Gas carburization

With the thermo chemical diffusion process, case hardening is highly significant. Prior to the actual hardening, the surface of the workpiece is enriched with carbon. This is carried out by heating the respective product in a carbon-emitting medium to an austenitising temperature and then more or less maintaining this. This carburizing process is carbon diffusion.

It is therefore clear that the carburizing depth is a function of temperature and time. The carburizing medium used may be powder or granulate, molten salts and gasses. The introduction of the carbon is controlled such that the surface layer generally exhibits an eutectoid composition (C=O.8 %). If excess carbon results (C>O.8 %), Fe3C separates out at the grain boundary. We obtain grain boundary cementite, which results in severe brittleness. Following carburization the workpieces are hardened. This can take place through direct quenching from the carburizing temperature (direct hardening). However, depending on the workpiece characteristic requirements other processes are also possible.

During carburizing, the core structure does not change in terms of its chemical composition. With extended holding times at the carburizing temperature grain coarsening occurs.

During direct hardening, reduction to the surface hardening temperature usually takes place prior to quenching. This avoids a superheated hardening of the C-rich surface structure.
With so-called single hardening, cooling to room temperature takes place slowly after carburizing. If necessary it is possible to carry out intermediate processing, e.g. partial processing of the case layer. After this, austenitising takes place once more, either to core hardness temperature or surface hardness temperature, before subsequent quenching.

With core hardening the surface is superheated and hardened, the martensite is coarsely granular and it is necessary to reckon with a greater retained austenite content. Surface hardening results in an optimum surface structure. However, the core structure contains martensite as well as residual ferrite.
A further option is double hardening. With this process, core hardening (usually direct hardening) is followed by a second hardening at surface temperature. The surface structure is optimum here. The core structure is re-refined, although it does retain some residual ferrite.
With staged hardening, following carburizing in a thermal bath (at approx. 650 °C) isolthermal cooling takes place in the perlite stage. Hardening subsequently takes place at surface hardening temperature. This process results in a very fine grained surface structure. After all hardening processes, subsequent tempering is carried out at 150 and 200 °C.

Case-hardened workpieces possess a hard surface layer and a significantly softer, tougher core. The surface hardness usually lies between 60 and 64 HRc. The case hardening depth (Eht) may vary within a relatively large range as a result of the selection of the carburization depth. The core hardness results from the chemical composition of the material used and the hardness conditions. In addition to increasing the wear resistance, the case hardening also improves the fatigue strength.
Today, gas nitriding is very common. Various processes exist for generating the suitable furnace atmosphere. On the one hand it is possible to use a gas carrier, to which carburizing gases such as methane, propane and similar are added. On the other hand it is also possible to carry out gas carburizing without a gas carrier. With the so-called drip-feed method, organic liquids and mixtures thereof are dripped directly into the charged furnace chamber, which is heated to the carburization temperature. These liquids evaporate and split in the furnace heat. During the splitting process diffusible carbon products are generated. Today, modern systems are available with control and regulation devices, which are able to guarantee specific furnace atmospheres, in order that faultless cases result. Gas carburization offers a clean, environmentally friendly process.