AN INVESTIGATION ON THE DEFORMATION AND CRACK FORMATION OF METALS IN CROSS FORGING AND CROSS ROLLING

The key to successful cross rolling is to understand fully the distribution of deformation and the mechanism of crack formation at the centre of the rolled material as well as the optimum parameters for the technological process. With these points in mind, the authors carried out investigations by means of microscopic examinations and hardness tests on the distribution of deformation and the mechanism of crack formation at the centre of certain steel specimens at various temperatures, various rates of deformation, different ratios between the length of the plastic zone and the specimen diameters as well as at different lengths of material at both ends of the plastic zone.As the deformation of metals in cross rolling is in many ways similar to that in cross forging, the authors carried out experiments on the cross forging of aluminium and lead which had been marked with concentric circles at the ends in a 35-ton mechanical press at room temperatures, and then compared the results thus obtained with those in the cross rolling of carbon and alloy steels.The results of these experiments show that:(1) With only one strike of the press in cross forging, the deformation is greatest at the surface and diminishes towards the centre of the specimen. When reduction is small, only elastic deformation occurs at the centre.(2) With several strikes of the press on rotating specimens, the deformation at the surface and in the centre is greater than that in the intermediate zone. When reduction is very small, plastic deformation also does not penetrate into the centre.(3) Similar to cross forging, the deformation of metal in cross rolling is also greatest at the surface, less at the centre and least in the intermediate zone. This distribution of deformation remains practically unchanged in different metals and various rolling temperatures as well as under other testing conditions mentioned above. Under conditions of the present experiments, when reduction is small, the deformation at the centre may be only elastic. In this case, the distribution of deformation is similar to that with only one strike of the press in cross forging.In cross rolling, the greater the reduction, the higher the rolling temperature, the higher the rate of deformation, the greater the ratio between the length of the plastic zone and the specimen diameter, and the shorter the length of the material on either side of the plastic zone, the greater is the tendency for crack formation at the centre. Alloy steels seem less liable to crack formation than carbon steels.