3 hardening

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Hardening and Tempering

Hardening is process in which steel is heated to a temperature above the

critical point, held at this temperature and quenched (rapidly cooled) in

water, oil or molten salt baths.

As earlier mentioned that if a piece of steel is heated above its upper

critical temperature and plunged into water to cool it an extremely hard,

needle-shaped structure known as martensite is formed. In other words,

sudden quenching of steel greatly increases its hardness.

After hardening steel must be tempered to:

1.reduce a brittleness,

2.reliev e the internal stresses, and

3.obtain pre-determined mechanical properties.

The hardening process is based on a very important metallurgical reaction

of decomposition of eutectoid. This reaction is dependent upon the

following factors:

1.Adecuate carbon content to produce hardening.

2.Austenite decomposition to produce pearlite , bainite and martensite

structures.

3.Heating rate and time.

4.Quenching medium.

5.Quenching rate.

6.Size of the part.

7.Surface conditions.

The rapidly with which the heat is absorbed by the quenching bath

has a considerable effect on the hardness of the metal. Clear, cold

water is very oftenly used, while the addition of salt still increases

degree of hardness.oil, however , gives the best balance between

hardness toughness and distortion for standard steels.


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In order to increase the cooling rate the parts may be moved around

the quenching bath, either by hand, or by passing them through the

tank in basket attached to mechanical conveyer. Large parts may be

lowered into the tank by a crane and kept moving while cooling.

It is often cheaper and more efficient, however , to circulate the

cooling liquid around the hot part.

The heating rate and heating time depend on the composition of the

steel, its structure, residual stresses, the form and size of the part to

be hardened, the more the intricate and large the part being

hardened, the slower it should be heated to avoid stresses due to

temperature differences between the internal and external layers of

the metal, warping, and even cracking. The practically attainable

heating rate depends upon the thermal capacity of the furnace, the

bulk of the changed parts, their arrangement in the furnace, and

other factors. The heating rate is usually reduced, not by reducing

the furnace temperature but by preheating the articles.

The heating time for carbon tool steels and medium-alloy

structural steels should be from 25 to 30% more than for carbon

structural steels. The heating time for high-alloy structural and tool

steels should be from 50 to 100% higher.

When steel is exposed to an oxidizing atmosphere, because of the

presence of water vapor or oxygen in the furnace, a layer of iron

oxide called (scale) is formed. Thin layer of scale has very little

effect on cooling rate, but that a thick layer of oxide (0.005 in.

deep) retord the actual cooling rate.

Quenching mediaThe quenching media in general use are :

Water, Brine, Oils, Air, Molten salt.

Water : it is probably the most widely used as it simple and effective, it

cools at the rate of 982C per second. It tends, however, to form bubbles

on the surface of the metal being quenched an causes soft spots, so a

brine solution is often used to prevent this trouble.


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Brine : it is very rapid cooling agent and may tend to cause distortion of

the parts , as will water.

Oil : it is used when there is any risk of distortion although it is more

suitable for alloy steels than plain carbon steels.

Air blast : when the risk of distortion is great, quenching must be carried

out air blast. Since the rate of cooling is then lower, more hardening

elements must be added to the steel , forming an air-hardening alloy. The

air blast must be dry, since any moisture in the air will crack the steel.

Molten salts : high speed steels are often quenched in molten salt to

hardened them.

Note : hypo-eutectoid steel containing very little carbon, say less than

0.25%, cannot be easily hardened by sudden quenching because of large

amount of soft ferrite which is contains and all of which cannot be

retained in solution even on very quick cooling. The hardening capacity

of steel increases with carbon content.

Hardening methods

The most extensively used method is conventional hardening or

quenching in a single medium. The disadvantage of this method,

however, is that the cooling rate in the transformation range will be very

high. It will differ only slightly from the rate on the upper zone of super-

cooled austenite of low stability and, therefore, cracks, distortion and

other defects may occur in this method.

Conventional Heat, Quench and Temper Process:

In this process, Austenite is transformed to Martensite as a result of rapidquench from furnace to room temperature. Then, martensite is heated to a

temperature which gives the desired hardness. One serious drawback is

the possibility of distorting and cracking the metal as a result of severe

quenching required to form Martensite without transforming any of the

austenite to pearlite. During quenching process, the outer area is cooled

quicker than the center. Thinner parts are cooled faster than parts with

greater cross-sectional areas. What this means is that transformations of

the Austenite are proceeding at different rates. As the metal cools, it alsocontracts and its microstructure occupies less volume. ]


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Extreme variations in size of metal parts complicate the work of the heat

treater and should be avoided in the designing of metal parts. This means

there is a limit to the overall size of parts that can be subjected to such

thermal processing. (Figure2.24) shows the conventional hardening,

tempering process.

Figure 2.24 Conventional quenching and tempering process.

Other hardening method, which shall be briefly described, are generally

employed to avoid these defects ad to obtain the required properties.

