Many industrial heat treating process require the use of “protective atmosphere.” This enclosure is intended to be a brief survey of the kinds of processes and atmospheres and related equipment that L&L is involved with. It is not meant as an exhaustive study of this subject but more as a basic guide to help insure that you select the proper equipment for your job.
WHAT IS PROTECTIVE ATMOSPHERE?
The most common atmosphere used is, of course, air. In many cases it is not important what chemical reactions take place on the surface of a part at high temperatures. This can be because it does not affect the performance of the part, it may be beneficial to the part, the surface of the part may be machined or cleaned afterwards anyway. The absence of air or vacuum is on the other extreme. This has the advantage of being highly controllable with the ability to produce very clean parts. The main disadvantage with vacuum processing is the high equipment and maintenance costs. In between these two extremes is the use of controlled atmosphere under normal or close to normal atmospheric pressure. That is the primary focus of this enclosure. In addition to this are some other techniques such as wrapping parts in stainless steel foil and operating a direct fired gas furnace with a rich gas to air mixture.
SEALED BOX VS ALLOY RETORT
The most common way to “contain” a protective atmosphere is to externally seal the furnace case and simply purge the interior of the furnace with the atmosphere. Typically, L&L does this by welding the case seams (when a combustible atmosphere is used we typically double weld the seams). The door seal is typically a combination of the standard brick to brick or fiber to fiber seal with a woven gasket of ceramic fiber. In electrically fired units the elements are exposed to the atmosphere (in most cases) and the element connection areas are sealed, gasketed with silicone rubber and purged. Gas fired atmosphere box furnaces require the use of radiant tubes which separates the products of combustion from the internal furnace atmosphere.
Advantages of this method are cost, the ability to easily put a fan into the furnace, the ability to more evenly distribute elements and no maintenance cost of retorts. This disadvantage is that the insulation is exposed to the atmosphere and the air and water vapor that get trapped in the insulation affect the integrity and purity of the atmosphere. It would be difficult to achieve a dew point in the furnace below 0°F and +20°F is probably more typical. Another potential disadvantage is that the atmosphere, depending on its composition, may affect the elements.
The other method of atmosphere containment is to put the atmosphere (and work) into an alloy retort. The retort (or muffle) is heated externally either by gas or electric power. Typically the seal on the retort is a water cooled silicone “O” ring gasket with bolt down clamps. This requires the seal to be made outside the heating chamber. This provides the ultimate in atmosphere purity. Another type of seal is a sand seal in a trough welded around the top perimeter of the retort. This allows the whole retort to be put inside the furnace. A variety of alloys and shapes are used depending on the temperature, work dimensions, and uniformity requirements. The retorts can be round, rectangular or “D” shaped. They can have corrugations for extra strength. Typical alloys used are 304, 316, 330, 600 and 601. These are all nickel based alloys with varying degrees of high temperature strength and corrosion resistance.
The main advantage of the retort method of containment is that it provides the cleanest, purist environment to contain the atmosphere. It is easy to achieve a -40°F dew point with a typical flow rate of 5 volume changes per hour. It is particularly suited for high purity hydrogen and argon atmospheres needed for many processes. It is possible to remove the retort from the heated chamber (or the heated chamber from the retort in the case of a Bell furnace) for relatively fast cool down under atmosphere (although there is a price to pay on retort maintenance costs.) It is not easy to have a fan in the retort (although it is possible) the water cooling is an extra utility cost (although usually minor). There are extra capital, maintenance and energy costs associated with the retort.
WHAT IS THE POINT OF A FLAME CURTAIN?
Many controlled atmosphere furnaces include a flame curtain at the door. When the door opens the flame curtain burner is activated and covers the door opening with a sheet of flame. This helps reduce oxygen inrush (but does not prevent it totally) and helps maintain internal temperature while the door is open. They also act to burn off any combustible gas in the furnace that escapes when the door is opened. Typically these are simple line burners powered with a small amount of compressed air and a relatively high pressure of natural gas or propane (usually 2 or more PSI). A compressed air regulator, pressure gauge and all piping are included. They include a pilot system that will not let gas flow without a proven pilot and usually include electronic ignition.
