The following procedures have been implemented in powdered-metal (P/M) parts manufacturing facilities successfully for many years but can also be adapted to almost any process that uses a continuous belt furnace and a nitrogen-diluted atmosphere.
|Fig. 1. Atmosphere injection points: The recommended atmosphere injection locations on a continuous-belt sintering furnace – atmosphere zoning concept|
Balancing the Furnace
Balancing the furnace is the first step and one of the most important steps toward conserving atmosphere flow. It is important that the location of the atmosphere injection and flow distribution be followed as shown in the schematics (Figs. 1 and 2). It is also important that there be several layers of thin fiber curtains located at the exit end of the furnace that extend down onto the stainless steel belt.
Next, make sure there is sufficient heat applied to the exhaust stack in the entrance end of the furnace to help draw the atmosphere in an upward direction to ignite any combustible gases exiting the furnace. Make sure there are no obstacles, such as doors or curtains inside the furnace, that would prevent a nice even flow of atmosphere from the points of injection toward the front of the furnace. There should be no external airflow toward the front or back of the furnace such as fans, air conditioners, open windows, doors, etc.
|Fig. 2. Atmosphere distribution and composition: The schematic shows the recommended atmosphere compositions and percentage range of total atmosphere flow rates at each injection point.|
Calibrating Flow Meters with Line Pressure
The one thing that many people tend to overlook is the calibration of the flow meters with the incoming atmosphere line pressures. Since most furnaces are equipped with Waukee flow meters with direct-read scales, it is important that the line pressure of the incoming atmosphere be the same as the calibration on the flow meter and that the proper flow meter is used for the designated gas. Once this is accomplished, you can be assured you are getting the proper flow of each gas to the furnace.
Determining Production Atmosphere Flow Rates
There are many theories on how to calculate atmosphere flow to the continuous-belt furnace. I have always found the following procedures to be very adequate. Since the lighter, higher hydrogen-containing atmospheres such as endothermic gas (40% H2) and dissociated ammonia (75% H2) have a lower specific gravity than the higher nitrogen-containing atmospheres, one must figure 100 cfh (cubic feet per hour) of atmosphere per inch of belt width. Since the specific gravity of nitrogen is approximately seven times heavier than hydrogen, by using the heavier atmospheres it is not unrealistic to have at least a 20% flow reduction and still have a very adequate atmosphere flow. This is especially important when processing parts where a carbon source is involved, such as sintering and brazing applications.
Even though you do not want to waste atmosphere, it may be more costly to operate with insufficient atmosphere flows. Starving the furnace could lead to increased scrap, deterioration of the furnace belt(s) and muffle(s) and a very unsafe working environment.
Determining Idle Atmosphere Flow Rates
The recommended atmosphere flow for the furnace is 100% nitrogen during idle hours. This is very easily accomplished when using a blend of nitrogen-hydrogen or nitrogen-dissociated ammonia. With these two blended atmospheres, turning the furnace hydrogen source off will save money and still keep the furnace conditioned for fast oxygen-free start-up. It is not recommended when using a blend of nitrogen-endothermic gas since the endothermic generator will continue to provide gas even though flow to the furnace is shut-off. Also, there are really no savings in this case.
The following procedure should be used during idle hours or even when a furnace is sitting idle for a short period of time waiting for parts to be processed. First, bring the front door of the furnace down as far as possible without touching the belt. Then, turn off the hydrogen source, whether it be pure hydrogen or dissociated ammonia, and reduce the nitrogen flow to approximately 30% of total atmosphere flow. This should include the nitrogen flow to the exit end of the furnace as well as to the injector bar in the preheat zone. Both of these injection points should be flowing 100% nitrogen even during processing hours. The only difference is that the nitrogen entering behind the fiber curtains will be dry nitrogen, whereas the nitrogen entering the preheat zone should contain some moisture (optional).
Always remember for safety sake “nitrogen in first and nitrogen out last.” With this in mind, everyone will go home safe when the shift is over.
Minimizing Atmosphere Flow for Idle Time
To minimize the atmosphere flow during idle time, one should take these following steps. Start a weekend shutdown using 100% nitrogen at a flow rate of 30% total processing atmosphere flow. Do not adjust the nitrogen flow to the rear curtain or the front injector bar, only the nitrogen injected between the high heat and cooling zone. The nitrogen flow in these two areas must be included in the total flow.
