It is near impossible to evaluate such things without looking at the valve, welding technique, preheat and postheat parameters. But, for cracks in the valve body away from the fusion line or HAZ, I would look at differential thermal expansion between the Stellite 6 seat and the 4130 valve body. You should start with a review of the postweld heat treatment charts to see what heating and cooling rates were used. We would typically require a max. heating/cooling rate of 250 F for an item such as this with a complex geometry and variation in wall thickness. I'm hoping Greg Roberts will chime in here and lend us his insight on welding of valve castings. It would definitely be advisable to maintain preheat without interruption until all welding is finished on a hardfacing repair. This could definitely be a contributor in your case. What size and pressure class is the valve?
For a valve refurbishment, we make every effort to avoid postweld heat treatment. Remember that (typically) preheat is based on base metal thickness, but postweld heat treatment is based on weld deposit thickness. There are some exceptions for certain base metals, but in general this is true. We also try to remove all of the old Stellite 6 or 12 and use Stellite 21 for the repair. The condition of the base metal prior to starting the repair may also be a factor. There may have been some damage present in the form of microcracks from creep or fatigue that propagate during the heating/cooling cycle.
Tom,
I just had a few thoughts on the cracks you are getting:
1. Are the cracks transverse or longitudinal? I am kind of assuming longitudinal.
2. What temperature was this valve running at? 4130 seems like a slightly odd material for steam service as its Cr and Mo content are pretty darn lean. If it was in service above its working limits, perhaps the material has creep damage or embrittlement that is being exposed by the extra stress places on it by welding? Most steam valves I have seen start at 1 1/2 Cr, 1/2 Mo, and go up from there. 4130 is around 1/2 Cr, 1/4 Mo, with high Nickel, which tends to embrittle during extended high temp service.
3. I have welded on lots of castings similar to 4130 without having to keep them hot until PWHT as has been suggested, but if I ran into one that had unknown cracking problems, that would be one of the first things I would try. It is worth a shot if you have to weld repair the cracked ares. You also need to make doubly sure that the cracks can't just be ground out and put back into service without any more welding since the cracks aren't in the seat itself. Since the cracks are shallow, perhaps you can grind, blend, and call it good.
4. It was suggested that the cracks could be from the different expansion coefficient of the two materials. Unless the stellite is very thick (which I would doubt), I think this would have a much greater chance of cracking the stellite than the casting merely due to the difference in size between the two.
5. You didn't mention what welding process you used for the stellite or going to use for the repairs of the cracks if necessary. If you can, I would use GTAW or SMAW as they tend to not harden the HAZ quite as much as FCAW or GMAW. Since the beads tend to be smaller, you can get better tempering effects from subsequent weld passes.
6. Since it is a casting, there is always a chance that there is microshrink or other defects in the area that the new stresses are opening up.
It could be any of the above problems, but my gut (depending on service temperature) wants to point at item #2.
GR-
You have given me a lot to think upon.
All of the smaller cracks are longitudinal to the axis of the valve. EXCEPT for one long bothersome crack that showed up that is transverse at the surface but when we started to grind it out, within ~3/64 or so below surface it changed direction and went longitudinal. This is the only one that was transverse out of about 75 cracks (total) in the four valves. Unfortunately I couldn't get to the camera before the grinding was underway, but perhaps what I have attached can tell the story. Also apologize for the poor clarity as i was sticking it in a 450 deg valve body.
Other particulars:
Steam valve operating at 600 psi saturated, temp I think at that pressure is about 486 deg F.
Cracks are too deep, numerous and clustered to attemp blending.
Stellite is only ~.063 max. thick and in great shape! considering all the other bad things.
Stellite was applied by GTAW, cracks will be repaired by GTAW.
Microshrink you say!? That sounds like a flaw that may have existed since the valve body was born, why would that come out to haunt me 15 years later?
If you can interpret these crack orientations and locations in any way, please don't hold back!
By the way, I should remention that all these cracks only were detected after all other repairs and final PWHT stress relief was completed.
Thankyou all for your interest.
Ok, so 486 degrees isn't that hot. So that might not be the problem.
As far as microshrink, that would have been there at the time the casting solidified. I have seen castings that have been heat treated multiple times without a problem in a particular area, then all of a sudden, the next heat treat opens up a crack to a previously unknown shrink pocket (not microshrink in this case). I thought of microshrink since it seems to be farily uniform around the perifery of the seat? And near the seat area is a thick section usually...and you are getting it in all the valves? Are they all the same pattern (i.e. pattern number from the foundry that made them?)
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