Hardening

Hardening treatment consist of heating to predetermined temperature usually known as hardening temperature ,holding at that temperature followed by rapid cooling such as quenching in water ,oil or salt water .

High tensile strength and hardness can be achieved by this process.

The high hardness is achieved by rapid cooling , rapid cooling means cooling rate is equal to more than the upper critical cooling rate.

Rapid cooling results in the transformation of austenite at considerably low temperature product called martensite which is a hard microconstituent of steel.

Introduction

Hypo eutectoid steel The hardening temperature depends on

chemical composition in plain carbon it depends on carbon only .

Hypo eutectoid steel is heated to about 30 to 50 ̊C above the upper critical temperature.

Ferrite and pearlite transform to austenite at hardening temperature.

HardeningOf Hypo And Hyper Eutectoid Steel

The austenite transform to martensite on rapid quenching .

If hypo eutectoid steel is heated to a hardening temperature equal to hyper eutectoid steel the structure will consist of ferrite and austenite .

This will transform to martensite and ferrite on quenching. Ferrite is very soft phase lows the hardness of hardened

steel . This is known as incomplete hardening .

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Hyper eutectoid steel is heated to about 30 to 50 ̊C above the lower critical temperature.

The preferred hardening temperature for this steel lies between A1 and Acm .

there are two advantages in this temperature range . The first one is related to the presence of cementite in

hardened steel . The presence of cementite in martensitic matrix increase

the wear resistance because both the constituent are hard . The second advantage is to attainment of fine martensite

in the final structure.

Hyper Eutectoid Steel

If we heat the hyper eutectoid steel above Acm for hardening the coarsening of austenitic grains and decarburization occur .

Coarse austenite ill transform to coarse acicular martensite which has poor mechanical properties .

Decarburized surface respond poorly to hardening treatment.

In addition to these factors quenching from such a high temperature will introduce several internal stresses into the hardened steel

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Important factors are Chemical composition of steel Size and shape of steel part Hardening cycle Homogeneity and grain size of austenite Quenching media Surface condition of steel part

Factors Effecting The Hardening Process

Critical hardening temperature depends on the chemical composition of steel.

In alloy steel hardening temperature has to be controlled closely and depends on the nature and amount of the alloying element.

Comparatively large amount of retain austenite is present in the hardened steel contain austenitic stabilizing element due to which strength reduces .

If there is carbide forming element in a given alloy steel the larger is the volume fraction of carbide in the hardened steel and higher is the wear resistance.

Other properties such as longer too life ,increase cutting ability can be achieved if carbide are present in martensitic matrix .

Effect Of Chemical Composition

Large parts with variable thickness of sections are heated at a very slow rate as to avoid thermal gradient between outer and inner layer

These parts should be held at the hardening temperature foe sufficient time to attain uniform temperature throughout the volume .

Cooling rate should also be appropriate. Sharp corner should be avoided .

For larger parts martensitic transformation will not proceed throughout the volume this results in production of high magnitude of internal stresses.

Effect Of Size And Shape

For complete hardening, hardening temperature should be such that homogenous austenite which minimum grain size.

Ideally cooling rate should be equal to the just upper critical cooling rate

cooling rate more than the critical cooling rate will result in development of large internal stress

Slow heating rate is desirable in some cases. However slow heating rates are to be avoided

where the problems of oxidation and decarburization is encountered

Hardening cycle

In such cases steel component is heated in two stages. To start with, it is heated to an intermediate temperature which is

lower than the austenitizing temperature. The component is socked at this temperature for a sufficiently long

time to avoid oxidation and decarburization . Now the steel part is transferred to other furnace to desire

temperature. In this way the problem of oxidation and decarburization is

minimum . Steel component which are heated to a hardening temperature at

slow heating rate required less holding time for homogenization

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The properties of hardened steel depends to a large extent to a nature of austenite because martensite in the quenched steel is formed by the direct transformation of austenite.

Homogeneity and grain size of austenite play an important role as far as final properties are concerned.

Quenching Media A quenching media is that is one which provides a cooling

rate higher than the critical cooling rate. Quenching media characteristics such as its temperature,

specific heat, thermal conductivity, and latent heat of vaporization effect the cooling rate to a great extent

Homogeneity And Grain Size Of Austenite & quenching media

The presence of oil grease scale and other foreign particles on the surface is not desirable.

Oil and grease burn during heating and leave behind residue which is bad conductor of heat.

Thus the spots where this residue exists will cool at slower rate than the rest of the surface.

This may lead to incomplete hardening. Presence of scale and foreign particles also

result in variable cooling rate within in the mass.

Surface condition of steel part

Following are the hardening method Convectional or direct quenching Quenching in stages Spray Quenching Quenching with self tempering

Hardening methods

It consist of quenching the steel component from hardening temperature to quenching media.

The part is allow to cool up to to the temperature of quenching bath.

In addition to sever to internal stress steel part also develop tendency towards the distortion and cracking due to drastic cooling rate .

Cooling rate can be controlled by other quenching media like oil only smaller sections can be hardened in quenchants having lesser quenching power

Direct Quenching

Quenching in stages in sequence in different media consist of quenching steel part from hardening temperature to a bath maintained at a predetermined temperature which is higher than the MS temperature.

The medium used generally is water than it is transferred quickly to a milder quenching medium where it s cooled to room temperature such as oil and air.

Internal stresses are less in this method due to two reasons

1. The severity of cooling is reduced2. Internal stresses develop are also reduced

Stage Quenching

In this method steel part is cooled rapidly from hardening temperature by spray quenchant continuously

In this case the rate of heat extraction from the steel part is much higher as compare to direct quenching process due to continuous stream of quenchant in contact with steel surface

This process is used for selective hardening Water is most commonly used quenchant

Spray Quenching

Certain application specially those involving impact loading require a soft and tough core with hardened and toughened case.

This condition is attained by quenching with self tempering. In this process steel part is quenched from the hardening

temperature. The part is withdrawn from the quenching bath after some

time without allowing it to cool completely so considerable amount of heat will be retained in this central portion.

The component is cooled in mild quenching media such as oil/air.

Hardening with Self Tempering

The first quenching results formation of martensite The depth up to which martensite forms will depend to a

great extend on the time period allowed to remain in the quenching bath.

Cooling during second quench will result in homogenization of temperature

The core will be cooled at much reduce rate. The core will undergo austenite to pearlite transformation in

place of martensite So already formed martensite will be tempered by without

any additional tempering during second cooling Thus the result case will get hardened and core get soft