PWHT cycles for C.S. test coupon 3

[jtseng123]

For Cr-Mo low alloy vessel, we normally ask for 3 PWHT cycles on test coupon for shop and future field reaper. Do we need to do the same request on heat treated carbon steel vessel?
I recently received a client spec asking minimum 3 PWHT cycles on heat treated CS vessels that I haven't seen this kind of request in the past.

 

[metengr]

Yes, because most likely the vessel was fabricated with a normalized and tempered HT for impact requirements and multiple PWHT cycles will affect notch toughness.

 

[jtseng123]

metengr,
I do understand your point. But I have never seen any spec either from many big engineering companies or oil companies have this requirements except for Cr-Mo. Or is it written in API somewhere similar to API 934-E, 3.1.6.?

So, do you think mill has the obligation or is it mandatory to PWHT coupon for several cycles (or so call " Maximum PWHT"), then do tensile, yield and impact test even the Maximum PWHT is not written in most specs for CS?

(We have a lot a field modification on PWHT'ed CS vessels, and no one bothers to check how many cycles on the original test coupon. The reason as told: for CS, it is good for many cycles, not a concern.)

 

[metengr]

jtseng123;
I should have asked earlier, what is the specific heat treatment condition of the pressure vessel steel and service conditions? If the vessel material was only normalized and not tempered, or the vessel was subject to PWHT because of service conditions with no impact requirement, there is no reason to run a PWHT repair cycle unless the client still requests it. For carbon steel either hot rolled or normalized, there was one study which showed you can weld repair the base material up to six times with no observable change in properties.

However, if you have for example SA 516 Grade 70 carbon steel and this material has been normalized and tempered because of impact requirements by design, then PWHT repair cycles are important.

 

[jtseng123]

metengr,

We order normalized SA516-70 for two main reasons: service (such as wet sour) or to meet impact property. Is it mandatory to "temper" SA516-70 after normalized to meet Fig UCS-66, curve D for impact property ?

If it is not, then the material can live up to 6 cycles from the study you point out, that can explain why no one cares the PWHT cycles when we are doing field modifications.

 

[metengr]

Is it mandatory to "temper" SA516-70 after normalized to meet Fig UCS-66, curve D for impact property ?

.

No.
See below

(4) Curve D applies to:
SA-203
SA-508 Grade 1
SA-516 if normalized or quenched and tempered

 

[jtseng123]

metengr,
Thank you very much. As we never order tempered SA516-70, 3 PWHT cycles is unneccessary, though no harm.

 

[pradipgoswami]

Hi JT Seng 123,

3 PWHT cycles maximum is a typical requirement for many oil companies. Typical requirement calls for 1 shop PWHT following shop fabrication and provisions for maximum 2 PWHT's for shop and field repair. weld joints undergong more than 2 repairs may be cut and rewelded at the discretion of the client/ owner's inspector. Such requirement is across the board for for all materials, including Carbon Steel also.

Under such circuamstances the best way to safeguard the design and fabrication code requirements is to " qualify the weld procedures" with maximum PWHT time.SA 516 /70 is normally supplied by mill with normalizing and tempering. If PWHT temperature does not exceed the tempering temperature (which in reality does not happen) then there should not be "concerns" on mechanical properties, e.g tensile and impact.

However if the design requirements are very stringent,say like -45 Deg C design requirement then getting the results of the simulation test coupons (base metal) from the steel mill could get the peace of mind.

Thanks

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

 

[jtseng123]

Pradip,
Thanks for your comments, but confuses me,

1. We hardly order SA516-70 normalized unless it is for low temp or for special process service. If we order "normalized", we never mention "tempering",. But you say material will be supplied normalizing and tempering, is it mandatory per code ?

According to metengr, if it is tempering, then PWHT cycles is a concern. Otherwise, CS without tempering can live up to 6 cycles according to a study. And that explains why our field contractors never concern PWHT cycles on CS modifications. My boss told me CS can live up to many cycles and he just could not explain why.

2. I read many engineering and oil companies' pressure vessel spec, and never see requirement for 3 PWHT cycles on CS except low Cr-Mo alloy which is normally tempered. When a CS vessel requires mandatary PWHT, do you think mill have the obligation to do maximum PWHT (3 cycles) on coupon even it is not written in the spec ?

 

[metengr]

It always pays to READ the material specification. Go back and review SA 516 Grade 70;

5. Heat Treatment
5.1 Plates 1.50 in. [40 mm] and under in thickness are
normally supplied in the as-rolled condition. The plates
may be ordered normalized or stress relieved, or both.

