A Heat Treater's Guide to Common Furnace Challenges
Every day, the Ipsen Aftermarket Support Helpline receives dozens of calls asking for troubleshooting assistance and technical insight on a wide range of manufacturers’ furnaces.
When asked what some of the most common vacuum-related challenges were, we decided to poll our technical experts and review the data from their call logs. What we discovered is that customers regularly ask five specific questions. They are:
- How do I leak check my furnace?
- Why won’t my vacuum furnace pump down?
- How do I determine the source of excessive outgassing?
- What should I do about poor ultimate vacuum?
- Why are my parts discolored?
Faced with these questions – and the frequency at which they are asked – we decided to compile a short guide that covers our experts’ answers to these common challenges.
There are two primary techniques used to leak check a vacuum furnace: the spray probe and the sniffer probe methods. The spray probe technique seen in Figure 1 is the traditional form of leak checking. It involves pumping down the furnace, applying helium to the exterior of the furnace (e.g., with a helium mass spectrometer connected) and looking for a response.
Then there is the sniffer probe technique. This is for applications where it’s not feasible to evacuate what you’re testing for leaks. To use this technique, you should charge – or pressurize – a test piece with a trace gas, such as helium. Test-piece examples include the heat exchanger, water-cooled flange, water-cooled fans and shafts, power feed-through, water jacket, etc. Once you charge the test piece with the trace gas, the probe sniffs around the part for any gas that may be escaping through a leak. This leak is then reported by the helium mass spectrometer.
It is also important to check for inert-gas leaks. This is because the tolerance of leaks on the entire backfill system – from the liquid storage system out to the furnace – must be zero. If inert gas is leaking in from the inert-gas system or from valving on the furnace, it can cause the furnace to exhibit signs of poor pumping, poor leak rates, poor ultimate vacuum, poor ability to pump against normal outgassing loads, backstreaming, etc.
There are also two common methods for locating inert-gas leaks. They are thin-film (soap-bubble) leak testing and vacuum leak testing. Thin-film leak testing is the first method, and it is often the easiest and quickest way to check for inert-gas leaks on the pressurized side. It involves spraying a leak-detector solution on known suspect areas. If it starts to bubble in any tested places, you know you have found a leak.
The more precise method is vacuum leak testing. It involves isolating the liquid-nitrogen or liquid-argon storage system from the furnace, ensuring the entire system is in vacuum and then using a helium mass spectrometer to leak check the system. To perform vacuum leak testing, you would:
- Close the outside valve. This isolates the outside liquid system from the evaporator stand, the backfill reservoir tank and the furnace and all of its ancillary partial-pressure and backfill lines.
- Pump down the furnace using the furnace’s normal pumping system.
- Depressurize the backfill reservoir tank.
- Manually open the backfill valve on the furnace by electrically energizing it open. This process should be performed by a qualified expert and according to safety guidelines.
Once you open the backfill valve and depressurize the backfill reservoir tank back to an atmospheric condition, the furnace will start drawing a vacuum on the lines. It will also draw a vacuum on the gas reservoir tank and the evaporator stand all the way back to the closed outside valve. Next, you can use the furnace’s pumping system to pump the entire backfill system and pull a vacuum on the complete system. You should then leak check the system using a helium mass spectrometer.
Once you have identified all of the vacuum furnace and/or inert-gas leaks, you must go back and address each of the individual leaks.
If your vacuum furnace won’t pump down, the first thing you should do is verify the functionality and blank-off of the mechanical pumping system. If the pumping system is operating correctly, then one of these failure modes is likely the cause: a damaged door seal, vacuum furnace leaks and/or hot-zone contamination.
The door seal on a vacuum furnace sees some of the highest wear-and-tear through normal operation. As such, it tends to be the cause of pump-down issues about 80% of the time. The flowchart in Figure 2 outlines the initial steps you should follow – starting with the door seal – when troubleshooting why your vacuum furnace will not pump down.
Outgassing occurs when contaminants present in the vacuum furnace system are released in the form of gas through the application of heat or reduced pressure. Such contaminants can include water vapor, grease, dirt, volatile liquids, etc. In addition, certain materials are more susceptible to outgassing than others, and they outgas at different rates depending on the vacuum level and temperature.
As such, some outgassing can be a normal part of heat-treatment operations. Yet there may also be times when you see a higher-than-normal outgas level, as well as some braze-quality and part-discoloration issues. When excessive outgassing becomes a concern, you should ask yourself a few fundamental questions.
- Has the part alloy or upstream cleaning process changed?
- What is the current linear cold leak rate of the furnace?
- What is the current hot leak rate of the furnace?
- Are the vacuum pumps functioning correctly and obtaining specified vacuum levels?
Excessive outgassing can also cause pump-down times to increase and the pumping system to look as if it is malfunctioning. Because of this, we recommend starting with question four. We find this to be the best use of your time because, at times, the pumping system is the culprit.
