Effect of Welding Into the Decarburized Layer

[tc7]

When welding on parts that are quenched and tempered, we would typically grind bevels and thereby eliminate most if not all of the decarburized layer and we never have to worry about this.

But what about fillet welds or buttering on flat surfaces where we would only clean off mill scale and sand of just enough to see shiny metal prior to welding? Are we compromising the weld by not grinding off the complete decarbed depth? And what is the depth of decarburization?

My case in point is 4340 plate that has been heated to 1500 deg F then quenched then tempered for 2.5 hours. We will weld in the Q&T state using 12018 SMAW electrode and stress relieve at 1100 deg F. We will be building up some surfaces and filet welding elsewhere. What effect will welding on the decarb areas have on final weld strengths?

 

[metengr]

tc7;
If you have a decarburized layer on the entire surface of the 4340 steel plate, fillet welding or weld overlay without complete removal of this layer will have little to no effect on overall performance relative to bulk properties of the plate. This is based on actual experience of Power Plant low alloy steel components in service.

IF the application of the plate is such that you have high local bending stresses that can result in fatigue crack initiation and propagation, a decarburized layer would need to be removed from the entire surface of the plate versus locally where welding will be performed. Why? Because the decarburized layer is lower strength (low carbon) and would have increased susceptibility to fatigue crack initiation and propagation.

 

[redpicker]

What effect will welding on the decarb areas have on final weld strengths?

I would say minimal effect. Typically, your decarb layer is less than 0.010" thick, and you are going to penetrate deeper than that. You can assume that all the molten metal will be fully mixed, so it's carbon content is going to be closer to your filler metal no matter what you do. I don't think you need to worry about it. Just make sure the surface is clean prior to welding and that you have adequate preheat/postheat.

rp

 

[tc7]

Red-
The C content of our weld rod is quite low to begin with, so further dilution with the carbon depleted surface layer may not have the restoring affect that you suggest.

Met-
since I really don't know how the equipment is loaded in service, I believe your suggetion to remove the decarb layer makes sense, although more work than we originally anticipated.

Thanks.