PROCESSES

Carburising - case hardening

A case hardening process that diffuses carbon into the surface of a solid ferrous alloy by holding at temperatures of 865-925⁰ C. The parts are then quenched to transform and harden the structure. The extra carbon enables a hard wear resistant outer layer to be formed while retaining a tougher core. Selected areas of the parts can be masked to prevent carburising for machining or design reasons (threads or thin sections).

Typical uses: Highly loaded gears, shafts, wear bushes
Steel selection: Case hardening steels - En36A/17CrNiMo6/8620

Carbonitriding

A case hardening process that diffuses carbon and nitrogen into the surface of a solid ferrous alloy by holding at temperatures of 820-880⁰ C. The parts are then quenched to transform and harden the structure. As in carburising the extra carbon produces a hard wear resistant outer layer to be formed but the addition of nitrogen increases surface hardness and allows lower alloyed steels to be processed.

Typical uses: Wear bushes, levers, hydraulic valves
Steel selection: S1214, S12L14, Mild steel, 1020, 1045

Tooling vacuum heat treatment

Vacuum heat treatment is a method of heat treating steel in a furnace without exposure to air. The furnace is manufactured to tight tolerances allowing the chamber to be evacuated of air. Tooling and General Heat Treatment have two vacuum furnaces with quench pressure up to 6 Bar allowing the treatment of lower alloyed tool steels and alloy steels. This prevents oxidation or decarburisation of work. Parts have minimal or no machining/grinding allowance. Allows tighter control over distortion through controlled gas quenching at up to 6 Bar.

Typical uses: Plastic moulding dies, press tools, die casting
Steel selection: Tool steels S, D, A, H, series, stainless steel 420, 431 440C and some alloy steels eg 4340

Nitriding

A low temperature surface hardening process that introduces nitrogen to the surface of a solid ferrous alloy by holding at temperatures of 480-560⁰ C. The case depth produced is typically from 0.1 mm to 0.6 mm. Distortion is kept to a minimum as quenching is not required to produce a hard, wear resistant layer (some growth can occur). The process also increases corrosion resistance on certain steels.

Steels should be through hardened and tempered to obtain the maximum surface hardness from nitriding. The nitriding process used by Tooling and General Heat Treatment is computer controlled to ensure maximum hardness and minimum distortion. Selected areas can be masked to prevent nitriding for machining or design reasons eg. threads or thin sections.

Typical uses: Gears, wear bushes, tools and dies, hydraulic valves
Steel selection: Chromium steels 4140, En40B; Tool and die steels; BisAlloy (selected applications)

Annealing/stress relieving

Annealing is a heat treatment process used to reduce the hardness and improve the machinability of materials. The procedure is to heat the components to the annealing temperature and then slow cool at a specified rate to produce the required structure and hardness. Annealing procedures will vary depending on the prior processing and the material grade.

Stress relieving is used on materials to relieve internal stresses caused by machining, welding, pressing or forming. Stress relieving can be carried out on ferrous and non-ferrous materials. Stress relieving is typically carried out at lower temperatures so that the structure and properties do not undergo large changes.

Shot peening

A cold working impact process that uses a high velocity blast stream of millions of hardened steel spheres. Each shot acts as a mini ball peen hammer striking the surface and forming a tiny dimple. The material below the surface tries to counteract the strain resulting in a layer of cold worked metal stressed in compression. The compressively stressed zone resists crack initiation and propagation, increasing the service life of the part.

For further information view pdf.

Shot peening will:
- Increase fatigue life of parts
- Prevent incidence/propagation of stress corrosion cracks.

Typical uses: Gears, shafts, corrosive environment parts (eg pumps), springs, torsion bars, pins, suspension parts

Useful links
www.everyspec.com
www.metalimprovement.com/shot_peening.php

Process control

The control of heat treatment processes is absolutely critical as parts are often finish machined. Our furnace equipment is regulated by temperature controllers linked to a master computer system. This provides 24 hour monitoring, control and traceability of all work.

Data logging of furnace parameters is recorded and can be supplied with completed work.

Controller calibration is carried out by a NATA certified company.

Through hardening

The through hardening process (quench and tempering) is used to harden steel parts by heating the whole part up to 820-900⁰ C and quenching to transform the structure. The quenched part is then tempered to increase toughness and ductility. The carbon level in the furnace is controlled to be equal to the carbon level in the part.

Typical uses: Lifting tools, axles, fasteners, shafts, bushes, pins, wear plates
Steel selection: 4140, 4340, EN25, EN26, EN40B, K245 (S101A), Gage plate (O1), 300M