CASE HARDENING: Carburizing, Nitriding, Cyaniding, Induction hardening, Flame hardening


Some times special characteristic are required in metal such as hard outer surface and soft, tough and more strength oriented core or inner structure of metal. This can be obtained by casehardening process. It is the process of carburization i.e. saturating the surface layer of steel with carbon or some other substance by which outer case of the object is hardened where as the core remains soft. It is applied to very low carbon steel. It is performed for obtaining hard and wear resistance on surface of metal and higher mechanical properties with higher fatigue, strength and toughness in the core. The following are the case hardening process.
(1)  Carburizing
(2)  Nitriding.
(3)  Cyaniding.
(4)  Induction hardening.
(5)  Flame hardening

These processes are discussed as under.


Carburizing can be of three types

1. Pack carburizing
2. Liquid carburizing and
3.  Gas carburizing
The above carburizing processes are discussed as under.

 Pack Carburizing

Metals to be carburized such as low carbon steel is placed in cast iron or steel boxes containing a rich material in carbon like charcoal, crushed bones, potassium Ferro-cyanide  or charred leather. Such boxes are made of heat resisting steel which are then closed and sealed with clay. Long parts to be carburized are kept vertical in -boxes. The boxes are heated to a temperature 900°C to 950°C according to type of steel for absorbing carbon on the outer surface. The carbon enters the on the metal to form a solid solution with iron and converts the outer surface into high carbon steel. Consequently pack hardened steel pieces have carbon content up to 0.85% in their outer case. After this treatment, the carburized parts are cooled in boxes. Only plane carbon steel is carburized in this process for hardening the outer skin and refining the structure of the core to make it soft and tough. Small gears are case hardened by this process for which they are enclosed in the cast iron or steel box containing  a material rich in carbon, such as small piece of charcoal and then heat to a temperature slightly above the critical range. Depth of hardness from 0.8-1.6 mm is attained in three to four hours. The gears are then allowed to cool slowly with-in the box and then removed. The second stage consists of reheating the gears (so obtained) to about 900°C and then quenched in oil so that its structure is refined, brittleness removed and the core becomes soft and tough. The metal is then reheated to about 700°C and quenched in water so that outer surface of gear, which had been rendered soft during the preceding operation, is again hardened.

Liquid Carburizing

Liquid carburizing is carried out in a container filled with a molten salt, such as sodium cyanide. This bath is heated by electrical immersion elements or by a gas burner and stirring is done to ensure uniform temperature. This process gives a thin hardened layer up to 0.08 mm thickness. Parts which are to be case-hardened are dipped into liquid bath solution containing calcium cyanide and polymerized hydro-cyanide acid or sodium or potassium cyanide along-with some salt. Bath temperature is kept from 815°C to 900°C. The furnace is usually carbon steel case pot which may be by fired by oil, gas or electrically. If only selected portions of the components are to be carburized, then the remaining portions are covered by copper plating. There are some advantages of the liquid bath carburizing which are given as under.
Greater depth of penetration possible in this process.


Cyanide may also be used to case harden the steel. It is used to give a very thin but hard outer case. Cyaniding is a case hardening process in which both C and N2 in form of cyaniding salt are added to surface of low and medium carbon steel. Sodium cyanide or potassium cyanide may be used as the hardening medium. It is a process of superficial case hardening which combines the absorption of carbon and nitrogen to obtain surface hardness. The components to be case hardened are immersed in a bath having fused sodium cyanide salts kept at 800-850°C. The component is then quenched in bath or water. This method is very much effective for increasing the fatigue limit of medium and small sized parts such as gears, spindle, shaft etc. Cyanide hardening has some advantages and disadvantage over carburizing and nitriding method. Cyaniding process gives bright finishing on the product. In it, distortion can be easily avoided and fatigue limit can be increased. Decarburizing can be reduced and time taken to complete the process is less. But the main disadvantage of this process is that  it is costly and highly toxic process in comparison to other process of case hardening. There are some common applications of cyaniding process which are given as under.

