Heating of Steels: Furnace Atmospheres and Its Conditions | Metallurgy

In this article we will discuss about the types of furnace atmosphere and process of choosing controlled furnace atmosphere for heating of steel.

Types of Furnace Atmospheres for Heating of Steels:

1. Exothermic Atmospheres:

This name has been given to these atmospheres because these are produced by the exothermic combustion (i.e. without the addition of heat) of gas, and air, and are low-cost prepared furnace atmospheres.

These are also of two types:

Rich Exothermic:

Nominal composition in Vol % is:

N₂ = 71.5%; CO = 10.5%; CO₂ = 5%; H₂ = 12.5%; CH₄ = 0.5%

There are used commonly for clean annealing and tempering of steel, brazing of copper and silver, and sintering of powdered metals. As the carbon potential of gas mixture is below 0.10%, only low carbon steels are heat treated in them, otherwise decarburisation takes place. Water vapours are removed by usual methods. As these are flammable gas mixtures, proper precautions should be taken while purging and also prevent air infiltration.

The gas mixture is commonly obtained by combustion of a hydrocarbon fuel like natural gas, or propane at a temperature at least 980°C in a controlled chamber with controlled air to fuel ratio of not lower than 6.6 volume of air to 1 of fuel, or one volume of fuel reacts with 6 volumes of air (1.25 O₂ + 4.75 N₂) to give 6.63 volume of gases after removal of water vapour. Water vapours are removed by cooling after equilibrium has been attained,

Lean Exothermic:

Nominal composition in Vol % is:

N₂ = 86.8%; CO = 1.5%; CO₂ = 10.5%; H₂= 1.2%

These atmospheres do not find applications in heat treatment, but used where intentional surface oxidation is needed, or for application like annealing of copper, or for specialised low temperature work.

The atmosphere is obtained by the combustion of a hydrocarbon fuel like natural gas, propane, or light fuel oil with controlled air to fuel ratio and then cooled. Normally one volume of fuel and 9.5 volume of air is used to give 8.7 volume of gases after removal of water vapours. Generally, gas has 1% minimum of CO + H₂. When total combustible gas is less than 4%, then the gas is handled as an inert gas, though its large CO% necessitates care against leakage, etc.

2. Endothermic-Atmospheres:

These derive the name as endothermic combustion (that is energy is added) of gas and air takes place.

These can also be classified in two types:

Lean Endothermic:

N₂ = 45.1%; CO = 19.6%; CO₂ = 0.4; H₂ = 14.6%; CH₄ = 0.5%

Rich Endothermic:

N₂ = 39.8%; CO = 20.7%; H₂ = 38.7%; CH₄ = 0.8%

Endothermic gas atmosphere can be used in any furnace requiring reducing conditions, but it is more commonly used as a carrier gas in gas carburising and carbonitriding applications. It finds applications in bright hardening and for restoration of carbon in forgings.

A hydrocarbon gas is mixed with air (just enough air not to produce CO₂ and H₂), then, compressed to about 7 to 14 kPa and then, passed on to heated nickel bearing catalyst packed in pressure-tight retort, which is heated from outside commonly by natural gas to approximately 980° to 1040°C. The gases coming out of catalyst are cooled to 315°C by having a water jacket at the top of the retort so that reverse reactions do not take place.

A clean active catalyst (a porous, refractory based and impregnated with nickel oxide) free of soot is a must for proper gas production. Fuel gas should be free of H2S gas. The carbon potential of the product gas in controlled in gas generator by controlling dew point (kept between – 7°C to 16°C) by adjusting gas/air ratio. Better exact control is obtained by measuring CO 2% with the help of Infrared method. The carrier gas usually has a carbon potential of about 0.40%.

Natural gas, being mainly methane (CH₄) generally gives an endothermic gas having composition:

H₂ = 40.4%; N₂ = 39%; CO = 19.8%; CH₄ = 0.5% H₂O = 0.2%; CO₂ = 0.1%.

Any other hydrocarbon can be used such as propane, butane etc.

