Weld Repairs And No PWHT? 1

[tc7]

We often are required to repair steel steam valve castings(typically of the normalized 4130 variety) which have developed small cracks (or surface porosity that may have always been there). Sometimes the weld repairs are deep and sometimes not. In every case, our standard WPS with ER80S or ER90S requires the typical 1 hour per inch 1150 degree PWHT. We are working on a temper bead WPS to permit us to forego a PWHT on small but deep area repairs.

There are occasions when the flaws are less than 1/4" deep, but still too deep for blending and too shallow and short for the temper bead. I would like to consider a repair with a very ductile but high tensile filler such as Inconel and avoid the PWHT altogether. Will the ductility (30%) of an Inconel (Inco 92 or Inconel's 52M) allow redistribution of welding stresses sufficiently to permit avoidance of PWHT? This idea is intended for shallow repairs only. If the Inconels wont accomplish this, is there another ductile alloy which will?

Thanks.

 

[metengr]

tc7;
PWHT is required to accomplish three things; (1)temper or soften the weld metal hardness, (2) reduce residual stress and (3) to reduce the hardness of the base metal heat affected zone.

If you wish to accomplish only one or two of the three items above, using a lower strength or more ductile weld metal will provide some benefit ( accomplishing 1 and 2 above). However, item three is still unaffected by using more ductile or lower stregnth filler metal. So, how important is item 3, in most cases it can be most important so I would use a local PWHT or temper bead method versus using a more ductile or lower stregnth filler metal.

 

[tc7]

Thankyou metengr,

Some of our valve castings are HUGE and so the idea of local PWHT has always bothered me, fearing that manual torch local PWHT will setup more thermal stresses than I am attempting to relieve! Resistance heat methods are very time consuming to setup because I always strive for symetric heating and this isn't efficient because we can't always get good contact with the casting surface. So the usual option is a 2-hour oven PWHT which just seems crazy if we are dealing with small surface repairs less than a 1/4" deep.

Sometimes the flaws are on the pressure boundary and sometimes they are on the flange or outside surface of the casting.

What other tricks might you suggest for repairs that are too shallow for temper beading?

 

[metengr]

tc7;
If the base material is 4130 low alloy steel, here is what I would do in your situation;

For excavation depths 1/4" or less,

1. Apply a local preheat of 400 deg F. Maintain preheat temperature for 15 minutes before welding.

2. Use low hydrogen E7018 or E8018 B2 electrodes. Use stringer beads with a 50% bead overlap.

3. After the completion of weld repair, blend grind the repair area and perform a wet fluorescent MT for final acceptance of the repair.

 

[tc7]

met-
I'm not clear on the 50% bead overlap. On a shallow grindout, the first pass might fill the void, then are you saying add a second bead, 1/2 on the first bead & 1/2 onto new base material? Then duplicate an overlap bead on the opposite side? This sounds like a simplified temper bead, but are we not increasing the HAZ by spreading the 50% overlap bead onto new base material?

Also we would typically use GTAW processs, so the equivalent rod would be ER70S-2 or -6 or a ER80S-2 or -6.

 

[metengr]

tc7;
The 50% bead overlap is for multiple beads. For example if you need to fill an excavation that is only 3/16" deep but extends say 3/8" in width on the surface, it may require more than one pass. In this case, if several shallow passes are required, the 50% overlap between passes would apply. Keep in mind that you need to grind the excavation to provide for access to fill it.

GTAW process is fine for using the above. Yes, this is a modified temper bead approach.

 

[tc7]

OK, so then the 50% overlap is providing that tempering treatment to soften the hardened HAZ. What if I can only get one additional full pass - is that sufficient to provide the temper?



Now the 70S is still providing low ductility (22% elongation), compared to the Inconel 92 (at 30% elongation). But the Inconel 92 is quite high in chrome at 14-17% whereas the 4130 castings are probably running at ~1-1.25% chrome. Any objection to the high crome Inconel 92 for this operation?

Thankyou very much.

 

[metengr]

tc7;
No, I don't have any technical objection. My own professional opinion is that I try to avoid dissimilar material combinations in service. Why? Because long term problems can develop, breakout of dissimilar materials along the fusion line of the weld repair, and having to remember that during NDT, the weld region can only be inspected by Liquid Penetrant versus wet fluorescent MT.

Regards

 

[tc7]

Points well taken and things I did not consider before. Thankyou for the advice.
T