ASTM A262 IGA Susceptibility Tests as P/O Section IX PQR? 3

[tc7]

I would like to find a test to detect and evaluate the amount of sensitization that may occur in the heat affected zone and the heat tinted area of SS welds. This will be an ASME Section IX GTAW pipe weld procedure qualification for a B31.3 corrosive fluid application. I realize that sensitization tests are not required by these Codes, but want to do this anyway for my own shop and intend to require it for potential welding subcontractors we may need to assist us. May also impose this on welder performance tests as well. (Note: during production PWHT will not be considered. Passivation will be required.)

The material will be 316L thin wall(.125")pipe (and I know the L-grades are said to be difficult to damage by welding, but I have seen some horrifying SS welds and I have my reasons). In reading over ASTM A262 it seems that the intergranular corrosion susceptibility test Practice F may offer exactly what I am looking for except it is stated to NOT give information on sigma phase. The specification recommends Practice B or C for sigma phase concerns.

So then, will both Practice F (for chromium carbide precipitation) concerns AND Practice C (for sigma phase concerns) be used, or can the one test by Practice C be equally effective for the sensitization concerns as well sigma phase?

If we are satisfied with the results of the ASTM A262 tests, can we conclude that pitting corrosion will not also be a problem? Of should additional ASTM G48 tests be called for?

Thanks for ideas and advice.

 

[metengr]

tc7;
Based on your stated requirements above, I would select the A 262 Practice F test and not the ASTM G48 test for 316 stainless steel. Your main concern is evaluating the degree of sensitization from welding.

 

[tc7]

Thankyou for the overtime reply Met.

When do you think that sigma phase becomes an issue to be concerned with for weldments?

Why do you suppose that Section IX or B31.3 Codes do not require such tests as p/o PQR or WPT?

 

[MikeMet]

tc7;

You can not necessarily relate the ASTM A262F to pitting corrosion. Sensitization and pitting resistance are due to different elements in the metal, with some overlap, such as chrome content. So if you are worried about both, you should run two different tests.

As to why ASME does not worry about corrosion resistance is the Code relies on the user to determine what to worry about, which is the right thing to do since you could not cover every possible case for material selection. There are cautionary sections in the B&PV Code talking about corrosion and metal degradation, which the user has to take into account when selecting materials of construction.

Based on what you supplied, if you are truly worried about pitting and HAZ attack, maybe you should consider a slight upgrade in alloy since Type 316L sst seems to be just on the edge of being unsuitable for your conditions. You can run all the production weld samples you want, but you can never be sure the welds you didn't test might be a problem in a marginal material. Even upgrading Type 317L or 317LMN sst can buy you some increase in pitting resistance due to the slightly higher moly with small cost increase.

 

[tc7]

Thanks Mike-
Use of 316L for the application was decided by others against the advice of many. And is why I want to be as close to 100% certain that our welding procedure is as ideal as it can be.

Any thoughts/suggestions in regard to sigma phase issues?

I thank you for your interest.

 

[metengr]

tc7;
Sigma phase really comes into play for prolonged elevated temperature exposure as a result of poor handling at the mill or plate fabricator versus weld fabrication. I don't believe it is necessary for your evaluation.

 

[SJones]

It is important to try and relate the test environment and the acceptance criteria to the actual service environment. For the tests that work on weight loss determination, what weight loss constitutes a pass criterion? Will you adopt the sensitisation heat treatment prior to the test? Personally, I would go for A262 Practice E in the as welded condition with a go-no go criterion for intergranular corrosion. Pitting corrosion is another issue and the use of G48 requires that some form of correlation with service environment performance is available for the test to have any relevance at all both in terms of the test parameters and the acceptance criteria.

Don't impose this testing for welder performance; it is the procedure that gives rise to the sensitisation. If a welder welds fully within the procedure why should they be penalised on a corrosion test?

Steve Jones
Materials & Corrosion Engineer

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

 

[tc7]

Thanks Steve-
You bring up a good point of establishing an acceptance criteria for the IGA Susceptibility tests and instead adopt go-no go criterion for intergranular corrosion. Instead of using Practice E with go/no go can the simpler Practice A be used for go/no go decisions?


And I agree that I should not include corrosion tests on welder perfomrance tests. So to place a control on the heat input I wiill have to establish a minimum travel speed in the PQR and hold our welders within that limit. I am sure this will also be considered unusual for a Section IX procedure on stainless steel.

 

[unclesyd]

I have to take a little exception to some comments noted above about the predictability of a material's performance based on a specific test in an extremely corrosive media as you seem to be presented with.
Based on many years experience with nothing but highly corrosive processes, the use of corrosion data books just gets one in the ball park in the selection of a material for use in a very corrosive media even if listed as most are essentially pure corrodents not process chemicals. It normally takes actual tests, like immersion and/or heatflux, with the actual media to get some measure of a particular materials performance.
The specific mention of IGC is more controlled by the chemistry, mainly Carbon, of the material rather than the welder or procedure. This is not saying the welder doesn't play a small part but it is very hard to differentiate between good TIG welders. I base this on actual testing of over fifty welders coupons and field welds by various laboratory methods and by actual examination of process welds in the field.

It would help immensely to know your process materials and conditions.

 

[tc7]

Unclesyd-

Process is GTAW, 316L base material, 316L welding rod, 100% argon shielding gas, argon purge. Not sure of amprage yet ( ~85-90 amps to start), travel speed TBD.

My goal is to establish a weld procedure that controls the heat input and doesn't ruin the SS material as far as chromium carbide precipitation goes, and I guess I don't need to worry about sigma phase. I understand the L grades are hard to injure from welding heat as long as we keep the torch moving at the right speed and we don't have the amperage higher than it needs to be for thin wall pipe.

I am aware that this material is not the correct selection for seawater environment but it was not my selection. So as not to be culpable when the material fails, I want to be able to prove that my weld procedure is not a contributor to corrosion failures. Thus, I started to worry about sensitization. What else would you advise?

 

[unclesyd]

I will temper my comments somewhat when it comes to the use of 316L SS in seawater as this presents many problems that are not manageable. From my experience IGC is not normally the major problem.

Can you come back with your exact conditions that the piping will see in the seawater. This will help as there are a number of members we versed in the use of Austenitic SS in sea water environments.

Have you looked at orbital welding for you weld joints.

 

[tc7]

Unclesyd-
My job is only to qualify procedures, welders and to get into production. We tried to persuade the designers to use duplexes, but were unable to do so. This is the hand I have been dealt. Don't know much about the application other than what has already been said - warm seawater (~180 deg F) flow is ~120gpm, max system pressure is about 80psi).

I looked at orbitals but are too expensive for me at this time. Besides they would be useless on the branch connections.

If as you say, IGC is not normally the major problem, then what else do I need to know?

 

[unclesyd]

What is the fluid velocity or size of the pipe?

Orbitals will do fittings. If the quantity is great enough at one time you could lease the equipment.

Again if the quantity warrants you could look at having someone make your branch connects.

http://www.t-drill.com/Ind_Mach_Main.html