Case Hardening

In case hardening, outer casing of low carbon steel is enriched with the carbon to approximately 0.85% carbon. So there is outer casing (called the “case”) eutectoid steel. Such steel. Such steel is heated about is upper critical limit and then quenched to form the “case” which hard, and the core which is tough.
Case hardening process then consist of two steps, namely:
a) Enriching the outer case with carbon to about 0.85% this is called carburizing.
b) Hardening

Carburizing
Carburizing can be done either by any of the following processes :
1. solid carburizing
2. liquid carburizing; and
3. gas carburizing

Solid carburizing –
In this process, the parts to be carburized are packed in boxes surrounded by carbonaceous material along with energizer. The boxes are closed type or closed by clay to prevent ingress of air. These boxes are then heated in a furnace at about 9000-950 0C . The carbon from the carbonaceous material combines with oxygen of the energizer to form carbon monoxide. This carbon monoxide decomposed into carbon dioxide and fresh carbon at the surface of the part, and this fresh carbon then diffuses into the steel. Depth of diffusion is dependent on the carburizing temperature and the time of carburization.
Various carbonaceous materials are charcoal, charred leather, crushed bone or horn. Energizers may be barium carbonate, soda ash, etc. Various readymade “packing” materials are available consisting of carbonaceous materials and energizers.

Liquid Carburizing-
In this case the to be carburized is immersed in a liquid bath of sodium cyanide and sodium carbonate maintained at 9000-9500C. The time of immersed depends on the depth of carbon penetration. The percentage of the solution is usually 16-40% depending on the steel and the depth of penetration. It is usual to check and maintain the strength of the cyanide every 24 hours. Cyanide salt bath in addition to providing carbon for carburizing, provides also some nitrogen. Sodium carbonate activates cyanide to increase carburizing action, and to decrease the nitriding action.

Gas Carburizing-
In this case, the parts are heated to about 9000-9500C insatiable gaseous atmosphere. Town gas , natural gas, propane and butane are some of the gases used. In this process the temperature of the gaseous atmosphere, the duration of the treatment and rate of flow of gas need to be control carefully. Advantage of gaseous carburizing is that it gives freedom decarburization.

Hardening-
Cooling the heat treated parts at the required rate is called quenching. Various quenching media are used. Some of them are:
Brine solution, oil bath and air. When a carburized part is directly quenched, coarser structure results. To refine this, first the core is refined by heating the part to 8750C and following by quenching. The case is then refined by heating the part to about 7600C (slightly above the critical point 7500C of the eutectoid steel of the case) and quenching it.

As per BS 970, there are various case hardening steel from En32 to En39, covering form lain low carbon steel (En32-En80A15) to alloy steels with nickel, chromium and molybdenum as alloying elements. These are low carbon steel with carbon less than 0.2% (except for en35-665m25, it is 0.2 to .28%).
SAE steels 1300, 2300, 2500, 4000, 4100, 4300, 4600, 4800, 5100, 8600, 8700, and 9300 series of steels are also suitable for case hardening.
17Mn1Cr95 is one of the case hardening steels, as per IS standard.