Steel components are used in many applications that require a hard and strong surface to resist wear and abrasion combined with a tough core to withstand shock and fatigue. This condition can be obtained by an alteration in the composition of the surface layer of a steel component followed by the application of a suitable “heat treatment. The surface region in which the properties and composition have been altered is known as the case, and the production of a hard surface is known as case hardening. In this process, the steel is caused to absorb such elements as carbon, nitrogen or a mixture of the two. When carbon alone is absorbed, the process is known as carburizing ; when nitrogen is added, the process is known as Nitriding, and when both are absorbed the operation is known as carbo- nitriding or cyaniding
Purpose of Case Hardening
The purpose of case hardening is as follows:
- To induce high carbon content on the metal surface while leaving the core of the metal soft.
- To increase the wear resistance of the metal.
- To increase the bending strength.
- To increase resistance to pitting.
- To increase resistance to scuffing or seizing.
Case Hardening Sequence
The various steps in the case hardening sequence are:
- Cementing (Carburising, Nitriding and Carbo-nitriding)
It is the first operation to be carried out in case hardening to induce extra carbon onto the required surface. It is done by heating the job to 900° C to 950°C and burying (or dipping) it in the selected carburising media. The penetration depends upon the temperature, time allowed for reaction and diffusion to take place and upon the composition of carburising media.
It is not carried out separately. After cementing, the articles are allowed to cool down slowly in the packed carburising media itself. This is done to anneal the newly formed case, which will have the effect of merging the outer high carbon case with the low carbon soft and tough inner core thus preventing the formation of a line between the two layers and reducing the tendency of the case to flake off or peel away during work.
This is most essential to counteract the ill effects of prolonged heating at high temperature during cementing. This consists of reheating the job to a temperature starting from 950°C and ending at 850°C and quenching each time either in oil or water. This is repeated twice or thrice depending upon the desired effects to be obtained, each time the heating temperature to be reduced by 50°C.
Finally the newly formed high carbon steel is hardened by heating to about 760°C and quenching it in water or oil.
Tempering is not carried out to all the case hardened jobs, unless it is liable to crack during usage. Plain carbon steels may be tempered by heating to its tempering temperature between 200° C to 320 °C and quenching it. In case of aero engine parts (alloy steels) the parts are to be heated up to 600° C and quenched.
Methods of Case Hardening
Various types of surface hardening or case hardening processes are.
The formation of a high carbon case is carried out by heating the steel in contact with a carbonaceous material. Carbon diffuses on to the surface layers of the metal, and the case is subsequently hardened by a suitable heat treatment. According to the nature of the carbonaceous material, three processes are recognized.
- Pack (solid) carburizing.
- Gas carburizing.
- Liquid carburising.
Pack Carburising. The steel components are packed in a container together with a carbonaceous material which decomposes at high temperatures with the release of carbon. The container is sealed with a lid and heated to above the upper critical temperature (900- 950°C) for 3-6 hours according to the depth of case desired. At this temperature, the austenite absorbs carbon and the penetration is about 0.030-0.060”. Higher carbon contents than this will lead to brittleness because of the excessive quantities of free carbide.
The carburizing material may consist of wood or bone charcoal mixed with up to 40 per cent of barium carbonate. Carbon is not absorbed from the solid state, but rather from the gaseous condition, the barium carbonate provides carbon dioxide, which reacts with the charcoal to form carbon monoxide. This gas decomposes at the surface of the steel and carbon is absorbed. When carburization is complete, the components are allowed to cool slowly. The cycle of reaction takes place as follows:
Ba CO3 → BaO + CO2
CO2+C ⇆ 2CO
Fe+2CO ⇆ CFe+CO2
CO2+C → 2 CO
Heat Treatment after Carburizing. When the parts are withdrawn from the containers they exhibit a coarse grain structure typical of overheated steels. In addition, they are generally soft and accordingly must be heat treated in order to obtain the optimum case and core properties.
In view of the wide differences in composition of the core and case, two heat treatments are required to produce the desired properties. The refining of the coarse core structure is accomplished by heating to just above its critical point (850-900°C) and then quenching in water. This refines the core, but leaves the case coarse and brittle. Reheating to just above the critical range (760-780°C) appropriate to a steel of high carbon content, followed by quenching results in a refining of the case and eliminates brittleness.
Gas Carburising. Carbon for case hardening is supplied by the decomposition of a gaseous carbon compound such as carbon monoxide, methane or propane. The reaction is as follows:
CH4+Fe → CFe +2H2O
2CO+Fe → CFe+CO2
The gas carburising atmosphere in addition to containing an adequate supply of carbon must possess a minimum of oxygen, carbon monoxide and water vapour to give rise to decarburising conditions at high temperature.
By adjustment of time, temperature and gas composition, the case depth can be varied. Circulation of the gaseous atmosphere throughout the furnace and effective sealing of the furnace to prevent excess of air are necessary in order to secure uniformity of the case.
Liquid Carburising. The bath consists essentially of a fifty-fifty mixture of sodium cyanide and sodium carbonate. The latter ensures a more efficient action by slowing down the disintegration of the cyanide at the temperature of operation (900 -950°C). At this temperature, sodium cyanide decomposes on contact with air. Two different reactions are possible.
(a) 2NaCN+202 → 2Na2CO3+CO+2N
(b) 2NaCN + 02 → 2NaCNO (Sodium cyanate)
(c) 3NaCNO → NaCN +Na2CO3+C+ 2N
Nitriding. Nitrogen, similarly to carbon, also hardens steel and gives rise to a process of case hardening known as nitriding. Ammonia is used as the source of nitrogen. The components are placed in a heat resistant metal container which is then filled with ammonia whilst cold. When it is completely purged, it is sealed, placed in a furnace and raised to a temperature of approximately 500°C. At this temperature the ammonia dissociates (NH3=3H+N) and nitrogen is absorbed in the surface layer of the steel. The treatment is continued until the required case depth is obtained, after which the component is allowed to cool in the container. The case obtained is extremely hard, resistant to corrosion and since no quenching is needed the amount of distortion encountered is negligible.
Iron nitride does not confer hardness to any extent and hence, steels suitable for nitriding must contain such alloying elements as Aluminium, Chromium and Vanadium, which form hard nitrides.
The nitriding process is suitable for the surface hardening of such components as crank shafts, pump parts, cylinder liners, valves, etc., where wear and heat resistance are essential.
Cyanide Hardening (Carbo-Nitriding). Absorption of both carbon and nitrogen may be affected by the immersion of the steel part in a molten bath of sodium cyanide held at just above the lower critical range (850 – 900°C). The composition of the bath is usually 20-40 percent NaCn, 30-40 percent Na2O3 and the balance of NaCI.
The Cyanide hardening process consists of two stages, nitriding and cyaniding. The hardness produced consists principally of nitrides in the surface layers. The case is not deep, but has the advantage of being quickly and cheaply applied.
In cyaniding, the parts are thoroughly degreased in dilute caustic soda, dried and stacked in baskets or trays and then immersed in the salt bath. Treatment time is usually about one hour.