Gas Carburising / Case Hardening / Carbonitriding
Gas Carburising (also known as case hardening)
- A carbon diffusion controlled process used to case harden steel, primarily lower carbon steel.
- A gaseous process carried out in pit or sealed quench furnaces.
- Enables a hard wear resistant case to be produced with a softer, tougher core.
- Carburising is carried out in the temperature range 900°C – 970°C to achieve case depths from 0.25mm to 5.0mm, dependent on material selection and metallurgical requirements, followed by quench (air, oil or salts) to harden the surface layer.
- Gleason Press Quenching is used mainly on flat cylindrical components and is available to ensure optimum dimensional control (ovality and flatness).
- Similar to gas carburising with the addition of ammonia to the carburising atmosphere which provides a source of nitrogen.
- Nitrogen is absorbed at the surface and diffuses into the work-piece, along with carbon, to increase surface hardenability.
- Working in a temperature range of 820°C to 940°C, typical case depths are 0.15mm – 0.7mm.
- Components are generally quenched in oil. Fast quench oils are available for lower hardenability steels.
- Molten salt is also available to minimise distortion.
- A tempering process may follow, dependent on specification and customer requirements.
- Gas Carburising produces a hard wear resistant case with ductility and toughness being retained in the core, but in some cases (thinner sections, higher hardenability case hardening steels etc) the core ductility can still be very limited.
- Carburising may be applied to near-finished components.
- Carburising may allow economical material selection.
- Carbonitriding is usually used on smaller components, more commonly with lean alloy or non-alloy steels of lower hardenability.
- Consideration must be given to preliminary high temperature normalising (and in some cases tempering) of the raw material to reduce distortion.
- A tempering process follows case hardening to partially stress relieve the hard lower ductility case and to control surface hardness.
- Case hardening treatments such as carburising and carbo-nitriding involve high treatment temperatures and quenching (rapid cooling), inevitably some size change and distortion will occur. It needs to be clear what the effective case depth and hardness requirements are at the heat treatment stage and to what standard/specification.
- Specifications and drawing/manufacturing requirements should allow for this dimensional change and post heat treatment machining such as grinding should have the allowance built into the case depth specification.
- Programmable Logic Controllers (PLCs), oxygen probes, three gas analysers and other software enable temperature, atmospheres and cycle times to be controlled accurately and repeatedly.
- Components up to 1120mm in diameter and 1730mm long can be treated vertically using special jigging to minimise distortion. Maximum weight 2000Kg. Please email to discuss: firstname.lastname@example.org Factors to be considered include weight, shape and accommodation on jigs.
- Components up to 680mm in diameter and 1200mm long can be treated vertically using special jigging to minimise distortion.
- Carbonitriding is most commonly used with lower hardenability steels and for shallower case depths.
- As carbo-nitride temperatures can be lower than gas carburising temperatures, this can help minimise distortion and size change.
- Typical examples of steels commonly case hardened: Not all are suitable for all processes or components.
Carbon case hardening steels – 080M15 (EN32B), 070M20 (EN3), AISI 1015
Lean alloy case hardening steels – 805M20 (EN362), AISI 8620, 635M15
Medium alloy case hardening steels – 655M13 (EN36), 16MnCr5, 18CrNiMo7-6
Higher alloy case hardening steels – 835M15 (EN36 or EN39), BS 5S 82
- Maximum weights: Carburising up to 2000Kg. Carbo-nitriding up to 1000Kg. Gleason capacity – 250mm high x 550mm diameter – maximum unit weight 50Kg.