Grade T for all fuel gases
Why are Grades R and RM for acetylene only? For many years, the predominant fuel gas in the industry was acetylene. Acetylene has little or no adverse effects on rubber and was only used at a maximum of 15-psi pressure. Therefore, no requirements for fuel gas compatibility were initially specified. Different types of fuel gases (propane, natural gas, methyl-acetylene-propadiene, propylene, hydrogen, etc.) became popular over time, particularly for cutting. Many of these fuel gases are detrimental to certain types of rubber. With the use of these fuel gases, combining the wrong hose and fuel could lead to premature hose failure.
Grades R, RM, and T are compatible with acetylene. If a fuel gas other than acetylene is used, Grade T hose must be used.
Fig. 3.8. Hoses for oxyacetylene welding must be of the correct type. The Compressed Gas Association defines three grades: Grade R, Grade RM, and Grade T. For fuel gases other than acetylene, only Grade T should be used.
Flame Types
In oxyacetylene welding, the torch tip never touches the material being welded; only the flame touches. The type of flame produced depends on the ratio of oxygen to acetylene.
A neutral flame is produced when there is a 1:1 ratio of oxygen to acetylene. This type of flame has no chemical effect on the weld metal so it does not oxidize the weld metal nor cause an increase in carbon. The excess acetylene flame is created when the proportion of acetylene in the mixture is higher than that required to produce the neutral flame. This is often called a carburizing flame.
An excess-acetylene flame causes an increase in the weld carbon content when welding steel.
An oxidizing flame is created when the proportion of acetylene in the mixture is lower than that required to produce a neutral flame. It ozonizes or “burns” some of the weld metal.
Chemistry of the Flame
When acetylene burns in the air, carbon dioxide and water vapor are the byproducts. It takes 2 cubic feet of acetylene and 5 cubic feet of oxygen or 2½ times as much oxygen as acetylene. Yet a neutral flame burns at 1:1 oxygen/acetylene ratio and a neutral flame does not have an excess of either gas. This is not a contradiction because the combustion process is more complex than simply the volume of oxygen and acetylene gas supplied to the torch. The actual combustion takes place in two stages. In the first stage, the mixture leaving the torch tip supplies the oxygen, and in the second stage, the air surrounding the flame supplies the oxygen.
In the first stage of combustion, the acetylene breaks down into carbon and hydrogen. The carbon reacts with the oxygen to form carbon monoxide, which requires one molecule of oxygen for each molecule of acetylene.
In the second stage of combustion, the carbon monoxide reacts with the oxygen from the air to form carbon dioxide. The hydrogen reacts with the oxygen from air to form water.
The two-stage combustion process produces the well-defined inner cone in an oxyacetylene flame. The first stage takes place at the boundary between the inner cone and the blue outer flame. The second stage takes place in the outer flame. If the proportion of acetylene supplied to the tip is increased, a white “feather” appears around the inner cone. This feather contains white-hot particles of carbon that cannot be oxidized to carbon monoxide in the inner cone boundary due to the lack of oxygen in the original mixture. On the other hand, if the proportion of oxygen fed to the tip is increased, the inner cone shortens noticeably and the noise of the flame increases.
Fig. 3.9. The neutral flame has a 1:1 ratio of oxygen to acetylene. Excess acetylene causes an increase in carbon content or carburizing when welding steel. An oxidizing flame is created when the proportion of acetylene in the mixture is lower than that required to produce a neutral flame, so it ozonizes or “burns” some weld metal.
Flame Adjustment
For most welding, a neutral flame is desired. Even a skilled oxyacetylene weldor has difficultly telling the difference between a true neutral flame and a slightly oxidizing flame. However, it is relatively easy to tell the difference between a neutral flame and a slight-excess acetylene flame. Therefore, it is always best to adjust the flame to neutral from a slight-excess acetylene flame.
Start with an excess-acetylene flame. Increase the flow of oxygen until the excess-acetylene feather is almost gone. This feather is visible in Figure 3.9, and it extends beyond the concentrated white flame cone at the torch tip.
Filler Rods
AWS classifies welding filler rods for oxyacetylene welding according to their chemical composition. AWS specification A5.2 designation of R45 is a common alloy. The rod contains low carbon and manganese alloy additions. It produces an all-weld tensile strength of about 45 ksi (1 ksi equals 1,000 psi. The use of ksi eliminates all of the zeros.)
For added strength, the R60 rod designation contains higher carbon, manganese, and some silicon.
In addition to meeting minimum chemical requirements, a weld must be made as defined in AWS A5.2 specification and produce a minimum of 60-ksi tensile strength. An even stronger alloy is available that contains other alloying elements and meets a minimum tensile strength of 65 ksi.
A note of interest is that R45 has very little alloy, which is probably similar to that of a coat hanger. For oxyacetylene welding, where only a low strength is needed a coat hanger could work. However, its chemistry may not be consistent and if you try it, be sure all of the paint or clear coating is removed.
Oxyacetylene is often more useful than other welding processes for braze welding. Unlike fusion welding, braze welding does not melt the base metal, so the melting point of the filler rod is below the melting point of the material being welded. When joining cast iron, for example, the joint does not have to deal with a mixture of the very high carbon-based material.
Fig. 3.10. One area in which oxyacetylene can be very useful over other welding processes is when braze welding. The melting point of 60-percent copper and 40-percent zinc filler rod is about 1,630 degrees F, well below that of cast iron, for example. This allows a quality joint to be made without having to melt the very-high-carbon cast iron.
A common brazing alloy with 60-percent copper 40-percent zinc has a melting point of about 1,630 degrees F, which is well below that of cast iron. Weld strength typically exceeds 45 ksi. The rod melts and wets the surface but does not melt the cast iron. A flux ensures a very clean surface and assists in the wetting process.
The other advantage of braze welding cast iron is that the high heat input and inherent slow cooling reduces the shrinkage stresses in the cast iron and avoids cracks. Braze welding is a preferred method of repairing cracks in cast-iron parts, particularly some types that are very difficult to fusion weld.
Welding equipment can be purchased from many sources, including the Internet. However, welding equipment operates in a difficult environment and sometimes requires repair, so make sure businesses are available to repair the product
