Welding is a process whereby two (2) metals are joined together. In today’s transportation industries welding is an integral part of both the manufacturing process and the service sector. There are two methods of welding used:
Fusion welding is a process where the edges of two (2) pieces of metal are brought together and melted. A filler metal or “welding rod” is added to the molten pool of the base metals. The welding rod is composed of a similar metal to the two (2) base metals and therefore unites with these to form a single homogeneous piece of metal.
Non-fusion welding or brazing is a process where the two (2) base metals are only heated to a dull red (tinning temperature). The filler rod (usually bronze) has a lower melting temperature than the two (2) base metals. The rod is melted and flowed onto the joint. When done correctly, this procedure is a strong as a fusion weld.
Oxygen and Acetylene
Both oxygen and acetylene gases are stored under pressure. The average pressure in an oxygen tank at room temperature (70°F / 21°C) is about 2200 PSI when full. The average pressure in an acetylene tank under the same conditions is about 250 PSI. Extreme care must be taken with both tanks. Refer to your study unit on WHMIS.
There are two (2) gauges on each regulator for both the oxygen cylinder and the acetylene cylinder. The first gauge tells the operator what the remaining pressure is in the cylinder. The second gauge lets the operator know what the line or working pressure is at the touch. The adjusting screw on each regulator allows for the adjustment of the working pressure.
Start by opening the valve on top of the oxygen cylinder all the way and the valve on top of the acetylene cylinder 1 ½ turns. With the acetylene valve on the torch slightly open, adjust the regulator pressure to the correct reading, then close the torch valve. Repeat the same procedure for the oxygen.
Once the regulator is set up correctly it is then safe to light the torch. Use the valves on the torch to adjust the pressure in order to obtain the correct flame. Always start by lighting the acetylene first and the oxygen second. Holding a friction lighter in on hand open the acetylene valve on the torch a ¼ turn, then ignite the gas. Adjust the flame so that it is smoke-free. Slowly open the oxygen valve until the three (3) zones of the flame are visible:
- the white inner cone
- the blue, feather shaped acetylene cone
- the bluish white outer envelope
Continue opening the oxygen until the acetylene cone disappears into the white inner core. If the pressures are adjusted correctly for the size of the torch opening, there should be little to no hissing noise coming from the torch. Once the torch flame is adjusted correctly, you are ready to braze.
Brazing is a form of non-fusion welding. The two (2) pieces of metal that are to be joined together should be heated until a dull red (tinning temperature). Once the pieces are heated, the filler rod is melted and flowed onto the joint. This procedure should be repeated for both sides of the joint. Be careful not to overheat the base metals. If the base metal is too hot, the bronze will sputter and valuable characteristics of the alloy elements in the filler rod will be destroyed.
If the base metal is not sufficiently heated, the melted bronze will form into balls, which will tend to roll on the surface of the base metal as water does on an oily surface. Only when the base metal has reached the correct tinning temperature, will the molten bronze flow evenly into the heated-expanded grain structure to form a bond with the base metal.