In addition to the mesh-belt and vacuum heat-treatment equipment outlined previously, there are other technologies used for processing fasteners. These are dictated by floor-space limitations, frequency of product changeovers, necessity to strictly segregate loads (where cross-contamination of parts is absolutely prohibited) and other factors.
Alternative Heat-Treatment Technologies to Mesh-Belt Conveyor Furnaces
Rotary-retort furnaces (Fig. 1) and cast-link conveyor furnaces are the most common alternatives to mesh-belt conveyors. The systems are typically automated and operate without operator intervention after parts are loaded into the hopper at the front end. Vibratory hoppers and weight-actuated skip loaders deposit precisely measured charges into the furnace to ensure the consistent loading of parts.
Typical system capacities for rotary-retort furnaces vary from around 225 kg/hour (500 pounds/hour) to 450 kg/hour (1,000 pounds/hour), but they can be manufactured to handle 1,800 kg/hour (4,000 pounds/hour) or more. The retorts are either cast or fabricated from high-temperature alloys. In general, cast retorts have superior mechanical-strength characteristics compared to wrought fabricated designs but often come at a cost premium. Auger flights with the retort convey the fasteners through the furnace. Variable-speed rotation of the retort provides flexibility of time-based processing cycles.
The actual quenching process of the part is typically completed within two to five seconds from the time fasteners drop into the quenchant, with 10 minutes being a typical residence time in the quench for continued cooling. The exception is austempering processes, which require extended times on the order of 20 minutes or more to produce a bainitic structure. It is critical that an adequate amount of fluid flow is delivered to the active quench area. Overall quench-tank capacity is not enough to ensure individual quenching of each part. The quench-tank design must eliminate any clumps and clogs to ensure that both proper technique and adequate quenching are obtained.
Tempering furnaces, whether mesh-belt or rotary-retort type, use a recirculation system designed to keep heated gases in contact with the fasteners as they progress through the oven. Convection heating is used to full advantage for efficient heat transfer in a minimum floor space. Operating temperature ranges vary from 150-650°C (300-1200°F).
Induction heating is also used to selectively heat treat fasteners at high production rates. For example, high-frequency (10-50 kHz) systems can draw back bolt heads after carburizing to improve toughness at rates in the order of 1.0-2.5 pieces per second. Similarly, seat and seat-belt retention bolts can be case hardened (200 kHz) to depths up to 0.508 mm (0.020 inch) and surface hardness of 40-45 HRC to impart both strength and toughness. Induction technology (Fig. 2) is considerably faster than convection-style furnaces, where the material heat-up rate is limited by the rate of heat conduction through the material. Induction equipment must necessarily be electrically powered and can require a considerable current draw, so energy costs and electrical infrastructure must be taken into account when considering such a system.
Fastener heat treatment is a robust, steadily growing sub-segment of the heat-treatment industry, with demand for high-volume production, reduced cost and increased quality driving the industry and its future growth.