Debbie Mellor, MD of metallurgical and laboratory service specialist Keighley Laboratories, is projecting 25% growth for the firm's Heat Treatment business this year. Much of this increased customer demand is being driven by its gas-fired pit furnace facility, one of only a handful in the UK, which accommodates metal components vertically and enables processes like carburising, case hardening, carbonitriding and tempering to be carried out with minimal distortion, in accordance with precise customer specifications.
The company has also commissioned a new jig to enable an even wider range of components to be processed, including large rectangular blocks.
Employing around half of Keighley Laboratories' sixty-plus staff, the heat treatment division offers a comprehensive range of surface engineering services to UK engineers, which also includes induction hardening, hardening and tempering, stress relieving, ferritic nitrocarburising or Tufftriding, austempering and martempering, and normalising, with nitriding to be added once the scheduled new building programme is complete. Working to BS EN ISO 9001, BS EN 9100 and aerospace standard AS9100 rev B, it serves a broadening customer base that take in the construction and mining, engineering, petrochemical, oil and gas, aviation, transport, defence, nuclear and energy markets, giving clients access to in-depth metallurgical experience and expertise.
"Our expertise really focuses on our highly accurate furnace controls, which we program in-house, and the immense knowledge and skills of our qualified staff, including our furnace operators who undergo rigorous training and continuous skills development," says Heat Treatment Commercial Manager, Michael Emmott, who has had hands-on experience of all processes and provides technical support in the field. "We work in close partnership with our customers, helping them to develop new components as they introduce new products or enter new markets, suggesting the most cost-effective heat treatments and even developing non-standard processes for complex products."
The precise control of all heat treatment processes is undertaken by dual-loop programmable logic controllers (PLCs) and measured by thermocouples and oxygen probes, which enable temperature and atmosphere cycle times to be monitored accurately. Endothermic gas, ideal for the heat treatment of metals, is generated on site and fed to the furnaces to reduce the scaling and decarburisation of steel at high temperatures and this can be enriched with natural gas to increase the carburising potential. The exhaust gas from furnaces is measured by an infra-red analyser, which applies a corrective factor as necessary to the PLC, for extremely accurate carbon control.
A variety of quenching media is employed by Keighley Laboratories, including fast and medium speed mineral oil and molten salt for processes like martempering and austempering, as well as air and fan cooling. There is also press quenching for cooling thin or complex parts such as gears, bearing races and cams, which would distort excessively if directly quenched in oil.
Thermochemical methods where a chemical element is combined with thermal energy, like carburising, carbonitriding and case hardening generally, produce a hard, wear-resistant surface on quenching, with a softer, tougher core that retains the material's inherent properties. Keighley Labs' pit furnace facilities provide for components up to 1.72m long x 0.965m diameter, in single pieces or batches up to 2 tonnes. Depending on size, parts are usually quenched in oil, or molten salt to minimise distortion, with case depths between 0.15mm and 4mm plus. Sealed quench furnace processing is also available for the heat treatment of smaller components in a protective environment, in sizes to 485mm x 355mm and maximum weights of 250kg.
Induction hardening involves the more rapid heating and quenching of components using high frequency electric fields and can be used for local surface hardening, without heating the whole product. This method is typically used for hardening shafts and pins up to 1.5m long, treating internal bores and the spin and contour hardening of individual gear teeth or complete gear wheels up to 4m in diameter, all options offering economical, low distortion processing.
Tufftride treatment, or salt-based ferritic nitrocarburising, is a comparatively low temperature, low distortion salt bath process, which can be applied to most ferrous materials and improves component quality by increasing fatigue strength, inhibiting wear, resisting corrosion, maximising hardness and generally enhancing appearance. Anticorrosive properties can be further enhanced by subsequent oxidation treatment, promoting a black finish, while a quench, polish, quench (QPQ) process gives a lustrous black finish with outstanding corrosion resistance. Process times are fairly rapid, typically in the region of 90 minutes.
Austempering is a comparatively unusual hardening process for ferrous alloys, steel and iron castings, including ADI (austempered ductile iron), in which the material is quenched from hardening temperature in a molten salt bath, producing greater durability, increased wear resistance and higher impact and fatigue strength. Available for maximum charges of 400kg in component sizes to 715mm diameter x 1320mm deep, this process offers low distortion and predictable stability, allowing parts to be cast close to finished shapes and minimising subsequent machining.
Martempering is another isothermal process, involving a molten salt bath, and effectively carries out hardening and tempering operations concurrently, keeping distortion to a minimum. Conventional hardening and tempering, to produce the optimum combination of hardness, strength and toughness in most commercially-available steels, is carried out in Keighley Labs' pit and sealed quench furnaces, for components up to 1.72m long, with a choice of quench media according to customer specifications. Other heat treatment processes include annealing for reducing the hardness of materials for subsequent machining, normalising to refine existing grain size and create a more homogenous structure for further processing, and stress relieving to remove residual stresses generated by previous manufacturing processes.
A fully independent service, Keighley Laboratories provides certification of all heat treatment processes, with the option of metallurgical inspection by its UKAS accredited, in-house testing facility; metallographic test pieces and records are archived for a minimum of six years, ensuring full traceability for customers. Its heat treatment team, supported by Technical Director, Keith Blower, can carry out failure mode and effects analysis (FMEA) studies for identifying and eliminating potential component reliability problems and is able to assist with the formulation of specifications for materials, heat treatment processing and testing.