The present invention relates to copper brazing and more particularly to the copper brazing of ferrous parts under particular furnace atmospheres which enable improved bonding and increased furnace material life.
Copper brazing is a commonly utilized technique for joining ferrous parts with close fitting joints (less than 0.1 mm wide gaps). Typically, such ferrous parts may comprise carbon steel, alloy steel and stainless steel and by melting a copper preform or paste material in a brazing furnace, molten copper is caused to flow into such gap and upon cooling, a copper bonded joint will be produced to provide liquid or gas tight seals. The preform which is essentially a solid copper bearing member or paste material is placed in controlled amounts near the close fitting joint to be brazed. The term "copper brazing" will include the use of pure copper and copper alloys containing relatively small quantities of tin, iron, nickel, phosphorus, as may be required to produce a particular bonded joint.
Copper brazing operations are typically carried out in a continuous conveyor (belt) furnace which consists of four notional zones or sections. Initially, the furnace includes a front throat or preheat section in which a temperature of up to approximately 700.degree. C. is established. The next furnace section in the direction of belt travel is the hot zone wherein the parts to be brazed are exposed to a maximum temperature and the copper or copper alloy of the preform or paste material melts and penetrates into the close fitting joint to be bonded. Typically, the length of a furnace hot zone or section is between 5-10 ft and the interior is lined with insulating brick or other suitable material. Commonly, the hot zone is heated by silicon carbide (Globar) heating elements and some furnaces are fitted with a metal alloy muffle in order to provide a more air tight interior of the hot zone. Muffle furnaces may be heated by use of natural gas from outside the muffle or from the use of silicon carbide heating elements either inside or outside the muffle. A slow cool section which is generally less than half the length of the hot zone of the furnace receives parts therefrom and enables a slow cooling of the parts to approximately 600.degree. C. in order to prevent warpage or distortion. A normal cool section follows the slow cool section in the direction of belt travel and enables a sufficient part temperature drop before such parts leave the exit end of the furnace while also preventing oxidation and discoloration of the parts and conveyor belt. Typically, the length of this section is at least as great as the length of the hot zone and may, in fact, be considerably longer.
Depending upon the particular copper brazing material (i.e. paste, etc.) utilized the brazing operation is performed in the furnace hot zone at a temperature of between 850.degree.-1150.degree. C. under a reducing atmosphere with a total residence time of less than 15 minutes in the hot zone. This relatively low residence time is effective to ensure a high production (i.e. throughput) rate of brazed parts. Typically, pure copper brazing is effected under hot zone temperatures of between about 1090 and 1150.degree. C. whereas copper-alloy brazing is generally carried out at temperatures of between about 850.degree.-1050.degree. C. In either case; however, the hot zone residence time is less than 15 minutes to ensure high throughput.
As mentioned above, the brazing operation is achieved under a reducing atmosphere although the particular composition of such atmosphere will be controlled by a set of critical requirements. In brief, these requirements may be referred to as clean surfaces, no surface decarburization, brazing material sources and furnace materials. Each of these requirements will now be discussed in some detail as will the inability of prior art brazing atmospheres in meeting these requirements.
Prior to the melting of copper or copper alloy (either from a solid preform or paste form) in the furnace hot zone, it is essential that surfaces to be joined together are metallurgically clean. The term "metallurgically clean" is utilized here to mean that the particular surface is "wetting" to molten copper or copper alloy and that this surface condition allows such molten metal to spread and penetrate into the joint to thereby form a strong brazed joint and good liquid or gas-tight seal. More specifically, the term "metallurgically clean" requires that the surfaces in and around the joint are virtually free of any oxide, carbide, soot (carbon) or any other material that reduces the "wetting" behavior between the molten copper or copper alloy and the ferrous part to be brazed. This initial requirement for successful brazing atmospheres dictates that such atmospheres be reducing but not highly carburizing to the ferrous part in order to ensure metallurgically clean surfaces. Thus, for plain iron and carbon steels under normal production conditions, a H.sub.2 /H.sub.2 O ratio (or equivalent H.sub.2 +CO/H.sub.2 O+CO.sub.2) of at least five is required. For low alloy steel parts, this ratio should be at least about 30 while for high alloy parts such as stainless steel, the minimal practical ratio is about 1,000. As a general rule, high alloy parts cannot tolerate any carburizing tendency of the brazing atmosphere.
The second requirement which must be satisfied by brazing atmospheres is the avoidance of surface decarburization. Certain steels such as medium to high carbon steels, low to medium alloy steels, certain high alloy tool steels, etc. contain about 0.3-1.0% carbon. When such ferrous materials are brazed, it is essential that decarburization be avoided in order to retain the metallurgical properties of such materials. Thus, the brazing atmosphere utilized must be essentially neutral in order to avoid decarburization and preclude carburization of the ferrous part surfaces in and around the joint as "wetting" and penetration of the brazing material will be impeded by carburized part surfaces.
