Surface hardening

Surface hardening, treatment of steel by heat or mechanical means to increase the hardness of the outer surface while the core remains relatively soft. The combination of a hard surface and a soft interior is greatly valued in modern engineering because it can withstand very high stress and fatigue, a property that is required in such items as gears and anti-friction bearings. Surface-hardened steel is also valued for its low cost and superior flexibility in manufacturing.

The various methods of surface hardening are:

1. Case hardening
• Pack hardening
• Liquid carburizing
• gas carburizing
2. Nitriding
3. Cyaniding
4. Flame hardening
5. Induction hardening.

          Mechanical means of hardening the surface of steel parts include peening, which is the hammering of the heated surface, as by iron pellets shot onto the surface or by air blasting, and cold-working, which consists of rolling, hammering, or drawing at temperatures that do not affect the composition of the steel.

Case hardening

Case hardening :

          As mentioned previously, only those carbon steels can be hardened whose carbon content is about 0.25% or more.

How do we harden dead mild steel? The answer is by case hardening. In this process, the work piece is packed in charcoal and heated as in annealing. It is kept at that high temperature for a few hours. The result is that carbon enters into the surface of the work piece to the depth of a mm or two depending upon the heating time. The work piece now has a case where carbon percentage is as per requirement for hardening. It is then heated and quenched in the usual manner. The result is a component whose surface acquires hardness, but core remains soft and tough.

The Objects of  case hardening are:

1. To obtain a hard and wear resistance surface on machine parts with enrichment of the surface layer with carbon to concentration of 0.75 to 1.2%.
2. To obtain a tough core.
3. To obtain close tolerance in machine parts.
4. To obtain a higher fatigue limit and high mechanical properties in the core.

          Case hardening consists in heating a steel in the presence of a solid, liquid or gas, rich in carbon in order to enable the surface to be hardened, while retaining a tough ductile core.

          There are three methods of adding carbon to the surface of the metal:

1. Pack hardening               2. Liquid Carburizing               3. Gas carburizing.

Pack Carburizing

          This method is the oldest. The steel to be carburized are packed in metal boxes, or pots completely surrounded by the compound, boxes that are usually made of a heat resistant alloy. The boxes are sealed with clay to exclude air and are placed in an oven, or furnace, where they are heated to temperature of between 900°C and 950°C , depending on the composition of steel. The carbon from the carburizing compound soaks into the surface of the hot steel to depth which depends on the time that the box is left in the furnace, so that the low-carbon steel is converted into high-carbon steel in the form of thin case. The internal section of the steel, and any parts, which have been protected by tinning, however, remain unaffected, the result being a piece of steel with a dual-structure. The steel is allowed to cool slowly in the box. 

          The steel is then removed from the box and reheated to a temperature just above its critical point, or appropriately 915-925°C for fine grain steel, followed by quenching in water, brine or oil. This hardens the skin and at the same time refines the core. Smaller articles and thin sections are heated to a lower temperature in order to avoid distortion. The steel is usually given a second heat treatment at about 760-780°C, in order to improve the ductility and impact resistance of the core and case.

          Small parts and single jobs are often carburized by heating them in forge, and covering them with a carburizing powder when the metal has reached a bright red heat. The carburizing compound melts and flows over the surface of the metal, which is then returned to the forge and maintained at a bright red heat for sufficient time to allow the carbon to penetrate the surface, quenching then follows as usual.

          Many commercial 'carburizing' compounds are available in suitable mixed form. Among the ingredients, combined in different percentages, are powdered charred leather, wood charcoal and horn. Wood charcoal is very largely used, although its value varies with the type of wood. Hickory gives the best results and a normal rate of penetration gradually decreases and ceases after eight hours. Wood charcoal gives the slowest rate of penetration of any of the carburizing materials. 

Liquid Carburizing

          Where a fairly thin case is required a more economical process is to carburize the parts in a liquid bath. A molten salt bath which will give up carbon to the steel, thus producing a carburized surface, has certain advantages, in that heating is rapid and distortion is minimized. The development of such salt baths is comparatively recent. Briefly, the salt baths usually contains sodium cyanide, barium chloride, sodium carbonate, and other salts, which is heated by electrical immersion elements or by a gas burner. For a constant time and temperature of carbonization, the depth of case depends on the cyanide content. The sodium cyanide content will not exceed 12% of the weight of the bath if a deep case is desired, and may go up to 23% for lighter cases. The temperature range is 1550°F-1750°F. With the lower temperatures more nitrogen will enter the case, which is benificial, and at higher temperatures more carbon will be taken in. This general process may be thought of as an extension of the cyanide process, wherein different compositions of the salt bath and higher operating temperatures are used. The parts leave the bath with a clean, bright finish, the scaling experienced during pack hardening being avoided.

Gas Carburizing

          It is another method of extra carbon into the surface of the steel; in this case by heating the metal in a furnace into which a gas which is rich such as methane, propane, butane is introduced. In gas carburizing, the gas may be generated from suitable liquids, as done in certain processes, or the gas such as methane or propane may be led directly into the container in which the work is placed. It is necessary to maintain a continuous flow of carburizing gas into the furnace, and to extract the spent gas. Some city gasses are quite suitable. Some source of oxygen is necessary for best results, either as air or CO2, but usually it is not necessary to add any oxygen.

          There are explosive ranges of gas composition, which should be avoided. The work is placed in a gas-tight container, which can be heated, in a suitable furnace, or the furnace itself may be the container. The carburizing gas is admitted to the container, and the exit is vented. It should be apparent that there are certain advantages in the gas carburizing process. The steel does not have to be packed, and there is no handling and storage problem of the solid carburizer with which to contend. The time required to bring the work to the carburizing temperature is less than is necessary with solid carburizers. On the other hand, the pack method requires less expensive equipment, and control of the composition of the gasses is simple. Continuous furnace can be used in either process. Continuous gas-carburizing furnace have been developed only recently. In continues gas carburizing, the quenching and tempering cycles take place in the same unit, that is the carburizing, quenching, and tempering processes are carried out in sequence in the same closed furnace as the work progresses on a conveyor from one operation to the next. There are several variations of gas carburizing;

          The horizontal, rotary type of gas carburizing furnace has a retort of muffle which revolves slowly so that the parts are rotated in the steam of gas; this is suitable for smaller parts such as ball and roller bearings, chain links, pins, axels and so on. Carburizing gas led into the retort, and the exit gas is burned. Case depths in relating to time and temperature. Larger parts are usually carburized in a vertically rotary furnace, in which gas is given a swirling rotary motion so that it circulates around the parts.