For example, if the diffusion pump and/or backing pumps are not performing to their specific standards, they may be unable to handle the normal outgassing loads. This, in turn, can give the perception of increased outgassing when it is actually the same as always. As a result, you might head down the wrong path of corrective actions when the real problem lies solely with the pumps.
When determining the answer to question four, you should separate the pumping system from the furnace to make sure it is functioning within specification. Only once you have ruled out the pumps should you move on to the other three questions to determine the root cause behind the excessive outgassing.
In the end, cleanliness is imperative for minimizing future instances of excessive outgassing. As such, we recommend keeping anything introduced to the furnace clean (e.g., parts, fixtures, part baskets), as well as keeping the door closed and evacuated whenever possible.
A typical vacuum furnace that is commonly used in the industry tends to reside in these normal ultimate vacuum ranges when clean, dry and empty.
- With diffusion pump: 10-6 with a leak-up rate of less than 5 microns/hour
- No diffusion pump: Less than 35 microns with a leak-up rate of less than 5 microns/hour
If you begin to experience poor ultimate vacuum – of which part discoloration is one of the first signs – there are a few common causes to consider. These include air leaks, gas leaks, poorly functioning pumping systems and hot-zone contamination-related issues (Fig. 3).
There are also certain processes that are more susceptible to poor vacuum levels. These include brazing (e.g., aluminum, copper, nickel and titanium), sintering, de-waxing and de-oiling. You might also see poor vacuum levels occur with parts that use high percentages of binders and furnaces that run over 2200°F (1204°C).
To determine the source of poor ultimate vacuum levels you should first perform a rate-of-rise test to determine the furnace’s linear leak rate. If the linear leak rate is truly linear and fails, this indicates the furnace is leaking, and you should perform a leak check.
If your furnace has a passing leak rate, however, then the next factor to consider is the pumping system. As discussed in the section on excessive outgassing, the pumping system can influence the furnace’s ability to hold vacuum levels against normal outgassing conditions, as well as how fast it can pump down.
Finally, if you have ruled out leaks and the pumping system, you should consider hot-zone contamination as the other possible cause. To address this, we recommend running a cleanup cycle. For best results, you might need to perform more than one general cleanup cycle.
It is important to note that other contaminants inside the hot zone (e.g., carbon in the form of soot or tar, fluxes from brazing pastes and excess braze alloy) can influence the time, temperature and pressure required for the cleanup cycle. You will need to adjust these three variables to target the known contaminant in the furnace. Overall, maintaining hot-zone cleanliness helps negate water retention within the hot zone and improve leak rates. This, in turn, leads to better ultimate vacuum.
One of the side effects of heat treating parts is that they sometimes emerge from the vacuum furnace a color they are not supposed to be. But did you know that different colors serve as indicators of what might be wrong with your furnace?
You can often judge the percentage of air, water and contamination entering your furnace by checking the color of your parts. Colors can range from yellow (least severe) to black (most severe), with others found between orange, green, light blue and dark blue. Figure 4 outlines what the three most common colors represent.
|Yellow (from straw yellow to gold):||When using argon, yellow parts signify either small air leaks, gas leaks and/or mixing of gas and air. The more severe the leak, the darker the yellow color. Once these leaks reach a certain level of severity, the parts emerge blue instead of yellow.|
|Blue (from sky blue to black):||When using argon, blue parts signify increasingly severe air leaks, gas leaks and/or mixing. When using nitrogen, blue parts signify air leaks. Small air leaks are light sky blue but, depending on the severity of the leak, they can transition into darker blues, navy blues and eventually black (which signifies the worst for your furnace).|
|Green (from lime to forest green):||Green parts signify a few potential problems, including:
1) The furnace door was left open between cycles and/or the furnace was not kept under vacuum for extended periods of time
2) Parts were put into the furnace while excessively wet and/or with wet braze alloys
3) The furnace was run at high temperatures while in a very deep vacuum; this will often liberate chrome from the parts and fixtures
4) The hot zone absorbed water vapor (refer to #1 for why)
5) Water-jacket leaks, heat-exchanger leaks, traces of water, etc.
Fig. 4. The top three colors often encountered among discolored parts and what this indicates about your vacuum furnace.
There are a few measures for preventing discoloration. These include running cleanup cycles configured for your specific contaminants, performing regular helium leak checks and using a liquid thin-film leak-detection fluid (for nitrogen/argon gas system feeds).
As a heat treater, we know there are many challenges you find yourself faced with every day. We don’t want your vacuum furnace to be one of them. Dealing with questions ranging from commonplace to unusual, the technical experts behind Ipsen’s Aftermarket Support Helpline have seen it all. Based on their experiences, we designed this guide to help you troubleshoot your equipment so you can focus on other important aspects of your business.
For more information: Contact Ipsen, Inc., 984 Ipsen Road, Cherry Valley, IL 61016; tel: 815-332-4941; fax: 815-332-4995; e-mail: sales@IpsenUSA.com; web: www.IpsenUSA.com. Author Jim Grann is technical director. For technical assistance, call 1-844-Go-Ipsen. For more maintenance advice, how-to guides and instructional videos, visit our blog at www.IpsenHarold.com.