HF-Modified austenitic stainless steel cast piping is utilized in hydro-processing units. Ultrasonic examination of HF-Modified material is difficult due to it’s inherent material properties. This paper outlines the results of trials conducted on test samples containing artificial flaws in the form of EDM notches. Use of manually manipulated Time Of Flight Diffraction (TOFD) ultrasonics has demonstrated the effectiveness of the technique to detect and size ID connected notches.
HF-Modified material has been used in hydro-processing units since 1963. (1) Applications include its use as reactor effluent piping. Failures have occurred in-service and polythionic acid stress corrosion cracking was determined to be the cause.(2) The cracking was detected using penetrant techniques, but due to access limitations and poor results from radiographic examination in detecting and sizing such cracks, it was decided to pursue ultrasonics as a possible non-destructive testing solution.
IESCO embarked on a series of trials early in 1992 to evaluate the potential of various ultrasonic techniques, or methods for detection and/or sizing of simulated cracks in HF modified piping. Similar studies were conducted by organizations such as EPRI, for cast stainless steel piping used in the nuclear utility industry.(3)
Various cast stainless steel piping components were purchased and had various notches inserted by electrode discharge machining (EDM). The components were welded as per the original welding procedure specification. Figures 1 and 2 show typical fabricated test samples. The notches were all surface connected and visible on the inner surface. Their final dimensions were measured in terms of length, depth and position and mapped as per Figures 3 and 4.
Unocal also provided some used piping and samples containing notches. These samples were initially tested blind. At a later stage in the trials, additional samples were provided by the client and all testing was witnessed by client representatives.
The ultrasonic testing technique had to be field portable, capable of detecting linear indications in base material and welds regardless of orientation, and reasonably accurate in terms of sizing in the through wall direction; no specification of detection and sizing limits were stated prior to investigation. Ultrasonic transducers and wedge combinations could only be placed on the external surface of the piping or piping components; no surface conditioning, in terms of weld cap removal would be allowed, but cosmetic surface conditioning, would be allowed. Area of interest was the remote face (inner pipe surface).
Shear wave ultrasonics using vertically polarized shear waves.
Angled longitudinal wave focused and unfocused.
Forward Scattering Time Of Flight Diffraction Ultrasonics.
The following represents the ultrasonic techniques initially tested:Automated and manual techniques were attempted. Initial results indicated that manually performed (Free Hand Scan) Time Of Flight Diffraction Ultrasonics was the most promising.
Time Of Flight Diffraction Ultrasonics works on the basis of two broadband transducers, facing each other, one acting as a generator of ultrasound, the other as a receiver. A pulse is transmitted through the material under test, normally compression waves are utilized as their arrival time is shorter.