We know about the many engineering components that have the internal toughness to protect them from any impact. e.g., Gear, cam, shaft, etc. Case hardening is the process of hardening the surface of a metal object while allowing the metal deeper underneath to remain soft. Thus, forming a thin layer of harder metal is called a case at the surface.
The internal properties ie., soft and tough are called core. The core provides the efficiency for case hardness to protect the metal from breakdown. Case hardening is usually done after the part has been formed into its final shape. Still, it can also be done to increase the hardening element content of bars to be used in a pattern welding or similar process.
This technique is used for steel with a low carbon content. The carbon is added to the outer surface of the steel to a depth of approximately 0.03 mm. In this chapter, we will discuss the different processes of case hardening.
Processes of Case Hardening
The case hardening process is the process of carburisation i.e., saturating the surface of the layer of steel with carbon or some other substance by which, the outer case of the object is hardened, whereas the core remains soft. It is applied to shallow carbon steel.Â
Many types of case hardening processes are as follows.
- Carburising
- Nitriding
- Cyaniding
- Flame hardening
- Induction hardeningÂ
1. Carburising
Carburising is a heat treatment process used to case harden the steel with a carbon content between 0.1 and 0.3%. In this process, steel is introduced to a carbon-rich environment at elevated temperatures for a certain amount of time period and then quenched. Mild steel or low carbon steel is heated at 925°C for 5 h approximately, due to which, carbon diffuses in the upper surface of steel up to 1-2 mm depth. In this way, case hardening is done by carburizing. By this method, the hardness of the case can be increased up to R 65 grade (ie., a grading system for measuring the case hardness of a material).Â
To obtain fine grain of core, firstly the metal is heated above its critical temperature (870-925°C) and then cooled in oil at 760°C. This results in developing a soft core and changing the whole structure in pearlite and martensite. The carburizing is a diffusion-controlled process and is used for case hardening of gear, cam and camshaft, bearing, etc.
Carburising can be classified as follows.
(i) Pack carburisingÂ
(ii) Liquid carburising
(ii) Gas carburising
(i) Pack Carburising
The carburizing in a solid medium is called pack carburizing. In this process, the job which is to be case hardened is placed in a cast iron box, with the power of carburizing substances such as powder of wood, charcoal, bone charcoal, etc. An energizer in the form of barium carbonate (BaCO3) is also used in the box.
The joints are sealed with fire clay and the box is gradually heated at a temperature of 900-950°C in a furnace for a specific period, depending on the case depth requirement. Then, the box is removed from the furnace and keep the box tight for slow cooling. This surface of the job becomes hard on cooling.
(ii) Liquid Carburising
It is performed by immersing the workpiece in a bath of molten salts, containing 20 to 50% sodium cyanide, 40% sodium carbonate, and a small amount of sodium or barium chloride. This process takes 0.5 to 2 h and requires a temperature of 850 to 1000°C. This process gives a thin harden layer up to 0.8 mm thickness. This process is mostly suitable for small parts. This process is also known as sodium carburizing.