weldability of case hardened parts

[vcebrad]

We are welding an assembly made out of 1018 and 1117 parts.

The 1117 part is actually heat treated prior to being welded. It is carbonitrided for wear resistance.

We have occasional porosity problems that I believe are related to an inconsistent heat treat process and the resulting chemistry of the parts.

1) Is it possible that the nitrogen content at the surface of the case hardened part is causing the porosity problem?
2) Is there a "safe" limit for nitrogen content of steel parts to assist in weldability?
3) Might the addition of nitrogen to our shield gas mixture (we currently use argon/co2) help with the problem?

 

[metengr]

I would not recommend any welding to the 1117 material. This is a resulfurized grade of carbon steel that exhibits poor weldability. Mostly, hot cracking will occur as a result of the sulfur content in welding the 1117 material.

 

[vcebrad]

I understand your recommendation. However, our company has managed to avoid the cracking issues common to the 1117 material for thirty years now. It's mainly the porosity issues we are curious about.

 

[metengr]

Nitrogen from the carbonitriding process should not produce porosity in the weld deposit. Porosity in the weld deposit is probably being caused by contaminants – either on the surface of the part(s) or in your shielding gas. These contaminants are absorbed during welding and as the weld puddle solidifies the contaminants are outgassing, resulting in porosity.

 

[vcebrad]

Our welding engieneer consultant came back with the following:

nitrogen should be held to less than 200ppm
<150 ppm preferred
anything >300 ppm will likely result in porosity

Thoughts?

 

[metengr]

Yes. The effect of nitrogen in the parent metal would be more of a concern to me in the formation of harmful (brittle) iron nitrides in the weld deposit coupled with increased susceptibility to hot cracking from increased sulfur content.

For gas –metal reactions in welding, the heat of the arc results in the dissociation of molecular nitrogen (present in the air or sometimes present in shielding gas) into nascent nitrogen that can be dissolved into the weld puddle. In most cases, the nitrogen does not result in porosity because there is insufficient quantity other than harmful iron nitrides. In your case, you already have nascent nitrogen in the parent metal from nitriding so from a thermodynamics viewpoint, if the weld deposit becomes oversaturated with nascent nitrogen, upon cooling, the excess nitrogen can outgas.

The only way you can attempt to weld nitrided steel is to either locally grind the hardened layer until it has been removed from the surface of the steel OR utilize a special formulated weld rod for tool steel weld repairs that can tie up the nascent nitrogen in weld metal slag. In this case strong nitride formers that can remove nitrogen from the weld puddle are Ti, Al, Si, and Zr.

 

[unclesyd]

A trick I use to minimize porosity during welding when using SMAW is bring the electrode as close to vertical as conditions permit. I use this quite often in welding CI and steel castings. I'm a short arc welder also.

I done a little work with GMAW where bringing the wire closer to a vertical entry helps with some porosity. The same goes for hot wire TIG.