Development of improved weld repair procedures for carbon steel castings

Abstract

Wear parts for mining equipment are sometimes in need of repair of cracks found in production. This experimental parameter study addresses the quality of repair welds made with process 111, Manual metal arc welding. The base material is a cast steel, the unalloyed electrodes have a basic coating. Parameters varied in the experiment include preheating, electrode diameter and weld current. Weld speed is adapted by the welder to generate a sound weld. Typically, five weld beads were needed to fill up the groove made with abrasive grinding, simulating removal of cracks. After the visual examination, cross sections were prepared for microscopy and hardness testing. The top layer welds were free from cracks and larger pores based on visual examination of the surface. Microstructural cross sections show heat affected zones with thickness from 0.8 to ca 3 mm, depending on weld parameters. Larger electrode diameters and higher currents lead to thicker heat affected zones. The weld metal exhibits a columnar dendritic microstructure. In overlapping welds, the columnar microstructure is recrystallized to a fine-grained heat affected zone. The hardness of the weld metal is slightly higher than the base metal hardness. Only a few pores, below 1 mm in diameter were found in the cross sections, but the sampling volume is too small to quantify pore distributions.