June 18, 2014
Q: We are welding several fabricated parts made from A514 (T1) steel using an AWS A5.29, 3/32-in-dia. E110 FCAW electrode. On one particular part we are experiencing longitudinal weld cracking. The structure is a 4-in. plate with a square cut out of the center and a 1.5-in. plate welded back in its place. We are following preheats recommended for the varying thicknesses, so what’s the problem?
A: It’s not uncommon to find longitudinal cracking on this type of fabrication. This type of scenario is considered one of the highest-restrained joints that can be created. If this were A36 steel, a simple increase in preheat temperature and longer slow cooling would resolve the problem. However, A514 steel is quenched and tempered, meaning excessive preheating can affect the material’s mechanical properties negatively, so you must use caution.
In many cases, if you adhere to proper weld preheat and slow-cooling practices and still are experiencing cracking, the cause may be one or a combination of things:
Typically with this wire classification, the welder tends to decrease the wire feed speed in an attempt to reduce the amount of spatter and smooth out the arc. Unfortunately, the net effect of this adjustment is a wider, shallower weld bead resulting in higher alloy dilution from the base material into the weld, which makes it more susceptible to centerline cracking. As the bead solidifies, there is not enough filler metal in the throat of the weld to accommodate the weld shrinkage. Be sure to follow the wire manufacturer’s recommended weld parameters.
Also be sure to stagger the weld bead starts and stops throughout the joint and avoid them in the plate corners. Always follow proper crater fill techniques when making the weld.
Since this is a flux-cored wire, make sure to use a “drag” technique. This technique will produce a weld with better penetration, and it also prevents the weld beads from getting too flat and shallow.
Finally, the weld beads should be staggered in an alternating fashion around the joint; that is, alternate between sides of the weld joint and between sides of the plate if it is welded on both sides. This effort will help balance out the residual stresses and prevent excessive stress buildup in one location.
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