Propane gives a gas:

N₂ = 45.3%; H₂ = 31.1%; CO = 23.4%; CH₄ = 0.2%; with water vapours less than 1% but with no CO₂.

3. Prepared Nitrogen-Based Atmospheres:

These are exothermic atmospheres prepared by combustion of a mixture of air and a hydrocarbon fuel (which is commonly natural gas) from which all the CO₂ and water vapours have been removed.

Two classes of these atmospheres with compositions are:

Lean Gas:

N₂ = 97.1%; CO = 1.7%; H₂ = 1.2%

Rich Gas:

N₂ = 75.3%; CO = 11%; H₂ = 13.2%; CH₄ = 0.5%.

As these atmospheres have very low dew point of around – 40°C and are free of CO₂, they are neither oxidising, nor decarburising and are also cheap but have high initial cost of equipment. Thus, these atmospheres find use virtually in all furnace applications that do not require highly reducing conditions commonly such as annealing, normalising and hardening of medium carbon and high carbon steels.

Lean gas is used for heat treatment of low carbon steels (care being taken that air, etc. do not infiltrate in furnace) such as for annealing of steel coils in air-tight bell-type furnace, which uses air/natural gas ratio of 9 to 1 and the prepared atmosphere contains 4% of combustible gases. Lean gas is used for purging explosive gases from furnaces and also in furnaces in idle periods, and in large semi-continuous, or continuous annealing furnaces, even after blending with, an endothermic atmosphere.

Rich gas atmosphere is enriched with methane, or other hydrocarbons as it is used very often as a carrier gas is gas carburising and annealing. Annealing, or brazing of steel or copper alloys, sintering of iron powder compacts is done in rich gas atmospheres.

4. Commercial Nitrogen-Based Atmospheres:

These are industrial nitrogen gas-based atmospheres in which commercially pure N₂ is used by blending with reducing gases, if required.

There are three types of them depending on the objective:

Protective Atmospheres:

Such an atmosphere prevents oxidation or decarburisation during heat treatment particularly in batch, or continuous furnaces, of ferrous or non-ferrous metals. The composition is 90-100% N₂ and up to 5% of a reactive gas like CH₄, H₂ etc. and thus, the gas mixture may even reduce the surface metal oxides, though finds applications for avoiding oxidation of clean metal surfaces.

Reactive Atmospheres:

It has more of reactive gas, 5 to 10% of H2 and CO and balance N2 to reduce metal oxides, or even to transfer a small amount of carbon to ferrous materials (N₂ = 85%; H₂ = 10%; CO = 5%). These find applications in powder metallurgy.

Carbon-Controlled Atmospheres:

These atmospheres are used for varied applications. The dew point is less than – 60°C.

The composition of gas atmospheres are:

For hardening- N₂ = 97%; H₂ = 1%, CO = 1%, CH₄ = 1%

For carburizing- N₂ = 40% H₂ = 40%; CO = 20%.

For decarburizing- N₂ = 90%; H₂ = 10%.

Thus, the applications include carburising and carbonitriding of machine parts, hardening (neutral); Decarburising annealing of motor and transformer laminations and powder metal sintering.

The commercial nitrogen-based atmospheres have advantages of varying the composition (by blending) at will when required and at different times during a cycle, and even at different locations (zones) in the furnace.

For example, in the beginning of a cycle pure N₂ may be purged to eliminate O₂ from the surface and furnace, and then during heat treatment, a very reactive mixture may be used to bright anneal heavily oxidised parts, and then a very less reactive atmosphere may be used during cooling of the parts, and before opening the furnace door, again pure nitrogen purge may be made to remove the combustible gases. In a continuous furnace, the composition can be varied in different zones of the furnace. This ability to control the composition makes them more flexible.

Methanol is commonly used as a source of H₂ and CO (to be blended with N₂ gas) which above 760°C gives:

CH₃OH à 2H₂ + CO …(2.33)

5. Dissociated Ammonia-Based Atmospheres:

The atmosphere has 75% H₂ and 25% N₂ with a dew point less than—50°C, and thus, is a carbon-free reducing atmosphere. The high H₂ content empowers high deoxidising potential, which removes metal surface oxides and prevents scaling during high temperature heat treatment.

