Case hardening

Case hardening is the diffusion heat treatment operation which involves two stages:

• Heating a steel part to a temperature above the upper critical temperature in a medium, containing an element capable to saturate the surface layer of the part through diffusion;

• Heat treatment of the part in order to obtain the desired combination of mechanical properties of the hard outer “case” and the ductile “core”.

As a medium for the case hardening solid, liquid and gaseous substances may be used.
The most widely used case hardening methods are: carburizing, nitriding and carbonitriding.

• Carburizing
• Nitriding
• Carbonitriding

Carburizing

Carburizing is the process of diffusion enrichment of the surface layer of a part with carbon followed by heat treatment of the part.
As carburizing medium the following substances are used:

• Charcoal or other carbon-containing solids mixed with sodium carbonate and barium carbonate accelerating the process of dissolving the carbon in steel.

The process is carried out in steel or cast iron boxes placed into a furnace at the temperature 1650°F - 1750°F (900°C - 950°C) resulting in formation of hard case of the thickness 0.02”-0.08” (0.5mm – 2mm) and containing 0.8-1% of carbon.

• Kerosene or benzene – liquid carbonizing mediums, which are usually used in dispersed form;
• Methane (CH4), propane (C3H8) – gaseous carbonizing mediums.

The process is carried out in a furnace (batch or continuous) at the temperature 1650°F - 1750°F (900°C - 950°C) for 3-4 hrs.
Thickness of the hard layer formed in the gaseous carburizing may reach 0.15” (4mm).
Heat treatment after carburizing involves hardening-tempering treatments with purpose of controlling structure and properties of both the hard layer and the ductile core.
Nitriding

Nitriding is the process of diffusion enrichment of the surface layer of a part with Nitrogen.
Gas nitriding is carried out at 930°F - 1110°F (500°C - 600°C) for 40-100 hrs.in the atmosphere of Ammonia, which dissociates to Hydrogen and nitrogen. The latter diffuses into the steel forming nitrides of iron, aluminum, chromium and vanadium.

Ion nitriding (plasma nitriding) is a surface Hardening heat treatment, in which Nitrogen is delivered to the workpiece surface in form of ionized gas (plasma).

The case formed as a result of nitriding has a hardness of about 1100 HV which is higher than the hardness after carburizing.
Nitrided part possess also better wear resistance, increased fatigue strength, enhanced toughness and good resistance to corrosion.
No additional heat treatment is required after nitriding.

Carbonitriding

Carbonitriding is the process of diffusion enrichment of the surface layer of a part with carbon and nitrogen.
Gaseous carbonitriding is carried out in gaseous medium, consisting of carburizing gas (methane, propane) with addition of 3-8% of Ammonia.
There are two principal methods of the gaseous carbonitriding:

• Low temperature gaseous carbonitriding, conducted at 930°F - 1110°F (500°C - 600°C). The steel surface is enriched mostly with nitrogen in this process.
• High temperature gaseous carbonitriding, conducted at1470°F - 1750°F (800°C - 950°C). The steel surface is enriched mostly with carbon in this process. This process is followed by heat treatment.

Cyaniding is the carbonitriding process, conducted in molten salt, containing 20-25% of sodium cyanide (extremely toxic substance).
The process is carried out at the temperatures 1500°F - 1580°F (820°C - 860°C) for 1 hour.
Carbonitrided parts possess better (than carburized parts) wear resistance.