Assessment of Residual Stresses via Charpy Test.

[ibf]

Hello Everybody !

We have a large component (fabricated via S.A.W in ASTM A 36) which needs some large welding to be developed at the port site due to transportation reasons. The weldment will be connecting two cilindrical bodies (like two "cans")which are 7.3 meters in diameter and 17 meters long (8.5 meters long each can). As neither localized stress relieving nor furnace stress relieving is possible to be developed at site (and even if it was possible , there would be distortion issues due to time and temperature) , I was suggested to run test plates under the same conditions of the actual weldment and then run charpy tests on each mock-up (one with and the other w/o. stress relieving treatment). The CVN values would then be compared in order to show whether residual stresses may or may not be detrimental to the actual components.

The question is ; Is that a reliable test to evaluate the level of residual stresses for a component which has not been stress relieved ? NOTE: Vibratory stress relief is not acceptable per the customer`s spec.

 

[mcguire]

I don't think this won't work. Any residual stresses would be relieved in the test specimen when you cut it from the mock-up.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[metengr]

The question is ; Is that a reliable test to evaluate the level of residual stresses for a component which has not been stress relieved

No. The level of residual stress cannot be determined in this application using CVN impact testing.

 

[metengr]

ibf;
What is the thickness of the cans? Post weld heat treatment of carbon steels is really based on wall thickness versus OD or geometry. Can you come back with some information on wall thickness?

Also, you can elevate preheat during welding to reduce susceptibility of residual stresses from welding.

 

[stanweld]

There are other methods to determine residual stress: X-ray diffraction, strain gauging.

 

[ibf]

Sorry for my delay in answering your questions.

Here you go :

1. Cans are 2.5" - thick. Full penetration weldment , with backgouging.

2. The connecting welding at site has not taken place as yet.

3. We are aware of those methods to measure R.S ; however, we want to avoid S/R due to high risks of distortion.

4. S.A.W is a welding process with high heat input , and that concerns a lot.

5. Vibratory S/R as mentioned before is not allowed.

6. Any ideas ?

Thank you.

 

[metengr]

Yes, can you elevate the preheat to a minimum of 300 deg F for SAW? If so, watch the start-stops, and using a temper bead style approach for welding you should minimize distortion and not require PWHT.

I just completed a review of an EPRI report for one of the code committees with regard to changing the current PWHT exemption for P-No 1 base material from ¾” to 1.5” with preheat. Even at a 2.5” thickness with increased preheat, the level of residual stress would be lowered and you would significantly reduce the hardness in the base metal HAZ from tempering benefits associated with the higher heat input AND preheat.

 

[SJones]

What is the governing standard for the welded fabrication? If the standard is prescriptive in the limits of having to apply PWHT (irrespective of preheat and bead deposition technique), the only way you can engineer your way out of it is to perform an engineering critical assessment assuming yield strength magnitude residual stresses. British Standard 7910 will provide you the necessary information.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[ibf]

The governing code is ASME SECT. IX.

In case the code does not allow me to use metengr`s suggestions , where can I obtain British Std. 7910 ?

 

[metengr]

ibf;
ASME Section IX does not provide any guidance for design engineering of welds. It is simply a guide for weld procedure and welder qualification. The elevated preheat/temper bead information I had provided to you is referenced in the National Board Inspection Code, which is an in-service repair code for pressure retaining items.

 

[SJones]

If you can't get hold of BS 7910, you may wish to struggle through API RP 579 instead.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[ibf]

Thanks SJones and metengr.

metengr - in case I follow your welding thecnique suggestions, I may avoid to s.relieving ; however , still very prone to higher distortion risks than the usual heat input and preheating temperature used during the conventional process of manufacturing of the "cans" - Is it correct ?

SJones - Did I understand correct that in case I get the British Std. 7910 , I will be able to estimate side effects by assuming yield strength magnitude of R.S ?

 

[SJones]

In a word, No. If the construction code you are using demands PWHT and you wish to avoid it, as an engineer you must prove that it is OK to do so. BS 7910 and API RP 579 advise you how to do this. Essentially, it means that you will have to determine the lower bound fracture toughness of the weld or HAZ using CTOD or similar, assess all the operational stresses (including fatigue, yield strength level residual stresses, and the effects of wall loss from corrosion amongst others), and then work out either whether the actual NDT flaws observed can be tolerated, or whether the maximum allowable flaw size of ASME IX or the design/fabrication standard can be tolerated.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[metengr]

Most likely, you do not have a specific construction standard that locks you into PWHT. As I stated above, using a higher preheat and bead deposition technique with P-No1 base materials (like A 36) may allow you to avoid having to PWHT. Distortion can certainly be managed by welding circumferentially as you have stated. In fact, this is the most desirable welding to balance the residual stresses that could cause noticeable distortion. The issue of base metal HAZ hardness can also be managed by what I had stated above (increased preheat and bead placement (in-situ tempering).