Application of Cyaniding

Cyaniding is generally applied to the low carbon steel parts of automobiles (sleeves, brake cam, speed box gears, drive worm screws, oil pump gears etc), motor cycle parts (gears, shaft, pins etc.) and agriculture machinery.


Nitriding is a special case hardening process of saturating the surface of steel with nitrogen by holding it for prolonged period generally in electric furnace at temperature from 480°C to 650°C in atmosphere of Ammonia gas (NH3). The nitrogen from the ammonia gas enters into on the surface of the steel and forms nitrides and  that impart extreme hardness to surface  of the metal. Nitriding is a case hardening process in which nitrogen instead of carbon is added to the outer skin of the steel. This process is used for those alloys which are susceptible to the formation a chemical nitrides. The article to be nitride is placed in a container (made  of high nickel chromium steel). Container is having inlet and outlet tubes through which ammonia gas is circulated. Ammonia gas is used as the nitrogen producing material. The alloy steel containing Cr, Ni, Al, Mo, V and Nitre-alloy are widely used for this process. Plain carbon steels are seldom nitirided. There are some common applications of this process which are given as under.

Application of Nitriding 

Many automobile, diesel engines parts, pumps, shafts, gears, clutches, etc. are treated with the nitriding process. This process is used for the parts which require high wear resistance at elevated temperatures such as automobile and air plane valve’s and valve parts, piston pins, crankshafts, cylinder liners etc. It is also used in ball and roller bearing parts die casting dies, wire drawing dies etc.

Flame Hardening

It consists of moving an oxyacetylene flame, over the part where hardening is required. Immediately after this, the heated portion is quenched by means of water spray or air passing over it. Temperature attained by the surface is controlled and the rate of cooling is controlled by selecting a suitable medium. Flame hardening is suitable for large sized articles where only some portions of the surface requiring hardening and hence there is no need to heat the whole article in the furnace. Metal is heated by means of oxy-acetylene flame for a sufficient time unto hardening range and than quenched by spray of water on it. The hardened depth can be easily controlled by adjusting and regulating the heating time, temperature, flame and water spray. The main advantages of the process is that a portion of metal can be hardened  by this process, leaving rest surface unaffected by confining the flame at relevant part only where hardening is required. This process is best suited to smal1 numbers of jobs which requiring short heating time. This method is highly suitable for stationary type of larger and bulky jobs.

 Induction Hardening

Induction hardening is accomplished by placing the part in a high frequency alternating magnetic field. It differs from surface hardening in the way that hardness of surface is not due to the increase in carbon content but due to rapid heating followed by controlled quenching. In this process, a high frequency current is introduced in the metal surface and its temperature is raised up to hardening range. As this temperature is attained, the current supply is cut off instantaneously water is sprayed on the surface. Heat is generated by the rapid reversals of polarity. The primary current is carried by a water cooled copper tube and is induced into the surface layers of the work piece. Thin walled sections require high frequencies and thicker sections must require low frequencies for adequate penetration of the electrical energy. The heating effect is due to induced eddy currents and hysteresis losses in the surface material. Some portion of the metal part is heated above the hardening temperature and is then quenched to obtain martensite on the metal surface. There are some advantages of this process which are given as under.

Advantages of  Induction Hardening

Induction hardening is comparatively quicker. A minimum distortion or oxidation is encountered because of the short cycle time. The operation is very fast and comparatively large parts can be processed in a minimum time. There are some applications of this process which are given as under.

Application of Induction Hardening

Induction hardening is widely used for hardening surfaces of crankshafts, cam shafts, gear automobile components, spline shafts, spindles, brake drums etc. It is also used for producing hard surfaces on cam, axles, shafts and gears.

0 thoughts on “CASE HARDENING: Carburizing, Nitriding, Cyaniding, Induction hardening, Flame hardening”

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