High-Temperature Environments: Pushing the Limits of the 28mm Cartridge Heater

What occurs when the operational temperature goes above 500°C, and how to maintain a cartridge heater working.


Most industrial heating jobs work well between 100°C and 400°C. When installed correctly, a basic stainless steel single head cartridge heater works well for years at these temperatures.

But certain uses need more. Die-casting, glass forming, some chemical processes, and plastics that work at very high temperatures all need to work at 500°C, 600°C, or even higher for a long time. Standard cartridge heaters break out quickly at these extremes.

Anyone who works with difficult thermal processes has to know what the 28mm big diameter single head cartridge heater can and can't do in high-temperature settings.

What Happens When the Temperature Rises?
As the temperature rises, every part of a cartridge heater has to work harder.

Material for the sheath. Above 500°C, standard stainless steel (304, 316) starts to lose its strength and ability to resist oxidation. The sheath may become brittle, and surface scaling may happen. Rapid degradation happens when you run it for a long time at 600°C. Incoloy® sheaths (grades 800 and 840) stay strong and don't rust up to about 800°C. Nickel-chromium alloys or unique superalloys are needed for the hottest temperatures.

Insulation made of MgO. Magnesium oxide insulation is quite stable and doesn't melt until it reaches 2800°C. But when the temperature is high, MgO can become a little conductive, which lowers the resistance of the insulator. This effect goes away when the temperature drops, but it might create annoying ground fault tripping in sensitive control systems. MgO with a controlled amount of moisture works better at high temperatures.

Wire that resists. The wire that resists heat is the part that is most sensitive to temperature. Above 600°C, standard NiCr (80/20 nickel-chromium) wire oxidises quickly. To keep working above this point, you need special alloys like NiCrFe or custom high-temperature grades.

Terminations. The place where the electrical leads join to the resistance wire is weak. At high temperatures, standard brazed or crimped connections may become weaker or break. You need welded connectors or high-temperature terminations that are made just for this purpose.

The 28mm Advantage in Hot Weather
In high-temperature situations, a 28mm cartridge heater is better because it has a bigger diameter. The resistance wire can be coiled at a lower watt density for the same total wattage since it has more space within. Even when the sheath is heated, lower watt density equals lower temperatures inside the wire.

A 28mm single head cartridge heater with a sheath temperature of 600°C and a watt density of 5 W/cm² may have an interior wire temperature of 750°C. A 12mm heater with the same wattage may raise the temperature of the inside wires to more than 900°C, which is just too high for any resistant alloy to handle.

This disparity is why bigger heaters often work in high-temperature situations while smaller ones don't.

Choosing Sheath Materials for High Temperature
Choosing the right sheath material is very important for a cartridge heater that works at 500°C.

Incoloy 800 can be used continuously at temperatures up to 760°C. Good mechanical strength and resistance to oxidation. The most typical choice for industrial uses that need to work at high temperatures.

Incoloy 840 is like 800, except it has more chromium in it, which makes it better at resisting oxidation. Works well up to 800°C.

Nickel-chromium alloys like 80/20 are called nichrome (NiCr). They can be used as sheath materials at very high temperatures (up to 1000°C), but they cost more and don't withstand corrosion as well as Incoloy.

Stainless steel 310 is an austenitic grade that has more chromium and nickel than 304/316. Good for short periods of time at temperatures up to 1000°C, but it oxidises faster than Incoloy when used continuously at high temperatures.

When working with corrosive gases or chemicals at high temperatures, you may need titanium or special coatings. However, these solutions are much more expensive.

Things to think about when installing in high heat
Thermal expansion is more noticeable at higher temperatures. When heated to 600°C, a 28mm cartridge heater that is already at room temperature may expand by around 0.1% of its length (about 0.2mm on a 200mm heater). If the mounting hole expands more than the heater, this can cause the heater to get stuck in a narrow hole or, on the other hand, make a gap.

For high-temperature applications, a diametral clearance at the higher end of the suggested range (0.04–0.05mm) is best since it allows for differential expansion. The mounting hole should be straight and smooth so that the heater can expand without bending.

You also need to think about the temperatures near the lead exit. Even if the lead end is not heated, heat can still move along the sheath to the ends. If the temperature of the lead exits goes beyond 200°C, normal lead wires and seals will break. You might need long, unheated lengths, cooling blocks, or high-temperature lead materials like ceramic fibre or metal-sheathed cables.

Most Common Ways Things Go Wrong at High Temperature
When a cartridge heater stops working at a high-temperature setting, it's usually because of one of the following:

Oxidation of the sheath: Scaling makes the sheath wall thinner, which finally leads to a breach that lets the outside world see the inside parts.

Insulation resistance drop: The MgO becomes conductive enough at high temperatures to trip ground fault protection, even when the heater is still working.

Oxidation of resistance wire: The wire slowly loses its cross-section, which raises resistance and generates hot spots that speed up failure.

Lead seal failure: The seal between the sheath and the lead wires breaks down, letting moisture or dirt into the heater when it cools down.

Helpful tips for success at high temperatures
When choosing a cartridge heater for high-temperature use, keep these things in mind. The 28mm size is frequently the best because it is the largest that may be used. Choose sheath and wire materials that can handle the temperature you need plus a safety margin for prolonged use. For long-term use at high temperatures, keep the watt density as low as possible, ideally below 5 W/cm². At the lead end, make sure there is enough unheated length to protect the terminations. If you need to, think about using active cooling (air or water) for lead exits.

Last Thoughts
When used at high temperatures, every part of a single head cartridge heater is pushed to its limits. To be successful, you need to choose your materials carefully, keep the watt density low, and pay attention to thermal expansion and lead exit temperatures. The 28mm large diameter single head cartridge heater is preferable for these tough conditions since it has a higher surface area and a bigger internal volume than lesser sizes.

Different thermal processes need different amounts of heat. A cartridge heater that works in a die-casting application at 550°C might not work as well in a glass-forming process at 700°C. For the heater to work reliably, it needs to be set to the right temperature for the job.