heat treatment equipments part 1

8/8/2019 Heat Treatment Equipments Part 1

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Dr. S. Rajendra Boopathy , Professor,Department of Mechanical Engineering,

College of Engineering, Guindy,

Anna University Chennai.

Unit 4

Heat Treatment Equipment -I (Various heating media used for heat treatment &

Temperature and atmosphere control)


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The equipment required for heat-treating consists

of furnaces or other heating devices, quenching baths, temperature controls and sensors, requiredfor the proper operation of the equipment or for the control of the process.

For a heat treating process the importantconsiderations to be focused are:

Various heating media used for heat treatment Temperature and atmosphere control Quenching media and their characteristicsVarious heat treatment furnaces


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VARIOUS HEATING MEDIA USED FOR HEATTREATMENT

I n order to obtain the required properties for a metal the heattreatment process is carried under the surrounding of severalgases.

O pen air heat treating.

Heat treating under the surrounding of some gases.

Vacuum heat treating.

These Are The Basic Media Used For Heat Treatment


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The heat treatment of metal components such as steelin air leads to surface oxidation. The type andthickness of oxide layers produced are dependent onthe temperature of the heat treatment, the duration of exposure, and, also on the type of steel.

(Open air heat treating)


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To avoid surface oxidation the air in the heattreatment furnace must be replaced by anatmosphere that does not contain oxygen.

Another way to avoid surface oxidation is toreduce the amount of air surrounding the

workpieces by vacuum heat treating.


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The use of an inert atmosphere, such as

nitrogen (N2), is one method to reducesurface oxidation.

Another is the use of a protectiveatmosphere that utilizes nitrogen incombination with reducing elements such ashydrogen (H 2) and carbon monoxide (CO).Thermal processing by evacuation of air to

a low level that the remaining oxygen is below the oxidation level of the material.This can be achieved by vacuum type heattreating.


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P rimary Furnace Gases

N itrogen

Hydrogen

Carbon Monoxide

Carbon Dioxide

Argon and Helium

Dissociated Ammonia

Steam

Hydrocarbons


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C HARA C TERISTI C S OF GAS MEDIUM

Nitrogen is considered to be chemically inert and is used as a carrier gasfor reactive furnace atmos pheres, for purging furnaces, and in otherprocesses requiring inert gases. However, at high tem perature, nitrogenmay not be com patible with certain metals such as molybdenum,chromium, titanium, and columbium.

Ex ample:

NITRIDING is one exam p le by which a case or skin of hardened steel canbe produced. The p iece to be case hardened is put into a furnace andheated to between 950 and 1,200F and, at the same time, is ex posed toammonia gas. The heat of the furnace causes the ammonia to break downinto nitrogen and hydrogen. Some of the nitrogen combines with theelements in the steel to form chemical com pounds called nitrides in theouter layer of the steel. These nitrides give the surface its hard, wear-resistant qualities.


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Hydrogen is a highly reducing atmosphere that is used both for preventing steel oxidation and for oxide reduction according to thesurface reactions hydrogen may be adsorbed by the metal at elevatedtemperatures, causing hydrogen embrittlement. Hydrogen is

potentially an extremely explosive and flammable gas. However, if proper safety precautions are followed, it can be used safely in heattreatment.

Ca rbon monoxide is also considered to be a reducing gas as it mayreduce iron oxide. Although CO is a reducing atmosphere, it is notas good a reducing agent as hydrogen. Carbon dioxide is a mildlyoxidizing gas. I t will form oxides upon reaction with iron at elevatedtemperatures.


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Helium a nd a rgon are also considered to be inert gases

for heat treatment processes because they will notundergo gassolid reactions, even at high temperatures.Water vapor (steam) is also an important component inheat treating.

Ste a m will react with steel at 343 oC650 oC to produce a blueing effect, which imparts a wear-resistant andoxidation-resistant surface furnish. This is due to theformation of either Fe 2O3, Fe 3O4, or FeO, depending on

the surface temperature of the steel and the ratio of water vapor pressure to hydrogen pressure in the atmosphere.


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VA C UUM HEAT TREATING

The term vacuumheat processingrefers to heattreatment processesin which ferrous andnonferrous

components aresubjected to theapplication of thermal heat energyin a vacuumenvironment.

A vacuum heat treatment furnace first evacuates a particularspace using vacuum pum p , then heats or cools the s pace.


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P ieces treated in atmospheric furnaces usuallyexhibit a light gray color even in reducingatmospheres, whereas the surfaces of vacuumheat-treated components remain bright and shiny.Heating an item in a vacuum reduces oxidation.Since a vacuum pump evacuates the space as the

item is heated, all gases attached to the item can be removed.

