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Penetrator Flaws in High Frequency Induction Welded Tube

 

Detection of Penetrator Flaws in High Frequency Induction Welded (ERW) Tube

TWI Industrial Member Report Summary 601/1997

A B Day

There is some evidence that ultrasonic inspection systems currently used for in-line tube inspection do not always detect short deep 'penetrator' type flaws. This work was aimed at assessing the effectiveness of sound beam focusing using axicon (conical) lenses for improving detection of penetrator flaws in high frequency induction (ERW) welds.

Background

Increasing use of ERW tubing in high pressure applications has imposed more stringent demands on quality of the seam weld, both in terms of toughness and the size of flaw which can be tolerated. Over the last few decades considerable progress has been made in manufacturing technology, both in terms of plate quality and control of the welding process itself. However, these advances have not gone so far as to eliminate completely the occurrence of weld flaws. This has increased the importance of in-line NDT, which is now required to detect even smaller flaws than in the past.

The most important inspection techniques, for detection of internal (sub-surface) flaws is the ultrasonic shearwave (angled beam) technique. This technique, when optimised, is capable of detecting quite shallow flaws (less than 5% of the wall thickness). However, relatively short flaws (less than around 5mm long) are known to be difficult to detect ultrasonically.

Work reported here was aimed at improving detection of short deep flaws by means of axicon (conical) lenses to focus the sound beam. This type of lens, developed by TWI in the eighties, [l] has been used with considerable success in systems for inspection of pipeline girth welds. However, axicon lenses have yet to be fully utilised for in-line testing of seam welds. In this application the tight control of beam width offers the possibility of improved detection of short flaws.

Objectives

  • The principal objective is to demonstrate improved ultrasonic detection of short flaws in ERW tube welds by use of axicon focused probes.

    Specific objectives were as follows:

    - To design and manufacture axicon focusing elements (lenses) suitable for inspection of a specific range of tube sizes
    - To determine improvement in detection of short flaws obtained on this range of tube sizes with axicon focused probes for both surface breaking and buried (totally enclosed) flaws.

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