Exemption of PWHT for SA537 Cl2 making wed weaker than base metal 1

[LJthien]

Dear all,
I am reviewing my Vendor's WPS and looking to your advice.
I understand that ASME VIII-Div1 UCS 56 table exempts from PWHT for SA537 Cl2 of 38mm & below thickness.
The Vendor's PQR passed all mechanical tests (impact test at -48C) and both tensile specimens broke at weld metal. The PQR weld is obviously weaker than Base Metal taking into account that SA537 Cl2 is the Quenched and Tempered steel (at 910C & 610C accordingly by steel maker).
Thus, please consult me if the design concept of making weld weaker than Base Metal is considered as the optimization in engineering view.
Thanks a lot.

 

[r6155]

You should read asme ix, did you read it?

Regards

 

[weldstan]

If the break in the weld metal met the minimum UTS of the base metal, it's acceptable and the design is acceptable.

 

[LJthien]

Thanks r6155
I have checked out ASME IX and understood that the PQR passed and complied with ASME. No concern about that.
I am only not so clear about the engineering point of view. I mean I have no idea if the optimization of engineering is "weld should be more strengthened than Base Metal" instead of weaker.

 

[r6155]

LJthien
In ASME II for SA-537 Class 2 UT is 80 - 100 ksi for 2.5in and under. If you see the material test report of your plate UT may be i.e 85 ksi.
The required UT for PQR is 80 ksi
If the UT in PQR is 82 ksi in the weld or in the plate then PQR is OK
If the UT in PQR is 79 ksi in base plate. WPS is rejected. Check the plate
If the UT in PQR is 79 ksi in the weld then WPS is rejected. See what went wrong.

Regards

 

[marty007]

Essentially ASME has a design margin on either yield or ultimate tensile (depending on material).

Material suppliers must meet the minimum strength requirements of the material specifications. Any additional strength above this, is actually an additional safety margin that just isn't taken credit for. For welding, the test only need to meet the minimum strength requirements. As long as that is proven through testing, you are still maintaining the full design margin required by the code.

Cheers,
Marty

 

The 'UTS' obtained from a reduced section tensile test cannot be directly compared with the UTS (or YS) obtained from a simple base metal tension test. It is not any kind of true UTS because you are testing a composite structure.

Section IX logic is to demonstrate that the weld strength meets or exceeds the minimum specified UTS of the base metal. To this end, the design of the coupon is such that it tries to force failure in the weld. However the Code recognizes that the actual base metal UTS may far exceed its minimum; asking the weld to meet or exceed that places an unfair standard on the weld, thus it only demands the apparent UTS meet the minimum UTS specified for the base metal. The more fundamental Code logic is that the metal meets a minimum strength level at every point in a structure. Conservatism is enhanced to the extent that materials of construction exceed their respective minimum requirements. So it appears the results reported on your PQR are acceptable to Code.

In my experience, reduced section tensile tests in carbon and low alloy steels usually fail in the base metal. On occasions where it failed in the weld it was because of discontinuities; the result in this case is still acceptable if it meets the min UTS for the base metal (although the bend tests may have a problem).

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[r6155]

Sorry, I forgot in my example:

QW-153 (d) if the specimen breaks in the base metal outside of
the weld or weld interface, the test shall be accepted as
meeting the requirements, provided the strength is not
more than 5% below the minimum specified tensile
strength of the base metal.

My apologies !!

Regards

 

[LJthien]

@ r6155
The requirements of QW-153(d) and minimum UT of SA537-Cl2 are all met. On the other hand, Vendor used E8016-C1 SMAW electrode from a reputed manufacturer which has good mechanical properties in contribution to weld metal strength.

@ marty007
The point of over safety margin may be a reason even though I believe that if the PQR coupon had been PWHT-ed, both tensile strength and impact value should have increased more. Obviously, successful non-PWHT PRQ allows production welding without PWHT which help save lots of cost.

But this design concept may be troublesome if Vendors use cheaper material from some countries for their cost down.

 

[r6155]

Good quality of plate and electrodes is not a guarantee for a good PQR. Think on the welder and the welding engineer.
I have rejected several PQRs (including nuclear) before they begin to weld the specimen: the electrodes were not stored correctly.

Regards

 

A successful PQR basically proves that your best welder was able to make a sound weld using hand-picked materials in a comfortable position on a sunny day.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[weldstan]

PWHT would have minorly decreased tensile properties of the base metal, not increased them. The weld metal would be less affected because its response to PWHT is much slower than that for the base metal.

Under ASME IX the purpose of PQR testing is to verify that mechanical properties (tensile strength, ductility and when required impact toughness) of the weld coupon meet the minimum requirements of the base metal without visual defects under the essential or supplementary essential variables. That is all.

 

[r6155]

@ weldstan You are wrong… ........“without visual defects”
Please see ASME IX
QW-163 ACCEPTANCE CRITERIA — BEND TESTS
The weld and heat- affected zone of a transverse weld-bend
specimen shall be completely within the bent portion
of the specimen after testing.
The guided-bend specimens shall have no open discontinuity
in the weld or heat-affected zone exceeding 1/8 in
(3 mm), measured in any direction on the convex surface
of the specimen after bending. ………..

Section IX requires each organization to qualify WPSs themselves.
PQR is a good way to detect problems in the organization.
This is so since if the PQR is rejected, the job cannot continue.

Regards

 

[weldstan]

r6155
I was referring to the visual examination of the completed weld as defined in ASME IX, which is not much.

If you assume that the up to 1/8" open discontinuity permitted in a completed bend test is a defect, you are mistaken. If it exceed 1/8", the test and PQR fails.

 

[r6155]

weldstan
Discontinuity is not a defect.

Regards

 

[weldstan]

Isn't that what I said? However, a discontinuity of size defined by a Code as being a defect is.

 

You say potayto, I say potawto.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[SJones]

To get back to the original point: @LJthien - try a Google search on "effect of undermatching weld strength" . Plenty of hits to get your teeth into.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

 

[LJthien]

@SJones
Thanks, I will look up some thing to work out why and how the unmatching could have been considered.

 

We're starting to seriously overthink what is a simple Section IX concept.

'Matching' filler metal can mean a variety of things in a variety of contexts. Strength is only one of those things.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."