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Niobium Content effect on weldability

Niobium Content effect on weldability

Niobium Content effect on weldability

(OP)
Good day all,

I am looking at an ASTM A519 4130 heat for welding (the only material we can find which is close to size). The heat has a far higher Niobium content than our spec - 0.029 (heat analysis) vs 0.010 (spec).

Fortunately the Carbon Equivalent (IIW) is within spec - 0.72 (analysis) vs 0.75 (spec). So this gives me hope for the out of spec Niobium. This will be welded by Tig root, and SAW fill and cap, if that is helpful.

Does anyone have experience/an opinion on this matter?

RE: Niobium Content effect on weldability

I have experience with Nb and other similar strong actors (e.g. Zr, V) in microalloyed steels. You may not post-weld heat treat those steels when welded (strain hardening in the HAZ leading to cracking) and I would have the same concern for Nb in more conventional low alloy steels. In the as-welded condition I don't have big concerns, but there's only one way to find out.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

RE: Niobium Content effect on weldability

I presume that your hardenability calculation included the Nb.
At that level I wouldn't be concerned about weldability (higher levels can cause hot shortness), but the properties of HAZ may be a concern.
All that you can do is make a test weld with your usual pre-heat, interpass temp, and PWHT and see what you get.
What is the condition of your tubes? HF, CF, normalized? That could also matter.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: Niobium Content effect on weldability

I have seen its effects on low carbon low alloy steels and the HAZ issues mentioned in both posts above may be present in your case as well. And your test weld will give you information you'll need

RE: Niobium Content effect on weldability

Quote (weldstan)

And your test weld will give you information you'll need

Not necessarily.
I would also prepare a micrograph and give the HAZ a severe interrogation, plus targeted microindentation hardness testing.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

RE: Niobium Content effect on weldability

ir
I would have done similar hardness testing and possibly impact testing depending on operating conditions and strength/heat treatment and PWHT pareemeters of 4130 material.

RE: Niobium Content effect on weldability

As far as carbon equivalent goes, I don't believe there is a single CE formula (there are several) that can adequately capture the situation for strong microalloy additions to low alloy steel like 4130. Your problem will be stress relief cracking, not so much hydrogen-induced cracking, which for 4130 is quite manageable.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

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