Atmospheres

The question is often asked, “What is an atmosphere?”  A simple description of an atmosphere is a gas that can be introduced into a thermal process furnace with the objective of providing:

• A surface protection environment
• A controlled oxidation environment
• The introduction of elements for surface modification

There are many ways that a heat treatment atmosphere can be a generated. The atmosphere can be a synthetic atmosphere, or simply gas that is pre-mixed/pre-stored.

Three retort Endo thermic gas generator

If one considers a wood fire and the flame being generated from that wood fire is seen as a bright yellow, then the flame is a carbon rich flame.  If one considers wood charcoal that is commonly used for a barbecue, then the gas generated as a result of the combustion is carbon monoxide, which is a carburizing gas and carbon dioxide which is considered as a decarburizing gas.

Because of the increasing demand for repeatable metallurgical results, great emphasis is being placed on the use of consistent gas analyses for various metallurgical processes. (Carburizing for example) It is necessary to understand the basic reactions of the industrial gases chosen for metallurgical processing.  Depending upon the application in question, one can use individual gases or combinations of gases.  The principal gases used in the heat treatment of steel are as follows:

• Oxygen is perhaps the most freely available gas that there is. It is present in many generated gases such as the endothermically generated gases.  Oxygen will react readily with iron in steel to produce iron oxide, more readily known as scale. It will also create Inter Granular Oxidation in the surface of an atmosphere carburized component

        In addition to this, oxygen will react with the carbon present in the surface of the steel and cause surface decarburization.  It must be said at this point, that some processes take advantage of the presence of oxygen in order to create a controlled surface oxidation.  This is accomplished to provide a corrosion resistant barrier on the surface of the steel.

• Nitrogen is usually present in an atmosphere as molecular nitrogen which is passive to ferrite and is most satisfactory for use in the processing of low carbon steels for annealing. The grade of nitrogen must be chosen very carefully due to the potential for the presence of moisture in the gas.  If the gas does contain moisture then the surface oxidation and potential decarburization will take place. 
• Therefore, clean dry nitrogen is necessary for successful annealing of low carbon steels. If atomic nitrogen is used (this means if the gas is cracked to provide atomic nitrogen for a fraction of a second) then the nitrogen will begin to react with the iron (and alloys) to form finely divided nitrides (Iron Nitrides) that will be present in the surface of the steel. 
• This will cause an increase in hardness of the surface and in some cases brittleness, particularly at sharp corners. Nitrogen is often considered by many to be a neutral atmosphere. This is a misunderstanding of the action of nitrogen under heat.  Nitrogen will prevent surface oxidation, but it will not stop surface decarburization. 
• It should be remembered that in order to prevent surface decarburization, the carbon potential of the furnace atmosphere needs to be in equilibrium to the surface carbon potential of the steel.

• Carbon dioxide. At elevated austenitizing temperatures, carbon dioxide will react with the carbon in the steel surface and will produce carbon monoxide as shown in the following the reaction:

                                                                    (C)  + CO2 = 2CO                                                        

The above reaction will continue to occur until there is no carbon dioxide available in the furnace atmosphere or until the steel surface has completely lost it’s carbon. In other words it can be seen that the carbon dioxide is an oxidizing gas.

• Hydrogen is considered to be a reducing gas which will reduce iron oxide to iron or copper oxide to copper. At temperatures above 1300 ° F. Hydrogen will have a decarburizing affect on the surface of the steel.  Below 1300 ° F. the decarburization is almost negligible.  If water vapor is present, then this will increase the decarburizing effect because of its dissociation and providing atomic nitrogen and oxygen. The reaction will also react with carbon in the steel surface to form methane in the following  manner:                                  

                                                                     (C )   +  4H  =  CH4

• Walter Vapor. If one is using an endothermic gas generator, then the air that is being used to form the endothermic reaction contains water vapor from the atmosphere. Water vapor will oxidize the iron in the surface of the steel and will combine also with the carbon in the steel surface to form carbon monoxide and hydrogen:                                           

                                                                 (Fe) + H2 O   = FeO +H2 (Reaction 1)

                                                                 (C)   + H2 O = CO + H2

This is why steam is often used as a bluing agent for motor laminations as it will oxygen eyes the surface of the steel and if the temperature is correct at around 700 ° F., then a blue color will be seen.

