to gain high surface hardness. Analysis of the microstructure, surface morphology, and micro-area composition found that these cracks result from thermal cracking at high temperatures, and the surfaces with these interior cracks are found to be eutectic with low fusion point and contain segregation of alloy elements and dendritic crystal cells. In order to enhance the plasticity of the deposited metal and eliminate such cracks, in the meantime still maintaining the strength, a new overlaying welding consumable was developed, applied, and tested, which has (1) reduced contents of carbide forming elements such as V and Mo, (2) increased contents of powerful carbide forming elements Nb and Ti, and (3) newly added RE elements for deoxidation and reducing the contents of impurity elements S and P. No crack was found by ultrasound NDT, and microstructure analysis after a tensile test show that plasticity of the new hardfacing metal is improved without affecting hardness and strength.