in several cycles before the maximum flame available from a given tip is achieved. Adjusting the flame below the minimum flow rate for the tip orifice permits the flame to ignite inside the nozzle. This is flashback and makes a popping sound. If you need a smaller flame, use a smaller torch tip. See the section on flashback.
Figure 1–2Shows flame adjustments from carburizing to a neutral flame
What it the hottest part of a neutral flame?
The tip of the inner cone is the hottest part of the flame. The inner cone is where the optimum mixture of oxygen and acetylene burn. The outer envelope where any unburned acetylene burns with oxygen from the atmosphere. A neutral flame is when enough oxygen is present in the flame to be burning all of the acetylene gas and is used for most welding processes. See Figure 1–3.
Figure 1–3Graph of an oxyacetylene flame temperature profile
What effect do oxidizing and carburizing flames have on molten metal in the weld pool?
An oxidizing flame contains more oxygen than the flame can burn and this oxygen combines (or burns) with carbon in the steel to carbon dioxide gas. The result is the weld metal has a change in carbon content and in its properties. Strength is always degraded and brittleness increased.
A carburizing flame contains more acetylene than the flame can burn and the carbon in the acetylene adds to the carbon in the weld pool causing gas bubbles in the weld. When the weld freezes these gas bubbles create porosity holes.
What are the proper steps to shut down an oxyacetylene torch and its cylinders?
•First turn off the oxygen and then the acetylene with the torch handle valves. Turning off the acetylene first can cause a flashback.
•Turn off the oxygen and acetylene cylinder valves at the upstream side of the regulators.
•Separately, open and reclose the oxygen and acetylene valves on the torch handle to bleed the remaining gas in the hoses and regulator into the atmosphere. Verify that both the high-pressure and low-pressure gauges on both regulators indicate zero.
•Unscrew the regulator pressure adjustment screws on both cylinders in preparation for the next use of the equipment. The regulator screws should be loose but not about to fall from their threads.
Gases
What is acetylene gas?
It is a clear gas having a specific gravity slightly lighter than air at 0.906 (air = 1.000). Acetylene’s chemical formula, C2H2, indicates that each molecule of this hydrocarbon compound contains two carbon atoms and two hydrogen atoms.
What is the odor of acetylene gas?
It has a distinctive garlic odor. Because the liquid acetone in the acetylene cylinder also has an odor, this acetone odor is frequently mistaken for that of acetylene, when in fact it is the odor of the mixture of both acetylene and acetone.
How is acetylene made?
Acetylene results from dissolving calcium carbide in water and capturing the resulting gas. One pound of calcium carbide generates about 10 cubic feet of acetylene (1 kg calcium carbide generates about 618 liters of acetylene).
Where does calcium carbide come from?
Calcium carbide results from an industrial process where lime and coke are smelted in an electric furnace. A gray, hard solid, it is supplied in a variety of forms: bricks, powders, pellets, or granules.
What is the chemical equation of combustion of acetylene in a neutral flame?
C2H2 + 2.5O2 → 2CO2 + H2O
What does this equation tell us?
One part acetylene and two and a half parts oxygen combine to produce a neutral flame (a neutral flame has just the right amounts of fuel and oxygen so there is neither an excess of oxygen or fuel after combustion). What the equation does not tell us is that equal volumes of acetylene and oxygen from the compressed gas cylinders combine with another one and a half parts of oxygen from the atmosphere to make the flame.
How is oxygen made for welding?
Atmospheric air is repeatedly cooled and compressed until it becomes a very cold liquid. This liquid is gradually warmed, and as each component gas of the liquid air reaches its vaporization temperature, it comes out of the liquid air, and separates itself. This is the fractional distillation of liquid air. Other gases important in welding—nitrogen, carbon dioxide, and argon—are also made using this process. Oxygen can also be made by electrolysis of water, but this is not a cost-effective process to make industrial quantities.
What are the two main ways of supplying welding shops with oxygen?
Compressed gas cylinders are used in smaller shops; liquid oxygen cylinders in larger shops. The liquid oxygen flows from its cylinder into a radiator that warms the liquid oxygen, and converts it into gaseous oxygen.
How are large welding shops supplied with acetylene?
Multiple cylinders are manifolded together and their output piped around the plant to each welding or cutting station.
What is peculiar about the filling and draining of acetylene cylinders?
Because the acetylene is dissolved in acetone, not just pumped into the pressure vessel, the filling process takes seven hours as the absorption process occurs. Similarly, an acetylene cylinder can only deliver one-seventh of its capacity per hour as the acetylene will not come out of solution in the acetone faster. More acetylene capacity will require cylinders manifolded together. This can become an important issue when using large multi-flame heating tips (in the industry called a rosebud tip) which consume many times more gas than a welding tip.
Why is acetylene potentially so dangerous?
Acetylene will form explosive mixtures with air at all concentrations between 2.5 and 80%. This is the widest range of any common gas and almost insures an explosion if leaking gas is ignited.
Can other fuel gases be used in place of acetylene?
Certainly, but their maximum heat potential is below that required for welding steel. Acetylene is the best gas for welding because it:
•Has the highest temperature of all fuel gases.
•Acetylene delivers a higher concentration of heat than other fuel gases.
•Has the lowest chemical interaction with the weld pool’s molten metal than all other gases.
However,