• Hydrocarbon gasses are those gasses which are rich in carbon, that can be easily cracked and shown in the following:                                      

                                                                   (CH4)   = Methane

                                                                   (C3 H8) = Propane

                                                                   (C2 H6) = Ethane

                                                                    (C2H2) = Acetylene

These gases will produce carbon rich atmospheres within the furnace process chamber.  The chemical activity which will take place at the surface of the steel will depend upon the surface temperature of steel in order to decompose the carbon rich gas into nascent carbon.  One needs to be very careful in the selection of the carbon rich gas in order to minimize the risk of sooting occurring within the process chamber.

• Ammonia is often used for one or both of its elemental gases (nitrogen and/or hydrogen). The ammonia can be used as a source of nitrogen for the nitriding process, or a source of hydrogen for a reducing process.  The ammonia can be produced as a generated gas, a bottled gas, or a bulk storage gas.
• Argon is truly an inert gas and that will not react with a metal surface. Its use is more often seen in the aerospace industry, providing a truly non reactive atmosphere gas.  The major drawback of using argon is the cost.

Process Troubleshooting

Protective atmospheres for heat treatment shops (captive or commercial) can fall into five categories:

Endothermic Atmospheres

Endothermic based atmospheres are manufactured in an endothermic gas generator.  The gas generator is a very simple furnace construction and its principle of operation is extremely simple.  The generated gas is simply a mixture of a hydrocarbon and gas and air in very specific ratios.  As one is aware, air will always carry moisture.  The two gases of air and hydrocarbon are mixed together followed by a compression.  The compressed gas is then passed through a nickel impregnated catalyst which will act as at catalyst to decompose and clean the gas of heavy carbon (soot).  The compress gas is passed through a heated chamber which holds the nickel catalyst at temperature of approximately 1900 ° Fahrenheit.  The gas output composition will be approximately in the following percentage by volume:

• Nitrogen (N2)              =       1 %
• Carbon monoxide (CO) = 6 %
• Methane (CH4)           =        3%
• Carbon Dioxide (CO2) =      4 %
• Hydrogen  (H2)             =        6 %

This is based on an atmospheric temperature of 72 ° Fahrenheit to produce a dew point at approximately 50 ° Fahrenheit. The carbon potential of the gas will be approximately 0.3%. The gas ratio of gas to air should be approximately 2.8 volumes air to 1 volume of gas. This will vary according to the ‘natural gas’ source.   

On exhausting the from the furnace, the process gas passes through a cooler which is there to condense out any heavy carbon and to prevent it from being carried over to the process furnace.  At the process furnace it is there where one can decide how to blend the gas with the enriching gas

Troubleshooting the Endothermic Generator

If atmosphere problems arise at the process furnace, it is prudent to check the performance of the endothermic gas generator. The following sequence is a suggestion to commence trouble shooting.

The function of the endothermic generator is based upon the performance of the nickel catalyst cubes, and it is necessary to keep the catalyst cubes clean and clear of residual carbon. A simple operation is necessary to ensure the cleanliness and the ability of the generator to produce good clean gas.  The procedure is called burnout. The sequence of the burnout procedure is simply to reduce the cracking temperature of the generator to approximately 1500 ° Fahrenheit, with the process gas turned off.  The generator is then run with only the air compressor operating.  The presence of air inside the process retort at 1500 ° Fahrenheit will cause the carbon to ignite. The time taken to complete the burned-out procedure will be determined by the amount of carbon present in the nickel catalyst. Generally, the time can be between one to four hours.

Below is given a chart which will show the relationship of dew point to process temperature in relation to the common potential of the atmosphere being sampled either at the generator or the process furnace.

Trouble Shooting the Process Furnace Atmosphere

If it has been ascertained that the endothermic generator is functioning correctly, then the next step is to observe the process furnace atmosphere conditions. There are five common methods of testing and controlling the furnace atmosphere which are as follows:

• Shim analysis, by weight or by controlled burning
• Three or four gas analyzer
• Dew point
• Oxygen probe
• CO2/Infra red analyzer

Perhaps the most common method of testing the furnace atmosphere will be by using the dew point method. This method is an old but tried and tested method of atmosphere control.  It is usually supported by one or two other methods of control which shim analysis (Fig 4) are using the formulae as follows:

Carbon Potential =   Final Weight – Original Weight X 100% + Original  Carbon Content.