ADVANTAGES OF VA C UUM HEAT TREATING


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TEMPERATURE C ONTROL

The desired properties of a metal during aheat treating process can be achieved byp ro per control over the tem perature of thefurnace. So its necessary for us to measure thetem perature more correctly.

For the high tem peratures met with in heat

treatment furnaces one or other of thetem perature measuring devices known as

pyrometers is required.


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Thermocouple pyrometer

This is the mostwidely usedtemperaturemeasuringdevice for heattreatmentpurposes.

If the junction of two wires made from dissimilar metals ( such as acopper wire and an iron wire) form part of a closed electric circuitand the junction is heated, an electromotive force (emf ) isdevelo ped whose magnitude and direction de pend on thecontacting materials and the tem perature difference between thetwo points. .


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The presence of this current can be indicated by a

sensitive galvanometer.If the galvanometer iscalibrated in degrees of temperature, we have a

temperature measuring device called a pyrometer.


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The thermocouple probe consists of a junction of two wires of dissimilar metals contained within a tube of refractory metal or of porcelain.P orcelain beads are used to insulatethe two wires and locate them in thesheath as shown


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The radiation pyrometer

The radiant heat from the furnace or the com ponent being heated in the furnaceis focused onto the thermocou p le by a parabolic mirror.Furnaces are frequently o perated above the required process tem perature, andthe work is withdrawn from the furnace when it has reached its correcttem perature as measured by a radiation pyrometer.


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Uses of RADIATION PYROMETER

Used for large hot com ponents that have beenremoved from the furnace.W here the furnace tem perature is so high itwould damage the thermocou p le p robe.W here the hot com ponent is inaccessible.W here the tem perature of the com ponent inthe furnace needs to be measured rather thanthe tem perature of the furnace atmos phereitself.


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Temperature assessmentThere are sim p ler ways of assessing theappro x imate tem perature; some of these will nowbe described.

Paints and crayonsThese are a pp lied to the surface of the com ponentto be heat treated. The mark left on the surface by their application changes in colour and appearance when the desired temperature hasbeen reached . The paints and crayons are availablein a range of com positions to suit the tem peraturerequired. They have the advantage of indicatingthe tem perature of the com ponent at the point of app lication.


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C eramic cones

These are also known as Seger cones and maybe conical or pyramidal in sha pe.

The cones are made with various com positionsso that they soften at different tem peratures. It is

usual to choose three cones, one slightly belowthe required tem perature (cone A), the second atthe required tem perature (cone B), and a thirdslightly above the required tem perature (cone C).


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I f the furnace is below the required temperature none of the cones soften and droop as shown in Fig. ( a).

I f the furnace is too hot, all the cones will droop as shown in Fig. ( b).

I f the furnace is at the correct temperature,cone A will droop a lot, cone B will just start todroop at the tip, and cone C will be unaffected.This situation is shown in Fig. ( c).

These are some of the Sim p lest ty pe of techniques used for tem perature assessment.


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ATMOSPHERE CONTROL

Re a son for Atmospheric controlWhen natural gas is burnt in a furnace, excess air is usually

present to ensure complete and efficient combustion. Theresulting products of combustion (flue gases) contain oxygen,carbon dioxide, sulphur, nitrogen and water vapour. These allreact to a greater or lesser degree with the surface of theworkpiece & will produce heavy scaling .

What is atmospheric control?

Air in the furnace chamber can be re p laced by alternative

atmos pheres, de pending u pon the process being performedand the metal being treated, in order to reduce scaling. This isknown as atmosphere control .


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CARBURIZING WI TH G ASES

A process in which carbon is added to the surface layersof low carbon steels or low alloy steels to a carefullyregulated depth. This addition of carbon is calledcarburizing.

Carburizing is achieved by heating the steel attemperatures in the homogeneous austenite phase field inan environment of appropriate carbon sources . Thecarburizing time depends on the de sired diffusion depth.The processes in use are classified according to their carbon sources in pack carburizing (solid compounds),salt bath carburizing (liquid carbon sources), and gas and

plasma carburizing (gaseous carbon sources).


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a) First, the work p iece isheated to between 900 Cand 950 C in contact withthe carburizing

com pound until theadditional carbon hasbeen absorbed to the

required de p th.


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b) Second, the work p iece isremoved from the

carburizing com pound &reheated to between 780 Cand 820 C


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c) Finally the com ponent isdipp ed in cold water for

quenching.

After quenching a component from a temperature above 780 C


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C arburizing atmosphere

Carburizing de pends u pon the fact that verylow carbon (0.1%) steels will absorb carbonwhen heated to between 900 C and 950 C.Various carbonaceous materials are used inthe carburizing p rocess.

Solid media

Molten saltsG aseous media


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M olten salts such as sodium cyanide, together with

sodium carbonate and/or barium carbonate and sodium

or barium chloride. Since cyanide is a deadly poison such

salts must be handled with great care and the cyanide

makes u p only between 20 and 50% of the total.Stringent safety precautions must be taken in its use. The

com ponents to be carburized are immersed in the

molten salts .


