Preheating a shaft-sprocket assembly
February 13, 2007
We are welding 1018 cold-rolled, 2.5-inch-diameter shaft to a sprocket made from 1045 steel. We preheat the shafts to about 200 to 250 degrees. We are having some failure of the shaft, and I'm wondering what the proper procedure is for welding the assembly together. The sprockets are welded full-diameter on both sides. The shaft usually breaks at the transition from the edge of the weld. Should we be preheating to a higher temperature, heating both the shaft and the sprockets, or not preheating at all?
John A. Andrew, Unity, Wis.
You're joining a low-carbon (less than 20 percent) shaft to a medium-carbon (more that 40 percent) sprocket. You're on the right track, but your preheat must include the sprocket. It also should be in the 200- to 400-degree range, depending on the process used and joint thickness.
Maintain this same range for interpass temperature. Use a low-hydrogen procedure and filler metal; hydrogen pickup through dilution will contribute to a high weld metal hardness, which will increase susceptibility to cracking and brittleness. Minimize dilution by depositing small weld beads in a multipass technique.
Finally, use a postweld heat treatment. If this isn't practical, maintain the preheat/interpass temperature for two to three hours per inch of joint thickness.