Hand Outs for Heat Treatment Equipments Part I

8/8/2019 Hand Outs for Heat Treatment Equipments Part I

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Unit 4

Heat Treatment Equipment Part I.

Various heating media used for heat treatment. Temperature and atmosphere control,

carburising atmosphere and carbon potential measurement, nitriding gas atmospheres.

1. INTRODUCTION:

The equipment required for heat-treating consists of furnaces or other heating devices, quenching baths or other cooling devices, temperature controls and indicators, and other controls and

indicators required or the proper operation of the equipment or for the control of the process.Some of the most important decisions that the heat treater will make are related to the selection

of furnaces and ancillary equipment. These decisions involve selection of the energy source, gasor electricity, which is vital to the overall profitability of the heat treatment process. Another is

the selection of the furnace transfer mode, batch or continuous, and the particular furnace type.

The focus of the discussion is on the furnace, furnace atmosphere generation, and ancillaryequipment.

For a heat treating process the important considerations to be focused are:

Various heating media used for heat treatment.

Temperature and atmosphere control Quenching media and their characteristics

Various heat treatment furnaces

Typical Furnace Components

(Open air heat treating)

Charge door: Work piece is introduced trough this charge door.



H earth: Work piece is placed over the hearth during heat treating.

Furnace chamber: H eat comes in contact with the work piece.

Burner: H eat may be supplied by liquid fuel, gaseous fuel or electricity .

Discharge door: The heat treated work piece is ejected through this door.

2. VARIOUS HEATING MEDIA USED FOR HEAT TREATMENT.

The heat treatment of metal components such as steel in air leads to surface oxidation. The type

and thickness of oxide layer s produced are dependent on the temperature of the heat treatment,the duration of exposure, and, also on the type of steel. To avoid surface oxidation there are

several options available. The air in the heat treatment furnace must be replaced by anatmosphere that does not contain oxygen. The use of an inert atmosphere, such as nitrogen (N2),

is one method. Another is the use of a protective atmosphere that utilizes nitrogen in combination

with reducing elements such as hydrogen (H2) and carbon monoxide (CO).

Another way to avoid surface oxidation is to reduce the amount of air surrounding theworkpieces during the thermal processing by evacuation to such a low level that the remaining

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

2.1 Mainly heat treatment processes are performed under the following conditions.

1 Open air heat treating.

2 H eat treating under the surrounding of some gases.

Primary Furnace Gases

Nitrogen

Hydrogen

Carbon Monoxide

Carbon Dioxide

Argon and Helium

Dissociated AmmoniaSteam

Hydrocarbons

Nitrogen is considered to be chemically inert and is used as a carrier gas for reactive furnace

atmospheres, for purging furnaces, and in other processes requiring inert gases. However, at high



temperature, nitrogen may not be compatible with certain metals such as molybdenum,chromium, titanium, and columbium.

Hydrogen is a highly reducing atmosphere that is used both for preventing steel oxidation and

for oxide reduction according to the surface reactions hydrogen may be adsorbed by the metal at

elevated temperatures, causing hydrogen embrittlement. Hydrogen is potentially an extremelyexplosive and flammable gas. However, if proper safety precautions are followed, it can be usedsafely in heat treatment.

Carbon monoxide is also considered to be a reducing gas as it may reduce iron oxide. Although

CO is a reducing atmosphere, it is not as good a reducing agent as hydrogen. Carbon dioxide is amildly oxidizing gas. It will form oxides upon reaction with iron at elevated temperatures.

Helium and argon are also considered to be inert gases for heat treatment processes becausethey will not undergo gas±solid reactions, even at high temperatures. Water vapor (steam) is also

an important component in heat treating.

Steam will react with steel at 343oC±650

oC to produce a blueing effect, which imparts a wear-

resistant and oxidation-resistant surface furnish. This is due to the formation of either Fe2O3,

Fe3O4, or FeO, depending on the surface temperature of the steel and the ratio of water vapor pressure to hydrogen pressure in the atmosphere.

3 Vacuum heat treating.

The term vacuum heat processing refers to heat treatment processes in which ferrous andnonferrous components are subjected to the application of thermal heat energy in a vacuum

environment. A vacuum heat treatment furnace first evacuates a particular space, then heats or

cools the space.



