Detection of welding defects

Visual inspection.

Prior to any welding, the material should be visually inspected to see that

they are clean, aligned correctly; that machine setting and filler selection are

checked etc.

At the first stage of inspection of all completed welds, visual inspection

under good lighting should be carried out. A magnifying glass and a straight

edge may be used as a part of this process. Undercutting can be detected with

the naked eye and, provided there is access to the reverse side, excess penetration

can often be visually detected.

Liquid penetrant inspection.

Serious cases of surface cracking can be detected by the naked eye but for

most cases some type of aid is needed and the use of dye penetrant methods is

quite efficient when used by a trained operator. This procedure is as follows:

 clean the surface of the weld and the weld vicinity;

 spray the surface with a liquid dye that has good penetrating properties;

 carefully wipe all the dye off the surface;

 spray the surface with a white powder;

 any cracks will have trapped some dye which will weep out and colour the white coating and be clearly visible.

X-ray inspection.

Sub-surface cracks and inclusions can be detected by X-ray examination.

This is expensive, but for safety critical points e.g. in submarines and nuclear

power plants – 100 % X-ray examination of welded joints will normally be carried

out.

Ultrasonic inspection.

Surface and sub-surface defects can also be detected by ultrasonic inspection.

This involves directing a high frequency sound beam through the base material and

weld on a predictable path. When the beam strikes a discontinuity some of it is reflected

back. This reflected beam is received, amplified and processed and from the

time delay the location of the flaw is estimated. Porosity, however, in the form of

numerous gas bubbles causes a lot of low amplitude reflections which are difficult

to distinguish from the background noise. Results of any ultrasonic inspection require skilled interpretation.

Magnetic particle inspection.

This process can be used to detect surface and slightly sub-surface cracks in

ferro-magnetic materials. It can not therefore be used with austenitic stainless

steels. The process involves placing a probe on each side of the area to be inspected and passing a high current between them. This produces a magnetic flux at right angles to the flow of the current. When these lines of force meet a discontinuity, such as a longitudinal crack, they are diverted and leak through the surface, creating magnetic poles or points of attraction. A magnetic powder dusted onto the surface will cling to the leakage area more than elsewhere, indicating the location of any discontinuities. This process may be carried out wet or dry, the wet process is more sensitive as finer particles may be used which can detect very small defects. Fluorescent powders can also be used to enhance sensitivity when used in combination with ultraviolet illumination.

Repair.

Any detected cracks must be ground out and the area re-welded to give the required profile and then the joint must be inspected again.

Text 6

WELD TESTING

The following requirements determine the testing conditions for gravity

tanks, including independent tanks of 5 m3 or more in capacity, watertight or

weather-tight structures. The purpose of these tests is to check the tightness and

/ or the strength of structural elements. Tests are to be carried out in the presence

of the Surveyor at a stage sufficiently close to completion so that any subsequent

work would not impair the strength and tightness of the structure.

IR InI particular, tests are to be carried out after air vents and sounding

pipes are fitted. The Society may accept that structural testing of a sister ship is

limited to a single tank for each type of structural arrangement. However, if the

Surveyor detects anomalies, he may require the number of tests to be increased

or the same number of tests to be provided as for the first ship in a series.

Shop primer is a thin coating applied after surface preparation and prior to

fabrication as a protection against corrosion during fabrication. Protective coating

is a final coating protecting the structure from corrosion. Structural testing is

a hydrostatic test carried out to demonstrate the tightness of the tanks and the

structural adequacy of the design. Where practical limitations prevail, and hydrostatic testing is not feasible (for example, when it is difficult, in practice, to

apply the required head at the top of the tank), hydro-pneumatic testing may be

carried out instead. Hydro-pneumatic testing is a combination of hydrostatic and air testing, consisting in filling the tank to the top with water and applying an

additional air pressure. Leak testing is an air or other medium test carried out to

demonstrate the tightness of the structure. Hose testing is carried out to demonstrate the tightness of structural items not subjected to hydrostatic or leak testing and of other components which contribute to the watertight or weather-tight integrity of the hull.

A sister ship is a ship having the same main dimensions, general arrangement,

capacity plan and structural design as those of the first ship in a series.

Structural testing may be carried out before or after launching, after application

of the shop primer or of the protective coating, provided that one of the following

two conditions is satisfied: 1) all the welds are completed and carefully inspected

visually to the satisfaction of the Surveyor prior to the application of the

protective coating and 2) leak testing is carried out prior to the application of the

protective coating. In the absence of leak testing, protective coating is to be applied

after the structural testing of all erection welds, both manual and automatic,

in all manual fillet weld connections on tank boundaries and manual penetration

welds.

When hydro-pneumatic testing is performed, the conditions are to simulate,

as far as practicable, the actual loading of the tank. The value of the additional

air pressure is at the discretion of the Society, but it is to be at least as defined

for leak testing. The same safety precautions as for leak testing are to be

adopted.

Literature.

1. ASM International (2003). Trends in Welding Research. Materials Park, Ohio ASM International. - ISBN 0871707802

2. Assessment of Exposure to Fume from Welding and Allied Processes, HSE Books, 1990.

3. Blunt, Jane and Nigel C. Balchin (2002). Health and Safety in Welding and Allied Processes. Cambridge Woodhead. - ISBN 1855735385

4. Brightmore A. D., Bernasek M. Moving Weld Management from the Desk to the Desktopю Using "expert" software packages, computers can make life easier for the welding engineer. - http//www.cspec.com/csp-paper.htm1.

5. Canadian Welding Association. - http//www.cwa-acs.org

6. Cary, Howard B. and Scott C. Helzer (2005). Modern Welding

Technology. Upper Saddle River, New Jersey Pearson Education. – ISBN 0131130295

7. Kalpakjian, Serope and Steven R. Schmid (2001). Manufacturing Engineering and Technology. Prentice Hall.- ISBN 0201361310.

8. Klingensmith, S., J. N. DuPont and A. R. Marder, Welding Journal, 84 (2005) 77s-85s.

9. Hicks, John (1999). Welded Joint Design. New York Industrial Press. - ISBN 0831131306.

10. Lincoln Electric (1994). The Procedure Handbook of Arc Welding. Cleveland Lincoln Electric. - ISBN 9994925822.

11. Modern Welding by Althouse, Turnquist, and Bowditch. The Goodheart- Willcox Co. 1970

12. Robot welding. - http//www.robot-welding.com