High-quality product usually requires a bright finish. That finish can be obtained easily and consistently using a high-quality atmosphere, but in addition to the production engineer’s quality goals, minimizing cost is also the objective. This optimization process will entail closer tailoring of the atmosphere to material and furnace requirements.
Basic PrinciplesThe basic function of the furnace atmosphere in copper-based-alloy annealing is to protect the product from oxidation. To achieve this, the oxygen partial pressure of the atmosphere must be less than that necessary to form the oxide. Even high-purity nitrogen contains one or two parts per million of oxygen, and furnace leaks are inevitable. Therefore, to reduce the oxygen, it is reacted with an active gas, such as hydrogen, as follows:
½O2 + H2 ® H2O
From this equation we can see that the oxygen partial pressure is related to the H2:H2O ratio. The higher the ratio, the more reducing the atmosphere. Thus, if the atmosphere composition is above the line in Figure 1, the oxide would be reduced and below the line the metal oxidized. The component requiring the most reducing atmosphere must be considered the critical element. For most practical purposes, however, elements with concentrations below 1% can be ignored because they do not form continuous oxide films.
As can be seen from Figure 1, for some alloying elements the H2:H2O ratio required to keep the alloy bright rises as the temperature falls, making the cooling zone the most critical part of a continuous furnace in these cases. If the furnace atmosphere of such a furnace is flammable and allowed to burn off at the exit, oxidation of the product can occur as it passes through the flame front. Such problems are best avoided either by using a non-flammable atmosphere or ensuring that any flammable component is diluted below the flammable limit before the exit. These differing requirements, together with the need to minimize the total quantity of atmosphere gas used (and hence minimize cost), are best achieved using zoning technology. This technique allows the use of different atmospheres in different zones in a continuous furnace and helps to minimize the overall atmosphere consumption.
Conclusions drawn from Figure 1 must be regarded with caution, particularly below about 300°C (572°F), since equilibrium conditions may not apply and reactions are slow. What happens is more related to reaction rates and availability. At low temperatures, non-equilibrium oxides of tin and copper may form, particularly when the availability of oxygen is low. At lower temperatures, the oxides formed on alloys tend to reflect the most abundant rather than the most active constituent.
Other reducing gases can be used to prevent oxidation. Exothermically generated gas (exogas) is a partially combusted natural gas mixture containing N2, CO, CO2, H2 and H2O. It is sometimes used in copper-alloy annealing. At typical annealing temperatures, the component gases are at equilibrium, and it is sufficient to examine the H2:H2O ratio to determine its oxidizing tendencies. A point to note about CO-bearing atmospheres, besides their toxicity, is the possibility of forming carbon soot, particularly in the 400-600°C (752-1112°F) range according to the producer gas or Boudouard reaction:
2CO = CO2 + C
An additional role of the atmosphere is to remove unwanted processing lubricants from a furnace. This can be achieved either by physical flushing or by reacting the lubricant with a component of the atmosphere. In continuous furnaces it is usually necessary to restrict the active atmosphere component to the entry areas because they tend to be oxidizing to the components at the annealing temperature. The same is true of batch-furnace operations where the process must be completed well before the annealing temperature is reached. For some lubricants, this can entail a dwell in the ramp-up to annealing temperature at some intermediate temperature at which volatilization or reaction is essentially complete.
CopperCopper is difficult to oxidize and, as can be seen from Figure 1, lies below most of the common nonferrous metals. It is common practice to anneal copper either in exogas or in steam. The former contains some CO which can decompose into soot and CO2, and the latter is expensive. A cleaner, non-toxic alternative is high-purity nitrogen with a small hydrogen addition (1-2% depending on furnace tightness). It should be noted that tough pitch copper should be annealed in atmospheres containing no more than 2% hydrogen to avoid embrittlement.
While high-purity nitrogen/hydrogen will undoubtedly result in the highest-quality finish, there are cheaper options that, with the proper precautions, can produce an equally good result (Fig. 2). If the high-purity nitrogen is replaced by lower-purity nitrogen from an ADSOSS® non-cryogenic nitrogen generator (Fig. 3), costs can be reduced. The small amount of oxygen present in the nitrogen will react with some of the hydrogen to form water vapor, which as stated previously is not oxidizing to copper. Unfortunately, however, the reaction is slow and steps must be taken to ensure that it is complete before the gas mixture interacts with the work. There are several ways of achieving this, but perhaps the most effective is to pass the gas mixture over a catalyst prior to its introduction into the furnace. The hydrogen level must also be adjusted to take into account that which reacted with the oxygen.
