PWHT of 4140

[jgwright]

I am preparing to perform a PQR for a WPS for the welding of 4140 casing. The WT of the coupon is .5". Processes used will be GMAW ER80S-D2 root and SMAW E10018M (H4) fill (butt weld). Pre-heat will be 450F. The "PWHT" that has been recommended is 950F +/- 25. The base material was quenched and tempered at 1173F. It has also been instructed that we put the coupon into the oven as soon as possible after welding, I think they are worried about cracking after welding. It's my belief that the worry about cracking stems from the how the production pieces are to be welded. There are components that will be welded to the external surface of the casings with 2 - 6" long, one pass 1/4" fillet welds.

There are a few issues that we feel are at play here. One is 950 is not really a PHWT temperature for this material, it's more of a "bake out" for hydrogen considerations. Another is that we're worried about not allowing the material to go through it's microstructure transformations as it cools after welding.

Opinions and advice?

 

[metengr]

The PWHT temperature is too low. I would recommend a temperature about 50 deg F below the original tempering temperature. I would strongly advise using low hydrogen consumables - H4 suffix designator to minimize the potential for hydrogen cracking. The welding consumables look acceptable. The preheat looks fine, the interpass temperature should be 600 deg F. I would recommend a post weld surface NDT - wet fluorescent MT of the welds.

 

[metengr]

Forgot to mention you do not need to immediately PWHT. You can but this is not required for this material and the fact that you are using low hydrogen consumables to minimize the risk of delayed cracking from hydrogen.

 

[jgwright]

The immediate PWHT is what we want addressed the most. Does putting it in the oven quickly have any serious detriments?

To further this, we plan on doing either a hot wire GTAW or pulsed GMAW WPS once we've met the requirements of the first procedure. Since both processes have almost no hydrogen present and the current PWHT is not high enough to stress relieve, would the 950F PWHT not even be required because there is no hydrogen present? Of course this is all hinging on if the PWHT is ACTUALLY currently being done at 950F. This is a detail that hasn't been shared as of yet with us as of yet.

If it is being completed in the 1100-1150F range then this part of the question is moot.

 

[metengr]

The immediate PWHT is what we want addressed the most. Does putting it in the oven quickly have any serious detriments?

No.

I think the 950 deg F PWHT was selected because someone wanted to remain well below the original tempering temperature, I have seen this before. I don’t believe this is a hydrogen bake because hydrogen can diffuse at or above 450 deg F. Don't believe any welding process is hydrogen free, hydrogen can be introduced from the wire; yes the wire manufacture, and surrounding atmosphere.

 

[OGMetEngr]

Looking at a CCT diagram for 4140, the only thing going straight to the oven would do is prevent martensite from forming. As far as if this is detrimental or not, it depends on what properties you are going after and whether you want tempered martensite or pearlite/ferrite.