Torch Brazing. This method requires very little equipment and is more frequently practiced. It involves the use of a type of gas blowpipe fitted with the combination of air and gas regulators by means of which both the size and nature of the flame can be adjusted to suit the work. The gas combinations used and the temperatures obtained from them are given below:
(a) Compressed air-coal gas : 1000C to 1151C
(b) Coal gas-oxygen : 2100C to 2200C
(c) Hydrogen-oxygen : 2400C
(d) Oxy-acetylene : 3200C
In general, slightly oxidizing flame should be used. The torch is usually kept in motion to achieve a general heat all over the work, rather than a local heat and the envelope of the flame is used for this purpose. Torch brazing is quicker in action than other brazing methods, and suitable for small repetition work such as assembly of instruments, wireless and electrical components.
Furnace Brazing. Furnace as a heating source for brazing is best suited for brazing large or similar assemblies of work of moderate sizes. In the controlled atmosphere, a conveyor belt (nickel chrome belt), whose speed is controllable, conveys the work to the heating zone. The temperature in the furnace is set at brazing temperature. Normally, one or two minutes at the brazing temperature are sufficient to make 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 hood, the brazed assemblies are cooled to a temperature at which parts will not oxidize when discharged to the air or room temperature. Similar in general design is pusher type furnace, in which trays of work are thrust into the heating chamber at predetermined time intervals, a tray of completed work being pushed into the cooling chamber as each fresh tray is introduced.
Dip Brazing. The main use of this process is to join tubular structures like cycle frame. The work must be properly jigged before lowering into the bath. The bath contains brazing alloy protected by a layer of flux.
The job is first lowered into the molten flux and held there till the assembly attains the brazing temperature. It is then passed into the molten filler metal, where a short period of immersion is sufficient to flush out the flux, which has penetrated the joint, and replace it by brazing material. On the way out, the work is held again in the flux layer to permit the excess brazing material to drain. The parts, which are not to be brazed, are protected by painting.
Dip Brazing and Dip Brazed Parts
Salt Bath Brazing. This process is not very common. The fused salt employed should be one, which in addition to providing heating medium, also acts as a brazing flux and thus protects the work and brazing alloy from oxidation.
Salt Bath Brazing
Forge Brazing. It is similar to torch brazing in technique, but the only difference is that the source of heat is a blacksmith forge or brazing hearth. The forge is a valuable means of heating large components such as castings where local heating is liable to cause cracking or distortion.
Some of the brazing fluxes available through service source are shown below:
|Borax powder||For general brazing work|
|Flux brazing low temperature||For use with silver solders
(DTD 900 and B.S. 1845 type 3)
|Flux brazing aluminium||For general use|
|Flux welding bronze||For bronze welding|
There are two types of fluxes used for brazing. Borax type flux is used for temperatures above 750C and Fluoride type flux is used for temperatures between 600C – 750C.