Carrying out thermochemical processes like carburizing and nitriding in vacuum furnaces at low pressures can be done using gases that produce high mass transfer rates where the thermal

dissociation of the process gas yields large amounts of reactive elements. In vacuum carburizing,for example, this is done only by the use of acetylene.

Advantages of Heat Treating in a Vacuum.

y Heating an item in a vacuum reduces oxidation.

y Since a vacuum pump evacuates the space as the item is heated, all gases attached to theitem can be removed.

y Pieces treated in atmospheric furnaces usually exhibit a light gray color even in reducingatmospheres, whereas the surfaces of vacuum heat-treated components remain bright andshiny.



3. TEMPER ATURE CONTROL:

The desired properties of a metal during a heat treating process can be achieved by proper control over the temperature of the furnace. So its necessary for us to measure the temperature

more correctly.

3 .1 THERMOCOUPLE PYROMETER

For the high temperatures met with in heat treatment furnaces one or other of the

temperature measuring devices known as pyrometers is required.

Thermocouple pyrometer

This is the most widely used temperature measuring device for heat treatment purposes. If the

junction of two wires made from dissimilar metals (such as a copper wire and an iron wire) form

part of a closed electric circuit and the junction is heated, a small electric current will flow. Anelectromotive force (emf ) is developed whose magnitude and direction depend on the contacting

materials and the temperature difference between the two points. The presence of this current

can be indicated by a sensitive galvanometer.



Increasing the temperature difference between the hot and cold junctions increases the current inthe circuit. If the galvanometer is calibrated in degrees of temperature, we have a temperature

measuring device called a pyrometer.

3 .2 THE R AD I ATION PYROMETER

The principle of this type of pyrometer is shown below

The radiation pyrometer Instead of the thermocouple probe being inserted into the furnace atmosphere, theradiant heat from the furnace or the component being heated in the furnace is focusedonto the thermocouple by a parabolic mirror.

The thermocouple probeconsists of a junction of

two wires of d issimilar

metals contained within a

tube of refractory metal

or of porcelain.

Porcelain beads are used

to insulate the two wires

and locate them in the

sheath as shown



Remember that as the temperature of a work reaches the furnace temperature, the rateat which the temperature of the work increases slows down. It is difficult to assess justwhen, if ever, the component reaches furnace temperature. Certainly, the soaking timeinvolved would give rise to excessive grain growth. Furnaces are frequently operatedabove the required process temperature, and the work is withdrawn from the furnace

when it has reached its correct temperature as measured by a radiation pyrometer.

This device is used to measure the temperature:

Of large hot components that have been removed from the furnace.

Where the furnace temperature is so high it would damage the thermocouple probe.

Where the hot component is inaccessible.

Where the temperature of the component in the furnace needs to be measured rather

than the temperature of the furnace atmosphere itself.

3.3 TEMPERATURE ASSESSMENT

There are simpler ways of assessing the approximate temperature; some of these willnow be described.

Paints and crayons

These are applied to the surface of the component to be heat treated. The mark left onthe surface by their application changes in colour and appearance when the desired

temperature has been reached. The paints and crayons are available in a range of compositions to suit the temperature required. They have the advantage of indicatingthe temperature of the component at the point of application. It has been statedpreviously that the temperature of the charge does not necessarily reflect thetemperature in the furnace. They can also be used to indicate the pre-heatingtemperature of components to be joined by welding. Another application is to mark theblades of gas turbines (jet engines) so, when undergoing routine maintenance, it can beseen if the blades have been overheated and therefore weakened.

Ceramic cones

These are also known as µSeger¶ cones and may be conical or pyramidal in shape. Thelatter have a triangular base. The µcones¶ are made with various compositions so thatthey soften at different temperatures. It is usual to choose three cones, one slightlybelow the required temperature (cone A), the second at the required temperature (coneB), and a third slightly above the required temperature (cone C).



4. ATMOSPHERE CONTROL:

When natural gas is burnt in a furnace, excess air is usually present to ensure complete and

efficient combustion. The resulting products of combustion (flue gases) contain oxygen, carbondioxide, sulphur, nitrogen and water vapour. These all react to a greater or lesser degree with the

surface of the workpiece when it is in the furnace. They will produce heavy scaling and, in thecase of steel, surface decarburization and softening. The situation is not so serious in the case of

a muffle furnace as the fuel is burnt in a separate chamber and cannot come into contact with thework. However, the oxygen and water vapour in the air are still present in the muffle chamber

and will cause some scaling and decarburization of the work. Little can be done to offset thiseffect in simple furnaces. However, in muffle furnaces air in the muffle chamber can be replaced

by alternative atmospheres, depending upon the process being performed and the metal beingtreated. This is known as atmosphere control . These controlled atmospheres can be based upon

natural gas (methane) and LPG gases such as propane and butane. For special applications,ammonia gas and µcracked¶ ammonia gas are used.

