CTOD testing

Date 06-26-2002 14:43

My company performs tons of CTOD testing each year. I am no expert, but I have a good deal of experiance with the testing of them and I know several experts in the field. What would you like to know?

Regards,
Mike Mason
Bodycote Materials Testing.

By GRoberts (***)

Date 06-26-2002 15:55

Specific questions that come to mind, right now are:

1. I have heard that there are a lot of invalid tests that occur when testing the HAZ. Is there anything that can be done during welding to minimize this?

2. Do you use the B x 2B x W, or compact specimens? Which is easier, or cheaper to use?

3. Any other pitfalls, or common mistakes for first time testers.

Thanks

By Mikeomni1 (*)

Date 06-27-2002 17:27

Here are the answers to your questions as has been my experience.

1. I have heard that there are a lot of invalid tests that occur when testing the HAZ. Is there anything that can be done during welding to minimize this?

Invalid specimens result from uneven crack growth during the fatiguing portion of the test process. Uneven crack growth is caused by varying degrees of mechanical properties across the plane being tested. As an example, visualize a 45 degree butt weld being sampled through the HAZ. The notch in the specimen would intersect weld metal, HAZ and base metal. This sets up the possibility that the crack front under the notch will not propagate evenly as it intersects the different areas of the weldment. The main reason for this occurrence is variation in yield strengths. Specimen pre-compression and repositioning of the specimen can be the difference between a valid or invalid crack front. The biggest problems are found in smaller weldments because the specification tolerances provide less latitude. Many company procedures now require the weldment to have a straight side on the groove so that the notch can be mostly contained in the HAZ. Also I see invalid specimens in weld metal CTOD specimens due to weld defects interfering with the fatigue crack growth.

2. Do you use the B x 2B x W, or compact specimens? Which is easier, or cheaper to use?

B x 2B is the preferred size. It is easier and less expensive, especially when testing at low temperatures. B x B specimens are normally only used when material is sparse or a surface notch is requested. Compact tension specimens have more complex machining and testing set-up requirements. They are also difficult to use for thicker weldments.

3. Any other pitfalls, or common mistakes for first time testers.

Be sure and choose a laboratory that is experienced with, and accredited for CTOD testing. The tests are too expensive and time consuming not to be handled properly first time. Always anticipate the possibility of invalid specimens. The lab can only do so much to produce a valid test. Do all the NDE you can on the weld to make sure the sampling area is clean-clean-clean.

I hope this helps and good luck with your tests.

Regards,
Mike Mason
Bodycote Materials Testing

Email: MMason@BodycoteUSA.com <

By GRoberts (***)

Date 06-27-2002 20:05

Thanks!

By GRoberts (***)

Date 07-10-2002 20:37

Mike,

I have a couple of further questions if you have the time. We are working to a bit of a mishmash of specifications. (Our customers like the phrase "Testing program similar to ______" Fill in your favorite CTOD test specification.

The main one I am looking at right now is API RP 2Z. My question about this one is in paragraph 3.1.2, it reqiures that the weld metal have exceed the HAZ CTOD value by .13mm at -10C. One of our customers wants .38mm CTOD for the base metal. if the weld metal has to exceed .38mm by .13mm, it needs to be at least .51mm. So far, filler metal manufacturers say that that's way too high. Will we have problems getting valid tests if the weld metal doesn't get that high of a value? I also have EN10225 that describes a CTOD testing plan, but it does not mention the weld metal exceeding the base metal as far as I have found. Is this important?

Thanks

By Mikeomni1 (*)

Date 07-12-2002 16:29

In answer to your last questions our opinions are as follows.

Point 1: CTOD’s for weldments tend to be project specific. Pipeline specs like API 1104 & BS 4515 do request them but are not always mandatory. It is our opinion that the British Standard for CTOD testing of Weldments (BS 7448 Parts 1-4) is the best specification as far as pure methodology is concerned. EEUMA 158 (North Sea Oil & Gas Spec) is our favorite for CTOD HAZ validation purposes.

Point 2: The fundamental point of API RP2Z to qualify weldability toughness properties for base materials at the HAZ & Fusion Line. Obviously the weld metal will be diluted in these regions, therefore you cannot really say the HAZ (weldability) properties are poor if the fracture toughness of the weld was low in it’s own right. We concur that it is difficult to get weld center line properties at 0.51mm, but not impossible. It is our experience that concessions in this area are usually made as long as the weld metal hits 0.38mm.

We are not familiar with EN 10225.

(I wish to thank the president of our company, Colin Speedie for the considerable assistance given me in answering these questions.)

Best regards,
Mike Mason
Bodycote Materials Testing

Email: MMason@BodycoteUSA.com