heat treatment chapter 2

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1

Heat Treatment

Case Hardening

and

Surface Treatment



2

Carburizing

Nitriding

Cyaniding

Carbonitriding

Flame Hardening

Induction Hardening

The processesused to create

hardened cases

are:-



3

CARBURIZING



4

Carburizing

Definition

Process

Characteristics

Typical useAdvantages

Disadvantages



5

NITRIDING



6

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 partiallydissociate (distance) ammonia or other suitable

medium

This process produces a hard case without

quenching or any further heat treatment.



7

Nitriding

CHARACTERISTICS:-

� Extreme Hardness� Case has improved its corrosion

resistance



8

Nitriding TYPICAL USES:-

� Gears

� Valve seats

� Guides� Gauges

� Bushing

� Air craft engine parts

� Aero engine cylinder

� Crank pin and journals



9

Nitriding

PROCESS:-

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



10

Nitriding - Nitriding: The components are placed in a heat

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

- At this temperature ammonia separates in tothree 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 thatrequirements the parts are kept for severalhours and temperature

- Then after allowed to cool the parts



11

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



12

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 thesurface may be lost and component must benitreted again



13

CYANIDING



14

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.



15

Cyaniding

CHARACTERISTICS:-

� Hardness after treatment is R C 65.

� Case depth is about 0.25 mm

� Negligible change in dimensions.

� Distortion may occur during heat treatment



16

Cyaniding

TYPICAL USES:-

� Screw

� Nuts and bolts

� Small gear



17

Cyaniding

PROCESS:-

Low carbon steel is heated at 800 Û Cto 870

ÛC 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.



18

Cyaniding

� In cyaniding, the bath usually contains30% NaCN,40% Na2 CO3 & 30% NaCl.

The mixture has melting point of 1140 Û F

and remains quite stable under continuousoperating conditions.

This mixture, when used at a temperatureranging from 787 Û C to 898 Û C,

decomposes to free carbon and nitrogenwhich are then absorbed into the steel toform a hardened carbide-nitride case.



19

Cyaniding

2NaCN+2O2 Na2CO3+CO+2N

2COCO2+C

2NaCN+O2 2NaCNO (sodium cyanide)

NaCN+CO2 NaCNO + CO

3NaCNO NaCN + Na2CO3 + C +2N

CHEMICAL REACTION



20

Cyaniding

In order to obtain hardness after cyaniding, itis necessary to quench directly in to oil or water from the cyaniding bath.

The great use of cyaniding is for parts that areto be subjected to relatively light loads and

that require improvement in the surfacewear resistance.



21

CARBONITRIDING



22

Carbonitriding

INTRODUCTION:-

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.



23

Carbonitriding

CHARACTERISTICS:-

� Hardness after heat treatment is R C 65.

� Negligible change in dimensions.

� Distortion is less than in carburizing or cyaniding.



24

Carbonitrding

TYPICAL USES:-

In the case of hardening

� Gears

� Nuts

� Bolts

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