The anhydrous liquid ammonia is- fed in an electrically heated, or gas fired chamber (at 900-980°C) having one, or more catalyst filled alloy retorts, and then the gas mixture is cooled. Two volumes of ammonia give four volumes of dissociated ammonia.

It is a medium-cost atmosphere which is used in bright brazing of copper and silver, for bright heat treatment of carbon steels, copper and nickel alloys. It is used as a carrier gas in certain nitriding processes.

6. Hydrogen Atmosphere:

Commercial hydrogen is 98 to 99.9% pure with traces of water vapour and oxygen, etc. It is obtained by electrolysis of water, or by decomposition of ammonia, or catalytic conversion of hydrocarbons, or by water-gas reaction, etc. For metallurgical purposes, hydrogen obtained by electrolysis of distilled water is best suited after removing O₂ by room temperature catalytic process to give pure H2 of dew point—50°C.

Hydrogen is a powerful deoxidiser but in dry-state, decarburises high carbon steels at high temperatures to form methane. H2 when adsorbed causes hydrogen-embrittlement specially in high carbon steels. The oxide inclusions in steels may be reduced by hydrogen and the water thus produced, may build up high pressure at high temperatures to cause intergranular fracture in steels.

Dry hydrogen is used as an atmosphere in annealing of low carbon steels, stainless steels, electrical steels, non-ferrous metals, sintering of tungsten carbide and tantalum carbide, direct reduction of metal ores, sintering of metal powder components. Whenever H₂ is used in a furnace, adequate inert gas is used to purge (N₂ or products of combustion) it.

7. Steam Atmosphere:

It is used for scale-free tempering and stress-relieving of ferrous metals in temperature range of 345° to 650°C. Steam produces a thin, hard, tenacious blue-black oxide film of 0.00127 to 0.005 mm thickness on steel surface.

Thus, the surfaces must be clean and oxide-free before steam treatment is given. Steam is admitted, when the temperature is higher than 100°C to prevent rusting and condensation. Cast irons and steel parts develop high resistance to wear and corrosion by this treatment.

High speed steel drills, reamers, milling cutters, taps if steam-treated after tempering and then finish ground have 50 to 100% increased life of cutting edges. The oxide-film also holds the oil well. The oxides formed in sintered iron compacts seal the pores and the surface, and thus have high compressive strength as well as resistance to wear and corrosion.

8. Charcoal Based Atmosphere:

It has composition- N2 = 65%; CO = 34.7%; H₂ = 1.2%. It is still used in production of malleable iron castings and for small tool-room heat treating furnaces. It has low initial cost and is reasonably good for small units, or where intermittent use of atmosphere is needed.

Air supplied by a blower is passed through a bed of hot charcoal. As soon as air comes in contact at the bottom most part, it burns to give N₂, CO₂ and water vapour. This exothermic reaction raises the temperature of charcoal of upper portion to incandescence. The highly-heated charcoal converts CO₂ to CO and H₂O to H₂ gas as the gas-mixture moves up. Fly-ash is removed by filtration.

Normally, the gas mixture may have slightly different compositions due to volatile matter and water in charcoal, such as:

CO₂ = 1 – 2%; H₂ = 1.5 – 7%, CO = 30-32%, CH₄ = 0 – 0.5% N₂ = balance.

The gas-mixture is neutral to high carbon steels and if needed, carbon potential can be raised by adding natural gas. These atmospheres find applications in annealing, normalising and hardening of high carbon steels without scale, or decarburisation. Due to low H2 content, it is suited to malleable cast irons. The high operating costs, inability to make it automatic and being intermittent (due to time of charging charcoal and discharging ash), these atmospheres are less used.

9. Exothermic-Endothermic Based Atmospheres:

These exothermic based atmospheres are less reducing than conventional endothermic-based furnace atmospheres.

