Welded joints in a structure (e.g., a bridge) are expected to possess certain service-related capabilities. Welded joints are generally required to carry loadings of various types in which the weld is subject to stress of either a simple or complex character.
A finished weld is not always as good or as bad as it may appear to be on its surface. Therefore, it is crucial to find out how sound the weld is. For this purpose, certain weld inspection and testing procedures have been discovered and standardized to estimate the expected performance of the welded structures.
Weld Inspection and Weld Testing are two different terms with their functions somewhat overlapping.
Weld Inspection is the observation of the processes and products of manufacture (e.g. weldments) to ensure the presence of desired qualities or properties.
In certain cases, inspection may be entirely qualitative and involve only visual observation of correctness of operation or dimensions, examination of surface defects of welded joints, etc., whereas in other instances, inspection may involve the performance of complicated tests to determine whether specifications required meet or not.
Weld Testing, specifically refers to the physical performance of operations (tests) to determine quantitative measure of certain properties such as mechanical which will be explained later.
Testing aims to determine quality, i.e., to discover facts regardless of the implications of the results, whereas Inspection intends to control quality through the application of established criteria and involves the idea of rejection of substandard products.
Stages of Weld Inspection and Testing
For fabricating a welded structure, inspection may be carried out in three stages:
- Inspection before welding.
- Inspection during welding.
- Inspection after welding.
1. Inspection before Welding
Inspection before welding will assess the following details to ensure the correct conditions are in place.
- Check the drawings in respect of weld details, dimensional tolerances, process specifications, etc.
- Examine the specifications given by the customer applicable to the class of work, quality required, end use of the product, etc.
- A suitable welding process is selected to obtain welds of the desired quality.
- Only the tested and defect free materials and as per the approved specifications or drawing should be used for the structure to be welded.
- Consumables such as welding electrodes, flux, gases, etc., to be used shall be as per standards. Deteriorated or spoilt consumables should not be used.
- Welding Procedure should be laid down. Preferably, it should conform to some standard, (e.g. ISI). The procedure should give preparations and tolerances to be achieved during production work. The procedure should ensure required quality of welds.
- Welding equipment should be in satisfactory working condition and be able to produce right quality welds.
- Welders and operators employed for the work shall be trained, tested and certified to the appropriate standards.
- Weld Testing Equipment should be in good operating condition and be handled by the well trained staff.
- Equipment for cutting, straightening, heat treatment, material handling, etc., should be in good working condition.
- Jigs, fixtures, manipulators etc., should be available depending upon the type of work. Adequate storage facilities should exist for storage of consumable and materials.
2. Inspection during Welding
Inspection that takes place while the welding process is underway will assess the following details.
- The weld groove should be free from dirt, rust, oil, slag or any other foreign matter which may affect the quality of the weld.
- Job edge preparation should be as per approved welding procedure
- The fitup, gap, orientation, welding position, method and sequence of assembly all should be as per approved welding procedure.
- Tack welds should be of adequate size, length and pitch.
- Fittings, clamps, fixtures, etc., should not interfere with welding.
- Methods should be adopted to minimize distortion.
- If during welding certain, consumables are found defective, they should be replaced by another brand of identical type.
- Welding procedure may be modified if it is found during welding that welds of acceptable quality are not being produced.
- Slag should be thoroughly removed from each pass in multipass arc welds and spot welding electrodes should be dressed periodically.
- Welds which would become inaccessible or more difficult to inspect at a later stage shall be inspected at this stage.
- Inspection if carried out during welding will help identify potential sources of defects and thus eliminate them at the early stage.
- Inspection during welding will help finding any deviation from approved procedures, consumables, etc. which if goes unnoticed may deteriorate weld quality.
- Visual inspection during welding will reduce the chances of rejecting the weldment at the final stage.
3. Inspection after Welding
Testing and Inspection is carried out after the jobs have been welded, with a view to:
- Assess the properties and quality of the welded joints.
- Asses the suitability of the weldment for the intended purpose.
All forms of testing and inspection of welds after fabrication can be grouped into two basic categories, namely:
- Destructive Testing.
- Non destructive Testing.