In general, the number of flaws in a weld depends on the skill of the welder and the specific parameters of the welding process and procedure. There can be considerable variations in skill and hence the number of flaws between two welders with the same welder approval. Welds fabricated using modern automated welding techniques with controlled set-up would be expected to have a lower number of flaws than those fabricated using manual processes.
Welding parameters influencing the number of flaws include the process variables, type of materials and consumables, heat input, pre-heat, access, fit-up, position, restraint, location (shop or field), section thickness and run length. Welds have different susceptibilities to flaws of different types (slag, pores, hydrogen cracks, solidification cracking, liquation cracking, reheat cracking, lack of fusion, lack of penetration etc). Flaws may be associated with an individual bead or, in the case of a multi-pass weld, with the weld itself.
The number of flaws is a weld is often expressed as a frequency of flaws per unit length of bead laid or per unit length of the completed weld. The susceptibility of the weld to each flaw type is determined by the underlying mechanisms by which they can be formed and the associated metallurgical and process factors. A number of studies have been made to determine flaw frequencies from inspections and examinations of welded equipment.
Much attention has focused on thick-section submerged arc welds used for early reactor pressure vessels in the nuclear industry. These studies suggests that the frequency of large flaws (>0.5in) in pressure vessels manufactured in the early 1970s to Class 1 standards is on average about 0.4 flaws per vessel containing around 20 weldments [e.g. Marshall (1982), Smith and Warwick (1981)]. More recent Class 1 pressure vessels would be expected to have a lower frequency of flaws as improvements in welding technology, quality assurance and mechanistic understanding of the ways in which flaws have translated into practice.
From examining inspection reports, Thurlbeck et al (1996) and Baker and Kountouris (1989) have derived estimates for the frequency of fabrication flaws in submerged-arc welds used for offshore jacket structures manufactured since 1970. Wagner and Watchett have published data for girth weld defects requiring repair in mechanised GMA field welded pipelines. They found the repair rate due to flaws was around 1 repair per 10m of weld laid.
Welds are routinely inspected after fabrication and this can lead to the detection of significant flaws and their removal and repair. New flaws can initiate before service by mechanisms such as reheat cracking or arrested brittle fracture. Further changes to the flaw population can occur in service as new flaws are created by mechanisms such as fatigue, creep or stress corrosion cracking. The frequency of flaws in a weld may therefore change with time.
TWI core research report 749/2002 (only available to TWI Industrial Members) gives an extensive review of the factors influencing the number of flaws in a weld differentiating according to flaw type and size. Chapman  has developed an expert system based on judgement and calibration to estimate flaw frequency of specified welds. TWI is able to offer its Industrial Members expert advice on how to minimize the number of flaws in a weld.
- Baker M J and Kountouris I S, Defect assessment analysis of the dimensions of defects detected by ultrasonic inspection of an offshore structure. CESLIC Report OR8:1989.
- Chapman V, Simulation of defects in weld construction, ASME PVP Conf. 1992
- Marshall W, Report of the Study Group on LWR Pressure Vessel Integrity. HMSO Publications 1976, 1982.
- Smith T A, Warwick R G, A survey of defects in pressure vessels in the UK for the period 1962 to 1978 and its relevance to nuclear pressure circuits, UKAEA Report SRD R203, 1981
- Thurlbeck S D, Stacey A, Sharp J V, Nichols N W. Welding fabrication defects in two offshore jacket structures. OMAE Conf., Florence, 1996
- Wagner M J and Watchett B M, Girth weld defects in mechanised GMA field welded pipelines. Welding Journal, 75 - 82, June 1991.
- Wintle J B, Sanderson R M, and Hart P H M, A review of methods for determining nthe frequency and size distribution of welding flaws in steel fabrications, TWI core research report 749/2002, June 2002.
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