Heat treatment in gaseous atmospheres falls into two categories: processes with the aim of avoiding a mass transfer between the gaseous atmosphere and the material, and processes with

the aim of achieving just such a transfer. Mass transfer occurs when there is a difference in the potential between the constituents of a gaseous atmosphere and those of the microstructure of a

component.

Air in the furnace chamber can be replaced by alternati ve atmospheres, d epend ing upon the

process being performed and the metal being treated , in or d er to red uce scaling. This is known

as atmosphere control.

If the f urnace is below the required

temperature none of the cones sof ten and

droop as shown in Fig. (a).

If the f urnace is too hot, all the cones will

droop as shown in Fig. (b).

If the f urnace is at the correct temperature,

cone A will droop a lot, cone B will just start

to droop at the tip, and cone C will be

unaff ected. This situation is shown in Fig.



4.1 C A RBURIZING ATMOSPHERE

Carburizing depends upon the fact that very low carbon (0.1%) steels will absorb carbon when

heated to between 900C and 950C. Various carbonaceous materials are used in the carburizing

process.

Solid media

Molten salts

Gaseous media

S olid media such as bone charcoal or charred leather, together with an energizer such as sodium

and/or barium carbonate. The energizer makes up to 40% of the total composition.

Molten 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 up only between 20 and 50% of the total. Stringent safety

precautions must be taken in its use. The components to be carburized are immersed in the

molten salts.

Gaseous media based upon natural gas (methane) are increasingly used. Methane is ahydrocarbon gas containing organic carbon compounds that are readily absorbed into the steel.

The methane gas is frequently enriched by the vapours that are given off when mineral oils are

µcracked¶ by heating them in contact with the metal platinum which acts as a catalyst.

It is a common fallacy that carburizing hardens the steel. It does not , it adds carbon only to the

surface of the steel and leaves the steel in a fully annealed (soft) condition. It is the subsequent

heat treatment that hardens the steel.

4.2 C A RBON POTENTI A L ME A SUREMENT

The electrical resistance of steel varies with carbon content.

WEIGHT MEASUREMENT OF EQUILIBRIUM SHIM STOCK Blumenthal and Hlasny describe a method for calibration of carbon sensors that uses a test for

true carbon potential, which is measured by equilibrating an AISI 1010 steel shim sample, 0.003in. thick and 2.5 x 3 in. in area in a furnace atmosphere and then determining the carbon content

of the shim by weight gain (or by chemical analysis). The shim-holding device is illustrated below.

A pparatus for d etermination of carbon content by shim-stock exposure and measurement.



Proced ure:

1. Wearing rubber gloves clean the shim test specimen with acetone and weigh it on a balance to

the nearest 0.1 mg.

2. Roll the specimen into a cylinder approximately 3/4 in. in diameter, and insert it into the shim

holder as shown.

3. Be sure the furnace is operating above 871 oC and that there is a load in it. Close the doors

with the atmosphere circulation fan running; usually 30 min is sufficient to attain equilibrium.

4. Remove the plug from the sample side of the furnace, and open the gate valve. Insert the shim

holder to the same depth as the carbon±oxygen sensor, usually approximately 20in.

5. Leave the shim specimen in the furnace for 30 min.

6. Record the weight percent carbon from the carbon controller, average furnace temperature,

and average probe millivolt output during the test.

7. Move the shim-holder cartridge from the furnace to the cooling chamber.

8. Wearing rubber gloves, remove the steel shim from the shim-holder cartridge.

9. Clean the shim with acetone.10. Reweigh the shim to the nearest 0.1 mg. Calculate the carbon potential:

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

4.3 Various NITRI D ING gas atmosphere are:

i. Gas nitriding

ii. Salt bath nitriding

iii. Plasma nitriding

i) Gas nitriding

In gas nitriding the donor is a nitrogen rich gas usually ammonia (NH3), which is why it is

sometimes known as ammonia nitriding . When ammonia comes into contact with the heatedwork piece it disassociates into nitrogen and hydrogen. The nitrogen then diffuses from the

surface into the core of the material. This process has been around for nearly a century thoughonly in the last few decades has there been a concentrated effort to investigate the

thermodynamics and kinetics involved. Recent developments have lead to a process that can beaccurately controlled. The thickness and phase constitution of the resulting nitriding layers can

be selected and the process optimized for the particular properties required.



