Study On The Structure & Properties Of Alloyed Steel Roll After Laser Melting

Roller is the most important part of the rolling mill, and also is one of the main consuming parts during production, which has a large size and is very expensive, so reducing the consuming of the roller is an important problem in the production. Fast solidification process on the metal material by Laser heating technology can produce the non-equilibrium and metastable microstructures such as minicrystal, amorphous, quasi-crystal and so on, which is fine, uniform and metastable, and different from the matrix in elements, microstructures and properties, and then improves the properties of the materials. Therefore, studying on "Research on the microstructures and properties of roller by laser surface melting" is provided with a large practical significance and theoretical value.In this paper laser surface melting (LSM) is performed on the roller steel with a 5kW continuous wave CO₂ laser, and the microstructures, hardness and cracking are studied; also numerical simulation was carried out about the process of laser surface melting with the software SYSWELD, what's more, the varying regularities of temperature field, stress field and hardness field has been achieved.After laser surface melting, the sectional region on roller steel is divided into three distinctive zones, namely, laser melted zone, heat-affected zone (HAZ) and the substrate, which can not be delimited strictly. Laser melted zone is composed of cellular structure and dendritic crystals, and the microstructures are martensite, residual austenite and disperse carbide, also HAZ is mainly composed of martensite, residual austenite and carbide. Hardness of the roller after laser surface melting has a gradient distribution along the depth of melted zone, which in melted zone is lower after laser surface melting, however, the maximum value appears at HAZ. And while the laser power and the diameter are given, the depth of the melted zone has an inversely-proportional relationship with the velocity. The macroscopic quantity is influenced by scanning velocity significantly. With the increase of the scanning velocity, the cracking susceptibility increases, and the depth of melted zone decreases gradually as well as the width, so the ratio ofΔH and W increases gradually. Cracks were found on both sides of molten pool under the experimental conditions. While overlapping and with a small overlapping fraction, cracks appear in the overlapping, but when the overlapping fraction reaches half of the diameter of the spot, and also the microstructures in the overlapping are uniform and free of cracks, but the softening occurs with the hardness decreasing.There-dimension model of the laser source was established, and the shape of the laser surface melting according with the depth and width can be simulated, which is concordant with the calculated values, so the model is applicable. The temperature field can be simulated with the model concordant with practiceAccording to the results of simulation, laser surface melting is a process of quick heating and cooling, in which the temperature can reach 10⁴℃/s; the hardness of the material after laser surface melting increases, and in the center of the melted zone is larger than that of the other zones; and the residual stress is compressive in the melted zone, but which is tensile at HAZ, being the dangerous zone of cracking; the non-uniform variation of the carbon content results in the non-uniform distribution of compression stress in the melted zone.Different distributions of residual stress appear under different progress parameters by laser surface melting. The distributions of the Mises stress and average stress in the surface of melted zone are similar to that along the depth of the melted zone, that is to say, the value of stress will decrease when the scanning velocity is too larger or too small. While the scanning velocity varies from 600mm/min to 1000mm/min, compression stress can be obtained which is in favor of improving the property of the workpiece. The generation of the compression stress in the melted zone is related to the hard brittle martensite created during the microstructural transformation...