As mentioned previously, copper or copper alloy brazing materials are typically applied to close fitting joints of parts to be bonded as solid preforms (100% dense) or as paste materials. When a copper paste is utilized, the brazing atmosphere must meet the aforementioned requirements of clean surfaces and avoidance of decarburization and additionally the paste must not leave residues which cause beading of molten copper which impedes the wetting properties thereof. Brazing pastes consist of a fine copper or copper alloy powder suspended in a suitable "vehicle" which contains a thickening agent and other additives. The thickening agent, etc. is necessary to prevent separation of the powder and thus assures the ability to apply a controllable amount of uniform paste to a particular joint to be brazed. Typically, the various vehicles, thickening agents, etc. are organic based (glycerine, glycol, petroleum, etc.) and these pastes contain no fluxing agents. The copper or copper alloy powder content is usually between 60 and 80% by weight of the paste to ensure good penetration of molten copper into a joint. If is mandatory that the vehicle, thickening agent, etc. be completely volatilized and burned to avoid leaving any residue prior to the melting of the metallic copper powder. In the absence of such burning, the molten copper metal tends to form in beads outside of the joint rather than penetrate the same and in addition, the residue reduces the esthetic appearance of the brazed joint. It will be understood that although the organic components of a paste may be thermally decomposed to yield carbon and hydrogen gas, carbon remains in a solid or soot form and tends to carburize part surfaces without wetting the same. Also, the solid residue (soot) interferes with plating or other operations and requires further cleaning of the part surfaces. Consequently, the brazing atmosphere must supply an oxidant in order to enable complete volatilization and burning of such organic components which then exist in a gaseous phase and leave no residue. However, it will be seen that those atmospheres commonly used for brazing which contain an oxidant also tend to oxidize furnace materials such as conveyor belts or the like and thus reduce the useful life of such materials.
The most widely used brazing atmosphere heretofore has been rich exothermic gas which is typically comprised of 15% hydrogen, 10% carbon monoxide, 2.5% moisture, 5% CO.sub.2 and the balance nitrogen. The organic components of brazing paste are effectively removed under rich exothermic atmospheres; however, this atmosphere is decarburizing to medium to high carbon steels and other carbon containing alloys. Also, it is not sufficiently reducing to high alloy parts such as stainless steel and it is highly oxidizing to various furnace materials such as muffles and conveyor belts thus severely limiting the useful life thereof. Other types of brazing atmospheres have been utilized and these include endothermic gas (38% H.sub.2, 19% CO, 0.82% H.sub.2 O,1% CO.sub.2 and balance nitrogen). Endothermic gas is not particularly effective in the burning of organic components of brazing pastes and other atmospheres such as wet and/or dry hydrogen are considerably more expensive and more flammable and explosive. Disassociated ammonia (75% H.sub.2, 0.003% H.sub.2 O, balance nitrogen) is not excessively expensive although it is very reducing to conveyor belts, furnace brick and heating elements. It is also not an effective "burner" of the organic components in the paste. Like hydrogen it is highly flammable and explosive.
As mentioned above, the brazing atmosphere can also affect various properties of furnace materials such as conveyor belts, insulating brick and heting elements. Typically, a conveyor belt is comprised of stainless steel, 80 nickel-20 chromium alloy, iron base with 30% nickel, 20 chromium and columbium stabilized and Incolloy 600 Series. All of these alloys which are effective to withstand high temperatures existent in a brazing furnace contain over 15% chromium which tends to oxidize easily unless the atmosphere is highly reducing. Also, these materials tend to carburize and become very brittle if the atmosphere is carburizing. Furthermore, when the atmosphere is too dry and reducing, the belt alloy lacks protective oxide layers and belt links tend to mildly weld together thus, as such as belt traverses a drum at the exit end of the furnace, the belt "crackles" and generally loses strength. Very dry and reducing brazing atmospheres also lower the life of silicon carbide or Globar heating elements. When such atmospheres are dry, reducing and carburizing, belt damage is even more rapid. Consequently, atmospheres which are too oxidizing, reducing or carburizing negatively affect the life of conveyor belts, related high alloy components of the furnace such as muffles and in some cases heating elements themselves.
Brazing furnaces typically are comprised of brick insulating materials in the interior of the hot zone which materials generally contain small amounts of relatively easily reducible oxides. Thus, these oxides will be reduced if the atmosphere is too reducing which in turn tends to decrease the useful life and effectiveness of such insulating bricks. For example, if the brazing atmosphere is both reducing and contains large amounts of carbon monoxide, bricks are weakened by decomposition of CO at about 500.degree.-650.degree. C. which results in carbon deposits interior of the brick which in turn leads to excessive internal stresses and cracking. Higher amounts of hydrogen result in more heat losses. Furnaces which include high alloy interior muffles will also have relatively short useful lives if the brazing atmosphere is either excessively carburizing or oxidizing or reducing as mentioned above. Furnace heating elements which are typically comprised of silicon carbide deteriorate under highly reducing atmospheres as silicon carbide is gradually reduced to lower levels thereof and eventually silicon. If this reduction of silicon carbide is non-uniform, the heating elements tend to develop "hot spots" and consequently uneven heat patterns are established in the furnace.
Accordingly, a clear need exists for brazing atmospheres and various brazing pastes such that during brazing of ferrous parts clean part surfaces are maintained, part surface carbon levels remain essentially unchanged and the life of furnace materials is increased.