Before resetting the furnace back to the processing mode, check the dew point in the high-heat zone of the furnace. Do not increase the nitrogen or temperature or add the hydrogen source. Using a dew- point analyzer, a pump and a ¼-inch stainless steel tube, run the tube down the inside of the furnace into the hot-zone area. The dew point should be between -60 and -80°F. Repeat this procedure, reducing the nitrogen by 100 cfh each weekend until the dew point starts to rise. When the dew point starts to rise, revert back to the previous setting. That is the amount of nitrogen you will need to use in that particular furnace during idle hours, whether idling during a weekend shutdown or only a few hours while waiting for work. When the furnace is ready to be returned to processing mode, just add the nitrogen to normal processing flows and then add the hydrogen. Within minutes the furnace is in the processing mode and the furnace operator can start sintering parts.
Atmosphere Reducing Potential
Another way to limit spending on your protective atmosphere is to understand the reducing potential of the atmosphere. Many times an excessive amount of reducing gas is added to the atmosphere simply because someone does not understand the amount of hydrogen required to reduce the oxide from the metal compact and/or tie up the residual oxygen present in the furnace. When hydrogen reacts with oxygen in the furnace it produces water vapor. The reduction of these metal oxides (MxOy) involves competing reactions by hydrogen (H2) and water vapor (H2O).
Example: MxOy + yH2 = xM + yH2O
The reducing potential of the atmosphere is proportional to the ratio of hydrogen to water vapor in the atmosphere (H2/H2O). So, by minimizing the amount of water vapor, lower hydrogen levels are required to complete the reduction process. For this reason, an atmosphere containing two oxygen-free gases such as pure nitrogen and hydrogen have a much higher reducing potential than the generated gases or a nitrogen-generated blended gas atmosphere.
The chart (Fig. 3) shows the recommended nitrogen to hydrogen and nitrogen to generated gas blends. The compositions of these atmospheres shown in this chart have been used successfully for many years in many P/M parts manufacturers’ sintering operations.
|Moisture control system: Hydration panel with variable moisture control|
Moisturized Nitrogen for Lubricant Removal
As shown in Fig. 1, the addition of an oxidant to the preheat zone of the furnace is optional but definitely recommended, especially for P/M sintering and in some brazing applications. In any continuous-belt furnace where carbon vapors are admitted, there should be an oxidant added to the furnace atmosphere. The oxygen will combine with these carbonaceous vapors to form carbon-rich gaseous compounds that can be removed from the furnace by the furnace atmosphere.
If these carbonaceous vapors are not removed from the furnace they will build up on the walls of the muffle to form high-carbon stalactites and stalagmites that will reduce the life of the stainless steel belt and high-heat muffle. Once the carbon precipitates either onto the walls of the high-heat zone or is dragged through the high-heat zone by the metal belt, the carbon will diffuse into the furnace components. This can result in brittle failure and cause tearing of the belt and/or sagging of the high-heat muffle.
Choosing an Oxidant
The oxidant can be added in the form of air or water. However, the addition of air is considerably less stable than the addition of water. For this reason, one should be very careful when choosing to add air as the oxygen source. The nitrogen blended atmospheres operate using very low dew points, which is excellent for controlling the carbon content of the part in the high-heat zone. It is not good when trying to get rid of the carbonaceous vapors that are produced either from lubricant removal from a P/M part or from the flux added for a brazing application.
It is also important when adding the oxidant to the atmosphere that one uses a system that maintains a continuous and constant dew point and operates on a 24/7 schedule. To add a system with a fluctuating dew point will certainly help, but eventually there will be a buildup of carbon deposits in the furnace.
The addition of a controlled-moisture system has proven to increase muffle life by 33-50% and belt life by 25% under actual processing conditions. These figures equate to a considerable savings in operating costs.
Since not only is every second or third muffle and every fourth or fifth belt essentially free, one must also consider the reduction in maintenance time and increased production time when figuring the actual cost savings.
There is no doubt that all the procedures mentioned – such as calibration of the gas line pressures with the flow meters, balancing of the furnace and following the procedures to reduce the atmosphere flows during idle time and production time – will save atmosphere costs. In addition, the injection of an oxidant to the preheat zone of the furnace will result in considerable cost savings when considering the cost of stainless steel muffles and belts and the reduction in maintenance time and increased production time. However, the only way there can be any noticeable cost savings is if these procedures are followed on a continuous basis.
Furnace & Atmosphere Service Technologies, Inc. in Brookville, Pa., provides a controlled moisture system that is guaranteed to maintain a stable dew point in your continuous-belt furnace. The unit is designed for this type of application, as well as for many other applications that require precise and continuous dew-point control. Not only has this system proven to increase belt and muffle life in P/M sintering furnaces, but it also reduces the stack particulate emissions by 66-74% as determined by a company that performs independent stack analysis. IH
For more information: Contact James G. Marsden II, president or James G. Marsden, FAPMI consultant; F.A.S.T., Inc. (Furnace and Atmosphere Service Technology), Brookville, Pa. 15825; tel: 814-849-2570; fax: 814-849-4951; e-mail: email@example.com; web: www.fast-inc-pmt.net