5.2 Plates over 1.50 in. [40 mm] in thickness shall be
normalized.
5.3 When notch-toughness tests are required on plates
11⁄2 in. [40 mm] and under in thickness, the plates shall be
normalized unless otherwise specified by the purchaser.

5.4 If approved by the purchaser, cooling rates faster
than those obtained by cooling in air are permissible for
improvement of the toughness, provided the plates are
subsequently tempered in the temperature range 1100 to
1300°F [595 to 705°C].

 

[pradipgoswami]

Hi JT Seng 123,

Would you kindly specify the following details(unless there're confidentiality issues)

-Design Material- SA-516/70, Thickness
-Design Codes+ Supplementary Specification Requirements(e.g NACE, API etc), Sour Service Requiremnt if any
-Minimum Design Metal Temperature.

Appreciate your response.

Thanks

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

 

[jtseng123]

Pradip,
This has nothing to do with any specific condition. This is just a general requirement that I want to clarify since I never see any spec requiring 3 PWHT for CS until now this client puts that in their spec.
Metengr cites from material spec stating tempering must be approved by purchaser. Since we never ask SA-516-70 to be tempered, I am sure mill will not do tempering. According to metengr, PWHT cycles become a concern for tempered CS.

My question left is # 2: is it mandatory mill has to do 3 PWHT cycles on coupon for PWHT'ed non-tempered CS vessel even it is not written in most companies' pressure vessel spec ?

 

[metengr]

When a CS vessel requires mandatary PWHT, do you think mill have the obligation to do maximum PWHT (3 cycles) on coupon even it is not written in the spec ?

No. The Purchaser must determine what tests above and beyond those required in SA-516 and SA-20. Please review SA 20. If Supplemental requirement S3 is invoked by the Purchaser


S3. Simulated Post-Weld Heat Treatment of
Mechanical Test Coupons
S3.1 Prior to testing, the test coupons representing
the plate for acceptance purposes for mechanical properties
shall be thermally treated to simulate a post-weld
heat treatment below the critical temperature (Ac3), using
the heat treatment parameters (such as temperature range,
time, and cooling rates) specified in the purchase order.
For tests using specimens taken from such heat treated
test coupons, the test results, shall meet the requirements
of the applicable product specfication.

 

[jtseng123]

metengr, thanks.

 

[TomEun]

No extra holding time for the future PWHTs of A516-70N is required because any precipitations (like Cr-Mo steels) of A516-70N (killed steel) and/or grain growth are not expected due to multiple PWHTs (under the low critical phase transformation temperature, around 715C for A516-70 and sound heating/cooling rates).
If N-T (normalized-tempered) is applied, the tempering temperature of base metal should be controlled higher than the PWHT temperature.
If it is in wet sour service, the most recommendable/critical practice is to keep the PWHT temperature at 621C as a minimum.

Normally the exposed time for several tens hours (at the PWHT temperature) may not reduce the tensile strength as well as low temperature toughness of A516-70 with thin or medium thickness.

Thomas Eun

 

[Guest102023]

I'd like to throw a spanner into the works here:

For sour service, is testing of the coupon done after one PWHT cycle AND after three cycles? Because the component will most likely spend its entire service life in a single PWHT condition, not three, so it seems to me testing for both extremes would make sense.

 

[metengr]

The issue as I see it is this - for carbon steel plate material there can be a cause for concern about degradation of material strength properties below the minimum specified, as well as an increase in notch toughness transition temperature. ArcelorMittal/USA has done extensive research work on their SA 516 and SA 387 bread and butter steel plate product lines.

For relatively “thin” plate steels, there is minimal effect of extended PWHT time on mechanical properties. Where the issue surfaces is on thicker plate steels where the carbon equivalent is not modified to account for time at temperature PWHT and the effect on mechanical properties, and notch toughness.

Please see the attached reference

http://www.arcelormittalna.com/Plat...uctBrochure/ARCELORMITTAL_PRESSURE_VESSEL.pdf

The bottom line is this, there is no blanket statement that can or should be made for SA 516 Grade 70 regarding effects on PWHT cycle time. The Purchaser has to decide if having simulated PWHT cycles is important enough to ensure the steel plate properties after PWHT do not fall below the original minimum specified, which depending on the carbon equivalent can happen. Given the fact that plate steels can be leaner in chemical composition does pose a threat that after ‘x’ number of repair cycles the material could have lower mechanical properties and have an adverse effect on notch toughness behavior. It is up to the Purchaser to beware of these issues.

 

[Guest102023]

Thanks for that excellent reference, Metengr!