If the exposure time of the copper to the annealing atmosphere is short (as, for example, in the continuous annealing of wire for electrical cable), then increasing amounts of free oxygen can be tolerated with increasing speed. For annealing of heavier cables in tube furnaces, high-purity nitrogen alone with 1 or 2 ppm of free oxygen will suffice. Where speeds are higher in the induction annealing of fine wires, oxygen levels of up to 3% can be used, allowing non-cryogenically generated nitrogen to be employed directly as the protective atmosphere.
BrassBrass is a very different problem than copper. Zinc is much more oxidizable than copper. To make matters worse, both zinc and its oxide volatilize at high temperatures. Zinc can volatilize from coils of brass strip and oxidize to form white discoloration at the edges of coils. If zinc oxide forms at high temperatures, it also volatilizes and reprecipitates in the cooler parts of the furnace, including the cooling work, forming an unsightly white bloom. None of the atmospheres generated from the partial combustion of hydrocarbon fuel gases are suitable for the bright annealing of brass, although endothermically generated atmospheres are sometimes employed. Once again, however, the Boudouard reaction must be considered as such atmospheres contain CO, which will break down to soot and discolor the components.
In most furnaces a mixture of high-purity nitrogen with 40% hydrogen is suitable, although many operators use 100% hydrogen in high-integrity bell furnaces for the highest-quality product (Fig. 4). It is recommended that the dew point is maintained at less that -40°C. A HYDROFLEX™ control system can be used to ensure optimum atmosphere conditions at all times in continuous furnaces. It may be possible to use slightly less hydrogen if the furnace is very tight (for example, for hump-back continuous furnaces or where a less than fully bright finish is acceptable). Cracked ammonia can also be used but with the caveats given below for bronzes.
BronzeTin, as well as being less easily oxidized than zinc, forms a stable protective oxide at annealing temperatures. This makes bronzes easier to protect during the process. Although a rich exogas can be employed, it contains even more CO than the lean gas used for copper annealing. Hence, the toxicity and propensity to soot are increased. The non-toxic alternative is nitrogen/hydrogen. One solution to the provision of nitrogen/hydrogen mixtures is cracked ammonia. As in this case, however, where the proportion of hydrogen required is well below the 75% level produced, the economics are often poor. They can be improved by diluting the cracked ammonia with high-purity nitrogen. If cracking equipment is not well-maintained, ammonia carryover can occur, resulting in corrosive attack on copper-based alloys.
The optimum solution is, therefore, to use a mixture of high-purity nitrogen and high-purity hydrogen. The proportion of hydrogen actually required is dependent upon the tightness of the furnace used. If possible, the proportion should be kept below 4.9%, which is the lower explosive limit. In the majority of well-sealed furnaces this is not a problem, and hydrogen levels as low as 2% have been successfully employed.
Other AlloysOther copper-based alloys contain active alloying elements such as aluminum, beryllium and zinc, but in relatively low amounts. These alloys can be annealed in protective atmospheres based on exothermically generated gas, as described above. However, nitrogen/hydrogen mixtures are preferable because of the ease of adjusting the reducing potential as well as considerations of toxicity and soot (Fig. 5 – lead photo). Monel 400 (66% Ni, 0.12% C, 0.90% Mn, 1.35% Fe, 0.15% Si, 31.5% Cu) and similar alloys remain bright and free of discoloration when heat treated in a reducing atmosphere.
SummaryAlthough the basic requirements of the atmosphere to keep any given alloy bright during annealing are fixed, the techniques available for meeting those requirements are numerous. The optimum method of supply in terms of cost, performance and environmental considerations depends on a number of factors, including furnace type and size. Several sources of advice are available, including the Copper Development Association (firstname.lastname@example.org), industrial gas suppliers such as Linde Gas (email@example.com) or specialist furnace manufacturers like Ebner-Industrieofenbau (firstname.lastname@example.org). IH
For more information: Contact Dr. Paul Stratton, CEng CSci FIMMM, heat-treatment and electronic-packaging application development, Linde AG BOC, Rother Valley Way, Holbrook, Sheffield, S20 3RP, UK; tel: +44 1484 328736; e-mail: email@example.com; web: www.linde-gas.com
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