The most underestimated and commonplace enemy to your vacuum heat-treating system is water vapor (e.g., excessive humidity). In fact, water vapor is the number one threat to your everyday heat treatment process. However, preventing it can help increase the longevity of your hot zone.
So, how do you eliminate or minimize this threat?
To start, follow good vacuum practices. This means keeping the vacuum furnace door closed in between loads (especially if it’s wet or humid outside) and pumping the furnace down in between cycles.
Some argue that if you pump the furnace down in between cycles, then you have to backfill it to get the door open again, which wastes gas. However, you have to weigh the cost of the gas versus the effect of water vapor on the furnace and hot zone.
If you pump the furnace down and backfill it, the wasted argon or nitrogen gas typically costs about 20 dollars every time. In comparison, if you allow water vapor to permeate the hot zone, it can result in reduced hot zone life, discolored work, poor brazing and longer cycle times. All of these negative outcomes that come with excessive water vapor retention significantly outweigh the cost of any wasted gas.
When the furnace atmosphere is full of water vapor, it will also cause excessive outgassing, which can extend the cycle time due to waiting for the vacuum to recover. As a result, you have maintenance needlessly checking the furnace, which takes it out of production and wastes more time.
If you are preparing for an extended shutdown, we recommend that you pump down the furnace and backfill it to -5 Hg to reduce the leak-up rate during long periods of inactivity. As a result, you are able to come back from shutdown to a clean, dry, empty furnace that can quickly resume normal operations.
Molybdenum versus Graphite
Overall, water vapor affects molybdenum the same way it affects graphite. However, graphite hot zones consume themselves more quickly due to water vapor retention. Regardless of the type of hot zone you have, the negative effects that occur during the process remain the same.
In the end, if you take care of the hot zone, perform scheduled cleanup cycles, keep the door closed between cycles and follow good vacuum practices (e.g., keeping water vapor out and leak rates down), you can, on average, expect to add at least two years to the life span of your capital investment.
Correcting Water Vapor Retention
If you leave the door open for an extended period of time, you have a few options:
- The most common (and recommended) practice is to put the parts in the furnace and pump it down without starting the heat cycle right away. Instead, adjust the cycle and extend the time it spends pumping to boil off excessive water while it’s under vacuum.
- Close the furnace door without the parts and run a mini drying cycle. It is important to note that one typically only performs this option during worst-case scenarios, such as a water leak in the hot zone.
One should also remember that water vapor doesn’t just affect the hot zone. Water vapor retention can also affect the pipes, pumping system, heat exchanger and vacuum sensors. Overall, it takes time, heat and vacuum to remove water vapor from the furnace. Keep in mind that water begins to boil at around 1 Torr. It’s also important to note that if you’re putting parts/fixtures in the furnace that have been sitting out in a humid or damp shop, the water vapor can also leap off the parts and affect the furnace.
If you did start a cycle without allowing adequate time to remove the water vapor, you might see parts emerge with varying discoloration depending on the material. Typically, discoloration will start off a sky blue and get progressively darker depending on how much water vapor and/or air was in the furnace.
It is important to note that most controls software, such as Ipsen’s CompuVac® or VacuProf® controls systems, should be able to prevent discoloration from occurring. By using the Outgas Hold feature in the software, you can control at what pressure the furnace heat turns on. As a result, the heat won’t turn on if the furnace is outgassing.
Roughing Pump and Water Vapor
To keep your roughing pump oil at the right temperature, we recommend the use of a roughing pump water miser. This device regulates the water flow to the mechanical pump predicated on the actual oil temperature. It also allows you to set it to the desired temperature so any water vapor coming in can’t mix or emulsify with the oil, thus preventing it from negatively affecting your process.
Not everyone has a water miser on their pumping system, though. As a result, they just pump water in and out of their roughing pump at all times. However, since this is not very conducive to good vacuum practice, you need to regulate the temperature of the oil in the roughing pump and make sure it stays between 140 °F (60 °C) and 160 °F (71 °C).
If you keep the oil at the correct temperature, it turns the water to steam. In turn, this prevents any water vapor from mixing with the oil in your roughing pump. This is important because your whole process ability declines if your roughing pump becomes waterbound.
However, if you do have a water miser installed and excessive water is noted, one solution is to open the gas ballast. This allows the roughing pump to temporarily increase its temperature so any excessive water can be removed.
Monitoring for Relative Humidity
The PdMetrics® predictive maintenance software platform will monitor the relative humidity and temperature and tell you when the furnace has been exposed to water vapor for prolonged periods of time. Essentially, it takes into consideration how long the furnace is taking to pump down compared to the ambient temperature and humidity (as well as how long the door is open). If there’s an issue, it will set off a vacuum integrity alert stating it’s taking too long to pump down. It will then determine if it’s humid outside. If it is, the software will provide a set of steps that you should first follow. If it’s not, it will provide a different set of steps for you to take. Overall, it will prioritize the corrective actions based on certain sensor readings and running algorithms.
If you require assistance with your heat-treating system, don’t hesitate to contact our experts by calling Ipsen’s Aftermarket Support Helpline at 1-844-Go-Ipsen (Toll Free: 1-844-464-7736; International: +1 815-332-2530) or fill out our Ask an Expert form at www.IpsenUSA.com/Ask-an-Expert.