                        Final Weight

In order to ensure that no contamination is carried into the process furnace, it is then necessary to ensure that the part is cleaned prior to entry into the process furnace.  Cutting fluids, cutting oils, and lapping compounds may contain chlorides, sulfides, silicones and hydrocarbon compounds. 

These surface contaminants may lead to a serious disturbance in the furnace atmosphere as well as at the work surface.  In addition to this it must be ensured that the work piece surface is free and clear of surface oxides prior to entry into the furnace. 

The presence of oxides on the work piece surface will lead to non uniform case formation (if carburizing).  It is then necessary to ensure that the parts are pre-cleaned or degreased prior to entry into the process furnace.

If high dew points are being experienced in the process furnace atmosphere, then it is likely that air is present in the furnace atmosphere.

There are many sources of air ingress into the furnace.  Check first that all door seals are not broken or damaged. The next area to observe would be all pneumatic cylinders that operate inner doors and elevators. 

If the furnace has an internal mechanical handling the device, then the external drive housing that is mounted onto the side of the furnace could be improperly sealed, or the seal could be damaged.  Another source of oxygen/air that is often overlooked and could contribute to high dew point levels is the external air itself. 

High air humidity levels will contribute to high furnace dew point levels.  If high water vapor levels are present within the furnace atmosphere, then there is a serious risk of causing the following reaction:

• Fe +  H2O Fe O + H2

It can be seen that water vapor will cause serious surface oxidation to any alloy steel at an elevated temperature and it will combine with carbon in the steel to form carbon monoxide and hydrogen as shown in the following reaction: 

• (C) + H2O  CO + H2

A further effect of the presence of moisture within the furnace atmosphere will be to cause a grain boundary oxidation.  Grain boundary oxidation can have a serious adverse effect on the steel surface particularly if there is no further machining to take place after the heat treatment procedure.

 The following graph shows the effect of dew point in relation to process temperature, in the relation to carbon percentage within the furnace.

Trouble Shooting the Carburizing Furnace Atmosphere

The carbon potential in the process furnace atmosphere to ensure a good carburized case should be determined by

• The type of steel to be treated
• The carburizing process temperature

Generally, one would use an atmosphere carbon potential between the eutectoid line on the Iron Carbon Equilibrium diagram at approximately 0.8 percent carbon and up to 1.2 percent it carbon (maximum). If control of the Carbon potential is not exercised then there are numerous problems that can occur on the resulting formed case:

• Retained austenite
• Grain boundary oxidation
• Intergranular cracking
• Surface cracking
• Low surface hardness
• Carbide networking

The carburizing furnace atmosphere is made up either of:

• Endothermic atmosphere carrier gas, plus city gas as the enrichment gas
• Nitrogen/Methanol carrier gas, plus city gas as the enrichment gas

If one is satisfied that the carrier gas is being generated in a satisfactory manner and there are no problems occurring with the carrier gas in terms of dew point, yet problems are occurring at the process furnace, then one should check the quality of the city gas. (Methane enrichment gas)

The following problems can occur as a result of the lack of control on the enrichment gas. Sooting.  This problem is as a direct result of too much carbon presence in the furnace atmosphere and can be visibly seen precipitating out of the atmosphere. 

Usually this condition would occur at carbon potentials that are approaching 1.6 % or greater. This condition will cause the furnace refractory to become overloaded with diffused carbon into the refractory brick, which will lead to difficulties in control of the furnace atmosphere.

 Further to this, the carburized surface of the steel will lead to a serious potential for the formation of retained austenite.  The obvious remedy is to cut back the enrichment gas or to dilute the furnace atmosphere with air. 

The problem with air dilution is that there becomes a greater risk for the formation of grain boundary oxides, and surface oxides.  Great care needs to be exercised when adding air into the enriched atmosphere that one does not create the problem of oxide formation. 

If the furnace has been operated at high carbon potentials for extended periods of time, it will be necessary to burn out the carbon from the furnace refractory.

