Aluminizing: A Great Coating for Metals Used in Heat Treat Processes

August 31, 2017 | Wire Forms, Steel Wire Products

Basket Holding Gears.jpgHeat treatment processes place severe strain on the equipment used in them. In particular, the baskets used in heat treatment take a lot of punishment because high temperatures can weaken most metals so that other stress factors, such as the weight of held parts, have an increased impact.

This can severely reduce the useful life of your heat treat baskets—meaning more frequent replacements and higher operating costs.

One way to extend the useful life of heat treat wire baskets and trays is to carefully pick the right base material for the frame of the basket. Some metals such as Inconel® 625 have extremely high performance characteristics, even in temperatures of 2,000°F (1,093°C).

However, even these metals will weaken if exposed to certain conditions. Which is why some of Marlin’s clients ask: “is there another way to prolong the useful life of a heat treat basket or tray? “

Why Heat Treat Trays and Baskets Fail

Before we get into solutions for heat treat wire basket/tray failures, it’s important to know why these metal forms fail.

The high temperatures of the heat treat process can weaken metal, making it more prone to losing shape or breaking, yes. But, stress from the weight of held parts isn’t the only cause of failure in these metal forms.

Other key causes of failure in heat treat processes include:

  • Metal Dusting. In carburizing processes, certain metallic components can break down into metal powder at temperatures in the 800-1,650°F (430-900°C) range in low-oxygen, high hydrocarbon (methane, propane, ethane, etc.) environments. The iron alloy in a metal form can be over-saturated with carbon and suffer buildup. This can cause embrittlement and loss of ductility.

  • Thermal Fatigue. Just because a metal form survived the heat treat process doesn’t mean that it’s still as strong as before it went in. Rapid heating and cooling of metal forms can create a temperature gradient between the inner core and outer shell of the metal—causing different rates of expansion and contraction between the inner and outer layers. Over time, the part begins to crack under the repeated strain.

A common means of fighting metal dusting is to use a high-nickel alloy (such as Inconel®). High nickel alloys resist the carbon saturation effect of metal dusting, and can resist oxidation at high temperatures as well.

However, thermal fatigue remains a problem, even for Inconel® baskets and trays. How can you diffuse the impact of thermal stresses to increase the useful life of your heat treat baskets?

One method is to apply a specialized coating to your heat treat metal forms—but not just any coating will do:

Prolonging Heat Treat Basket Life with Aluminizing

Coatings for heat treat baskets have to be exceptionally resistant to heat, so most polymer-based coatings are not suitable for this use. The reason is that most polymers have a melting point measured in the hundreds of degrees Fahrenheit, while heat treat processes can exceed 1,000-2,000°F.

One coating treatment commonly used for protecting heat treat baskets and trays is aluminizing.

In some ways, this process is similar to electroplating. Here, a mixture of aluminum, halide salt, and an inert filler material is used to transfer aluminum particles onto the surface of the metal being coated. This helps to create a protective aluminum oxide layer for heat treat baskets that will survive high temperatures.

Aluminizing has been successfully used to enhance the oxidation, corrosion, and wear-resistance properties of high-nickel alloys such as Inconel® for heat treat processes. Ironically enough, a basket that has been aluminized may need to go through a heat treat process itself to recover the properties of the substrate alloy.

It's important to note that eventually, even the toughest, most well-built heat treat baskets will fail. The stresses involved are just too great for any basket to completely withstand all the strain forever.

However, by using aluminizing to coat your heat treat baskets and improve their resistance to oxidation, heat stress, and corrosion, you can extend the useful life of your baskets to get a better return on your investment.

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