RESTORATION AND STRENGTHENING OF THE SURFACE OF HIGH-CHROMIUM CAST IRON PARTS BY LASER TREATMENT

doi: 10.25712/ASTU.2072-8921.2021.02.035

Abstract

The abrasive and corrosive effects of the sludge and the rapid changes in the working speed intensively affect the wear process of the pump elements, especially the impeller. It is estimated that the service life of the pump is from 212 to 248 hours. In just one year, approximately 400 pumps are removed from the line for repair at the repair plant. Most of them failed due to the wear of the impeller only. For example, according to our study, 80 percent of the 400 pumps failed just because of impeller wear. The production costs of the helix, impeller, and protective plate are 10.5, 5.1, and 1.5 million tugriks (a unit of Mongolian currency equal to approximately 2000 US dollars by 2015), respectively. Only one restored pump costs about 20 million tugriks (10 thousand US dollars). This study analyzed the effects of laser processing, such as welding, surfacing, and surface melt treatment, on the microstructure, hardness, and wear resistance of high-chromium white cast iron, which is used to make large slurry pumps. Wear-resistant high-chromium white cast iron is a material of high hardness and abrasive wear resistance. Any repair action for wear-resistant white iron must provide a hardness of more than 50 HRC and equal or greater wear resistance than that of the base cast iron. Laser surfacing of only a metal-ceramic powder (similar in composition to INCO-702) could satisfy the above requirement, but it gives the cast iron pores and cracks. The research was aimed at reducing the formation of cracks and pores during the processing process. Wear-resistant white cast iron can be laser-welded with preheating. Thin sections of high-chromium white cast iron, welded with samples of cast iron and steel, have no defects in the welding zone. Surface melt treatment is probably less effective for high-chromium white cast iron due to the low mechanical properties of the molten surface. In addition, the high cooling rate during laser surface melting does not allow the austenitic phase to transform and does not provide the thermal conditions necessary for the desired formation of secondary carbide precipitates or martensitic transformations.