Tempering and PWHT 2

[5pointslow]

We welded some SA333 Gr.6 pipe and subsequently performed post weld heat treatment at 1100F per UCS-56. The actual temperature maxed out at just under 1175F. The material test reports for the material show that it was tempered at a minimum temperature of 1050F. We have a customer that is saying we must provide new test results for the mechanical properties on the MTR because the PWHT temperature was higher than the tempering temperature. I have never heard of this before and it doesn't make sense to me. I've looked through Sections II, VIII Div.1 and IX and cannot find supporting evidence for either side. Can anyone help me?
thanks.

 

[metengr]

We have a customer that is saying we must provide new test results for the mechanical properties on the MTR because the PWHT temperature was higher than the tempering temperature.

No, this is not the case. ASME code rules for PWHT are based on establishing a minimum ductility and strength level. Tempering at lower temperature does provide increased strength above the minimum but ASME does not give credit for increased strength above the minimum requirement in the material specification. So, your customer is incorrect in their assumption.

 

[5pointslow]

PWHT will increase the mechanical properties of the materials vs material that is just tempered?

 

[DVWE]

I tend to agree with your customer on this. You have now re-tempered the base metal, so how do you know what the mechanical properties are, especially with a 125 F differential?

Can you demonstrate that your PQR test coupon underwent the same heat treatments? If so, they might buy that off as proof enough.

 

[metengr]

ASME BV&PV Code does not concern itself with having to prove actual properties after following fabrication rules unless material is subjected to re-austenitization from either exceeding the lower critical transformation temperature during PWHT OR re-heat treatment. Since you did neither of the above, you are on solid ground and your customer has no basis for requiring a new set of MTR's.

If material is manufactured to an SA specification with no specified tempering temperature in the specification other than what is agreed upon as suitable (review SA 333) why would I need to void this material or conduct mechanical property tests for an MTR?

Nowhere is there mentioned in ASME Code where if a lower tempering temperature is selected to gain higher strength that I need to conduct mechanical property tests.

In this case, a slightly lower temper was selected. The UTS for the plate will be much higher than the minimum specified. Again, one cannot take credit for this and the PWHT temperature that was used was selected based on current rules in UCS-56.

 

[metengr]

By the way I forgot to mention that if you carefully review SA 333 you will find that for seamless or welded pipe, other than Grades 8 and 11, you can leave the pipe in a normalized condition with no temper.

So guess what - suitable reheat temperature does not mean much in this case, does it?

 

[SJones]

By the way I forgot to mention that if you carefully review SA 333 you will find that for seamless or welded pipe, other than Grades 8 and 11, you can leave the pipe in a normalized condition with no temper.

So guess what - suitable reheat temperature does not mean much in this case, does it?

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[DVWE]

ASME Section VIII provides the MINIMUM requirements. The end user always has the final say. If there was something above and beyond the code such as this in their specifications, you have to comply.

Were such specs agreed upon during or before the issue of the PO?

 

[SJones]

Let's try that one again!!

It might mean something if the purchaser's technical specification has detailed the requirement. It might also mean something if the responsible engineer deems it to be outside of the specified PWHT temperature range as per QW-407.2. Naturally, if it is not addressed in any supplementary way, and the purchaser maintains the requirement, then the purchaser would pay for the privilege.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[metengr]

Maybe. QW 407.2 only applies to production weld PWHT and aggregate time associated with production welds. If I have a qualified WPS with 1100 deg F as my minimum PWHT temperature I am in compliance with UCS-56 up until I reach the lower critical transformation temperature or I place a maximum limit on upper bound PWHT.

Lets see, maybe I can present it this way. In SA 333, heat treatment shall be by one of the options in 4.2.1 - normalization OR normalization followed by reheating to an agreed upon tempering temperature.

I do not believe that this reheat temperature is used other than to establish some form of stress relief. If the user is concerned with welding and PWHT, they should have specified a higher agreed upon reheat temperature.

Of course the end user can do what they wish at their cost.

 

[Metalmeister]

I have long had the same concern as 5pointslow since metallurgically we know that the steel softens over time at temperature. So if your production piece just barely made minimum mechanical properties, there is a possibility that the described PWHT lowered these properties to below minimum. But since there is no clean way to make and apply the rule beyond what the Code states or doesn't state, and since there is usually sufficient margin to the specified minimum strength value, this has not been a problem. Changing the time and temperature range certainly affects the Charpies much more than tensile, since that is an essential variable change for many welding processes.

My solution is simply to avoid a problem by tempering as high as possible and PWHT 50-100F below that. I have still to come to grips with a P-1 to P-5 weld. PWHT for the P-5 sake certainly abrogates the P-1 CMTR properties. What do you do in this case? I recommend a design or process change for critical components.

 

[vesselfab]

We normally PWHT this material at 1150*+/- 50*F
and often 1175*F +/-25*F

it has always been our contention that as long as a pwht cycle per ucs-56 was maintained and we did not go above 1200*F we were fine

Now if you got the temp into the normalizing range, then yes, new tests are required.

But at 1175 not required

 

[metengr]

My solution is simply to avoid a problem by tempering as high as possible and PWHT 50-100F below that.

Agreed.

I have still to come to grips with a P-1 to P-5 weld. PWHT for the P-5 sake certainly abrogates the P-1 CMTR properties. What do you do in this case? I recommend a design or process change for critical components.

.

No design or process change is required. For a P1 to P5 weld, the lower alloy base metal will dictate PWHT requirements OR you butter the P-No 5 base metal side with B3 filler metal composition, PWHT the buttered P-No 5 base material, and finish the weld with B3 to carbon steel and perform a final PWHT at lower temperature for the P-No 1 base material. The Code rules are flexible.

 

[vesselfab]

we do a simmilar thing on carbon steel that requires pwht with a ss section added on.

309 butter (heavy pass) to c.s. and proceed with pwht

then weld on the SS section

yep...flexible

 

[Metalmeister]

Buttering the P5 is the process change I was alluding to. I'm not sure that the lower alloy material dictates the PWHT. It is more critical metallurgically to make sure that a P91 alloy for example, gets the proper PWHT with the P1 chips falling where they may. In most cases with this situation, the P1 is a nonpressure retaining attachment, but still has some structural purpose.

 

[stanweld]

Metalmeister,
We simply prohibit welding carbon steel to P91 alloy and use a transition P-5A or P-4 part.

 

[metengr]

By the way, creep strength enhanced ferritic steels will be now called P-No 15 E Group 1 in the 2009 addendum.

 

[Metalmeister]

That is a good heads-up, metengr. What does that change do to welding PQRs?

 

[metengr]

Nothing other than an editorial revision to existing WPS for the base metal P-No designation from P-No 5B to P-No 15E Group 1. No re-qualification is required!