ATMOSPHERE SAMPLE PORTS
A sample port is usually provided on a controlled atmosphere furnace or retort that uses either a combustible atmosphere or a complicated atmosphere that may need to be monitored. For instance it is good practice to take dew point readings from a box furnace using endothermic or nitrogen/methanol atmosphere. This can be a useful control and/or troubleshooting technique. In hydrogen retort furnaces samples are taken of the atmosphere to determine when it is safe to introduce hydrogen or open the door. Typically there is a valve to close this off when not in use.
STAINLESS STEEL WRAP: PROS AND CONS
Many people use stainless steel foil wrap to protect small tools from oxidation and decarburization. This is actually a very effective method of protection particularly for the popular air hardening tool steels (because the wrap does not interfere with the air quenching and still protects the steel during cool down). Its main disadvantage is that it can be quite costly if you are doing lots of heat treating not only in terms of material cost (it can usually be used only once) but in terms of labor cost. Generally the people who perform this labor are skilled and expensive craftsman whose time might be better spent. It can take a good bit of time to effectively wrap the tools and it must be done properly or it will not work well. It is ideally suited to situations where “atmosphere protection” is only occasionally needed. Putting some carbonaceous material such as charcoal inside the steel wrap can give you further protection from decarb.
Inert atmospheres such as nitrogen and argon will prevent most decarburization and oxidation in most tool steels. However, it does not provide complete protection. The higher the carbon level of the steel the less protection. This is because the carbon in the steel is highly reactive at high temperatures with even the trace levels of oxygen and water vapor in a furnace purged with inert gas. To provide complete protection the work needs to be wrapped in stainless steel foil or some carbon monoxide and free carbon needs to be added with either a hydrocarbon gas or carburizing compound. If some surface grinding is going to be done on the work after heat treating then the inert gas may be sufficient. Typical flow rates of gas are between 5 and 10 furnace interior volume changes per hour.
Carburizing adds carbon to the surface of carbon steel. The more carbon in steel the harder the steel is. This process allows you to make a part that is soft and ductile on the inside and hard on the outside. The typical atmosphere used is an endothermic or synthetic endothermic gas (see the section on Endothermic gas and Nitrogen/Methanol). To this is added a hydrocarbon such as methane, propane or butane to increase the carbon potential and air to decrease the carbon potential. The carburizing process is dependent on time and temperature. The higher the temperature and the longer the time the greater the case thickness that will be created. It typically takes place at around 1600°F to 1700°F.
Carbonitriding is similar to carburizing except that in addition to the carbon added to the case, nitrogen is also added to the case. In the past this process was usually accomplished with a cyanide salt bath furnace. For obvious environmental reasons this process is almost obsolete and gas carbonitriding is being used. This is a very useful process for imparting a very thin hard case on low carbon steel. A typical case thickness is 10 thousands of an inch. In addition to the base endothermic atmosphere and the hydrocarbon enrichment a small amount of ammonia is added to the atmosphere (typically 5% to 10%). This dissociates into hydrogen and mononuclear nitrogen (N rather than N2) which is a more reactive form of nitrogen. It typically takes place at around 1450°F to 1550°F.
Brazing is often done in air with various special fluxes to prevent oxidation and good flow of the brazing compound. Copper and silver brazing is, however, often done in a reducing atmosphere of either pure hydrogen or dissociated ammonia.
Bright annealing of stainless steel is usually done in pure hydrogen.
Atmosphere Control and Analysis
“Carbon control” is the automatic “in situ” control of carbon potential. Carbon potential is the ability or potential of the furnace atmosphere to either carburize or decarburize steel. You add carbon to the steel when you carburize and you subtract carbon when you decarburize. All steel has some carbon in solution in the steel. This is usually some fraction of a percent such as .4%. 4040 carbon steel, for instance, has .4% carbon. The typical carbon control system that L&L uses consists of an oxygen probe and carbon controller. The oxygen probe reads the oxygen level and furnace temperature at the probe. This information along with a constant based on an assumed carbon monoxide level (typically 20% of the furnace atmosphere) is used to calculate a carbon potential. This is expressed as a number approximating the percent of carbon in the atmosphere. .4 would represent an atmosphere neutral to 4040 steel. Hydrocarbons are added to the atmosphere to increase the carbon potential and air is added to decrease it.
