With the aim of saving fuel, some industrial aluminum heat treat operations have tested the replacement of direct fire by indirect fire in their heat treatment operations.
The level of effectiveness of direct heating will depend largely on the susceptibility of aluminum products to a form of atmospheric attack known as HTO (high temperature oxidation).
Below, we will talk about this reaction and how it influences the success of direct heating applications in aluminum.
About HTO reaction
HTO is not really an oxidation reaction, but consists of the absorption of atomic hydrogen on the surface of an aluminum product, where once inside the metal, hydrogen atoms are associated again as molecules, causing:
- Loss of tensile properties
- Crack formation on the surface
Aluminum normally has a thin, tight oxide film that protects the integrity of the metal. However, at elevated temperatures, water vapor can break down this film and dissociate on the metal surface, producing hydrogen atoms, which then diffuse into the metal.
Considerations when using direct heating
Although combustion products will not protect aluminum in air/fuel ratios useful for heat treatment, some furnaces can be converted for direct heating.
Some points to consider before recommending direct heating are:
1. Heat treatment temperature
- HTO mainly occurs at heat treatment and solution homogenization temperatures around 455°C–620°C (850°F–1150°F).
- Rarely do problems occur with low temperature operations (aging, annealing and stress relieving).
2. Product shape and composition
- Forged products (sheets, plates, extrusions, forged parts) are more susceptible to HTO.
- Castings are almost immune; susceptibility to HTO does increase with the magnesium content in the alloy.
- HTO has even been found in some molds made of high magnesium alloys.
3. Post processing
- HTO is primarily a surface defect, and it can likely be tolerated if the product surface is subsequently removed by machining.
- HTO in extruded material does not appear to present a problem; the affected area is seemingly removed from the billet during extrusion.
- Lamination will not cure the formation of surface cracks caused by HTO, so the material intended for the laminator must be kept free of HTO.
4. Protective compounds
- In many cases, HTO can be reduced or eliminated by adding certain fluorine compounds to the furnace atmosphere. Commonly used are ammonium, potassium and sodium fluoborates and boron trifluoride.
- These compounds prevent the diffusion of hydrogen in aluminum by depositing a protective layer on the surface. These fluorine compounds have two drawbacks:
a) If used in excessive quantities, they discolor aluminum.
b) The furnace’s combustion gases will contain fluorine compounds that can be an environmental problem.
At Nutec Bickley we are highly experienced in understanding the thermal equipment needs of our customers and providing the appropriate solution for their needs at the most competitive price with our heat treatment furnaces. Contact our consultants and get them to review your next project.
