Welcome to Heat Treating: 101 Series

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Choosing an Internal or External Quench Vacuum Furnace

One of the first decision points that vacuum furnace buyers encounter is the decision whether to purchase an internal or external quench furnace. At Ipsen, we consult with each customer prior to making this decision to ensure we fully understand their needs and can recommend the best furnace for them. Our MetalMaster line represents our external quench solution, while the TurboTreater and TITAN lines are our internal quench designs. Ipsen Technical Director Jim Grann and Chief Engineer Craig Moller discuss some of the factors that contribute to deciding which type of furnace is best for you.

“It’s important to note that there are reasons to use both categories of furnaces for different purposes. There’s a reason we continue to make both external-quench MetalMasters and internal-quench TurboTreaters and TITANs. Both designs have merits for different processes.”
— Jim Grann, Ipsen Technical Director

What Defines Internal vs External Quench?

The key functions that define the difference between internal and external quench are the heat exchanger and the gas cooling system (cooling fan & blower motor). Internal versus external quench, by definition, is determined by whether they operate within the pressure vessel, protected by a water-cooled barrier from the hot zone, or whether they operate outside the pressure vessel, connected with piping to the back of the furnace and the sidewall to complete the gas-cooling circuit. Though both designs are versatile, each design is optimized to handle different specific processes, workloads, metallurgy, and part geometry.

The Case for External Quench

At Ipsen, our MetalMaster series of external quench furnaces have a lineage that goes back decades. Their popularity comes from their longevity, their low maintenance costs, and their flexibility for a variety of quenching processes up to 2-bar pressure. In a MetalMaster, once the final heating segment has completed, the furnace injects the inert gas (argon or nitrogen) into the chamber until it achieves the desired quench pressure, and then it turns on the cooling system. The gas begins to circulate out through the back of the furnace, through the hot gas piping to the heat exchanger. Once the gas is cooled, the fan pushes the gas through the cool gas pipe and through the sidewall of the chamber to charge the gas plenum.

A plenum is an enclosed chamber where a treated substance collects for distribution, such as heated or cooled air in a ventilation system. In a vacuum furnace, it is located in the gap between the water-cooled jacket of the vessel and the exterior of the hot zone.

The plenum directs the gas through the cooling nozzles, through the hot zone, and back into the chamber, where it recirculates again, quenching the parts and carrying off the remaining heat through the back of the furnace repeatedly until the final quench temperature of the parts has been achieved. External quench systems like the MetalMaster are optimized for operating at 2-bar quenching pressure and below, which means they’re excellent for brazing, tempering, annealing, and a variety of other processes.

Limitations of External Quench Furnaces

While the external quench design is optimized for quenching pressure of 2 bar or lower, we can achieve 6-bar or even 10-bar qualified furnaces for higher pressure quenching. However, the external quench blower and heat exchanger housing, in addition to the piping, all have to be able to withstand that level of cooling gas pressure. When compared to an internal quench furnace for comparable high-pressure quenching, it can make these high-pressure external quench systems cost-prohibitive.

The extra volume of the pipes going to and from the heat exchanger and blower motor means that the process of drawing a vacuum in the furnace takes longer. Similarly, the system will require a larger consumption of gas than a comparably sized internal quench furnace. Given the lower quenching pressure levels within the furnace, external quench furnace designs are not ideal for most hardening processes.

The external systems also affect the overall footprint of the furnace. The gas cooling system and heat exchanger will take up floor space next to the furnace and may contribute to a higher operating noise level when the motor is running.