The various hardening method are:

1. Quenching in two media.

2. Hardening with self tempering.

3. Stepped quenching or martempering.

4. Isotermal quenching or austempering.


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1.Quenching in two media:

Articles hardening by this method are first quenched in water to a

temperature from 300C to 400C and then quickly transferred to a less

intensive quenching medium (for example oil or air) where they are helduntil they are completely cooled. The purpose of the transfer to the

second quenching is to reduce internal stresses associated with the

austenite to martensite transformation. It is not advisable to quench first

in water and then in oil as this may lead to partial decomposition of the

austenite in its zone of the least stability (500C to 600C) and to the

development of high residual stresses due to rapid cooling in martensite

transformation range.

Quenching in two media is widely employed in the heat treatment of

carbon steel tools (taps, dies, milling cutters etc.) of a shape unfavourable

as cracking and warping.

2.Hardening with self tempering:

Here the article is held in the quenching medium until it is completely

cooled but is withdrawn to retain a certain amount of heat in core which

accounts for the tempering (self tempering). Frequently, more heat is

retained in the core than is required for tempering and, when the

tempering temperature is reached, the article is reimmersed in the

quenching liquid.

This hardening is applied for chisels, sledge hammers, hand hammers,

centre punches, and other tools that require a high surface hardness in

conjunction with tough core.

3.Stepped quenching or martempering:

After heating the steel to a hardening temperature, it is quenched in the

medium having a temperature, from 150C to 300C. the article is held

until it reaches the temperature of medium and then its cooled further to

room temperature in air and sometimes in oil, the holding time in the

quenching bath should be sufficient to enable a uniform temperature to be

reached throughout the cross section but long enough to cause austenitic

decomposition. Austenite is transformed into martensite during the

subsequent period of cooling to room temperature.


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This treatment will provide a structure of martensite and retained

austenite in the hardened steel. (the cooling is stopped at a point above

the martensite transformation region to allow sufficient time for the

center to cool to the same temperature as the surface. Then cooling is

continued through the martensite region, followed by the usual

tempering) [Figure 2.25 ] .

Figure 2.25 Martempering process.

Retained austenite there is a large volume expansion when

martensite forms from austenite. as the martensite plates form during

quenching, they surround and isolate small pools of austenite (Figure

2.26), which deform to accommodate the lower density martensite.

However, for the remaining pools of austenite to transform, the

surrounding martensite must deform. Because the strong martensite resist

the transformation, either the existing martensite cracks or the austenite

remains trapped in the structure as retained austenite.


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Figure 2.26 Retained austenite (white) trapped between martensite

needles (black) ( 1000).

Retained austenite can be a serious problem. Martensite softens and

become more ductile during tempering. After tempering, the retained

austenite cools below the Ms and Mf temperatures and transforms to

martensite, since the surrounding tempered martensite can deform. But

now the steel contains more of the hard, brittle martensite. A second

tempering step may be needed to eliminate the martensite formed from

the retained austenite. Retained austenite is also more of a problem high

carbon steels.

The martensite stars and finish temperatures are reduced when the carbon

content increases (Figure2.27). High carbon steels must be refrigerated toproduce all martensite.


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Figure 2.27 Increasing

plain-carbon steels.

Residual stresses

produced because of the

stress relief anneal can

due to cold working.

expansion and contracti

causes stress.

When steels are quenche

and transforms to mart

transforms, the hard su

compressed. If the resi

cracks form at the surfac

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carbon reduces the Ms and Mf tempe

and cracking residual stresses

volume change or because of cold w

e used to remove or minimize residu

tresses are also induced because o

n. In steels, there is one more mecha

d, the surface of the quenched steel coo

nsite. When the austenite in the ce

face is placed in tension, while the

ual stresses exceed the yield strengt

(Figure 2.28)

atures in

are also

rking. A

l stresses

thermal

nism that

ls rapidly

ter later

center is

, quench


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Figure 2.28 Formatio

produced during quenc

stresses as the austenite t

Martempering has thquenching:

1. less volume changes

retained austenite and p

2. less warping since the

of the article.

3. less danger of quenchi

On the other hand, the e

from 500 to 600C requi

this range to obtain supe

is much slower than i

austenite in carbon stee

500C, without decomp

thickness). Such articles

steel articles hardened by

38

of quench cracks caused by residua

ing. The figure illustrates the develo

ansforms to martensite during cooling.

following advantages over co

occur due to the presence of a large a

ssibility of self tempering of the marte

transformations occur simultaneously i

g cracks appearing in the articles.

tremely low solubility of austenite in

res a cooling rate of 200 to 500C per

rcooling. at the same time, cooling in

water or oil at room temperature

can be cooled through the zone fro

osition, only in thin articles (upto 5

are expediently hardened by this meth

this method, may be considerably thic

l stresses

pment of

ventional

mount of

site.

all parts

his range

second in

ot media

herefore,

600 to

.8 mm I

od. Alloy

ker.


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4. Isothermal quenching or austempering:

This is the second method that can be used to overcome the restrictions of

conventional quench and tempering. The quench is interrupted at a higher

temperature than for Martempering to allow the metal at the center of thepart to reach the same temperature as the surface. By maintaining that

temperature, both the center and the surface are allowed to transform toBainite and are then cooled to room temperature ( Figure 2.29).

