June 25, 2014
Q: When welding a thick section of a forged 4130 ring to a thinner A36 plate in a T-joint configuration, we experience longitudinal cracking issues that originate from the ends of the welds. The required fillet size is ½ inch. It is skip-welded 12 in. on 18-in. center-to-center spacing. The material is being preheated to 250 degrees F and welded in the 2F position with two passes of 1/16-in.-dia. E71T-1 flux-cored wire and 100 percent CO2 shielding gas. How can we prevent this cracking?
A: The preheat temperature that you are using on the 4130 material is too low for heavier sections. When the preheat temperature is too low, the weld cools too quickly, which produces an undesirable microstructure that is prone to cracking. If the material is more than 1 in. thick, your preheat and interpass temperatures should be between 400 and 600 degrees F. Also, you will want to slow down the cooling process by covering the weld area with a fireproof insulation or heavy blanket immediately after welding.
The problem with two passes for a ½-in. weld is the first weld bead is typically focused on the flat part of the joint and doesn’t allow for sufficient penetration at the root. Additionally, to get the required ½-in. weld, you have to decrease the travel speed to allow the weld bead to build up, which typically leads to a heat input above recommended values.
Instead, use a true multipass technique to reach the required ½-in. fillet size. To do this, run one weld bead directly where the two pieces of metal meet at the root of the T-joint, and then weld corresponding layers using multiple passes to build up the weld from the flat to vertical material.
Each individual weld bead should be no wider than the requirements for a ¼-in. to 5/16-in. weld bead. Since this is typically a slag-producing process, drag the weld along at about a 30-degree angle and use a 40-degree work angle.
Also, be sure to leave the craters of the weld in the middle of the weld bead rather than at the weld ends, which will help eliminate crater cracking.
Since you are welding in the 2F position, I recommend changing to an E70T-1 wire. This will improve your bead appearance, profile, and weld-toe wetting to help reduce stresses in the weld.
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