Carburising

Carburising is the absorption and diffusion of carbon alone into the surface of steel to give a hard surface and softer core after hardening by quenching.

Carburising is the oldest of the case hardening methods. Case hardening, as the name implies, produces a hard surface to the treated component whilst at the same time producing a softer, more ductile core, which provides support for the harder case. It has been known from pre-history that it is possible to increase the quenched hardness of steel by first increasing its carbon content. This fact was utilised to produce hard and therefore keen cutting edges by heating the articles in a carbon rich material such as charcoal, prior to quenching.

If carburising has been correctly carried out, the core material will have an unchanged carbon content, whilst that of the surface or ‘case‘ material should be in the region of 0.8%. The precise case carbon content for optimum results varies slightly with the steel analysis. Higher carbon contents than this produce the cementite phase at grain boundaries which, if not subsequently rectified, would lead to case embrittlement with attendant dangers of spalling. Lower carbon contents can lead to ‘lean’ case compositions which fail to harden properly on quenching. Also, due to the prolonged heating in the austenitic range during carburising, the grain size of the steel can increase, leading to a reduction in strength and increased brittleness.

In order to obtain the optimum combination of case and core properties, carburised parts are subjected to a sequence of post carburising treatments, culminating in the quenching operation to induce hardening. The grain size of the core material can be refined by heating to above the transformation austenitising temperature, which for the low carbon material of the core is approximately 870°C, and quenching. It is then necessary to refine the grain size of the case structure. This is achieved during the hardening stage by heating to approximately 760° C, which is just above the transformation austenitising temperature for the case material. This procedure is known as the ‘double quench’ treatment and is normal practice in pack carburising.

With grain refined steel it is possible to achieve satisfactory hardening with acceptable grain size and microstructures by using a ‘single quench’ treatment. Although this can be done by quenching straight from the carburising temperature, it is common to carburise at 900/950°C, furnace cool to 840/850°C and equalise at this temperature (to give some case diffusion and core refinement).

As an alternative to furnace quenching, previously carburised components may be hardened by induction hardening or flame hardening, where their geometry dictates that a localised surface heating method is preferable.