Cracking of tungsten carbide rolls during hot rolling of branded rebars in a bar mill

Premature failure of Tungsten Carbide (WC) rolls, due to cracking during hot rolling of branded rebars was found to raise the cost of rolling due to unscheduled stoppage of the mill. Hence, a detailed investigation was conducted to reduce above problem using non-destructive analytical technique such as Ultrasonic velocity measurement, X-ray diffraction (XRD). Finite Element Analysis (FEA) was carried out to know the rolling stress and its adverse effects. Optical technique Scanning Electron Microscopy (SEM) and fractography also carried out to know the root cause of the cracking problem. XRD results indicated high tensile residual stresses at branded area (420.5 MPa) and unbranded area(279.2 MPa). FEA results revealed that the rolling stress was much lower than the yield stress of WC roll. SEM and Fractography revealed beach and striation marks which confirmed fatigue failure. Whereas ultrasonic velocity measurements indicated density variations of 0.49 g/cm3 which is very high in comparison to the desired permissible variation of 0.05 g/cm3. The rolling stress together with the impact stresses developed in the branded area due to surface unevenness and causes variation in compactness (density) which in turn causes fatigue failure. This phenomenon was also confirmed with SEM and fractography.