You must give more information about what you are trying to do. UT below 5mm is a pretty broad statement. Are you trying to do 0 degree or angle beam inspection? Please let us know more about exactly what you are trying to accomplish and we can help you with our comments.
I am also highly interested in this subject as. I also have cast steel pipeline network: wall thickness from 4mm to 13mm in 6", 8", 12", 16", 18" & 20".
My immediate concern is the 12" pipe, thickness between 6,35mm and 8mm, I want to locate and size planar anomalies (incl cracks, lack of fusion, etc) in the longitudinal seam weld and heat affected zone; in surface, sub-surface & internal wall.
I do this to verify events pointed out by in-line inspection surveys (pigging). the seam weld is ERW of pre-1970. I normally insist on both a zero & an angle probe.
Velu and I are anxiously awaiting your responses.
I know you are getting and are going to get many more responses. I can only answer you in broad terms right now - the answer is yes. We were doing UT of very thin plates and tubes, right upto 0.4 mm and we can go down futher if needed. I am talking of tubes, sheets and welds in zirconium alloys, titanium, austenitic stainless steel etc. for various applications.
Pl. feel free to contact with more details. I can help you if you have the right equipment.
The answer for simple question: "Can we do UT below 8mm?" is YES. We have been performing that kind of testing for over 20 years now. Also Polish power industry is using this method. (method is described in article from ECNDT 2006 http://www.ndt.net/search/abstract.php3?AbsID=4071 ).
Anyone feel free to contact me for more informations...