The desire to conserve fuel has lead many aluminum plants to consider substituting direct firing for indirect firing in their heat treating operations. Success or failure of direct firing depends largely on the aluminum products susceptibility to a form of atmospheric attack known ad HTO (high temperature oxidation).
HTO is not really an oxidation reaction, it is the absorption of atomic hydrogen into the surface of an aluminium product. Once inside the metal, the hydrogen atoms reassociate as molecules, causing internal porosity, surface blistering, discoloration, and loss of tensile properties.
Aluminum normally has tight, thin oxide film that protects the metal benealth. At elevated temperatures, however, water vapor can disrupt this film and dissociate on the metal surface, producing atomic hydrogen, which then diffuses into the metal.
This is the reason that many aluminum solution heat treat and homogenizing furnaces are indirect-fired to exclude water vapor present in the combustion products. As a further precaution, makeup air to the recirculating fans of many of these furnaces is dried before use. Tests have shown that as little as 0.8% water vapor in air (40 F dew point) will cause appreciable loss of tensile properties en 2024 alloy steel heated to 920-930 F for 20 hours.
On the other hand, CO2, in the furnace atmosphere inhibits the action of water vapor. Studies showed that the combustion products of natural gas contained enough CO2, levels were too low to prevent HTO, even tough water vapor content on the combustion products had also decreased. Unfortunately, 200% excess air corresponds to about 1600 F hot mix temperature, too high for most aluminum heat treating processes.
Although combustion products will not protect aluminum at air/fuel ratios useful for heat treating, some furnaces can be converted to direct firing anyway. Here are some points to consider before recommending direct firing:
1. Heat Treatment Temperature: HTO occurs primarily at solution heat treating and homogenizing temperatures (850 - 1150 F). It is rarely a problem with lower temperature operations (aging, annealing and stress relieving).
2. Products Form and Alloy: Wrought products 8sheet, plate, extrusions, forgings) are most susceptible to HTO. Castings are nearly immune, Susceptibility to HTO increases with the magnesium content of the alloy. In fact, HTO has even been found in some castings made from high magnesium alloys.
3. Subsequent Processing: HTO is primarily a surface defect, and probably can be tolerated if the product surface will later be removed by machining. HTO in extrusion stock doesn’t seem to present a problem -- apparently the affected area is stripped off the billet during extrusion. Rolling will not heal surface blistering caused by HTO, so stock destined for the rolling mill 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 atmosphere. Ammonium, potassium, and sodium fluoborate and boron trifluoride are commonly used. These compounds prevent diffusion of hydrogen into aluminum by depositing a protective coating on the surface. These fluorine compounds have two drawbacks.
- If used in excessive amounts, they discolor aluminum.
- Furnace flue gases will contain fluorine compounds that may be an environmental problem.
Sulfur and HTO. Avoid direct firing where fuels contain any measurable amount of sulfur. Only traces of SO2 in furnace atmosphere can have a devastating effect on product quality, producing severe HTO, even in relatively dry atmospheres.