The compositions of two types of these are:

Lean Mixture:

N₂ = 63%; CO = 17%; H₂ = 20%

Rich Mixture:

N₂ = 60%; CO = 19%; H₂ = 21%

These atmospheres can be used as carrier gas for carburising and carbonitriding. Lean gas is used for clean hardening, and rich gas for carburising. The gas-mixture is produced by having a perfect combustion of fuel gas with air in a refractory lined combustion chamber (has 0.5% CO or O₂). Natural gas gives 11 volumes of combustion product. These gases are dehydrated by passing through water-spray coolers.

The cooled combustion products are then blended with a measured volume of hydrocarbon fuel. The mixture is then introduced under pressure into catalyst-filled U-shaped externally heated alloy retort. It is then sent to atmosphere cooler and then, piped to furnaces.

10. Inert Gas Atmospheres:

It is a protective gas that, as regards its carbon, oxygen and nitrogen contents, remains unreactive to the steel. From chemical reaction point of view, the only gas is argon that truly satisfies the criterion of inert gas. The most commonly used unreactive gas consists mainly of nitrogen.

11. Vacuum Heating:

This has found increasing applications in the last three decades. The cold furnace in which the parts are placed is evacuated with the help of vacuum pumps to a commonly acceptable operating pressure of 10^-2 torr, although the use of diffusion pumps can give higher vacuum, but the cost of furnaces becomes high.

If annealing is to be done, the parts are heated, austenitised and cooled along with furnace to room temperature usually in a nitrogen atmosphere but in special cases, argon is used. For oil hardening, good furnaces are available, where charge can be quenched straight into oil in underlying tank, while in vacuum. For this purpose, there are special oils (Silicon oil etc.) which can be heated to about 200°C.

12. Vacuum Furnaces Gas Carburising Atmospheres:

Any of the carburising atmospheres can be introduced in a vacuum furnace to do carburising at appropriate temperature, commonly between 870 to 980°C, though nitrogen based hydrocarbon enriched gas is commonly used. Here, carburising can be done to have high carbon case, and then, on vacuum diffusion gives the required surface carbon and case depth.

Advantage of this furnace is that diffusion process can be done at the end of the carburising period, or in a series of interrupted periods during the carburising. Though the surface carbon content and thus, the surface hardness remains almost the same in both cases, but the interrupted step gives a less severe drop in carbon content and a higher hardness relative to case depth. Ion carburising and Ion-nitriding atmospheres are dealt in sections about carburising and nitriding later.

Conditions for Choosing Controlled Furnace Atmospheres:

There are quite a few factors while choosing a particular type of atmosphere, the cost of operation being generally the foremost consideration, although the initial cost of equipment with safety devices too plays quite a dominant role. But these factors are of secondary importance when stringent metallurgical specifications are to be achieved. In many cases, when multiple heat treatments are needed with differing atmospheres, large time gap cannot be allowed in between.

For example in a batch furnace, if clean hardening is done after carburising, then it is good practice to use nitrogen-based atmospheres in which during carburising cycle hydrocarbons and oxidants could be added and then, clean hardening may be done after removing these additions. Many times, more volume of gas is needed for chemical reactions to take place during the process than just to exclude unwanted infiltration of air. For example, in sintering, the volume of H₂ required is based on the amount of oxide to be reduced.

Commonly, choice has to be made depending on requirement of the process, the furnace already present, the atmosphere generator already present, the specifications needed to be adhered and of course, the cost.

Control of Furnace Atmospheres:

A furnace atmosphere is required to produce a desired result under the given conditions, and the control of it means to maintain required levels of various constituents of it.

All the controls can be broadly divided into two classes:

1. To control the atmosphere, once it is inside the furnace,

2. Control the supply of gases before it is added into the furnace.

Three main devices are used for controlling the furnace atmospheres:

1. Infrared.

2. Dew point measuring,

3. Oxygen probe.

The infrared method needs drawing of a sample of gas and measure CO and/or CO₂ levels in it. It is used commonly for endothermic- exothermic generator operations. Dew point method also needs analysing a sample drawn from the furnace particularly having N₂ – H₂ atmospheres. Oxygen-probe is in situ device that is it directly analyses the sample of gas inside the furnace to give carbon potential of it to an accuracy of ± 0.05%.

