Failure Analysis On Cracked Hardfacing Layer Of Metallurgical Roller And Finite Element Calculation

A failure investigation has been conducted on a submerged-arc surfacing roller. Crack morphology, macro and micro-fracture morphology of failed roller were observed by stereo microscopy and scanning electron microscopy (SEM). The hardness profiles in various regions were evaluated by Rockwell Hardness Tester (HRC). Microstructure of the hardfacing layer, transition layer and matrix was observed by OPM and SEM. The amount of retained austenite in hardfacing layer was measured by X-Ray diffraction (XRD). Chemical composition in various regions was determined by Optical Emission Spectroscopy (OPS). Energy-dispersive X-ray (EDX) analysis was carried out on the fracture surface to identify micro-chemical composition in various regions. Detailed metallurgical analysis and observations on the cracked surfaces of roller were conducted to reveal the possible reasons for the failure.In this paper, a Finite element analysis (FEA) model predicting the temperature field distribution, axial, circumferential and radial thermal stress field distribution in and around the bead was developed. Additionally, the axial, circumferential and radial stress fields under various welding energy inputs were also simulated.The main results are concluded as follows:(1) Occurrence of dispersively distributed transverse short cracks, transverse intergranular micro-cracks on the surface of hardfacing-repaired backup-roller and intensively distributed transverse short cracks, transverse intergranular micro-cracks inside the outer layer of hardfacing region is caused by hot cracking, which are crystallization cracks(1-10mm length) and reheat cracks respectively. Crystallization cracks are located in weld center, while, reheat crack through the ferrite grain boundary located in coarse grain area near weld.(2) The metallurgical factor responsible for hot cracking on the outer layer is the presence of general excessive content of P (0.025%) in the outer layer and P, S segregation in the interdendrite space. The metallurgical factor responsible for appearance of reheating cracks in the outer layer is presence of high content Mo, V carbide formation elements and P (0.04%), S(0.03%) weakening boundary elements.(3) ANSYS analysis indicates that the axial thermal stress peak area is presented in center of weld, while, the circumferential and radial thermal stress peak area in the heat-affected zone. The axial thermal stress (242MPa,1100℃) is much higher than the circumferential thermal stress (171MPa,1100℃) and radial thermal stress (48MPa, 1100℃). The higher axial thermal stress and thermal stress peak area in center of weld is mechanical factor leading to forming transverse crystallization cracks and reheat cracks.(4) Thermal stress becomes smaller as decrease in the weld bead width. It is suggested to minimize the energy input for decreasing the tensile stress.