The Research Of Welding Materials And Procedures Of Partial Repair Supported By Cr3 Back-up Rollers

Rollers work in poor conditions and they are rather liable to be partially invalidated. If build-up welding can be used to repair the partial invalidation, and what is more, the functional performance of the repaired rollers can reach the performance of the original rollers, then the economic benefits can be quite considerable. In the view of environmental protection, the resource can be saving largely, and the pressure on environment will be reduced. The cold crack susceptibility of Cr3 back-up rollers is extremely high for the quite high carbon content, so it is rather difficult to repair the partial defects by build-up welding. Because rollers are usually made by forging or casting, the weldability of Cr3 steel is seldom studied. According to the properties of the base metal and the working conditions, the build-up welding materials has been studied and the welding procedures for Cr3 back-up rollers partial repair has been completely analyzed in this thesis.The basic requests of the designed electrodes are the hardness (HRC50⁵5) and the crack resistance. Five kinds of electrode are designed and made, the core wire of all the electrodes is H08A,Φ4, and the type of all the coating is low-hydrogen. There are C, Cr, Mo, Si and Mn as the main alloy elements. The crack resistance, composition, hardness and resistance to wear, temper resistance, diffusible hydrogen content and etc of deposited metal of these five kinds of electrode have been tested and analyzed, and the properties of deposited metal of one of these five kinds of electrode have been verified in the welding trial which has been made on an abandoned Cr3 back-up roller. Furthermore, a method which combines the intuitive analysis of spot welding with the quantitative analysis of HAVOVER ANALYSTOR is used to test the welding processing properties of these five kinds of electrode.The carbon content of Cr3 steel is high, according to the traditional procedures, the partial repair by build-up welding on large workpieces such as rollers always needs to preheat to 300℃～500℃. The working conditions for workers is poor and the consumption of energy is much, so lowering preheating temperature is very necessary. In the partial repair for the Cr3 back-up rollers, most of the cracks are cold cracks which are in the heat affected zone (HAZ). So the key to determine welding procedures is to avoid the appearance of the cold cracks in HAZ. Welding thermal simulation is used to study the welding procedures, 10 welding procedures are simulated. Based on analyzing the dilatometric curves, metallographic structures of test samples of these simulation procedures, four factors which have influences on the tendency of the appearance of the cold cracks in HAZ have been studied, these factors are preheating, slow cooling, expanding the cooling time (t_8/5 and t_8/3), and heat preservation. Validating tests have been made on an abandoned Cr3 back-up roller to make sure the correctness of the results of the welding thermal simulation.The results show that, at least three kinds of the designed electrode have good deposited metal properties and welding processing properties, they can fit for the requirements of the partial repair of Cr3 back-up roller. If proper measures (such as slow cooling, heat preservation and etc) can be adopted in cooling process, it is possible to put the preheating temperature down to 200℃or even lower. So, in this thesis, the design of the build-up welding materials which can be used to repair the partial invalidation of Cr3 back-up rollers is successful, and the traditional welding procedures has been optimized.