In Summary

“The MetalMaster has become a workhorse in our industry, often serving customers for decades with minimal maintenance costs. While the MetalMaster can harden some D2 steels and some other simple alloys, I wouldn’t classify it under the umbrella of a ‘hardening furnace.’ Cooling parts very quickly isn’t an external quench furnace’s forte. But when you want a furnace with a pedigree of running for years without issues, and you’re using it for brazing, annealing, or tempering, the MetalMaster is my favorite furnace.”
— Jim Grann

The Case for Internal Quench

Ipsen’s lines of internal quench furnaces are the TurboTreater and TITAN brands as well as the Turbo²Treater and the VUTK. In most cases, both the heat exchanger and gas cooling system reside entirely within the pressure vessel, protected by a water-cooled wall from the hot zone. The TITAN has a mounted cooling motor on the back of the vessel, directly driving the cooling fan within the vessel by a shaft that’s sealed using a patented inflatable shaft seal.

Internal quench furnaces reduce the volume of quenching gas consumed by keeping it all within the walls of the pressure vessel while in use. For example, the TurboTreater recirculation system draws in the hot gas through the front door gap, back into the heat exchanger at the back of the pressure vessel, and then the cooling fan pushes the cooled gas back into the hot zone by charging the plenum and pushing it out through nozzles.

One of the biggest advantages of internal quench furnaces is the capacity to handle high-pressure quenching processes within a small footprint. There’s no need for floor space for the heat exchanger and gas circulation motor. Containing these systems within the vessel also makes for quieter operations.

Keeping all the gas within the pressure vessel means there are fewer places for leaks to occur, which also means these furnaces can reliably draw a higher vacuum level. The closed system allows internal-quench furnaces to quench with much higher gas pressures – covering pressure ranges from 2 bar to up to 20 bar — which makes quenching happen at a much faster rate; essential for most hardening processes.

TurboTreaters operate using a much larger horsepower motor to ensure that the gas moves quickly. Cooling gas speeds exiting the cooling nozzles can range from 100 miles per hour to 140 miles per hour. These high-velocity gas speeds can deliver reduced cycle times, featuring a faster quenching cycle. By increasing the gas pressure, operators can significantly decrease quench time.

“For parts with very complex geometries, when it comes to quenching, pressure is king. Where wind flow may cool a complex part unevenly, potentially causing distortion, part geometry is indifferent to pressure.”
— Jim Grann

TITAN’s advantages come with easier access to handle maintenance on the external blower motor. By operating outside the chamber, TITAN’s blower motors have the advantage of avoiding contamination from the chemical reactions or flying debris occurring within the chamber and are further removed from the dramatic thermal oscillations that are happening within the vessel.

Limitations of Internal Quench Furnaces

Because TurboTreaters with high-pressure quench systems are designed like a high-performance automobile — making lightning speed quenching possible with motors pushed to and beyond their limits by design, they require a similar amount of maintenance attention. High-pressure quenching processes will overamp the cooling motor by as much as 25 percent during the first five to ten minutes of a quench, to ensure that the hardening process is completed. The motors are special service factor motors that are designed to handle these heavy loads and overamping, but to keep the motor running optimally will require regular maintenance schedules and occasional replacement parts for areas of increased wear.

While the TITAN line helps reduce motor wear and tear by locating the blower motor outside of the chamber, the shaft seal must be maintained in accordance with the published procedures.

Internal quench systems may not consume as much gas as the external quench systems do, but to ensure the proper operations of higher horsepower motors in a system designed to move as many thermal units out of the furnace as quickly as possible, more robust power resources and water-cooling circulation systems may be required for these systems to work optimally.

In Summary

“The TurboTreater is super-fast, pumping down significantly faster than the MetalMaster, it backfills faster, uses less gas, and less floor space. While pushing a system like this to its limits may require a regular schedule of service, and users will need to prepare for the power and water demands that high-pressure rapid-quench systems require, the TurboTreater does a large number of things exceedingly well.”

“In the end, the best furnace for the job will be determined by process, part geometry, metallurgy, the overall volume of parts processed, and the available operating space.”
— Jim Grann

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Choosing between internal or external quenching is just one of the many important decisions to consider when investing in vacuum furnace equipment. At Ipsen, we take pride in our ability to design and manufacture specialized furnaces tailored to meet unique customer applications. Contact us at sales@ipsenusa.com to explore how we can support your specific needs.