TABLE B shows the minimum mandatory scope of inspection required by 5.5.1. Part A stipulates that a 100% visual inspection is required, and Part B advises the material thickness, or fillet weld size, when MPI and ultrasonic examination [U/S] become mandatory and the frequency of testing.
It should be noted that if MPI and ultrasonic examination are specified they are not alternatives;
both must be carried out since MPI can only discover surface discontinuities, and ultrasonic examination is required to check for sub-surface discontinuities.
In Part B are the table, the criteria for mandatory testing by MPI and ultrasonic inspection methods is based upon the thickness of the thickest element in the connection or the size of the weld. It should be appreciated that the failure of some welds could be catastrophic, whereas others, generally the smaller welds, are less critical. It is matters of this nature which have influenced the decisions at which testing becomes mandatory as well as what is practicable.
It is incumbent upon the Engineer to recognise if a particular connection may require NDT of welds to steels which are thinner than those specified in TABLE B; and reference to Annex A of BS 5950-2 may be used as a guide. Examples may exist of testing being needed beyond the limits given by the NSSS and the BS, for example, a cruciform weld situation where there is an in-line tensile force to be carried through the parting plate. In this case testing may be necessary for steels as thin as l0mm, with ultrasonic testing being applied to the plate as well as the weld.
Conversely, as noted in 5.5.1, there may be circumstances where an Engineer may agree to reduce the inspection requirements provided the Steelwork Contractor can demonstrate that he can maintain a satisfactory level of process quality against the requirements of BS EN 729:
Quality requirements for welding. Fusion welding of metallic materials. Parts 2, 3 and 4 of BS
EN 729 offer three levels of quality requirements – Comprehensive, Standard and Elementary – and Part 1 provides guidelines for selection and use. Obviously the level offered needs to be matched to the work being undertaken, and Annex E of DD ENV 1090-1 provides guidance on matching the level of welding coordination to the type of work.
The frequency of MPI and U/S testing in the fabrication shop is 50% for full penetration butt welds, 20% for partial penetration butt welds and 10% for fillet welds. These percentages should be taken from each batch of welds of similar type to avoid bias in the sampling. If the welds executed are repetitive, Note (iv) to TABLE B explains how the systematic procedure may be modified to reflect this by varying the sampling process towards regular weekly checking against usage of each particular welding procedure. The reference in Note (iv) to weekly checking is deliberate as it is not considered that routine sampling on a less frequent basis would generally be suitable.
The simplicity of this scheme is intended to facilitate the regular checking of welds during production irrespective of which particular projects are being fabricated at the time. The assumption is that the Steelwork Contractor is using the results of the tests as part of a quality system to monitor process quality on a continual basis. If, however, problems are identified, 5.5.1 places an obligation on him to increase the frequency of testing and extend the scope until the cause of the problems is identified and corrected.
The quality system specified in Section 11 requires that records be kept of tests and inspections;
5.5.2 emphasises that records are to be made of weld testing. The clauses on NDT have been drafted as stand-alone requirements which, together with the Fabrication Drawings, enable the Steelwork Contractor to inspect and correct the work in-house without referral to the Engineer for decisions. If nonconformities are found, generally the Steelwork Contractor is allowed to repair provided that all records of repairs are available for inspection by the Engineer. However, for serious defects which are difficult to repair these need to be referred to the Engineer for approval if the Steelwork Contractor does not wish to scrap and/or remake the nonconforming component.
The type of discontinuities that can occur in welds and their most likely causes should be understood. The inspection and measurement of defects can then be made with better interpretation and assessment of how important a defect is. Common fabrication discontinuities and their- causes are shown in Figure 5.1 which is reproduced from M. H. Ogle's paper Weld Quality Levels (Ref. 10).
Picture of In-line butt weld Picture of Cruciform (butt and fillet)
Discontinuity Type Most Common Location Most Likely Cause
Hydrogen cracks
Heat affected zone [HAZ] in thick joints (can be in weld metal)
Damp or contaminated consumables or joint faces, too low heat input or preheat, high restraint. High carbon or manganese in steel.
Solidification cracks Weld metal (on centreline)
Deep narrow welds, high sulphur and phosphorus in weldment, high current. High restraint, mainly in submerged arc.
Lamellar tearing
HAZ in Tee and Cruciform joints
Non-metallic inclusions in steel, large welds, high restraint.
Lack of fusion Side walls and root Incorrect welding parameters, too low arc energy, travel speed too fast, incorrect electrode position.
Lack of penetration Root Incorrect fit-up, incorrect welding parameters (as per lack of fusion), inadequate back gouging.
Slag inclusions Weld metal
Incorrect electrode manipulation, poor bead shape and inadequate slag removal, damaged electrode coatings.
Porosity Weld metal
Gas originating from dampness, grease, rust, air entrainment in gas shield, insufficient deoxidant in consumable or steel.
Excess weld metal (overfill) Weld bead cap
Excessive arc energy, insufficient preparation, incorrect electrode size, faulty electrode manipulation.