Quite often, a control instrument determines the carbon potential of the atmosphere but simultaneously checks with actual carbon potential of the atmosphere as determined by shim analysis. A shim is a thin low carbon metal sample which when placed in the furnace gets homogeneously carburised quickly to level equal to the carbon potential of the atmosphere.

The instrument used is then calibrated with the result of shim. As metallurgical reproducibility of any given process becomes more important, the need for atmosphere control to produce chemical stability becomes a necessity. Good control of metallurgical atmospheres requires gas analysis at the furnace as well as in the generator.

The main idea of .control of furnace atmosphere is to control its composition i.e., % of gases like, H₂, CO, CO₂, H₂O, N₂ etc. The extent of control done also depends on the applications. Carburising and hardening need a carbon probe and control systems. Infrared gives better results.

In many cases, it is important to control atmosphere stability because the room winds, drafts due to open doors can shift the atmospheres. In much cases door curtains can contain the atmospheres. But when large parts enter, or leave the furnace, blanketing the furnace doors with an inert gas flow, can avoid air entering the furnace.

For N₂-based atmospheres, as no reaction or combustion takes place outside the furnace, there is no need to supply analytical instruments, but for generated atmospheres, the generator needs CO₂ and dew point measurement and control.

Some important hints are- Dew point will rise approximately 6°C between a sample taken at the endothermic gas generator and a sample taken at the furnace before enriching gases are admitted to produce desired carbon potential within the furnace chamber. Increasing air to endothermic generator increases CO₂ and the dew point. Increasing air to exothermic generator decreases CO, or total combustibles in gases.

Control of Surface Carbon in Heat Treatment of Steel:

There are quite a few factors while choosing a particular type of atmosphere, the cost of operation being generally the foremost consideration, although the initial cost of equipment with safety devices too plays quite a dominant role. But these factors are of secondary importance when stringent metallurgical specifications are to be achieved. In many cases, when multiple heat treatments are needed with differing atmospheres, large time gap cannot be allowed in between.

For example in a batch furnace, if clean hardening is done after carburising, then it is good practice to use nitrogen-based atmospheres in which during carburising cycle hydrocarbons and oxidants could be added and then, clean hardening may be done after removing these additions. Many times, more volume of gas is needed for chemical reactions to take place during the process than just to exclude unwanted infiltration of air. For example, in sintering, the volume of H₂ required is based on the amount of oxide to be reduced.

Commonly, choice has to be made depending on requirement of the process, the furnace already present, the atmosphere generator already present, the specifications needed to be adhered and of course, the cost.

Control of Furnace Atmospheres:

A furnace atmosphere is required to produce a desired result under the given conditions, and the control of it means to maintain required levels of various constituents of it.

All the controls can be broadly divided into two classes:

1. To control the atmosphere, once it is inside the furnace,

2. Control the supply of gases before it is added into the furnace.

Three main devices are used for controlling the furnace atmospheres:

1. Infrared.

2. Dew point measuring,

3. Oxygen probe.

The infrared method needs drawing of a sample of gas and measure CO and/or CO₂ levels in it. It is used commonly for endothermic- exothermic generator operations. Dew point method also needs analysing a sample drawn from the furnace particularly having N₂ – H₂ atmospheres. Oxygen-probe is in situ device that is it directly analyses the sample of gas inside the furnace to give carbon potential of it to an accuracy of ± 0.05%.

Quite often, a control instrument determines the carbon potential of the atmosphere but simultaneously checks with actual carbon potential of the atmosphere as determined by shim analysis. A shim is a thin low carbon metal sample which when placed in the furnace gets homogeneously carburised quickly to level equal to the carbon potential of the atmosphere.

The instrument used is then calibrated with the result of shim. As metallurgical reproducibility of any given process becomes more important, the need for atmosphere control to produce chemical stability becomes a necessity. Good control of metallurgical atmospheres requires gas analysis at the furnace as well as in the generator.

The main idea of .control of furnace atmosphere is to control its composition i.e., % of gases like, H₂, CO, CO₂, H₂O, N₂ etc. The extent of control done also depends on the applications. Carburising and hardening need a carbon probe and control systems. Infrared gives better results.

