Many different types of equipment are used for the heat treatment of fasteners, depending on the production volume, the alloys being processed and the required material properties. Batch systems are suitable for smaller production rates, and continuous systems are required for high-volume production. Among the different technologies are applied energy (flame, induction and laser), batch processing (box furnaces, integral-quench systems, pit furnaces, tip-up furnaces, car-bottom furnaces and gantry furnaces), continuous processing (cast link conveyors, mesh-belt conveyors, humpback, pusher, rotary drum, rotary hearth, shaker hearth, walking beam, vibratory tube), both batch and continuous atmosphere processing (using a protective, inert or reactive gas), and vacuum processing (Fig. 1).
Mesh-Belt Conveyor Furnaces
Mesh-belt conveyor furnaces are the most common technology for heat treatment of fasteners. They utilize a woven conveyor belt to carry the parts, with the weave dependent on the size and weight of the fastener being processed. Designed for high-production environments, they are typically part of an automated heat-treating system that includes loaders, pre- and post-washers, a hardening furnace with quench tank and a tempering furnace (Fig. 2). Accessories such as soluble oil tanks, endothermic-atmosphere generators and nitrogen/methanol systems are required.
Mesh-belt heat-treating systems for fasteners must process small and large production runs and, therefore, need to be flexible. Production lots can be as small as 5-10 kg (11-22 pounds) and as large as 5,000 kg (11,000 pounds) or more. Standard production capacities in mesh-belt furnaces typically range from 100-3,000 kg per hour (250-7,000 pounds per hour). The fasteners are loaded 12.7-63.5 mm (0.5-2.5 inches) deep on the conveyor belt at 73-146 kg per square meter (15-30 pounds per foot).
Hardening and case hardening (carbonitriding and carburizing) are typical heat-treat processes for fasteners. Case depths are shallow, on the order of 0.0038-0.038 mm (0.0015-0.015 inches). Different quench media are used, depending on the desired material properties. Brine, water, polymer, oil and molten salt are commonly utilized.