Study on the distortion of steel worm shafts

This paper analyses the root causes for the distortion observed in some C45E grade steel worm shafts during the last steps of machining. Research carried out on two round bars of the steel where distortion was observed and two other ones of a batch machined without distortion were studied. Moreover, two worm shafts in the last steps of the machining process were also analysed. Residual stresses measurements revealed lower and more homogeneous distribution in the bars which do not exhibit the problem. Yield stress and ultimate tensile stress values recorded in the steel of these bars were also lower than those measured in the bars which presented the problem. Metallographic study of the first ones revealed a broken morphology of cementite in the pearlite pointing towards a material stress relieved after the rolling process, eliminating, at least partially, those stresses generated during the deformation of the bar. On the other hand, the pearlite in the steel which suffers distortion looked to be formed during the cooling process after rolling and no given posterior stress relief heat treatment. A stress relief heat treatment of the bars and/or a kinder machining are recommended.