PWHT ASTM A516 gr70 - why 1305°F if there is low carbon content?

Lampropeltis · May 18, 2016

I am having a bit of a problem getting an exact answer from our PWHT Vendor on bringing some 516 gr70 up past 1340°F/1305°F (1340 is max allowable before the carbon transforms, 1305 is a safety factor) I understand his logic, that embrittlement will occur as the carbon content shifts to austenite. However, when I read 2006 version of ASM International "Fundamentals of Heat Treating Steel", their examples make it seem to me, that, the lower your carbon content in steel, the higher can bring your max soak time and temp during PWHT (inverse relationship - I know using "higher and lower" sounds stupid).

The point of this is, the 516 gr 70 I am using has a mill cert showing .19% Carbon content, which seems pretty low for a Carbon steel, when the examples ASM uses are in the 0.4% range (double the Carbon content of this specific sample of 516g70 I have).

Is there any justification, based on the .19% Carbon content, for which I can bring the 516 gr70 up to 1400°F? Is 1340°F/1305°F a max temp the industry abides by because "that's how it's always been done"?

There are specific reasons why this needs to be done, and it really has less to do with the 516g70 and more to do with a dissimilar metal being welded to it, which needs a higher temp.

Thanks for any information or references you can point to which might help shed some light on this.

metengr · May 18, 2016

There is no reason to be near anywhere near the lower critical transformation temperature for a subcritical PWHT. The minimum PWHT temperature provided in ASME B&PV Code for this material is 1100 deg F. Typically a range of 1100 to 1200 deg F works best to remove forming strains and to soften the weld heat affected zone. The lower critical transformation temeprature for this material is around 1330 deg F.

If you are performing a normalizing heat treatment to remove the effects of the weld heat affected zone, you need to be above 1600 deg F.

pradipgoswami · May 18, 2016

What PWHT you're trying Stress Relieve or Normalizing? Could you be more specific?

1340/1305 Deg Certainly not the recommended stress relieve PWHT temperature for this steel.Sec VIII, DIV-I requirement is 1100 deg F/595deg C, minimum.If project or client has additional requirements, say hardness control, one may have to go past the ASME range.However the temperature mentioned in this post is not right.Exposure to this temperature range would cause poor notch toughness of the weld(by grain coarsening).

My understanding is that PWHT vendor follows what's written in the contract spec(ASME or Client's Requirements) usually they do not try to re-invent the wheel.

Pradip Goswami,P.Eng.IWE
Welding & Metallurgical Specialist
Ontario,Canada.
ca.linkedin.com/pub/pradip-goswami/5/985/299
All provided answer are personal opinions or personal judgements only. It's not connected with any employers by any means.

Lampropeltis · May 18, 2016

Thanks Metengr, I completely understand what you're saying.

Is there any merit/relevance in referencing the iron-cementite phase diagram and taking .19% C straight up to the lower limit of the austenite transformation phase line, then moving left and coming to about 1500°F? (see image below, I marked it up in red)
What is the significance of the horizontal 1333°F line? I ask because P91/T91 (P15E) has carbon content to it as well, and as you move DOWN in carbon, you can move UP along this line (lower transformation boundary limit).

Like I said, it's the dissimilar metal that needs the higher temp heat treated (P15E welded to P1). But since they are welded together, the P1 (516 g70, will be heated to the P15E temp).

516-70_at_.19pct_C_on_Carbon_Iron_Diagram_ftozym.jpg

EdStainless · May 18, 2016

The point that you marked is relevant, just not to PWHT. You need to stay below critical temperature.
What is the min. Code temp for the P15E? If it is near 1300F then you need to re-think this.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

Lampropeltis · May 18, 2016

Thanks guys.

Ed, to answer your question - Minimum code temps per ASME B31 are "approximations" which I thought were based on this table and carbon content % (Table 129.3.1 in the ASME code book and table 132 PWHT).
As referenced by METENGR previously, Table 129.3.1 lists Approximate Lower Critical Temp for P1 (ALL Carbon Steel) at 1340. It lists P15E at 1470.

The issue I have with this is that ASME groups ALL carbon steel together and treats it as if all carbon steel were substantially homogenously the same.

Anyhow, I am just trying to run this to ground but I may be opening a can of worms due to my lack of proper knowledge regarding materials & metallurgical science.

I do appreciate the help though.

metengr · May 18, 2016

First off, this type of dissimilar metal weld joint is doomed to failure. You need a transition piece to accommodate the creep strength mismatch between carbon steel and Grade 91.

Second, Grade 91 material is a creep strength enhanced ferritic steel that contains 9% Cr - Mo, V and N alloying elements among other spices. The lower critical transformation temperature for this material can vary from as low as 1470 deg F to as high as 1500 deg F. So, using carbon steel with no transition material is not effective unless you butter the carbon steel side and use a multi-weld deposit region, which is not good design. Instead, use a transition material between the carbon steel and Grade 91, like Grade 22 material to allow you to make butt welds and perform the necessary subcritical PWHT.

The subcritical PWHT temperature range for Grade 91 material is typically 1385 to 1425 deg F. This is well above the minimum of 1300 deg F in ASME B&PV Code.

DVWE · May 18, 2016

Agree with metengr on that point, not a good idea to join the two by welding.

If inevitable, depending on the process, a bolted flange connection would be another possibility.

weldstan · May 18, 2016

The dissimilar weld of P1 to P15E should be avoided as per metengr. I have prohibited it entirely and use transition joints as described. Having said that a welding procedure properly qualified with a PWHT over the transition temperature of the 516 material can and has been done. I have had to do it in the past when unable to dissuade the Engineer of his foolhardiness and the minimum PWHT as defined by Code was 1350F.

Lampropeltis · May 19, 2016

@ metengr/DVWE/Weldstan - thank you, your experience and knowledge is greatly appreciated.
To your point Weldstan(right, wrong or indifferent) we have submitted PQRs/WPSs and samples of P1 to P15E which passed tensile/pull/bend tests in case we HAD to do this per the customer's specification (soaked 4 hours @ 1250°F) - with the understanding that it probably still isn't a good practice necessarily, just because it passed testing one time.

I guess this point has been put to rest (we have submitted an RFI to switch out the P91 in lieu of P22, which works from a calculation standpoint).
However, I am still very curious why a pseudo scholarly article is referencing steel/iron/cementite phase diagram "A3 line" in their "PWHT" discussion. This is more a curiosity & educational point from here on out.
`

metengr · May 19, 2016

Lampropeltis
The reference to the phase diagram was most probably to show the lower critical transformation temperature and what happens when it is crossed if the PWHT temperature is too high. Nothing more than this. Also, you are not dealing with a simple Fe-C alloy system, you have other alloying elements in SA 516 and other carbon steels that play a role in establishing the lower critical transformation temperature or A₁.

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