Developments in renewable energy, mining and quarrying, oil exploration, marine and other applications have led to significant increases in the size of power transmission components like gears, shafts, pinions and bearings, to handle greater productivity demands. As these large components are exposed to severe duty cycles, there is renewed emphasis on the heat treating methods needed to enhance their wear resistance and strength properties.
Estimated by industry sources to represent around 10 per cent of component manufacturing costs, heat treatment modifies the microstructure of metallic materials, influencing mechanical characteristics like strength, ductility, toughness, hardness and wear resistance. Thus, it increases the service life and technical performance of metallic parts, which is even more critical for the new generation of larger components that are crucial to mechanical function and usually difficult to replace.
Anticipating growing customer demand for processing larger gears, shafts and other components, sub-contract heat treatment specialist Keighley Laboratories of West Yorkshire has enlarged the size of one of its largest pit furnaces, increasing dimensions to 1110mm working diameter and almost 1800mm maximum length, in the process gaining 30 per cent in overall capacity. At the same time, it has upgraded the lifting capacity of its overhead crane, which services all seven pit furnaces, a salt bath and tempering equipment, to 3.0 tonnes maximum lift.
Enlarging the No 1 pit furnace involved fabricating a custom-made inner shell or retort, made from exotic duplex stainless steel, but this considerable investment has already been offset by orders for heat treating 1108mm diameter gear wheels for an offshore application and 1755mm long, 1 tonne shafts for a renewable energy project. Indeed, the company is already considering upgrading other in-house pit furnaces and its programmable process controllers.
“This has opened up a new market for us amongst engineering companies and OEMs, who are looking to heat treat larger components. This involves physical issues regarding accommodating the size of these parts, then lifting them out of the furnace and into the quench tank, as well as demanding special skills for treating very large workpieces without damage or distortion,” says Divisional Commercial Director, Michael Emmott.
Keighley Labs employs its pit furnaces for carburising, carbonitriding, hardening and tempering, stress relieving, homogenising and carbon restoration, working at temperatures up to 980°C. It handles steel, iron, high chrome iron, cast iron, ADI (austempered ductile iron) and alloy steel materials, for aviation, marine, defence, rail, mobile plant, energy and general engineering companies.
Pit furnaces are vertically-oriented batch furnaces, with the furnace section buried in a pit and extending up to floor level and a hydraulically-operated sealed cover extending above the surface. Workpieces are held in a jig or charge basket in the furnace, the inner retort protecting them from the direct radiation of heat; this configuration is particularly suitable for treating long parts such as shafts, tubes and rods, although a wide variety of shapes and sizes can be accommodated, singly or in batches.
Apart from offering significant savings in floorspace, with one sealed quench occupying the same floor area as four pit furnaces, this furnace type offers very high repeatability, extremely precise thermochemical treatment, economic effectiveness, proven flexibility and the capacity to treat both long components and large rounded parts like gear wheels. Pit furnaces are also renowned for producing minimal distortion and this is particularly critical for larger components, where the same percentage of deformation equates to a much greater absolute distortion, possibly leading to the scrapping of parts.
Defined as irreversible and usually unpredictable dimensional changes during the process, distortion occurs when heating the workpiece above the transformation temperature results in it losing the inherent strength it possesses at room temperature, often causing it to distort under its own weight. Thus, long parts are preferably heated in a vertical pit furnace, to ensure that they are properly suspended during heating, while other parts must be carefully supported to suit their geometry, employing well-designed jigs and fixtures.
Keighley Labs has plenty of expertise in this area and its skilled furnace operators are responsible for jigging up the workpieces, uploading the computerised process routines into the furnace plc, loading and unloading the workpieces, then transferring to the salt quench, before washing off the components and carrying out any subsequent tempering processes. Depending upon the required case depth, component geometry and specific customer requirements, heat treat cycles can take anything from 2 or 3 hours to 2 or 3 days.
“With our new heat treatment department, modern furnaces and advanced low temperature, low distortion, thermochemical processes coming on stream later this year, we wanted to renew the focus on our pit furnace capability, where we rank amongst the leaders in the contract heat treat industry. When it comes to minimising distortion in heat treated components, pit furnaces still have a lot to offer and with gears, shafts and other parts increasing in size, the distortion challenge is becoming even greater. It’s an area where we can usefully contribute, drawing upon our many years’ experience,” says Michael Emmott.
“These days, many customers have highly technical resources and can specify unambiguously the heat treatment processes they require,” he adds. “We are continually investing in our heat treatment resources to keep abreast of their exacting requirements and offer alternative solutions across the whole spectrum of surface engineering. Where customers do seek our expertise, we are confident that we can provide in-depth practical and technical knowledge that is second to none,” he concludes.