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Heat Treatment

Case Hardening

and

Surface Treatment

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Carburizing

Nitriding

Cyaniding

Carbonitriding

Flame Hardening

Induction Hardening

The processes used to create

hardened cases are:-

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CARBURIZING

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Carburizing DefinitionProcess

CharacteristicsTypical useAdvantages

Disadvantages

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NITRIDING

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Nitriding DEFINITION:-

It is the introduction of nitrogen in to the surface of certain type of steels by heating it And holding it at a suitable temperature in contact with partially dissociate (distance) ammonia or other suitable medium

This process produces a hard case without quenching or any further heat treatment.

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Nitriding

CHARACTERISTICS:-

• Extreme Hardness

• Case has improved its corrosion resistance

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NitridingTYPICAL USES:-• Gears• Valve seats• Guides • Gauges • Bushing• Air craft engine parts• Aero engine cylinder• Crank pin and journals

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NitridingPROCESS:-

Before being nitreted, the components are heat treated to produce the required properties in the core. The normal operations are…

• Oil quenching between from 850 to 900 C followed by tempering at 600 to 700 C

• Rough machining followed by a annealing at 550 C for five hours to remove internal stresses

• Finish machining followed by nitriding

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Nitriding- Nitriding: The components are placed in a heat

resistant metal container which is then filled with ammonia. And sealed it, place in the furnace and rise the temperature up to 500 C

- At this temperature ammonia separates in to three pars of hydrogen and one part of nitrogen

- Nitrogen is absorbed in the layer of the steel - At required depth of the case based on that

requirements the parts are kept for several hours and temperature

- Then after allowed to cool the parts

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Nitriding- ADVANTAGES:-- Very high surface hardness - Minimizes distortion or cracking (no

quenching) - Good corrosion and wear resistance - Good fatigue resistance- No machining is required after nitriding- Complex parts can be done without difficulty - The process is economical for large number of

production requires

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Nitriding- DISADVANTAGES:-- Long cycle time (40 to 100 hrs)- The brittle case - Only special alloys can be done (Containing

Cr, Al and V)- High cost of nitriding- Technical control required - If over heated nitreted then hardness of the

surface may be lost and component must be nitreted again

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CYANIDING

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Cyaniding DEFINITION:-

In cyaniding, carbon and nitrogen are introduced into the surface of steel by heating it to a suitable temperature and holding it in contact with molten cyanide to form a thin skin or case which is subsequently quench hardened.

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Cyaniding CHARACTERISTICS:-

• Hardness after treatment is RC 65.

• Case depth is about 0.25 mm

• Negligible change in dimensions.

• Distortion may occur during heat treatment

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CyanidingTYPICAL USES:-

• Screw

• Nuts and bolts

• Small gear

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Cyaniding PROCESS:-

Low carbon steel is heated at 800 ˚ Cto 870 ˚C in a molten sodium cyanide bath for a period of between 30 min and 3 hrs depending upon the depth of case required.

The quenching in oil or water from this bath hardens the surface of the steel.

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Cyaniding • In cyaniding, the bath usually contains

30%NaCN,40%Na2 CO3 & 30%NaCl.The mixture has melting point of 1140 ˚ F

and remains quite stable under continuous operating conditions.

This mixture, when used at a temperature ranging from 787 ˚ C to 898 ˚ C, decomposes to free carbon and nitrogen which are then absorbed into the steel to form a hardened carbide-nitride case.

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Cyaniding

2NaCN+2O2 Na2CO3+CO+2N

2COCO2+C

2NaCN+O2 2NaCNO (sodium cyanide)

NaCN+CO2 NaCNO + CO

3NaCNO NaCN + Na2CO3 + C +2N

CHEMICAL REACTION

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Cyaniding In order to obtain hardness after cyaniding, it

is necessary to quench directly in to oil or water from the cyaniding bath.

The great use of cyaniding is for parts that are to be subjected to relatively light loads and that require improvement in the surface wear resistance.

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CARBONITRIDING

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CarbonitridingINTRODUCTION:-

Surface contain both carbon and nitrogen are where as produced by liquid salt bath in cyaniding, they are produced by the use of gas atmospheres in carbonitriding.

The carbonitrided alloy is usually quenched-hardened.

Metals usually hardened by carbonitriding.

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CarbonitridingCHARACTERISTICS:-

• Hardness after heat treatment is RC 65.

• Negligible change in dimensions.

• Distortion is less than in carburizing or cyaniding.

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Carbonitrding TYPICAL USES:-

In the case of hardening

• Gears

• Nuts

• Bolts