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

The advantage of this process is that hardness is achieved without the oil, water or air

quench. As an added advantage, hardening is accomplished in a nitrogen atmosphere that prevents

scaling and discoloration.

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.



The advantages of gas nitriding over the other variants are:

y All round nitriding effect (can be a disadvantage in some cases, compared with plasma

nitriding)y Large batch sizes possible - the limiting factor being furnace size and gas flow

y With modern computer control of the atmosphere the nitriding results can be tightlycontrolled

y Relatively cheap equipment cost - especially compared with plasma

The disadvantages of gas nitriding are:

y Reaction kinetics heavily influenced by surface condition - an oily surface or one

contaminated with cutting fluids for example will deliver poor resultsy Surface activation is sometimes required to successfully treat steels with a high

chromium content - compare sputtering during plasma nitridingy Ammonia as nitriding medium - though not especially toxic it can be harmful when

inhaled in large quantities. Also, care must be taken when heating in the presence of oxygen to reduce the risk of explosion

ii) 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 workpiece surface making salt bath anitrocarburizing process. The temperature used is typical of all nitrocarburizing processes: 550±

590 °C. The advantages of salt nitriding are:

y Quick processing time - usually in the order of 4 hours or so to achieve



y Simple operation - heat the salt and workpieces to temperature and submerge until theduration has expired

The disadvantages are:

y

The salts used are highly toxic - Disposal of salts are controlled by stringentenvironmental laws in western countries and has increased the costs involved in usingsalt baths. This is one of the most significant reasons the process has fallen out of favor in

the last decade or so.y Only one process possible with a particular salt type - since the nitrogen potential is set

by the salt, only one type of process is possible

iii) Plasma nitriding

Plasma nitriding, also known as ion nitriding , plasma ion nitriding or glow-discharge nitriding ,

is an industrial surface hardening treatment for metallic materials.

plasma ion nitri d ing

In plasma nitriding, the reactivity of the nitriding media is not due to the temperature but to thegas ionized state. In this technique intense electric fields are used to generate ionized molecules

of the gas around the surface to be nitrided. Such highly active gas with ionized molecules iscalled plasma, naming the technique. The gas used for plasma nitriding is usually pure nitrogen,

since no spontaneus decomposition is needed (as is the case of gas nitriding with amonia). Thereare hot plasmas typified by plasma jets used for metal cutting, welding, cladding or spraying.

There are also cold plasmas, usually generated inside vacuum chambers, at low pressure regimes.

Usually steels, alloy steels etc. are very beneficially treated with plasma nitriding. Plasmanitriding advantage is related to the close control of the nitrided microstructure, allowing

nitriding with or without compound layer formation. Not only the performance of metal parts



gets enhanced but working lifespan gets boosted. So does the strain limit, and the fatiguestrength of the metals being treated.

A plasma nitrided part is usually ready for use. It calls for no machining, or polishing or any

other post-nitriding operations. Thus the process is user-friendly, saves energy since it works

fastest, and causes little or no distortion.

In plasma nitriding processes nitrogen gas (N2) is usually the nitrogen carrying gas. Other gasses

like hydrogen or Argon are also used. Indeed, Argon and H2 can be used before the nitriding process during the heating up of the parts in order to clean the surfaces to be nitrided. This

cleaning procedure removes effectivelly oxide layer from surfaces and may remove fine layers of solvants that could remain. This also helps the thermal stability of the plasma plant since the heat

added by the plasma is already present during the warm up and hence once the processtemperature is reached the actual nitriding begins with minor heating changes. For the nitriding

process H2 gas is also added in order keep the surface clear of oxides.

4.4 C A RBONITRI D ING: This process involves with the diffusion of both carbon and nitrogen

into the steel surface. Methane or paropane serve as the source of carbon, the ammonia serves asthe 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.



Carbonitriding gives less distortion than carburizing. Carbonitriding is performed at temperaturesabove the transformation temperature of the steels (760

oC -to 870

oC)

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.

Hand Outs for Heat Treatment Equipments Part I

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