SS vs CS calibration blocks for UT

[arunmrao]

There is an interesting thread in Inspection forum on the relevance of using CS blocks for calibration while testing SS pipes.

The reason provided by one of the members is that SS is anisotropic material,which implies that CS is isotropic. I find it difficult to understand. Can someone please explain this anomaly?

"Stainless steel weld can respond drastically different than carbon steel. Stainless steel is an anisotropic material, which makes it very dificult to penetrate with ultrasound"

I have not failed. I've just found 10,000 ways that won't work." — Thomas Edison
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[redpicker]

I think someone has their terms mixed-up. With stainless steels, blanket statements such as this are usually assuming a specific type of stainless steel (austenitic, martensitic, ferritic, etc...). Without knowing which type the original author is referring to, it is hard to determine what he means.

From the other post, that is, using CS calibration standards for stainless steel inspection, if the stainless steel is an austentitc stainless, the differences in the speed of sound in CS versus the austenitic will make the CS calibration worthless. Generally, the austentic stainless will have a much larger grain and suffer much more attenuation. Both of these would make it very difficult to get any meaningful inspection on the stainless if you calibrated on CS. Neither of these are specifically based on the material's isotropicity (if that is actually a word). However, I can see how an inspector could see the situation as a matter of it being difficult to get the sound to "penetrate" the stainless.

rp

 

[metengr]

arunmrao;
Calibration blocks for use in Code ultrasonic test examination work requires materials that behave nearly identical to material you are testing. For carbon steel, carbon steel calibration blacks should be used. For low alloy with a specific heat treatment or thermo mechanical history, similar material should be used to manufacture calibration blocks. If base material contains welds, the welds used for the calibration block must match the process used and heat treatment history to produce desired results. The main reason is sound attenuation.

For ultrasonic examination of austenitic stainless steel pipe and welds calibration blocks must match the material. I have been involved with fabrication of calibration blocks back in my days with nuclear and the NDT examiners needed to match the fabrication history and weld method used in production or field welding to provide for meaningful UT examination results. The fusion zone of austenitic stainless steel weldments can result in significant sound attenuation and even sound reflection to give false indications.

So, the discussion in the other post about isotropic and anisotropic behavior is not correct. Calibration standards should always try to be manufactured from similar material and the material’s thermo mechanical history to avoid problems with interpreting indications.

 

[arunmrao]

I agree with you metengr. I have been involved in manufacture annd supply of calibration blocks. I was surprised,when I read the discussions and thought it prudent to seek knowledgeable opinion here.

Thanks for the clarifications.

I have not failed. I've just found 10,000 ways that won't work." — Thomas Edison
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[TVP]

redpicker,

FYI, the term is isotropy.

 

[0707]

For “simple thickness” measurements if you know the velocity of the material being tested you can use standard CS blocks.

To search for defects in austenitic stainless steels and other alloy materials, UT scanning, requires special blocks of the same material being scanned.

These special blocks shall have the same thickness and geometry and shall have “artificial” defects like those expected in what we are searching.

lm

 

[ndeguy]

Hi guys

Not only special blocks are required for UT inspection of austenitic welds, but also specific probes. These are low-frequency, longitudinal wave (or horizontally-polarised shear wave) probes which are less susceptible to scattering due to the longer wavelength of the sound in these materials. Additionally they should be twin-crystal (improved S/N ratio) and high angle to minimise beam-skewing by the columnar grain structure of the weld metal.

Regards



Nigel Armstrong
Lloyds Register
Independent Verification Body Surveyor

 

[rustbuster]

Where do I find the Inspection Forum?

 

[arunmrao]

QC,Inspection and Testing Engineering forum under Industrial/Manufacturing Engineers group.

I have not failed. I've just found 10,000 ways that won't work." — Thomas Edison
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