Some of the modern-day integral quench furnace manufacturers will build the furnace with a built in burnout system.  This means that the furnace operator only has to go to the program mode for burnout and the burnout will be completed automatically.   With the older type furnaces, one simply reduces the furnace temperature to approximately 1600 ° F. and removes any ferrous atmospheres that might be present within the process chamber. 

The doors of entry and exit to the furnace are opened and air is simply blown into the process chamber.  The air can be supplied either by and external air blower or by lines of compressed air simply blowing into the process chamber.  This will cause any carbon present in the refractory brick to ignite and burn.

The temperature control instrument should be observed as a rise in temperature will be seen to be occurring, and the temperature will continue rise until all the of the refractory carbon is burned out. Generally the burnout time would be approximately two to three hours.

• Low Hardness. Low hardness can be caused as a result of too low a carbon potential in the process furnace. The causes of low carbon could be as a result of too low a dew point in the process gas, too low a carbon potential, and too slow a quench (if the atmosphere is within the required carbon potential). Too much residual retained austenite as a result of too slow a quench or too much carbon in the surface of the steel. The remedy would be to check the furnace atmosphere carbon potential and adjust accordingly, also to check the quench medium temperature that it is not too hot and causing a slack/slow quench
• Grain Boundary Oxidation. This can be caused by the presence of oxygen/moisture that is present within the furnace atmosphere. The remedy would be to check the furnace for potential air leaks, and to check the volume of dilution air being used and adjust accordingly. Close and careful control of the dilution air is necessary to reduce the risk of the formation of the grain boundary oxide formation

These are just some of the potential problems that can occur as a result of minimal control of the furnace atmosphere. Control of the atmosphere is critical to the success of heat treat practices, especially with the carburizing process. It is also necessary that the furnace operator has an understanding of the process, its control feature and the cause and effect of corrective and non corrective actions. There are many aspects to the control of the endothermically generated gas as well as the blended process gas both for hardening as well as carburizing. The endothermic gas generator is a simple unit, to both operate and maintain, however the maintenance is so often overlooked, in particular the burnout procedure. When the question is asked when an atmosphere problem is present,” when did you burn out the generator?’

The answer is usually, “We don’t need to” or ‘What do you mean?’ It is a necessary maintenance procedure, and the recommended frequency of burn out can be seen in the Operation and Maintenance Manual supplied by the equipment manufacturer.

It is also necessary to do the procedure on the process furnace, particularly when operating with high carbon potentials. The high carbon potentials will very quickly load up the refractory brick with carbon, making it difficult to control the process and also the final product quality.

          Trouble Shooting the Exothermic Gas Generator.

The Exothermic gas generator is a unit that is designed as lower cost producing atmosphere generator and has been used for many years. The rich exothermic generator is a combustion chamber that is usually filled with a catalyst.

The internal process chamber is usually constructed with a gas combustion burner supplied with a gas air mixture for combustion. The important part of the unit is the combustion burner that must ensure a close tolerance ration of air to combustion gas.

The general problem that occurs with the exothermic generator can usually be traced back to the air to gas mixing system.  Or to the gas cooler on the reacted gas discharge side.

In some instance depending on the gas quality required, the exothermically generated gas may pass through a refrigerant dryer.   The refrigerant dryer will require the periodic recommended preventative maintenance as directed in the manufacturers Operating and Maintenance manual. Without maintenance, one can expect problems to arise.

  Conclusion

The trouble shooting of furnace atmospheres can be dramatically reduced if standard operating procedures are written for both incoming material inspection as well as for the furnace equipment.

The material for heat treatment can cause considerable atmosphere problems if the incoming material is not prepared and pre-cleaned prior to the process treatment.

In addition to this, any residual surface contaminant can cause surface problems on the steel being processed in terms of corrosion, pitting, appearance as well as quality. Oils, greases, lapping compounds, marker ink, paint and cutting fluids should be removed by washing or degreasing.

The standard operating procedures for the generators and furnaces should include the burn out procedures as well as the method of furnace operation. If the operator/furnace technician understands both the process and operational procedures, then they would become as valuable an asset as the equipment is. It is because of their practice, that management’s capital investment will be well protected.