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.

Nitriding

          The nature of the nitriding process used to obtain a case hardened is very different from that of the carburising process. Nitrogen, instead of carbon, is added to the surface of the steel. Carbon does not play any part in the nitriding operation but influence the machinability of steel. The temperature used in nitriding are much lower than those used in carburising and below the critical temperature of the steel.

          Simple carbon steels, which are often used for carburising are not used for nitriding. Steels used in the process are specially alloy steels. With the nitriding developing rather thin cases, a high core hardness is required to withstand any crushing loads. High tempering temperature call for a steel with a higher carbon content in order to develop this increase in core hardness. In addition to higher carbon content various alloying elements are called for in the steel to bring about an increase in the formation of these nitrides. Aluminium seems to display the strongest tendency in the formation of these nitrides. Chromium, molybdenum, vanadium and tungsten., all being nitrides former, are also used in nitriding steels. Nickel in nitriding steels hardens and strengthens the core and toughens the case but slight loss in its hardness.

Nitriding operation:

Advantages:

1. Better retention of hardness at elevated temperature.
2. Greater fatigue strength under corrosive conditions.
3. Less warping or distortion of pans treated.
4. High endurance limit under bending stresses.
5. Greater resistance to wear and corrosion.
6. Greater surface hardness.

Disadvantages:

1. Necessity of using high alloy containers to resist the nitriding.
2. High furnace costs due to the long lime of treatment.
3. Necessity of using special alloy steels.
4. Medium used is expensive.

Cyaniding

          Cyaniding is a superficial case-hardening process that utilizes carbon and nitrogen. The part to be cyanide case hardened is heated in a molten sodium cyanide at about 850°C  followed by quenching. The cyanide-casing operation is the fastest type of casing that can be applied to a mild steel core, less distortion due to use of salt bath; however, its disadvantages are that the cyanide salts are extremely toxic and the process is messy. Special protective equipment must be on by personnel operating in the vicinity of the cyanide salts. Parts must also be limited to a size that can be handled by one or two people. Not suitable for components subjected to shock, fatigue and impact because nitrogen has adverse effect on these properties.

Process:

Medium: Parts immersed in liquid bath containing NACN varying between 25% and 90%.

Bath heated in a range of 800 to 950°C.

Measured amount of air passed through the molten bath.

Reactions: 

                   2NaCN + O2 ----> 2NaCNO 

                   2NaCNO + O2 ----> Na2CO3 + CO + 2N

                   2CO  ---->  CO2 + C 

C and N2 so formed diffuse into steel and give thin wear resistant layer of carbonitride  ϵ  phase.

Quenched in oil or water.

Cyaniding time of 1.5 to 6hrs for case depth of 0.13 to 0.35mm @ 850° C 

Higher the temperature, higher the C diffusion (0.8 to 1.2%) on surface as compared to N (0.2 to 0.3% )

Case hardness: 850 VHN

Flame hardening

          Flame hardening is a surface-hardening method that uses an oxyacetylene flame to heat treatment the surface of the metal. Flame hardening can be performed on only medium or high carbon steel or cast iron. When flame hardening is applied to steels with over 70 points of carbon, extra care must be used in order to prevent surface cracking of the high-carbon steel.

          The process is based on the rapid heating of the outer surface of a ferrous metal to or above its transformation temperature.The minimum distance that the oxyacetylene flame must be held from the base metal is approximately 9/16 inches. If the oxyacetylene flame is closer than 9/16 inches., the base metal will be deformed by the flame. The metal is moved rapidly under the flame, allowing the flame to heat the base metal only on the surface. This surface heating creates two heat-affected zones, a primary heat-affected zone, where the transformation of the metal has taken place, and a secondary heat affected zone, where grain growth has been developed or where the grains have been enlarged or decreased by the application of heat to the base metal. Immediately after it is heated, the metal is subjected to a quenching spray, generally water, that hardens the metal area that has undergone transformation.

          The depth of hardness depends entirely on the hardenability of the material being treated since no other elements are being added or diffused, as in case hardening. With proper control, the interior of the metal will not be affected by this process. Often an average application of this process involves heat treating a complete piece to a certain specified softness or toughness. The exterior then may be flame hardened so that the finished piece resembles an item that has been case hardened. It is quick and the hardening is restricted to parts which are affected by wear.

          Fig shows, a flame hardening of gear teeth. A flame from an oxy-acetylene or similar burner is played on to the teeth so as to arise temperature rapidly above the hardening temperature. Hardening results when the austenised surface is quenched by spray that follows spray.

Induction hardening

          Induction hardening is a method similar to flame hardening, with the exception that the heat is generated in the metal by an induced electrical alternating current. The only metals that can be induced hardened are those that are conductors or semiconductors. The ACHF (alternating current high frequency) obtained from a pulsating magnetic field about a wire produces the heat in induction hardening. The heat results from molecular agitation induced by the electricity. The high-frequency, low-voltage, high ampere current produce a great number of eddy currents which are primarily responsible for the heating of the metal, although hysteresis is another source of heating.

          An inductor block, similar to a primary coil in a transformer, is placed around the part to be hardened. This coil does not touch the metal. A high frequency current is passed through the block or the coil and induces a sympathetic current in the surface of the metal, creating heat, a process called hysteresis. As the temperature of the metal reached the transformation range, the power is turned off, the heat source is removed, and the area is quenched, usually by a spray from a water jet built in to the inductor block. The most important aspects of induction heating is its rapid action. For example, it requires only a few seconds to heat steel to a depth of 1/8 inches.

          The heat produced by induction is the result of both current and frequency. Higher currents produce stronger magnetic fields, while higher frequencies produce more pulsation of the field within a given time. A specific degree of heating can be obtained either by using high current at low frequencies or low current at high frequencies. However, high-frequency induction heating of metals requires a device that can convert 60 hertz (Hz) power to a high frequency of several hundreds or more cycles per second. the frequency most used for metal treating applications is 450 kilohertz per second(KHz/S).

Applications:

          The induction hardening is at present extensively used for producing hard surface on crank shaft, axels, and gears. the principle advantages are as listed below:

1. Increases the labour productivity by reducing the time requirement for this heat treatment operation.
2. Deformation due to heat treatment is considerably reduced.
3. No scale effect.
4. By controlling the current the hardening of the surface can be easily controlled.
5. By changing frequency of supplying voltage the depth of hardness can be controlled easily.