Furnace Brazing

Furnace Brazing

Furnace Brazing - Furnace brazing is most suited for mass production of brazing components 1 to 1.5 kg each. The use of furnace brazing is preferred over other methods of brazing when: (i) A number of joints are to be brazed simultaneously. (ii) Components to be brazed can be preassembled. (iii) Filler metal can be preplaced in contact with the joint before brazing. (iv) Many like assemblies are to be brazed. Furnace brazing requires the use of a suitable furnace for heating and a suitable atmosphere to protect the steel assemblies against oxidation, or oxidation and decarburization, during brazing and during cooling, which is accomplished in chambers adjacent to the brazing furnace.
Proper atmosphere also makes possible the proper wetting of the joint surfaces by the molten copper filler metal, usually with use of a brazing flux. The following types of furnaces are commonly used for brazing purposes: (i) Box-type (ii) Wire mesh belt type (iii) Roller hearth type. Furnaces are heated using: (i) Electric-resistor type heating elements (for high temperatures). (ii) Gas-fired radiant-tube heaters (for lower temperatures). The furnace should be operated at a temperature above the liquid us of the filler metal, or at the minimum value for the brazing temperature range of the filler metal.
	Hydrogen and dissociated ammonia (75% hydrogen and 25% nitrogen) are excellent atmospheres for brazing furnaces. However, much cheaper gas can be produced by partial combustion of natural gas, coke-oven gas, propane or butane. Much of the furnace brazing is done with pure copper as a filler material and hydrogen as the furnace atmosphere without the use of any flux. A flux, in addition to the suitable atmosphere inside the furnace, is generally not necessary; it is employed only for brazing those metals whose oxides cannot be readily reduced in the available furnace atmosphere at brazing temperature. For furnace brazing, the components are self-assembled or using a jig. The filler metal and the flux (if necessary) are preplaced adjacent to the joint before the assembly is put in the furnace. The heat in the furnace is increased until brazing temperature is reached.
During heating the filler metal melts, wetting the surface and creeping on the surface until drawn into and through each joint by capillary attraction and alloying with the parent metal of the assembly of parts being joined. Any excess of the brazing medium usually forms a neat fillet at the outer extremity of each joint, which adds to the strength of the joint. Normally, one or two minutes at the brazing temperature is sufficient to make the braze. After heating, the assembly is cooled within the reducing atmosphere in a cooling hood attached to and forming an integral part of the furnace equipment. In the cooling operation the alloy of brazing medium and parent metal solidify, developing a strong union of the parts. In the cooling hood the brazed assemblies are cooled to a temperature at which the parts will not oxidize when discharged to the air or room atmosphere.
Advantages (i) Brazing temperature can be controlled accurately in the furnace. (ii) Heating being uniform, residual stresses are low in the furnace brazed components. (iii) The joints possess good strength at elevated temperature. (iv)Filler metal is distributed uniformly in the joints which therefore can be relied upon for gas tightness. (v) Bright surfaces free of scale are obtained. (vi) Silver brazing, phosphorus copper brazing and copper brazing can be carried out in the furnace.

Limitations
(i) Members having different coefficients of expansion or masses* will be heated differently in the furnace land may call for the adjustment of joint clearances.
This problem is solved by employing a filler metal that has a long range between liquid us and solid us and at the same time exhibits a sluggish flow.
(ii) The initial cost of the furnace and atmosphere generator is high, compared with that of most other types of brazing equipment.

Applications
Among the steel components that are commonly furnaces brazed are
(i) Machine parts
(ii) Light stampings
(iii) Small forgings
(iv)Deep drawn sheet metal parts
(v) Castings, etc.

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