Pen et Rant - NDT's first agony aunt

Dear Pen,

“It is generally accepted that ultrasonic examination of thick austenitic steel welds is difficult and at times virtually impossible due to changes in grain structure leading to random variations in acoustic impedance within the material.” – written in 1972^[1].

“It seems that acoustic anisotropy also makes ultrasonic wave velocities vary locally, which means that the beam can appear to be bent; thus locating and sizing a reflector was found to be very difficult.“ – written in 1974^[2].

“Practical experience and published work clearly show that longitudinal wave techniques are superior to the shear wave techniques commonly used on ferritic weldments.“ – written in 1980^[3].

The above extracts are taken from the British Journal of NDT, now called Insight, and copies of the papers can be readily obtained by making a brief telephone call and payment of a small fee. Now, I know that the journal is considered by some to be too academic and therefore may not be read by non-academics, but then there is always Google, which at the stroke of 27 keys pulls up 114,000 results on ultrasonic inspection of austenitic welds.

What I would like you to tell me, Aunt Pen, is why, when the information has been known for over 40 years and is so readily available, do some practitioners, NDT vendors and companies who buy NDT still not know that the inspection of austenitic welds is different to inspecting ferritic welds?

Ross the Boss

Dear Ross
I haven’t the foggiest! But as it is my job to answer difficult questions I will have a go. Last September I told one of your colleagues that everyone knows how to look for things. So it is likely that the culprits don’t see the need to pay money to check that they are doing it right or provide additional training when what they have will do. And, of course, if they treat a thick section austenitic weld like a ferritic one, they are unlikely to detect any defects and everyone is happy (and none the wiser)!

However, the NDT profession as a whole has to carry a lot of the blame. It has not bothered to publicise how to inspect austenitic welds except through journals and conferences. In fact, if the profession had deliberately set out to keep the difficulty of inspecting austenitic welds quiet, then it would have been hard pushed to do a better job than that which has been achieved. Generally, the first step for any company with respect to NDT is to look to use a national standard and a nationally certified operator. There is PCN certification for the ultrasonic inspection of welds. In PSL 26, ‘Info sheet: PCN certification available’, only the single word ‘welds’ is mentioned. PCN/Gen Appendix C1 is the relevant document and the title on the PCN web page states: ‘…Ultrasonic Testing of Fusion Welded Ferritic Steels’. However, the document itself is entitled ‘Certification of Personnel for Ultrasonic Testing of Welds’. The only mention of ‘ferritic’ is hidden in the scope of the procedure to be used in the exam. As far as I know, the syllabus does not mention austenitic welds and the PCN certificate issued also only mentions welds. An austenitic weld is a weld so it is not unreasonable to assume that the certification is valid for these.

Similarly, with the national standards: BS EN ISO 17640 (the replacement for BS EN 1714) is entitled: ‘Non-destructive testing of welds. Ultrasonic testing. Techniques, testing levels and assessment’. The scope does state: ‘for…testing of fusion welded joints in metallic materials…which exhibit low ultrasonic attenuation…It is primarily intended for…welded and parent material are ferritic’. But it is not exactly an unambiguous warning for it not to be used on austenitic welds. The picture is further clouded by PD 5500^[4], which gives the rules for the design of pressure vessels. It provides lots of materials data for austenitic material, but when it comes to inspection it recommends the use of BS EN 1714 (now superseded). Again, it states that ultrasonics is ‘very suitable for…planar defects…in ferritic steels’, but gives no guidance for austenitic welds. The case for national standards is rescued slightly by BS EN ISO 17635^[5], which, in the table of generally accepted methods, has UT in parentheses for austenitic welds greater than 8 mm thick; parentheses means that the method is only applicable with limitations!

However, Ross, if all NDT vendors, and all companies who buy NDT, follow ISO 9001 and treat NDT as the QA special process that it is, then they would find out themselves whether their procedures and personnel qualifications are adequate before any damage is done. But that is another subject which is not promulgated as it should be.

Pen et Rant

[1] C J Abrahams, ’The ultrasonic examination of thin-wall tube-to-tube butt welds in austenitic steel
     using assisted hand-scanning methods’, 5, p 66, 1972.
[2] J P Pelseneer and G Louis, ‘Ultrasonic testing of austenitic steel castings and welds’, 7, p 107,           1974.
[3] R J Hudgell and H Seed, ‘Ultrasonic longitudinal wave examination of austenitic welds’, 3, p 78,
     1980.
[4] PD5500:2006, ‘Specification for unfired fusion welded pressure vessels’.
[5] BS EN ISO 17635: 2010, ‘Non-destructive testing of welds. General rules for metallic materials”.

Please note that the views expressed in this column are the author’s own personal ramblings for the purpose of encouraging discussion within the NDT Newspaper. They do not represent the views of the IVC, Serco Assurance or the HSE who funded the PANI projects.

Letters can be mailed to The Editor, NDT News, Newton Building, St George’s Avenue, Northampton NN2 6JB. Fax: 01604 89 3861; Email: ndtnews@bindt.org or email Bernard McGrath direct at Bernard.McGrath@sercoassurance.com