·Table of Contents
New Experience in the ultrasonic examination for stainless steel welds of heavy wall pressure vesselY. S PARK, N. G KWAG, K. C. SHIN HANJUNG - SOUTH KOREA
G. NARDONI, P. NARDONI I & T NARDONI INSTITUTE - ITALY
Experimental work of research started in the HANJUNG NDT laboratories to set up test blocks, probes, equipments, scanning techniques, trainning for NDT personnel to establish an ultrasonic procedure to satisfy the code request(BS5500).
In the present paper we underline recomandations based on our experience gained so far.
Hysteresis and scattering losses are the major cause of attenuation.
The variations of ultrasonic velocity are depending from the angle between beam axis and cristallyte axis. (see fig.1, Table 1)
|Fig 1: Variation of ultrasonic longitudinal waves as a function of beam direction to the crystallite axis.|
|direction||Longitudinal wave velocity|
|<110>||[(1/2r)(C11 + C12 + 2C44)]1/2|
|<111>||[(1/3r)(C11 + C12 + 2C44)]1/2|
|Table 1:Velocities in the major directions of the crystallographic axis.|
The consequence is that the ultrasonic beam will be skewed during its passing through the weld.
Skew angle can be determined experimentally on test blocks; its value is very important for the accurate localization of defects.
Spacial probe with double crystal emitting angled longitudinal waves have to be used.
Each probe has a proper focal distance(see fig. 1)
Shear waves are contemporary generated at the interface.
The weld section has to be divided in multiple depth zones for each of these a proper focal distance have to be used. (see fig. 2)
|Fig 2: Schematization of ultrasonic beams, focal distance, and amplitude distance function||Fig 3: Principle of the creeping wave; waves pattern|
For surface/subsurface defects creeping wave have to be used. Creeping waves are unaffected by surface roughness.
Creeping waves changes it selfs in shear waves along its path. Indications may appear on CRS from shear waves hiting the back surface. Probability to have such indications is related to the thickness examined, surface condications, surface/subsurface cracks present
|Depth zone||Probe||Technique||Scanning pattern
||1 - 5
||2 - 3 - 4
F : 70
|Angled compression |
waves & RTT
F : 40 or 35
F : 35
F : 40
|Fig 4: Example of scanning direction in the range of thickness 70 - 100 mm
|Block No.||SIZE (mm)|
|Thickness||A||B||Hole dia.||Hole depth|
|USS - A01||40||150||150||Ø2.4 x 7ea||50||SAW|
|Fig 5: UT block for angle beam and creeping wave technique calibration|
|Fig 6: Reference block for round trip technique (RTT), LTT probes, tandem technique. (The use of one of these techniques is related to thicknesess to be examined.)|
Two difference DAC curves have to be determited;
|Fig 6: DAC curve on CRS/monitor|
|Fig 8: Indication detected by Round Trip Tandem Technique(60° LLT)|
|Fig 9: Indication detected by 28° shear wave angle beam|
In fig. 9 ,10 an example of this coincidence is represented.
Other type of coincidence may happen when position of the defect relative to the angle beam give to two indications.(see fig. 10)
One relected by the longitudinal angle beam and the second reflected by RTT
|Fig 10: indications detected by RTT and longitudinal angle beam(60°)|
|© AIPnD , created by NDT.net|||Home| |Top||