Brittle Fracture of a Welded Shaft
A 3 in. diam shaft was found to have suffered excessive wear on one of the journals and was built up by welding. While it was in the lathe prior to turning down the built-up region, a crack was discovered in the root of the oil-seal groove and subsequently the end of the shaft was broken off with hammer blows. The fracture surface was duplex in nature, there being an annular region surrounding a central zone, which suggests that the fracture developed in two stages. Microscopic examination confirmed that the fracture was of the brittle type. The shaft material showed a microstructure typical of a medium-carbon steel (carbon approximately 0.4%) in the normalized condition, a material not weldable by ordinary methods. It was concluded that the post-welding crack arose primarily from the thermal contraction which developed in the weld metal on cooling. It is probable that if the built-up zone had extended beyond the oil seal groove, failure in the manner would not have occurred. Experience indicated however, that failure from fatigue cracking would still have been likely to occur.