Advantages of Austempering:

(1) Less distortion and cracking than martempering,

(2) No need for final tempering (less time consuming and more energyefficient)

(3) Improvement of toughness (impact resistance is higher than the

conventional quench and tempering)

(4) Improved ductility

Limitations of Austempering:

Austempering can be applied to parts where the transformation to pearlite

can be avoided. This means that the section must be cooled fast enough to

avoid the formation of pearlite. Thin sections can be cooled faster than

the bulky sections. Most industrial applications of austempering have

been limited to sections less than 1/2 in. thick. The thickness can be

increased by the use of alloy steels, but then the time for completion of

transformation to bainite may become excessive.


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Figure 2.29 Austempering process.

In Austempering process, the end product is 100% bainite. It is

accomplished by first heating the part to the properr austenitizing

temperature followed by cooling rapidly in a slat bath which is

maintained between 400 and 800 oF. The part is left in the bath until the

transformation to bainite is complete. The steel is caused to go directly

from austenite to bainite.

Quench rate In using the TTT diagram, we assume that we could

cool from the austenitizing temperature to the transformation temperature

instantly. because this does not occur in practice, undesired

microconstituents may form during the quenching process.

For example, pearlite may forms as steel cools past the nose of the curve,

particularly because the time of the nose is less than one second in plain

carbon steels.

The rate at which the steel cools during quenching depends on several

factors. First, the surface cools faster than the center of the part. In

addition, as the size of the part increases, the cooling rate at any location

is slower. Finally, the cooling rate depends on the temperature and heat

transfer characteristics of the quenching medium (Table 2.2 ).


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Quenching in oil, for example, produces a lower H coefficient, or slower

cooling rate, than quenching in water and brine.

The H coefficient is equivalent to the heat transfer coefficient. Agitation

helps break the vapor blanket (e.g., when water is the quenching medium)and improves overall heat transfer rate by bringing cooler liquid into

contact with the parts being quenched.

Sub-zero treatment

The resultant microstructure of a fully hardened steel should consist of

martensite. In practice, it is very difficult to have completely martensitic

structure by hardening treatment. Some amount of austenite is generally

present in the hardened steel. This austenite existing along with

martensite is referred to as retained austenite.

The presence of retained austenite greatly reduced mechanical properties

and such steels do not develop maximum hardness even after cooling at

rates higher than the critical cooling rate. The amount of retained

austenite depends largely on the chemical composition of steel. For plain

carbon steels, the amount of retained austenite increases with the rise in

carbon contents.


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The problem of retained austenite is more complex in alloy steels. Most

of the alloying elements increase the content of retained austenite.

In hardened steels containing retained austenite, the strength can be

improved by process known as sub-zero treatment or cold treatment.Retained austenite is converted into martensite by this treatment.

This conversion of retained austenite into martensite results in increased

hardness, wear resistance and dimensional stability of steel.

The process consist of cooling steel to sub-zero temperature which

should be lower than Mf temperature of the steel. Mf temperature

for most steels lie between -30C and -70C.

During the process, considerable amount of internal stresses aredeveloped in the steel, and hence tempering is done immediately

after the treatment. This treatment also helps to temper martensite

which is formed by decomposition of retained austenite during sub-

zero treatment.

Sub-zero treatment must be performed first after the hardening

treatment. Mechanical refrigeration units, dry ice, and some

liquefied gases such as liquid nitrogen can be used for cooling

steels to sub-zero temperature. This treatment is employed for : high carbon and high alloy steels

used for making tools, bearings, measuring gauges and components

requiring high impact and fatigue strength coupled with

dimensional stability-case hardened steels.

Example 2.4 Design of a Quench and Temper Treatment

A rotating shaft that delivers power from an electric motor is made from a

1050 steel. Its yield strength should be at least 145,000 psi, yet it should

also have at least 15% elongation in order to provide toughness. Design a

heat treatment to produce this part.

Solution:

We are not able to obtain this combination of properties by annealing or

normalizing (Figure2.23). however a quench and temper heat treatment

produces Aa microstructure that can provide both strength and toughness.


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(Figure2.30) shows that t

is tempered below 46

tempering is done above

The A3 temperature for t

1. Austenitize abov

appropriate tempe

2. Quench rapidly to

martensite will for

3. Temper by heati

sufficient if the ste

4.Cool to room temp

Figure 2.30 The effe

p

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he yield strength exceeds 145.000 psi ioC, whereas the elongation exceeds

425oC.

e steel is 770o

C. A possible heat treatm

the A3 temperature of 770oC for

ature may be 770 + 55 = 825oC.

room temperature. Since the Mfis abo

.

g the steel to 440oC. Normally, 1

el is not too thick.

rature.

t of tempering temperature on the mec

roperties of a 1050 steel.

the steel

15% if

ent is:

1 h. An

ut 250oC,

will be

anical

3 hardening

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