There is no short cut or improved best practices that would differ between a single PWHT cycle or repeated PWHT (2) cycle. This is why you have two PWHT cycles demonstrated on this material, if you anticipate repeated weld repairs. Depending on the original hardness of the Grade 91, you may indeed get favorable hardness results (at or above 190 BHN) for a 2 cycle PWHT.

Keep in mind the minimum hardness is not documented in literature as being rejectable. It is a place holder for some clients.

A saved PWHT cycle is pretty common in the industry.

Although per ASME IX, multiple PWHT cycles can be performed in a single heating, see QW-407.2 (even if impact
testing is not required), it can be done both ways as you suggest in your plan.

Gr 91 is pretty robust material, and I wouldn't have concerns for hardness after one additional PWHT cycle.

Thanks for the input guys.

I have tried to take exception to the two PWHT cycles with no luck. The customer is adamant that he wants it even though there is very little chance that we will have repairs after heat treat as we alway RT after the bakeout and prior to the final PWHT.

Thanks again.

If the customer is paying a fair price, the customer is always right. Until he is no longer your customer.

We have seen no problem with meeting 190 BHN in Grade 91 with multiple post weld heat treatments during qualification. As metengr warns, the base metal must have appropriate initial hardness. Our Grade 91 coupons were always above 210 BHN prior to qualification. We also required that heat treated P91 pipe spools supplied to us had a similar minimum hardness at the beveled ends for field welding. You may also wish to perform your PWHT at a somewhat lower temperature especially the after repair PWHT. One can tailor PWHT when hardness in on the cusp prior to the repair PWHT.

Thanks weldstsan.

Hi iam42,
What's the rationale behind qualifying PQR with a PWHT cycle of 2 hours at 1400°F. Don't you foresee any cracks or other defects in the weld being noticed after PWHT which would call for weld repair. Even minor grinding to remove defects following PWHT with consequential weld build-up would involve additional(2nd) PWHT.
I would say the rationale put forth by your client is totally correct.
Keeping those issues in the forefront you should:
-- qualify a new PQR, with at least 2 PWHT cycles.
--or qualify a new PQR with an extended soaking time at 1400°F, to cover at least 1 or 2 repair following PWHT.

Generally rule of thumb clients may not allow more that 1 or 2 weld repairs following PWHT.Hence additional PWHT or PWHT with extended soaking cycle should cover those, during procedure qualification.
Also one needs to check the Tempering Cycle adopted for base metal in the mill.PWHT of welds should not exceed the tempering temperature of adopted for base metals in the mill.

Repeated or prolonged PWHT at above the tempering temperature could cause 200 HB or below hardness in weld/HAZ.

API-938B, clause 6.4.6 claerly states the guidelines for PWHT. If this spec is not mandatory for your projects, you may refer this for good guidelines.

Thanks.

Pradip Goswami,P.Eng.IWE
Welding & Metallurgical Specialist
Ontario,Canada.
ca.linkedin.com/pub/pradip-goswami/5/985/299

Hi Pradip,

Thanks for the input.

My rationale for NOT needing a 2nd PWHT is that we RT all Grade 91 joints after bakeout and prior to PWHT. We also RT the joint after PWHT. In the 5-6 years that we have adopted this process we have never had to repair a weld after the PWHT. I do not think that we have had to even repair a joint after the first RT prior to PWHT.

We are also in a fortunate position that we are shop based and we have a dedicated bay and welder for all grade 91 welds. The welds we make are all very similar in size and thickness. It took us a few year to fine tune our process but I am now very confident in what we do. I am a firm believer in that if it is not broken, don't fix it.

iam42,

What you need to consider is that your customer is probably requesting the extra PWHT for future use, and not implying that you will need it at your shop.

Many vessels and pipe spools will need some type of weld repair in the future after being put into service. This gives the end user firm confidence that the PWHT can be carried out and not affect the material properties - they have been proved with the extra PWHT.

At our shop, we have had customers in the past requiring TWO saved PWHT cycles for their future use.

Requirements like this have been the common over the last 10 years or so.

Thanks DVWE,

That is my thoughts exactly. I have no problem running the extended PWHT for the customer, I am just concerned that my Hardnesses will drop but after doing more reading I am maybe being more concerned than I should be. (I hope!!!)

With the proper material as advised in the above posts, you most definitely are.

I have done P91 pipe with up to 4 PWHT cycles and still met properties with flying colors....so much so that I feel more could be done.

Hi Iam42,
Generally it's always a help for the welding engineer to have PQR qualified with additional PWHT cycles( or soak time).In shop environment also one is not assured of failures after PWHT. Hence that's the additional resource to be kept at the back pocket.
Sec-IX recognized this, hence QW 407.2(as stated above) talks about the minimum time(80% of production welds)to be adopted for the PQR coupons. Sec-IX provides flexibility about the maximum time on the PQR.

As I mentioned above, in order to keep the hardness under controll even with extended time ,one need to ensure that PWHT temperature does not exceed the tempering temperature adopted for the BM in the mill.
This is customary in addition to the precautions adopted during welding with respect to heat input and interpass control.

I've come across multiple(at least 2 after PWHT) repairs with Grade-91 steel, still meeting all the requirements.

Thanks.

Pradip Goswami,P.Eng.IWE
Welding & Metallurgical Specialist
Ontario,Canada.
ca.linkedin.com/pub/pradip-goswami/5/985/299

Regarding need of repair after PWHT RT versus that performed prior to PWHT, we have more than once seen defects with final RT when pre PWHT proved acceptable. Most of these instances have been due to differences in technique. In one case, an Owner used fast film as a normal check of weld soundness prior to PWHT. He then employed fine grain films for the Code mandated RT after PWHT. Needless to say, this method does not always eliminate the need for a repair after PWHT.