DEW POINT CONTROL
In some applications it is critical to know what the dew point of the furnace atmosphere is. L&L has used a variety of instruments for this task. One system is the Model 580 hydrometer. This includes two set point alarms, calibrated range of -130°F to +70°F in 2°F increments, digital read out and recorder output. Various filters are available to protect the sensor. We have used this system to measure dew point at the exit of a retort. The alarm allows the furnace to start the heating cycle when the dew point gets below a certain level.
L&L sells a portable oxygen/hydrogen analyzer which will perform both the “purge in” and “purge out” test requirements of the NFPA 86C guidelines for introducing and removing hydrogen from an enclosed retort. This includes separate digital readouts for hydrogen and oxygen, built in sample pump, filters and flowmeter, rechargeable battery and AC operation, and 10 second warm up time. Range is 0-25% Oxygen and 0-40% Hydrogen. Recorder outputs are available.
HYDROGEN LEAK MONITOR
An analyzer to monitor ambient atmosphere for hydrogen leaks is available. This is a nice extra safety precaution. The system is calibrated for 1/4 of the LEL (1% H2). An alarm output from this sensor can be made to shut off at its source (with a separate solenoid).
Types of Atmospheres
Nitrogen, argon and helium are considered “inert” atmospheres. Their degree of inertness varies with the temperature, type of gas, purity of the gas and the material being processed. Nitrogen may be completely inert to many steels but will react with other ones above certain temperatures. Argon and helium are more inert than nitrogen in general. They are also more expensive. There are varying grades of purity. For instance an inexpensive grade of nitrogen would be 99.5% pure. A more expensive grade would be 99.9% pure. That small amount of impurity (typically oxygen or water vapor) may be enough to ruin your work. These gasses can be purchased in cylinders, banks of cylinders, dewars bottles or in liquid bulk. For costs and options, it is best to talk to your local gas supplier once you have an estimate of usage. There are also on site generators available which may be economical in many cases. For example, a relatively inexpensive molecular sieve type nitrogen generator.
ENDOTHERMIC GAS VS NITROGEN/METHANOL SYNTHETIC ENDOTHERMIC GAS
Endothermic atmosphere is generated by cracking methane in a retort into 40% nitrogen, 40% hydrogen and 20% carbon monoxide. In addition, the minimum flows generally available in such a generator are 500 CFM which would can waist natural gas in many applications. They also take awhile to stabilize so they are really only useful for continuous use. They also require considerable maintenance. A nitrogen/methanol atmosphere will provide a more controllable atmosphere with more consistent carbon-monoxide and hydrogen levels. This is particularly suited for applications where a carbon controller is required for precise and repeatable carbon levels that may need to get changed for various steels. Methanol dissociates into 67% hydrogen and 33% carbon monoxide. Along with 40% added nitrogen this provides the base atmosphere (very similar to endothermic gas but more uniform and with a lower dew point) ideal for carburizing, carbonitriding or neutral hardening. Hydrocarbons such as methane or propane are added to increase carbon potential. Filtered compressed air is used to lower carbon potential when necessary and for carbon burn out. Nitrogen is also used as an emergency purge. The percentage of methanol vs nitrogen can be varied to adjust for varying conditions. There are disadvantages of nitrogen/methanol. The cost per cubic foot of gas is higher than that of an endothermic generator (although, with all the other costs of the generator this is not so straight forward a calculation). Methanol is a flammable and hazardous liquid (on the order of gasoline) and requires careful handling and storage. Bulk nitrogen must be provided.
NITROGEN/NATURAL GAS OR PROPANE
A small percentage of carbonaceous gas mixed with nitrogen maintains a neutral carbon potential with most tool steels. The carbon based gas also dissociates into some free hydrogen and carbon monoxide which helps prevent oxidation along with the protective inert gas. The atmosphere mixture is normally kept below (or close to) the LEL (lower explosive limit) of the combustible gas. This is done by an inert gas low flow switch and limiting valve for combustible gas. The LEL of natural gas (mostly methane) is 5% and propane is about 2.5%. Concentrations of these gases below these limits will not support combustion in air, let a lone in inert gas. Although this atmosphere mix will not produce a totally bright finish, it will result in a hard part that is not decarburized. The control of the carbon potential is strictly manual with this system. This is because the small amount of carbon monoxide present does not lend itself to accurate readings of the carbon levels with an oxygen probe and carbon controller. Should such automatic control be desired or necessary a nitrogen / methanol or endothermic atmosphere will be necessary. ( See NM Series and EN Series Bulletins.) The advantage of this gas is low cost of equipment and gas, and the fact that the atmosphere is not classified as combustible. It is important to operate a furnace with this system with adequate ventilation.
