A backfire is a small explosion of the flame at the torch tip. The biggest hazard is that the detonation from the tip may blow molten weld metal five to ten feet from the weld and injure someone. Also, a series of repeated, sustained backfires, which can sound like a machine gun, may overheat the tip or torch, permanently damaging them.
How can backfire be prevented?
The most frequent cause of backfire is pre-ignition of the mixed acetylene and oxygen. Here are the most common causes of pre-ignition and their solutions:
•The mixed welding gases are flowing out through the tip more slowly than the flame front burns and the flame front ignites the gas in the tip and/or mixing chamber causing a pop. Solution: Slightly increase both the oxygen and acetylene pressures and if this results in too large a flame for the job, reduce the torch tip size.
•The tip may be overheated from being held too close to the weld or from working in a confined area like a corner. Solution: Let the tip cool off and try again holding the tip farther from the weld pool.
•Carbon deposits or metal particles inside the tip act like spark plugs prematurely igniting the mixed gases. Solution: Let the tip cool, then clean it thoroughly with your tip cleaning kit.
You are using a multi-flame (rosebud) tip that has a large flame for heating metal prior to welding, bending, or brazing. Soon after the torch is lit, it starts to pop (either once, or in a series of pops), or begins to squeal. What is the most likely problem and how is it best corrected?
This is flashback. Most likely low acetylene gas pressure is not pushing the oxygen/fuel mixture out of the tip faster than the flame can burn back on itself inside the tip. This allows the flame to burn inside the torch either in a single pop, a series of pops, or in a rapid series of pops that sounds like a squeal. Not only can one ruin a tip by allowing this to continue unchecked, but if the flame burning inside the tip reaches back into the hoses, these can explode and/or burn off and leave the welder holding a burning rubber hose, a very serious condition. To avoid this hazard: Immediately turn off the torch, oxygen first, then the acetylene. Allow the torch to cool down for several minutes, increase the acetylene regulator pressure setting to 15 psi (1 bar), reignite the torch and open the acetylene valve to obtain full flow, followed by adjusting the oxygen.
What is the proper way to set-up, light, adjust and use a multi-flame?
When using a multi-flame tip you first set the acetylene pressure at or just below 15psi (1 bar) and the oxygen pressure at 30psi (2 bar); open the acetylene torch valve far enough to light the acetylene flame once the flame is ignited open the acetylene valve until you have full flow of gas; now you can open the oxygen torch valve and adjust the flame to slightly carburizing. You may now use the multi-flame (rosebud) to heat materials but keep the sharp inner cone flame away from the material and only touch the carburizing flame to the material being heated. A heat sensing device such as a pyrometer or temperature sensing stick can be applied to the material to indicate the temperature of the material being heated.
Flames
What are the three types of flames that different ratios of oxygen and acetylene can produce and what are the characteristics of these flames?
•Oxidizing flames result when there is an excess of oxygen over acetylene. This flame will change the metallurgy of the weld pool metal by lowering the carbon content as it is converted to carbon dioxide.
•Neutral flames result when there is just enough oxygen to burn all the acetylene present. This flame has the least effect on weld pool metal as only carbon monoxide and hydrogen combustion products result and is most frequently used in welding common materials.
•Carburizing flames result when there is an excess of acetylene gas over the amount that can be burned by the oxygen present. The opposite of an oxidizing flame, it adds carbon to the weld pool and can change its metallurgy, usually adversely.
Which of the three types of flames, oxidizing, neutral, and carburizing produces the hottest flame?
An oxidizing flame is significantly hotter than the other two flames, but is less useful as it will introduce more contaminants into the weld pool.
For what applications is an oxidizing flame used?
An oxidizing flame is often used in braze welding or in fusion welding of heavy, thick parts with brass or bronze rod. In these applications, we are not concerned with weld pool contamination by carbon. An oxidizing flame is required for oxygen-fuel cutting.
Applications
For what type jobs is OAW best suited?
•Repair and maintenance where one type of equipment can perform many different repairs
•Welding of thin sheet, tubing, and small diameter pipe
•In field operations for natural gas distribution systems up to four inch diameter schedule forty pipes
For what type jobs are OAW definitely not a good choice?
OAW welds of thick sections are not economical when compared with shielded metal arc welding, flux-cored arc welding, or gas metal arc welding.
What disadvantages does OAW have over other welding processes?
In general most other processes are faster: they can apply more weld filler metal in a given time.
What metals can the OAW process readily weld?
•Copper
•Bronze
•Lead
•Low alloy steels
•Wrought Iron
•Cast steel
What materials can be welded by the OAW process if additional steps are taken?
Aluminum and stainless steel may be welded, provided one or more of the following steps are taken—preheat, postheat, use of fluxes, or special welding techniques.
What are three major problems associated with welding aluminum?
•Does not change color prior to melting, so it requires extra welder skill to control heat input.
•Has hot shortness—lacks strength at high temperatures.
•Exposed aluminum has a very thin oxide layer that requires the use of flux and also the oxide surface does not let the welder see a wet-looking molten weld pool.
Besides welding, what other processes can an oxyacetylene welding assembly perform?
With minor additional equipment, it may perform:
•Brazing and soldering
•Case hardening
•Descaling