Undercut Weld toe Excessive current, too large a weld bead.
Overlap Weld toe Arc energy too low, travel speed too low, incorrect
position, incorrect electrode manipulation.
Misalignment Incorrect jigging, inadequate tacking.
Excess root gap Incorrect jigging, inadequate tacking.
Figure 5.1 — Common fabrication discontinuities and their causes
Visual inspection of welds, at a level of 100%, is required by 5.5.3 before other flaw detection methods are used. This inspection is to ensure that the right type of weld is made in the right location and no weld is missing. The dimensions of throat and leg lengths of fillet welds, overfill and undercut, concavity and misalignment can be measured with the aid of a hand-held gauge.
TABLES C.1 and C.2 make clear the remedial measures to employ if any discontinuities are present.
When welds will become inaccessible by later workmanship, it is necessary to perform such inspections and take corrective measures before they become inaccessible. Later work that may make welds inaccessible for testing would include attachment of subsequent components and surface treatments such as paint and galvanizing. Other than observance of hold times (see Commentary on 5.5.4), it is generally wise to undertake testing as soon as possible after welding.
BS EN 970 provides useful guidance on visual inspection that is appropriate for the work being inspected and the type of weld.
Weld defects arising from hydrogen cracking may take hours to develop after welding has been executed, albeit this period would be reduced if the weldment were held at an elevated temperature. Hence, if hydrogen cracking is likely, 5.5.4 requires that a period be allowed between welding and testing. TABLE A gives guidance on the appropriate hold time in relation to material thickness and carbon equivalent value [CEV]. Commonly a ‘next day’ or ‘day after that’ approach to testing is adopted as necessary. If cracks appear due to an error in procedure, e.g. moisture contamination of electrodes or failure to provide proper preheat, it is likely to affect many of the welds in the batch. The prime purpose of the inspection is to spot the basic problem.
When the weld procedure requires the weld to be tested at an intermediate stage of the fabrication process, the work piece will have cooled and will have to be reheated to the temperature recommended in BS EN 1011-2 before welding continues.
The NSSS gives preference in 5.5.5 to surface flaw detection by magnetic particle inspection [MPI]. MPI works on the principle that a magnetic field in a piece of steel will leak out to the surface if a crack is present. The particles of iron powder are attracted to the crack location, producing a visual indication of a crack which might otherwise be invisible to the naked eye.
For carbon steels, magnetic particle testing for surface cracks is preferable to dye-penetration testing methods. However, the NSSS allows the use of dye penetrants if MPI is not available.
BS EN 1290: Non-destructive examination of welds. Magnetic particle examination of welds gives advice on the materials to be used, safety precautions, testing procedures, magnetization, surface preparation, application of detecting material, viewing, recording, reporting and demagnetization.
A welder who has been suitably trained, and has been tested by a nationally recognised authority, can gain competence in surface flaw detection. It is acceptable for a welder who is so qualified to examine his own work.
I n 5.5.6, ul trasoni c exami nat ion is specified for subsurface examination of welds and no mention is made of radiography. Ultrasonic examination is favoured for all except thin material (up to 6-8mm thick) and small fillet welds. Radiography is slow and disruptive to other operations in the vicinity and carries severe health and safety hazards.
Ultrasonic examination is to be made in accordance with BS EN 1714 covering manual methods of weld examinations of fusion welded butt joints (BS 3923-2 covers automatic examination).
Various levels of examination of in-line butt welds are covered; the NSSS sets out the levels for reference and evaluation.
All ultrasonic examination requires considerable skill and training. It must be carried out by operators who hold a current certificate of competence in making an examination and interpreting the results.
ANNEX C — WELDS - ACCEPTANCE REQUIREMENTS & MEASUREMENT DEFINITIONS
Table C.1 shows the acceptance criteria and corrective actions required by 5.5.7. Since the table is based upon 'fitness for purpose' criteria, a lesser standard is not permitted. The welding standards specified are achievable by competent Steelwork Contractors.
The clause indicates once again that the acceptance criteria are for structures subject to static loading only. In this context wind loading is considered as a static load unless the wind can set up a frequency of oscillation similar to the natural frequency of the component. In general this is only likely in certain tubular structures, towers and chimneys (See BS 4076: Specification for steel chimneys and BS 8100: Lattice towers and masts).
If the designer specifies that fatigue loading is to be applied to parts of a static structure, such as an overhead crane gantry, or a building housing reciprocating machinery, it is recommended that the advice given in A.5 of BS 5950-2 be followed that weld acceptance criteria should be in accordance with ISO 10721-2. This is because the development of the acceptance criteria for static welds given in the ISO was based on the earlier work undertaken by TWI for BCSA to develop the NSSS.
The acceptance criteria are not difficult for a competent welder to achieve, but TABLES C.1 and C.2 need careful study for them to be fully understood. The following notes are made to assist in the interpretation of the tables.