Commercially available hydrogen is 98 to 99.9% pure. All cylinder hydrogen contains water vapor and oxygen. In addition some methane, nitrogen, carbon monoxide and carbon dioxide may be present in trace amounts. Hydrogen is, of course, a strong deoxidizer, limited only by the amount of water vapor content. Its thermal conductivity is 7 times that of air. It is a decarburizing gas with high carbon steels. It will cause hydrogen embrittlement in many steels. It is often used in annealing stainless steel alloys, magnetic steel alloys, sintering and copper brazing.
Ammonia can be dissociated into 75% hydrogen and 25% mononuclear nitrogen (N vs N2). This is done in a dissociator. The dew point is -60°F with a purity of 99.9% or better. Metallurgical grade ammonia must be used. This consists of a retort heated to about 1800°F with a controlled furnace. Inside the retort is a catalyst to help break down the ammonia. This makes a cheap protective atmosphere for applications such as brazing or bright annealing. It is a decarburizing atmosphere so it is not normally used with high carbon steels.
Steam is used for scale free tempering and stress relieving of ferrous metal in the range of 650 to 1200°F (345 to 650°C). The steam causes a thin, hard, and tenacious blue-black oxide to form on the metal surface. This decreases the porosity of the steel and adds to wear resistance of certain tool steels.
Burning methane or propane in a tightly controlled ratio produces either rich or lean exothermic gas. Rich exothermic gas has between 10% and 21% carbon monoxide and hydrogen while lean exothermic gas has between 1% and 4% of these reducing gasses. This is a cheap atmosphere typically used in continuous processes including tempering and annealing of steal, copper brazing, and sintering of powdered metals.
Atmosphere Control Equipment
L&L ATMOSPHERE CONTROL PANELS
L&L makes all its own atmosphere control panels. All necessary safety devices are built into these panels. On relatively low flow panels we use copper and steel tubing with brass fittings and valves and flair connectors which are easy to disassemble for maintenance. Larger flow units will typically be piped with hard carbon steel piping with unions as required. Special applications may require all stainless steel piping (such as for electronics manufacturing) or oxygen cleaning for pure oxygen use.
- (H2) Hydrogen Control Panel
- (MPN) Nitrogen/Hydrogen Mixing Control Panel
- (NM) Nitrogen/Methanol Control Panel
- (MPN) Nitrogen/Gas Propane Mixing Control Panel
- (EN) Endothermic Gas Control Panel
VARIOUS KINDS OF STANDARD ROTOMETERS AND MASS FLOWMETERS
Most applications do just fine with standard rotometers which is a simple ball and tube. The pressure of the gas moves the ball up to indicate flow on a calibrated scale. The accuracy of these is in the 2% to 3% range. The ones typically used are simple Dwyer rotometers with valves attached to the rotometer. These are all calibrated for air and need to be converted when using other gasses. We also use Porter precision flowmeters for special circumstances and Waukee flowmeters (which are much more expensive) when specified or required for larger flow rates. A nice feature of the Waukee meters is that they can be calibrated for the specific gas being used. For highly precise applications and where control and recording or indication of gas flow is critical then mass flowmeters are used. These measure the mass flow of the gas (rather than volume), allow precise mixing of gas ratios and allow digital indication and recorder outputs. They are also expensive.
References and Sources of Information
List of Sources
- ASM HANDBOOK, VOLUME 4, HEAT TREATING, Section on Atmospheres
- STEELS HEAT TREATMENT AND PROCESSING PRINCIPLES, George Krauss, 1989, ASM International
- THE HEAT TREATING SOURCE BOOK, 1986, ASM International
- MATHESON GAS DATA BOOK, 1980, Matheson Gas Products, Inc.
- HANDBOOK OF COMPRESSED GASES,, 1990, Compressed Gas Association, Van Nostrand Reinhold, NY
- CARBURIZING, PROCESSING AND PERFORMANCE, George Krauss, ASM International
- National Fire Protection Agency Booklets (See above under Safety Information)