No abstract

Bimetallic rolls are widely used in hot rolling mills because of excellent hardness, wear resistance, and high temperature properties. Considerable residual stresses are produced for the bimetallic roll during quenching. Moreover, severe thermal stresses are caused by heating-cooling thermal cycles during subsequent hot rolling process. Fracture from the roll center may occur due to the residual stress adding to the thermal stress, and therefore, it is desirable to investigate the residual stress to improve roll service life. Therefore, FEM simulation of the bimetallic roll is performed for the quenching process. It should be noted that a large number of experimental data of the core and shell material are utilized for the wide range of temperature considering the quenching. The generation mechanism for the residual stress is discussed focusing on the effect of temperature gradient and phase transformation. Furthermore, the effects of shell-core ratio and diameter on residual stress are considered. Results show that the residual stress only slightly increases with increasing shell-core ratio, while significantly increases with increasing diameter.

Bimetallic rolls are widely used in steel rolling industries because of the excellent hardness, wear resistance, and high temperature properties. Controlling the residual stress distribution is important to improve the roll fatigue life due to the compressive residual stress at the roll surface. Recently, to reduce the tensile residual stress appearing at the roll center, quenching heat treatment is performed just after heating the roll non-uniformly instead of heating the roll uniformly with enough time. In this paper, therefore, the residual stresses are compared after between the uniform heating quenching and the non-uniform heating quenching on the basis of the FEM simulation. The results show that tensile stresses at the roll center for non-uniform heating are smaller than that for uniform heating by 400 MPa, although the same compressive stresses appear at the surface. The effect of creep on stress relaxation is also considered in this study. By considering creep, the maximum tensile residual stress decreases by 8% for uniform heating and by 15% for non-uniform heating.

Ultrasonic welding of pure aluminium sheets was observed with a high speed video camera. The dynamic vibration behaviours of a welding tip and aluminium sheets were analysed using the digital image correlation method. The welding process consisted of the following three stages. First, the upper specimen in contact with the weld tip vibrated. The formation of partially welded regions was confirmed at this stage. Second, the vibration amplitude of the upper specimen decreased, while friction between the weld tip and the upper specimen increased. Growth of the partially bonded region was confirmed in the second stage. Third, the welding part began to plastically deform owing to the clamping force. The joint strength reached its maximum value at the third stage. The analysis demonstrated that the relative motion between the weld tip and the upper specimen predominantly affected the increase in joint strength.

Bimetallic rolls are widely used in steel rolling industries because of the excellent hardness, wear resistance and high temperature properties. Controlling the residual stress distribution is necessary since the compressive residual stress at the surface may improve fatigue life though the tensile residual stress at the center may reduce the strength. Therefore, it is necessary to measure the residual stress distribution from the surface to center correctly to ensure the roll quality. The disk method has been widely used in predicting the roll residual stress by measuring the stress of the thin sliced disk from the roll. In this study, therefore, the relation between the original roll residual stress and the sliced disk residual stress is investigated for the single material roll and bimetallic roll on the basis of thermo-elastic-plastic FEM analysis. The effect of the quenching time is discussed as well as the effect of the sliced disk thickness on the residual stress.