Cutting Torch Oxy fuel Safety - The Burning Process

1. Technically speaking, when using an oxy fuel cutting torch to separate metal, are you ‘cutting’ or ‘burning’ the metal?

Burning.

Although commonly referred as cutting, the actual chemical process is burning.

3. When BURNING metal, why can’t an oxy fuel cutting torch be used on aluminum or stainless steel successfully?

Lack of iron content in the metal. The combustion process of burning steel is actually oxidation.

Aluminum and stainless will not rust due a lack iron content.

4. What is the kindling temperature of steel?

1600 degrees F. (approximately) Mild carbon steel needs to be a minimum of 1600 degrees before the burning process will begin.

5. Why are cylinders of oxygen required, rather than using oxygen in the atmosphere, to operate a cutting torch?

Cutting or burning of steel, requires an oxygen purity content of 99%. When steel is brought to its kindling temperature, the combustion process will initiate when oxygen purity of 99% is added directly to the steel.

6. If when cutting or burning steel using a oxy fuel cutting torch, the cut is ‘lost’, why does this happen?

One of two things happened. Either the kindling temperature was not maintained, or the oxygen purity was compromised over the full profile of the metal profile.

To initiate a cut or burn, the metal must be pre-heated to its kindling temperature. At that point, pure pressurized oxygen is introduced in the flame, causing the metal to oxidize and the velocity of the oxygen blows the burned metal away.

If impurity in the metal is present, a different kindling temperature may be required. Speed of travel may vary from dirty to clean metals. If traveling too fast, the kindling temperature will not be maintained and the cut will be lost.

The amount of oxygen pressure set at the regulator influences the volume of oxygen that passes through the cutting tip. The greater the thickness of metal to be cut, the greater the amount of oxygen volume required.

Oxygen purity must be maintained over the full profile of the cut. The reason that difficulty is commonly found when cutting ‘thick’ steel, is that oxygen purity is compromised.

Insufficient pressure (volume of oxygen) causes the cutting jet oxygen stream to lose concentricity before exiting the bottom of the cut. If the kindling temperature is maintained, and sufficient oxygen is flowing through the full cut profile, virtually any thickness of steel can be ‘cut’.

Cutting tips are designed to cut a certain thickness of material, with a certain amount of oxygen and fuel gas. The operator should refer to the manufacturer's suggestion for pressure settings for any given cutting tip.

Oxygen is used in the preheat flame and in the cutting jet. If a machine torch is to be used, as in a motorized carriage, pressure settings for both pre-heat and cutting oxygen need to be followed.

The speed of oxygen through the orifice of the cutting jet is approximately Mach 1 or approximately the speed of sound.

A center oxygen orifice of a cutting tip needs to be extremely smooth. Any roughness will cause the oxygen stream to be turbulent, which in turn causes the oxygen stream to lose concentricity prematurely. Excess oxygen will also cause excessive slag at the bottom of the cut, and the slag may be difficult to remove. Slowing the cutting speeds may also produce excess slag. Torch designs that use two versus three tubes to supply gases to the cutting tip, provide a large difference in performance, if cutting ‘thick’ material. Three tube designs allow for a separate tube for the fuel gas and the pre-heat oxygen, with a third tube for the cutting oxygen. Two-tube designs combine the pre-heat oxygen with cutting jet oxygen. On thicker materials, when large volumes of oxygen are required, engaging the cutting lever may change the pre-heat flame settings, and cause the kindling temperature to be lost.

7. How do you set a neutral flame on a cutting torch? What is the difference between an oxidizing flame and a carburizing or reducing flame?

A neutral flame is a flame that consumes all of the fuel gas and oxygen. An oxidizing flame has excess oxygen. A carburizing flame contains excess fuel gas.

The correct procedure to follow when lighting an oxy-fuel torch, with acetylene, consists of the following steps:

• After all connections are made and no leaks are detected, set regulators to correct pressures for the tip to be used.

• Open the fuel valve at the butt of the torch a 1/8 to 1/4 turn.

• Let the fuel flow through the hose for 2 to 3 seconds, to purge the line.

• With a cutting torch striker (not an open flame), light the fuel gas. No oxygen should be flowing yet.

• The fuel gas will ignite, producing a yellow to orange flame, with soot.

• Increase the flow of fuel until the flame is ‘bushy’ at the end, with little or no soot.

• Now, open the oxygen valve, slowly increasing the volume of oxygen being added to the fuel flame.

• Add oxygen until the visible secondary cone is reduced, exactly matching the primary cone of flame. This is a neutral flame setting.

An oxidizing flame is set by continuing to add oxygen, producing a flame that has a primary cone with very sharp flame points. A carburizing flame is the opposite, where the secondary cone has not yet matched the primary cone. All three flame settings are determined by the amount of oxygen being added to the fuel flame.

The value of the pre-heat flame is important to raise the temperature of the steel to its kindling temperature.

Plugging of the pre-heats, by allowing the tip to contact molten steel, will damage the tip. Plugged cutting torch pre-heats slow the time required to heat the material to be cut, and will contribute to flashback.

A flashback is created when gases are ignited before they leave the tip. The quality of the oxygen cutting jet orifice is more important to cut quality than the pre-heat flames.

Ready for Section 5? see below

• MAIN PAGE ON CUTTING TORCH OXY FUEL SAFETY

• SECTION 1. Understanding the fundamentals of combustion

• SECTION 2. Proper handling of compressed gas cylinders.

• SECTION 3. Oxy-Fuel gas characteristics

• SECTION 4. The burning process

• SECTION 5. Proper operation of oxy-fuel heating tips

• SECTION 6. Oxy-fuel Equipment inspection

• SECTION 7. Oxy fuel Safety torch - tips