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CARBON P O TE N TI AL ME AS U R EME N T (The electrical resistance of steel varies with carbon content)

M easuring Procedure:AISI 1010 steel shim sam p le is used for measurement. A ISI 1010 is aStandard grade Carbon Steel.Clean the shim test s pecimen with acetone and weigh it on a balance tothe nearest 0.1 mg.insert it into the shim holder as shown and Be sure the furnace isoperating above 871 oC.Leave the shim s pecimen in the furnace for 30 min. (Carburizing takesp lace)

A pparatus for determination of carbon content by shim-stock exposure and measurement


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Move the shim-holder cartridge from thefurnace to the cooling chamber.Remove the steel shim from the shim-holdercartridge.Reweigh the shim to the nearest 0.1 mg. Andcalculate the carbon potential using:

where wt% C is the original weight percent carbon content.


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Nitriding process

N itrogen is diffused into the surface of thesteel being treated.

The nitrogen source is usually Ammonia (N

H3).At the nitriding tem perature the ammoniadissociates into N itrogen and Hydrogen.V arious NITRIDIN G gas atmosphere are:

Gas nitridingSalt bath nitriding

Plasma nitriding


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Gas Nitriding processThe reaction of nitrogen withthe steel causes the formationof very hard iron and alloynitrogen com pounds.

N itriding tem perature is below the lower critical tem perature

of the steel and it is set between 496 oC and 565 oC.


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Advantage of this process

The resulting nitride case is harder than toolsteels or carburized steels.

The advantage of this p rocess is that hardnessis achieved without the oil, water or airquench.As an added advantage, hardening isaccom p lished in a nitrogen atmos phere thatp revents scaling and discoloration.


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The white layer has a detrimental

effect on the fatigue life of nitrided parts, and it is normally removed

from parts subjected to severe

service. Two stage gas-nitriding

processes can be used to prevent

the formation of white layer

As the nitriding time increases the

effective case depth also increases


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Ca rbonitriding: This process involves with the

diffusion of both carbon and nitrogen into the steelsurface. Methane or paropane serve as the source of

carbon, the ammonia serves as the source of nitrogen.

Quenching is done in a gas which is not as severe as

water quench. As a result of les severe quench, there is

less distortion on the material to be treated.


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The process is performed in a gas atmosphere furnace

using a carburizing gas such as propane or methane

mixed with several percent (by volume) of ammonia.

Carbonitriding gives less

distortion than carburizing.

Carbonitriding is performed at

temperatures above the

transformation temperature of

the steels (760 oC -to 870 oC)


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Salt bath nitriding

In salt bath nitriding the nitrogen donating medium is a

nitrogen containing salt such as cyanide salt. The salts used

also donate carbon to the work p iece surface making salt

bath a nitrocarburizing process. The tem perature used istyp ical of all nitrocarburizing processes: 550 590 C.

The advantages of salt nitriding are:

Quick p rocessing time - usually in the order of 4 hours or soto achieveSimp le o peration - heat the salt and work p ieces totem perature and submerge until the duration has ex p ired


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The disadvantages of Salt bath nitriding are:

The salts used are highly toxic - Dis posal of salts are controlled by stringent environmentallaws in western countries and has increasedthe costs involved in using salt baths. This is

one of the most significant reasons thep rocess has fallen out of favor in the lastdecade or so.

Only one p rocess possible with a particularsalt ty pe - since the nitrogen potential is set bythe salt, only one ty pe of p rocess is possible


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Plasma nitriding

P lasma nitriding, also known as ion nitriding , plasma ion nitriding or glow-discharge nitriding ,is an industrial surface hardening treatment for

metallic materials.As an alternative to gas nitriding, p lasma (ion)nitriding process has been develo ped toovercome the shortcomings of the earlier

traditional uncontrolled gas nitriding p rocessesand to offer certain o perational advantages thatgas nitriding does not have.


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The gas used for p lasma nitriding is usually

pure nitrogen.Electric fields are used to generate ionized

molecules of the gasCharged particles are res ponsible for the

nitriding effect.

This figure shows how the metallurgicalpro perties of the nitride layer and the whitelayer can be controlled in the p lasma nitridingprocess by adjusting the process gas

com position.


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P lasma is essentially a gas nitriding treatment in which the

method of delivering nitrogen atoms to the surface of nitrided

components is quite different from the standard gas nitriding

processes. I t occurs at a very low pressure and under high

voltage.

P lasma offers certain distinct advantages such as:The ease of masking the component surface where nitriding is

to be avoided.

Ability to nitride low density powder metallurgy parts.


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heat treatment equipments part 1

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