Unusual But Interesting PWHT Problem 1

[tc7]

I need to make a weld repair to a massive 4130 casting. I believe the flaw will require ~1/4” deep excavation x ~3-inches long. Normally we would TIG the small repair using an 80S or 90s filler, then PWHT (locally) for 2 hours at 1150 deg F (I hate local PWHT !!!).

This casting includes a nickel bushing located about 8-inches away from the flaw to be welded. This bushing has a max allowed operating temp of ~1000 deg F. We are prohibited from removing the bushing. We are prohibited from temper beading the repair (due to ignorant ultra conservative engineers). So I may only local PWHT the flawed area by means of a torch. (the forging is p/o a steam valve which sees frequent temp & pressure cycles).

My questions: To help protect the bushing during the 1150 deg F PWHT, I want to insert a close fitting cooling coil and run 60 deg F tap water through it. My concern is that I am creating a rather large temp differential between the region of high local PWHT and the bushing (1150 – 60 = 1090 deg difference). Will this temp difference defeat the purpose of PWHT, i.e., which is to anneal the weld-HAZ hardened area? Will this temp difference create an additional hardened area in the base metal due to temp difference and quench effects?
Please help and thankyou.

 

[metengr]

tc7;
For a 1/4" deep excavation, I would blend grind and run a minimum thickness calculation to assure that you did not violate minimum wall of the casting. IF what you report is correct that this is a large casting, from my expierence, you should have adequate margin to blend grind using a 3:1 taper to reduce stress concentration, and LEAVE IT ALONE.

PS; I don't like the option of trying to work around this bushing and attempting to locally cool it. I have done many PWHT's on valve bodies and flaw evaluation, and attempting local PWHT with trying to keep an adjacent area cooler is risky.

 

[tc7]

Met-
I should have mentioned that the flaws are in a critical sealing suface and so blending is not possible. We have also already skim cut this surface to the max permissable dimension.

Aside from the risk to the bushing (which, we may very well find that we distort the bushing and the shop will dance and cry "TOLD YOU SO!!!!) (little frustration here), but aside from the possible bushing damage, is there any validity to the quenching concern that I tried to describe above?
Thanks

 

[metengr]

tc7;
In this situation, I would not use a neutral flame torch for PWHT. It will be very difficult to control the temperature and to control the soaking band. This takes expertise and practice.

I would probably use two electrical resistance heaters. Keep the bushing at 1000 deg F with the one heater and use the second heater over the weld repair area. The PWHT time is always based on weld deposit thickness NOT base metal thickness. In this case at 1150 deg F you would need only 15 minutes at temperature to PWHT a 1/4" deep excavation.

You need to work around the bushing. Attempting to cool the bushing will not result in a quenching affect of the valve base material. What you need to do is to zone the heating to reduce thermal stress (which is more of a concern to me).

 

[unclesyd]

Is this a code vessel or are you using the PWHT as a stress relief?

The reason I ask is that we weld on heavy sections of 4130 without any stress relief or PWHT without having ever experienced any problems.

 

[tc7]

Met
I never would have thought to heat the bearing to reduce the temperature difference. It does make sense from a thermal stress point of view. Is it permissable to interrupt PWHT and continue on multiple phases? in other words, if I have to shut down the local PWHT when the bushing is reaching max temp, can I cool down, then re-PWHT for the remaining period?

Unclesyd-
It would be UNTHINKABLE for us to not PWHT 4130 under any Code requirement. I view PWHT as accomplishing stress relief AND annealing for HAZ hardened area. Isn't 4130 very crack sensitive? I'd like to know more about what you do, but my reading always recommended PWHT, even if it was an normalized or annealed variety.

 

[metengr]

tc7;
Yes, you can do an interrupted PWHT. The zone heating is something that I used when we needed to PWHT a valve body that was installed on a IP steam turbine shell w/o affecting the installed studs. The concept was to heat most of the valve body at 900 deg F and zero in on a local PWHT in the repair areas. I never had to shut any of the heaters off and the local PWHT went very well.

Regarding welding of 4130, I have heard and experienced welding with and without PWHT. It all depends on service application. My preference is to use thermal treatment, if practical, because this is the most conservative approach in the event of hydrostatic or pressure testing of components in the field.

 

[tc7]

Brilliant! and thankyou.

 

[unclesyd]

I wasn't against any PWHT or a stress relief if it is required for any reason. I was trying to convey the fact that welding of 4130 is carried out everyday without benefit of same in things like race cars, bicycles, and airplanes.

If you can consider a thermal treatment a "stress relief" you can lower the temperature and time at temperature.

I'm going to get hammered for this, but if you have a pure stress relief operation at a temperature at or above 900F you will relieve 90% of the stresses in first ten minutes when the metal reaches temperature.

 

[redpicker]

I tend to agree with unclesyd. If you can get away with it from a code standpoint, I think you'd be just fine with a 1000 deg preheat, weld and slow cool, and a 1000 F post heat.

With a large section of 4130, the 1000 F preheat should prevent any significant amount of martensite from forming on cooling. Then, the 1000 F post heat is mainly for stress relief of the deposit.

rp

 

[metengr]

rp;
I believe you meant only a 1000 deg F post weld heat treatment. A preheat of 1000 deg F will be unnecessary and a waste of energy. The typical preheat for 4130 is 300-400 deg F.

 

[tc7]

All interesting points of view, but I continue to think that stress relief is only a small part of the problem when welding 4130. Are we not also concerned, even more-so with the hardening that occurs in the HAZ?

I can see that the idea of a super high pre-heat (1000 deg., WOW!) might diminish this hardening effect due to quenching. Is that your point??????

 

[metengr]

The idea of a super preheat is not correct for welding. There are a minimum of three essential variables for welding that need to be controlled based on alloy composition and filler metal;

preheat,
interpass temperature and
post weld treatment (if applicable)

If the preheat is too high, this can significantly alter the bulk properties of the base materials that you are joining.

So, simply increasing preheat without adjusting the maximum interpass temperature can result in a completely altered material(s).

Diminishing the hardening affect is controlled thru proper application of preheat and controlling the interpass temperature.

 

[redpicker]

Well, I was being a bit excessive with the 1000F preheat, my point was that you could go this high and not damage the bushing. But, a preheat in the 600/800 F range would be very effective in preventing hardening of the 4130 HAZ (this is a very common preheat range for 4140/4145).

This is commonly used on oil drilling tools with no post heat (not my recommendation, but I know it is commonly done). And, of course, you will want to limit your interpass temperature to the same range (which shouldn't be a problem with TIG).

Of course, you will want to qualify the procedure with the temperatures actually used, but a 600/800 F preheat, TIG filler (I'd consider just using an E4130 rod) folowed by a 1000 F post heat should result in a acceptable hardnesses in the weld metal and HAZ and low residual stress without affecting the properties of the base material.

Now, getting the welder to actually get it to 600F before he starts to weld could be a problem. A common procedure on large parts is to check the temperature after removeal of the heat and again 3 minutes later. If the temperature has not dropped more than 50 deg F, you have adequate preheat.

rp