igh-quality product usually requires a bright fi nish. That fi nish can be obtained easily and consistently using a high-quality atmosphere, but in addition to the production engineers quality goals, minimizing

cost is also the objective. This optimization process will entail closer tailoring of the atmosphere to material and furnace require-ments.

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 fur-nace leaks are inevitable. Therefore, to reduce the oxygen, it is reacted with an active gas, such as hydrogen, as follows:

O2 + H2 H2O

From this equation we can see that the oxygen partial pres-sure is related to the H

:H

O 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 fi lms. As can be seen from Figure 1, for some alloying elements the

Bright Annealing Copper and its AlloysPaul F. Stratton Linde Gas; Sheffi eld, U.K.

The annealing of the majority of copper-based alloys is not very demanding in terms of protective atmospheres. However, the protective atmosphere used can be critical to the apparent quality of the fi nished product.

IndustrialHeating.com - February 2010 51

FEATURE | IndustrialGases/Combustion

HH

Fig. 5. Copper alloy after bright annealing(Photo courtesy of Ebner-Industrieofenbau)

10+14

10+11

10+9

10+5

10+2

10-1

10-4

10-7

10-10 0 200 400 600 800 1000 1200

Al

CrZn

FeSnNi

Pb CuHyd

roge

n:w

ater

ratio

Temperature, C

Fig. 1. The oxidation limits for various elements

52 February 2010 - IndustrialHeating.com

H:H

O ratio required to keep the alloy

bright rises as the temperature falls, mak-ing the cooling zone the most critical part of a continuous furnace in these cases. If the furnace atmosphere of such a furnace is fl ammable and allowed to burn off at the exit, oxidation of the product can oc-cur as it passes through the fl ame front. Such problems are best avoided either by using a non-fl ammable atmosphere or en-suring that any fl ammable component is diluted below the fl ammable limit before the exit. These differing requirements, to-gether with the need to minimize the to-tal 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 atmo-sphere consumption. Conclusions drawn from Figure 1 must be regarded with caution, particularly below about 300C (572F), since equi-librium conditions may not apply and re-actions are slow. What happens is more re-lated 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 refl ect the most abundant rather than the most active constituent. Other reducing gases can be used to prevent oxidation. Exothermically gener-ated 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 anneal-ing temperatures, the component gases are at equilibrium, and it is suffi cient 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 form-ing carbon soot, particularly in the 400-600C (752-1112F) 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 ei-ther by physical fl ushing or by reacting the lubricant with a component of the atmosphere. In continuous furnaces it is usually necessary to restrict the active at-mosphere component to the entry areas because they tend to be oxidizing to the components at the annealing tempera-

ture. The same is true of batch-furnace operations where the process must be completed well before the annealing tem-perature is reached. For some lubricants, this can entail a dwell in the ramp-up to annealing temperature at some interme-diate temperature at which volatilization or reaction is essentially complete.

CopperCopper is diffi cult 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% de-pending on furnace tightness). It should be noted that tough pitch copper should be an-nealed in atmospheres containing no more than 2% hydrogen to avoid embrittlement. While high-purity nitrogen/hydrogen will undoubtedly result in the highest-quality fi nish, 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

FEATURE | IndustrialGases/Combustion

Fig. 2. Copper tube annealed using a non-cryogenically generated nitrogen and hydrogen atmosphere Fig. 3. ADSOSS-N nitrogen-generation plant

IndustrialHeating.com - February 2010 53

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 com-plete before the gas mixture interacts with the work. There are several ways of achieving this, but perhaps the most ef-fective 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 suffi ce. Where speeds are higher in the induction annealing of fi ne wires, oxygen levels of up to 3% can be used, allowing non-cryogenically gener-ated nitrogen to be employed directly as the protective atmosphere.

BrassBrass is a very different problem than cop-per. Zinc is much more oxidizable than cop-per. 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, form-ing 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 at-mospheres 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 suit-able, 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 -40C. A HY-DROFLEX 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 fi nish 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 pro-cess. Although a rich exogas can be em-ployed, 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 alterna-tive is nitrogen/hydrogen. One solution to the provision of nitrogen/hydrogen mixtures is cracked ammonia. As in this case, however, where the proportion of

Fig. 4. Brass wire after annealing in 100% hydrogen in a high-integrity HICON/H2 bell furnace (Photo courtesy of Ebner-Industrieofenbau)

54 February 2010 - IndustrialHeating.com

FEATURE | IndustrialGases/Combustion

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 depen-dent 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 ac-tive alloying elements such as aluminum,

beryllium and zinc, but in relatively low amounts. These alloys can be annealed in protective atmospheres based on exother-mically generated gas, as described above. However, nitrogen/hydrogen mixtures are preferable because of the ease of adjusting the reducing potential as well as consid-erations 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 re-ducing atmosphere.

SummaryAlthough the basic requirements of the atmosphere to keep any given alloy bright during annealing are fi xed, 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 Develop-ment Association (helpline@copperdev.co.uk), industrial gas suppliers such as Linde Gas (heat-treatment@linde-gas.com) or specialist furnace manufactur-ers like Ebner-Industrieofenbau (sales@ebner.cc). IH

For more information: Contact Dr. Paul Stratton, CEng CSci FIMMM, heat-treatment and electronic-packaging application devel-opment, Linde AG BOC, Rother Valley Way, Holbrook, Sheffi eld, S20 3RP, UK; tel: +44 1484 328736; e-mail: heat-treatment@linde-gas.com; web: www.linde-gas.com

Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: reducing atmosphere, exothermically generated atmosphere, bright annealing, induction annealing

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