Case Hardening (or) Surface Hardening Processes

Case hardening (or) Surface hardening

Case hardening (or) Surface hardening is a process of heating the metal over its surface so as to harden it. This process is adopted, as many of the applications like gears, cams, and crankshafts desire high hardness on the outer surface and softer core, which is tough enough to withstand the shocks. So to attain such properties it is very difficult to employ low carbon steels or high carbon steels as low carbon steels cannot be hardened where as high carbon steels have poor toughness. So for obtaining the required properties in general medium carbon steels are used. These steels have intermediate properties of both the above-mentioned steels. So the hardness of these materials over the surface is increased by any of the following ways
1) So as to increase the surface of hardness of the material it is subjected to heating so as to reduce the carbon in free form and helps in forming of cementite.
2) Nitriding is done so as to form nitrides which are very strong in nature
3) Hardening with out change of composition can also be done by some surface hardening processes like flame hardening, electric arc hardening etc.
4) Steels can also harden by sending both nitrogen and carbon at a time along with heating.
Surface hardening is done in many methods if the composition of the surface is changed then the hardening is called as case hardening and if there is no change in surface composition then the hardening technique is called surface hardening.
The following are the processes in case hardening
1) Carburizing
2) Nitriding
3) Cyano Nitriding
4) Carbo nitriding
The following are the processes in surface hardening
1) Flame hardening
2) Induction hardening

Carburizing

Method of increasing the surface hardness of the steel material by increasing the amount of carbon over it is called as carburizing process. The component, which is to be hardened, is kept in an oven and is heated to a temperature of around 9000C and is exposed to carbon containing environment. There should be no oxygen while carburizing as the oxygen reduces the carburizing effect. In general carburizing process can be done in three ways basing on the type of carburizing agent used
1) Solid Carburizing
2) Gas carburizing
3) Liquid carburizing

Solid carburizing

In solid carburizing the component, which is to be surface hardened, is to be kept packed in a carburizing medium and heated to a temperature around 9000 C. At that temperature the carburizing agents gets decomposed and carbon monoxide is released. Carbon monoxide has greater affinity to move inwards in the steel. So this released carbon monoxide penetrates in to the surface of the steel and decomposes in to carbon and carbon dioxide. Hence the carbon content over the surface gets increased and high surface hardness can be obtained. This process should be carried in a closed environment as the carburizing agents release the carburizing gases these gases have higher affinity towards oxygen so they react with the oxygen present in the environment and does make the surface of the material harder.

Roll carburizing

This is a special type of carburizing technique in which the steel material is heated up to a temperature of 9000C and is rolled over a carburizing agent. The carbon gets impregnated in to the surface of the material and hence the hardness gets increased.

Gas carburizing

In this process the steel is heated to austenitic region and then gases like ethane methane, butane are passed over the heated material. To obtain uniform depth of hardness the gases to be sent are controlled. The gases gets impregnated in to the surface of the material and hence hardens the material

Liquid carburizing

In this process the steel material is immersed in bath of carbon fused salts. For doing this process the material should be preheated to austenitic region.
The main drawback of this hardening process is the depth of hardness is not uniform and cannot be controlled easily.
After carburizing the heat treatment processes, which may be adopted, are
1) Quenching
2) Double quenching

Nitriding

In this process the steel material is heated to a temperature of around 5500C and then exposed to atomic nitrogen. This atomic nitrogen reacts with iron and other alloying elements and forms nitrides, which are very hard in nature. By this process both wear resistance and hardness of the product can be increased. Atomic nitrogen only has the property to penetrate in the steel but where as molecular nitrogen lacks in that property. So molecular nitrogen is never used as a nitriding medium while hardening of steels. The atomic nitrogen required for this process is generated by the decomposition of salt bath like NaCN.
Nitriding processes can also have three types of media. Based on those they are classified in to
Solid nitriding
Liquid nitriding
Gaseous nitriding

Some of the advantages of this process are

1) All the materials, which had undergone nitriding process, require no special heat treatment process. Hence the oocurance of distortion is very less.
2) As there occurs nitride precipitation there will be an expansion in which makes the surface prone to high residual compression so due to this the notch sensitivity factor for that material gets reduced to a large extent.
3) Through nitriding process higher degree of hardness can be obtained
4) The products formed after nitriding have less coefficient of friction.

Some of the disadvantages of this process are

1) Depth of hardness is less
2) This process cannot be applied for all the alloy steels
3) An expensive grinding process is to be adopted after nitriding due to the formation of a thin layer which is in white color
4) Since there will be no post heat treatment process lot of care should be taken about the inner core

Carbo nitriding

In a survey it is found that when the carbon percentage in the nitrided parts is high then the life of the component is dramatically higher than those parts that have less carbon percentage. So by taking this point in to consideration this process is developed. In process both the above processes are done simultaneously. In this process methane and ethane mixed with 5% to 10% ammonia acts as the hardening agent. The steel material is heated to a temperature between A1 and A3¬. These gases are allowed to pass over it for a period of time. Carbo nitriding process is of two types
1) Liquid carbo nitriding
2) Gaseous carbo nitriding
In liquid carbo nitriding process the media used for hardening is of liquid and gases in gaseous carbo nitriding

Liquid carbo nitriding is also called as cyaniding. This processes is nearly equal to that of liquid carbo nitriding process. Some of the major differences between these two processes are
1) No alkaline slats are present in cyaniding
2) Bath having high percentage of Sodium Cyanide is used for cyaniding process
3) Nitrogen content in the parts generated by cyaniding is higher where as carbon percentage is low
4) Thin depths are possible by cyaniding

Flame hardening

Flame hardening is one of the surface hardening processes, which is in use now a days. In this process an oxyacetylene flame is used to harden the surface of the material. The surface material is heated to a temperature above upper critical temperature and quenched by using water. So due to this effect there occurs a transformation of austenite to martensite. In general there are different ways to do flame hardening
1) Spot method
2) Progressive method
3) Spinning method
4) A combination of the later two

Some of the parameters on which the depth of hardening depends are

1) Effective distance between the component surface and the flames
2) Ratios and gas pressures
3) Speed of movement of flame or the material with relative to other
4) Type of quenching media
5) Volume of quenching media
6) Way of application of quenching media

Advantages

1) Depth of hardening can be easily controlled
2) Less distortion of the material
3) Oxidation is minimum
4) Loss of carbon is also minimum
5) Clean working environments can be maintained
6) Low energy consumption
7) High out put rate
8) Only light machining is required after this process
9) These products can be used directly without any additional processes

Some of disadvantages of this process are

1) Lot of care is required while tempering
2) High investment cost

Induction hardening

Induction hardening is one of the advanced processes, which saves lot of energy and time. In this process a coil known as inductor coil is used to heat the surface layers of the material. The energy given to this inductor coil is electrical energy. In general the material used for the inductor coil is a water-cooled copper tubes. When the high frequency alternating current is passed over the inductor high frequency alternating magnetic field is generated due to this eddy currents are produced and the surface starts getting heated. So due to this reason the material gets heated up in very less time.

Some of the advantages of this process are

1) Fast heating rates
2) No time wastage
3) No scaling
4) No decarburization
5) Less distortion as only the surface gets heated
6) Depth of hardening can be very easily maintained
7) Clean working environments can be maintained
8) Low energy consumption
9) High out put rate
10) Only light machining is required after this process

Some of the constraints of this process are

1) High cost
2) Irregular shapes cannot be hardened easily by this process