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New Experience in the ultrasonic examination for stainless steel welds of heavy wall pressure vessel Y. S PARK, N. G KWAG, K. C. SHIN HANJUNG - SOUTH KOREA G. NARDONI, P. NARDONI I & T NARDONI INSTITUTE - ITALY Contact

1 Introduction

HANJUNG in 1999 has been involved in a large project designed by BPMCO(Bonny Project Management Company Limited) for the construction of pressure vessels in stainless steel for the LNG Nigeria plant. Due to the heavy thickness special ultrasonic techniques have been developed to guaranty the requested integrity of the welds.

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.

2 Interaction of ultrasound with the austenitic structure.

Attenuation, scattering, change in velocity are the main factors that characterized the propagation of ultrasonic waves in the stainless steel welds.

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
<100>	(c11/r)1/2
<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.

3 Probes

Angled Longitudinal waves have to be used to penetrate the austenitic structure of the weld section maintaining a resonable value of intensity.

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.

Depth zone
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

Surface/subsurface defects
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

4 Scanning directions

Multuple scanning directions have to be used to achieve good value POD(probability of detection)

Depth zone	Probe	Technique	Scanning pattern
1 - 5	TRL CR	Creeping waves
2 - 3 - 4	TRL-60° F : 70	Angled compression waves & RTT
	TRL-60° F : 40 or 35	Angled compression waves
	TRL-70° F : 35
	TRL-45° F : 40
Fig 4: Example of scanning direction in the range of thickness 70 - 100 mm

5 Mode conversion at the boundary

Both longitudinal and shear waves beam are under the critical angle and consequently mode transformation occurs at the back surface. Accurate geometries of the ultrasonic path have to be calculated for the interpretation of indication. The mode transformation is based on the RTT (round trip technique) technique for the detection of planar defects

6 Calibration Blocks

Calibration blocks need to have the following characteristics;
1. Have the same base material of the weld to be examined;
2. Have the same weld of the one to be examined welded, with the same WPS/PQR;
3. Have holes for calibration in the weld center line and along the fusion line;
4. Have subsurface holes for creeping wave calibrations;
5. Have notchs for shear waves beam intensity calibration at the back surface;
6. Have side flat bottom holes for RTT, LTT probes, tandem technique calibration.

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.)

7 Sensitivity, DAC curves

Sensitivity is set-up on cilindrical holes of 2.5-3mm diameter.

Two difference DAC curves have to be determited;

1. DAC 1 on holes along the centerline of the weld;
2. DAC 2 on holes along the heat affect zone passing with the beam through the full weld section.

Fig 6: DAC curve on CRS/monitor

8 Coincidence

In relation of the thickness under examination, the indication detected by RTT technique may appear in the same posion on the CRS/monitor as back reflection of the shear weaves due to surface defects or shaped weld cups.

Fig 8: Indication detected by Round Trip Tandem Technique(60° LLT)

Fig 9: Indication detected by 28° shear wave angle beam



1. Indications shown on fig. 8 are detected from f5.0 flat bottom hole located at 60° mm depth of RTT UT block by round trip tandem technique using the 60°(FS 70mm) compression angle beam probe.
2. Indications shown on fig. 9 are detected form notch located on the surface of the UT block by 28° shear wave using the 60°(FS 70mm) compression angle beam probe.

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°)



1. Indications shown on fig. 10 are detected simultaneously by compression angle beam and round trip tandem technique using the 60° compression angle beam probe
2. The fist indication(#1) is detected by 60° compression wave angle beam and the second indication(#2) is detected by round trip tandem technique.
3. The defect type : lack of fusion located on the 20-23 mm depth of stainless steel weld
4. Figure 6 : UT block for tandem technique
5. Figure 5 : UT block for angle beam and creeping wave technique

9 Conclusion

The application of the procedure presented in this paper on production welds has given reliable results. Indications have been evaluated with good confidence. Repaires of unacceptable indications have confirmed accuracy in the localization and estimation of defects size.

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