Arc brazing of stainless steel to stainless steel and galvanised mild steel
Brazing is the joining process which occurs at a temperature above 450 degrees celsius and below the solidus of the parent material. Below 450 degrees celsius the process is known as soldering and above the solidus of the parent material the process is known as welding.
In conventional brazing techniques the heat source may be an oxy-fuel torch or a furnace. The braze material is prepositioned or fed into the joint during the heating process and a flux is used to both improve the wetting of the parent material and to protect the braze from the environment. Finally the process is difficult to automate for large structures.
Arc brazing differs from conventional brazing techniques in the following ways. Firstly the heart source is an electric arc. No Flux is required as a shielding gas is used to protect the joint from atmospheric contamination and the process is relatively easy to automate for large components. Finally the braze material is deposited by a process known as short circuit transfer.
Short circuit transfer
As the arc is initiated it causes a drop of molten filler metal to grow on the tip of the electrode. As the current passes through the electrode a compressive magnetic force, known as magnetic pinch, is exerted on the wire. The wire feed then causes the drop to contact the work piece and as a result of the short circuit the current increases. The increased current results in an increase in the magnetic pinch force exerted on the electrode and the droplet is detached. This re-initiates the arc and the process is repeated.
Using the correct conditions it is possible to arc braze stainless steel (304) and produce joints with both adequate strength (above the 0.2 per cent proof strength of the parent material), satisfactory appearance (in terms of lack of distortion and spatter) and complete penetration of the joint can be achieved with the appropriate gap between the faying surfaces prior to brazing.
However it has also been noted that arc brazed joints can suffer from brittle fracture during tensile testing but the susceptibility of an arc brazed joint to this is dependant on the combination of filler material and shielding gas
Process variables of Arc Brazing
- shield gas - Argon, Argon +1 per cent Oxygen and Argon 2 per cent Oxygen
- shielding gas flow rate
- braze alloys - BS:2901 C9, C11 and C28
- wire feed rate
- torch velocity
- torch height
- torch angle
- gap between faying surfaces of a butt joint
- pulsed current variables - base current, peak current, frequency, pulse width, rise and fall rate