In many cases, it is important to control atmosphere stability because the room winds, drafts due to open doors can shift the atmospheres. In much cases door curtains can contain the atmospheres. But when large parts enter, or leave the furnace, blanketing the furnace doors with an inert gas flow, can avoid air entering the furnace.

For N₂-based atmospheres, as no reaction or combustion takes place outside the furnace, there is no need to supply analytical instruments, but for generated atmospheres, the generator needs CO₂ and dew point measurement and control.

Some important hints are- Dew point will rise approximately 6°C between a sample taken at the endothermic gas generator and a sample taken at the furnace before enriching gases are admitted to produce desired carbon potential within the furnace chamber. Increasing air to endothermic generator increases CO₂ and the dew point. Increasing air to exothermic generator decreases CO, or total combustibles in gases.

Control of Surface Carbon in Heat Treatment of Steel:

When steel is heated for heat treatment, a furnace atmosphere is required which is in equilibrium with the steel. Depending on the steel, the carbon could vary from 0.1% to the saturated austenite at the heat treating temperature.

The furnace atmosphere shall have both carburising and decarburising gases, whose compositions must be controlled at definite levels by analysing continuously and deficiencies made up as and when required, by adding methane, or air to the furnace atmosphere.

The carbon potential is controlled in principle by analysis of atmosphere by its either water vapour concentration (dew point), carbon dioxide concentration, or oxygen partial pressure.

The control of carbon potential can be illustrated by reaction:

where, C is the carbon dissolved at the surface of the steel.

The equilibrium constant, K₂ for reaction (2.15) is given by:

where, a_c is the activity of carbon of atmosphere and Pco₂, and Pco are the partial pressures of CO₂ and CO in the furnace atmosphere, respectively.

The equilibrium constant, K₂ is a function of temperature only and is given by:

where, ΔG° is the standard Gibbs free energy for the reaction 2.15, and R and T are gas constant and temperature, respectively.

From the thermodynamic data, K₂ is given by:

From equation 2.15, the activity of carbon, a_c, can be written as:

Hence, at a given temperature of carburising if Pco and Pco2 in the gaseous atmosphere are known, then activity of carbon of furnace atmosphere, ac, can be calculated, or if the activity of carbon of the atmosphere is known, then the required partial pressure of CO and CO₂ can be calculated, knowing that % CO + % CO₂ = 20%.

Now, if steel being heat treated is to be protected against carburising or decarburising of the atmosphere, the activity of the carbon of the furnace atmosphere i.e., carbon potential of atmosphere should be same as activity of carbon in the steel. The activity of carbon in a steel not only varies with the carbon content of steel and temperature, but also varies with the alloy content of the steel.

The activity of carbon in steel is related to weight percent carbon in austenite with the activity coefficient of carbon (f_c) by the following relationship:

where, fc is activity coefficient of carbon in steel which varies with temperature and composition of austenite, that is, alloying elements in austenite. Fig. 2.8 gives, for carbon and low-nickel steels (experimental) relationship between activity of carbon in steel and carbon in austenite for some isothermal temperatures between 815°C to 1040°C.

Thus, for plain carbon steels and low alloy steels, knowing the carbon content of steel, Fig. 2.8 could be used to know the activity of carbon, ac and then equation 2.34 could be used to get the partial pressures of CO₂ and CO, that is, the gas composition to maintain that particular carbon content on the surface of steel without carburisation or decarburisation.

Harvey’s tables could be used to obtain the value of activity coefficient in equation 2.35 depending on the alloying elements present in the steel.

The activity of carbon in a steel containing a number of alloying elements can be written as:

Table 2.4 given values of θ_c for some common alloying elements in steel:

Thus, depending on the composition of the steel to be protected, i.e. its carbon, and alloying elements, the activity of the carbon can be calculated (equation 2.36). The activity of carbon, ac, thus calculated, can then be used to calculate the required gas composition with the help of equation 2.34. For plain carbon steels and low alloy steels, Fig. 2.9 could be used to obtain % CO₂ in an endothermic furnace atmosphere to a good approximation.