Friday, 28 September 2012

Test & inspection service ensures welds are fit for purpose

Usually regarded as the most economical way of joining two or more metal components, in terms of fabrication costs and materials usage, welding technology is central to many engineering and manufacturing processes, from producing wings and fuselages in the aerospace industry and platforms and pipelines in the energy and petrochemicals sector, to automotive and rail components, white goods and metal furniture. Since these welded joints are subject to various loads and fatigue during their service life, possibly giving rise to safety and quality issues, it is vital that rigorous testing and inspection procedures are applied, to examine the structure of completed welds and their conformation to specification, as well as determining the skill levels of the welding operators.

Keighley Laboratories offers a comprehensive weld testing and inspection service, including welding procedure consultancy and approval, welder qualification tests, on-site weld investigation and, through its newly-upgraded Test House, a complete range of destructive, non-destructive and metallography testing facilities.

Under the direction of Divisional Technical Services Director, Matthew Mellor, the weld test and inspection team’s resources are broadly divided between the specialist aerospace field, led by Peter Hanson, and general commercial welding, headed up by Jeremy Duignan, both of whom are fully qualified metallurgists. Unusually, Matthew, Peter and Technical Director, Keith Blower, are also approved by the Civil Aviation Authority as Weld Specimen Supervisors, able to witness and verify critical aircraft-related welding on the authority’s behalf and invigilate at customer sites.

Keighley Labs is UKAS accredited for weldment testing and certification across a growing list of professional specifications, including relevant commercial BS EN ISO and ASTM standards and aerospace primes like BAE, Rolls Royce, Westland Helicopters, Airbus, Bombardier and the CAA, with the final assessment process now underway for NADCAP approval.

The team is familiar with testing weld coupons only millimetres thick in titanium, aluminium, nickel and cobalt alloys and other exotic metals for aerospace fabricators, as well as carbon steel and stainless steel test plates several inches thick for commercial welders and manufacturers. It covers all forms of welded joint, including butt or groove welds, fillets, lap, edge and tee joints, corners and cruciform, both plate-to-plate and tube-to-tube in similar or dissimilar materials, which are produced using all manual and automated welding methods, from stick and oxy-acetylene techniques, to TIG, MIG, MAG and plasma arc, even brazing and soldering.

Keighley Labs employs a complete array of test and inspection procedures to qualify welding procedures, welding operators and welding processes, by examining completed welds to establish their quality, integrity and compliance with specifications. Non-destructive testing techniques extend from visual inspection to check for surface defects and weld sizes, through to magnetic particle, liquid penetrant and ultrasonic examinations for detecting surface and near-surface discontinuities invisible to the naked eye, locating weld leaks and any internal flaws, in addition to meeting specific codes and specifications. Radiographic inspection for determining the internal soundness of welds is also provided on a sub-contract basis.

Weld coupons are also examined for soundness, strength and toughness using a range of mechanical testing equipment in Keighley Labs’ on-site Test House, which includes a dedicated CNC machine for producing weld test plates from customer-submitted samples. Options here include Brinell, Rockwell, Vickers and Knoop hardness testing, for providing information about metallurgical changes caused by welding, weld bending for evaluating the ductility and soundness of welded joints, Charpy and Izod impact testing for measuring resistance to mechanical shock, and digital equipment for fatigue life testing.

In its newly-enlarged Optical Suite, Keighley Labs offers metallographic inspection of sections of welded joints, at macro levels up to 50x magnification and micro examinations up to 1000x, employing powerful stereo microscopes. Metallurgy is used to check the extent of the heat affected zone and any weld defects such as cracks, pores, swivels and lack of fusion. Meticulous surface preparation of samples is essential for revealing details of the microstructure and the suite boasts semi-automated precision cutting, grinding and polishing equipment.

With customers as far afield as South Africa and the Mediterranean, as well as closer to hand amongst the North West Aerospace Alliance, of which it is a member, Keighley Labs enjoys strong demand for its weld test and inspection services, with a track record of repeat business.

“We have a good reputation within the aerospace industry and amongst commercial fabricators and, given the increasing need to comply with welding procedures and employ skilled welding operators, it is an expert service that many customers and their clients value,” says Keighley Labs’ Peter Hanson. “Being independent, we have no vested interest in any particular welded joint, we’re just concerned with whether it passes or fails. If it fails, we can identify precisely why it did so and offer remedial advice for meeting the required specification.”

“A good looking joint was often considered a reliable indicator of a high quality weld, but surface appearance alone does not guarantee good workmanship, internal integrity and compliance with welding procedures,” he adds. “Nowadays, you need access to non-destructive mechanical and metallurgical examination techniques to judge whether a weld is fit for purpose and achieves the necessary acceptance criteria. And that’s what we do.”

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