Heat treatment of metals and alloys is used to produce parts having the desired mechanical and surface properties, as well as to relieve residual stresses after mechanical deformation. Today, the atmospheres required to conduct these heat treatments are generated using exothermic and endothermic generators and ammonia dissociators. These generated gases have serious disadvantages compared with atmospheres composed of industrial gases such as nitrogen and hydrogen. For this reason, many heat treatment companies use high-quality atmospheres based on nitrogen.
There are several types of industrial gas atmospheres used for the heat treatment of aluminum. Air Products provides industrial gas-based atmospheres including pure nitrogen or pure hydrogen and nitrogen/hydrogen mixtures for use in conventional applications such as annealing, as well as for brazing and sintering. Because oxidizing atmosphere constituents are commonly considered harmful for heat treatment processes, most heat treaters use very high purity nitrogen, which is delivered and stored in a liquid form in cryogenic tanks and then vaporized on site for gaseous use.
It is becoming increasingly important to produce high quality parts at competitive prices. In a number of heat treatment operations, it is technically possible to use nitrogen of a lower purity, which means higher oxygen levels compared with liquid nitrogen. This nitrogen can be generated in a gaseous form on the customer's site in so called on-site generators. Depending on the flow conditions and acceptable purity, on-site generated nitrogen can be significantly less expensive than liquid nitrogen.
In general, atmospheres are used to provide an atmosphere neutral to the metal, to prevent oxidation on the surface and to provide a bright surface finish. Therefore, air has to be removed from the furnace and, depending on the type of metal, the remaining traces of oxygen have to be reduced by a reactive component in the atmosphere blend. However, the reducing reaction should not produce too many oxidizing components, such as moisture in the form of water vapor (reaction 1) or carbon dioxide (reaction 2), so it is important to provide a sufficient flow rate of a protective atmosphere to prevent air ingress (see equations 1 & 2).
However, heat-treating atmospheres always have residual amounts or traces of oxidizing components. The resulting metal oxidation can be represented by following reactions (see equations 3-5).
Reactions (4) and (5) show that the oxidizing or reducing potential always depends not only on the amount of oxidizing components present, but also on the equilibrium of pH2/pH2O or pCO/ pCO2. The driving force for reaction (3) is simply the partial pressure of oxygen.
This reaction is the most important for pure nitrogen atmospheres. The Ellingham- Richardson-Diagram (Fig. 1) shows the reducing or oxidizing potential of the reactions (3), (4) and (5) for several materials and for different temperatures.
Heat treatment of aluminum
As shown in Table 1, aluminum reacts with very small amounts of oxidants and, therefore, produces a very thin passive oxide layer. For that reason, aluminum was traditionally heat treated in an exothermic generated atmosphere or in air. But annealing in an oxygen rich atmosphere has two drawbacks: (1) increased surface oxidation and (2) the possibility of explosion due to rolling oil vapor mixing with oxygen at a high temperature.
Exothermic generated atmospheres also have the disadvantage of being toxic and combustible. Furthermore, these generators have high maintenance costs and sometimes cause production losses due to equipment breakdown. Consequently, nearly all heat treaters of aluminum use a pure nitrogen atmosphere. Nitrogen purges the oxygen out of the furnace, eliminates the risk of explosion due to oil vapor and avoids the formation of flammable atmospheres containing toxic components such as CO, as found in exothermic atmospheres. Since aluminum forms a corrosion resistant protective layer, both with oxygen (O2) and water vapor (H2O), there is no need to use high purity nitrogen. Trials have shown that there are no differences in the surface quality or oxide layer by using nitrogen of purities 99.5 % and above for pure aluminum. In the case of aluminum alloys, there might be the requirement for higher purities of nitrogen, but there is still the opportunity to use cost-effective on-site generated nitrogen.
Nitrogen on-site systems
Nitrogen can be supplied in different ways and having different purities. Bulk or on-site supply can be used for the quantities used normally in heat treatment companies. Bulk, or liquid stored nitrogen has the highest quality with a 99.9995% purity, and on-site generated nitrogen can be produced in different ways and with different purities. Three typical technologies used for on-site nitrogen generation are membrane systems, pressure swing adsorption (PSA) systems, and high purity nitrogen (HPN) generators. Table 2 provides an overview of the different systems including the range of nitrogen purities, flow rates and pressure for optimal use.
Because nitrogen flow rates and operational procedures are different in every plant, there can be no general recommendation for any mode of supply. The investment costs for an on-site system increase with higher purities and flow rates. In general, on-site systems are very competitive for continuous and high flow rates, but the break-even point is different for every customer. In some cases, it can be more economical to stay with liquid nitrogen.
Figure 2 shows a typical nitrogen flow profile for a heat treatment plant that has several furnaces. On-site systems work most economically when they deliver a constant 100% design flow. Therefore, it is recommended that the heat treater specify the on-site system for the base load (gray line) and to supply the peaks by liquid nitrogen (LIN - green line). The buffer vessel typically used with on-site systems allows short-term peaks in demand to be catered for (orange line). Practical experience on customer sites has shown significant costs savings of up to 50% on total nitrogen costs.
Exothermic generated atmospheres or nitrogen atmospheres are used for normal aluminum heat-treating processes. Due to several disadvantages of exothermic generated atmospheres, such as high maintenance costs, and toxic and explosive atmospheres, most heat treaters have replaced the generators with nitrogen-based atmospheres. In this case, the nitrogen is commonly supplied in a liquid form. However, in many heat treatment processes it is possible to use on-site generated nitrogen having a lower purity compared with liquid nitrogen. Furthermore, in some applications, although the high quality of liquid nitrogen is required, it still makes sense to convert the liquid to an on-site supply. This is true if the atmosphere requirement is continuous and at a constant flow rate. Practical experience on customer sites has shown significant costs savings of up to 50% on total nitrogen costs. Technical gas suppliers can assist in calculating the real commercial benefits for specific applications.