Welding Gas Flames: Neutral, Carburizing, Oxidizing [PDF]

What are Gas Welding Flames? 

Gas welding is the process of welding two metals together using a flame, which is generated by the burning of gases with oxygen. The flames generated during the process of gas welding are known as welding flames. Filler metal may or may not be used in the process of gas welding. Gas welding is an economical and effective welding method that is used widely. 

Different gases used in the generation of flame

The flame generated during the gas welding process is due to the mixture of fuel with oxygen. The fuel gases used can be acetylene, propane, butane, hydrogen, or natural gas. Out of these gases, acetylene is the most widely used fuel gas for the production of welding flame.

Oxyacetylene Flames

What is Oxyacetylene Flame?

The gas mixture used for producing welding flame is called oxyacetylene flame. Oxyacetylene welding was first developed in 1903, by two French engineers Edmond Fouché and Charles Picard. During the early 20th century before the development of Arc welding, oxyacetylene gas welding was the only method for welding steel. It was an effective and economical process for welding during that period. 

calcium carbide + water → oxy-acetylene + slaked lime

CaC2 + 2H2O → C2H2 + Ca (2 OH )

Characteristics of oxyacetylene flame

• The oxyacetylene flame temperature is much higher than other flames. For example, the oxyacetylene flame temperature is 3500^oC. On the other hand, the flame temperature of the oxyhydrogen flame is 3000^oC. 
• It is used for welding ferrous and non-ferrous metals and can weld metals up to 6 mm in thickness. Also used to weld materials with low Melting points. 
• Oxygen is used for burning action with acetylene as a fuel. It provides a better medium for combustion than air. This is because the thermal efficiency of oxygen is more than that of air. Oxygen also provides better-quality weld and higher welding speed.

Production and supply of oxygen and acetylene

Following are the 2 methods of producing oxygen:

By electrolysis of water

By Electrolysis the water breaks into oxygen and hydrogen by passing a current through it. Condensation of air is considered to be a better process for producing oxygen. Oxygen is stored in steel tanks compressed to a pressure of 15MPa at 20^oC. About 7m³ of oxygen is stored in a cylindrical tank having a height of 1.3m and a diameter of 0.23m. 

By liquefaction of air

Acetylene is an endothermic compound that absorbs heat during production and gives out heat during decomposition. Commercial production of acetylene is done by mixing water with calcium carbide CaC₂. Acetylene is an explosive compound hence its storage must be safe and secured. It is stored in a steel cylinder containing emulsion of charcoal and some other substances, to provide a porous medium. Acetone is used as a solvent for storing acetylene because of its nature to absorb acetylene. Acetylene cylinders contain up to 9m³ of acetylene compressed to 1.7MPa at 20^oC

Zones of Oxyacetylene Flames

Three basic types of oxyacetylene flames are used in commercial welding processes. These flames not only differ in chemical nature but also differ in appearance. All three flames consist of 3 zones. 

1. The inner luminous zone, where hot oxygen and acetylene mix physically. 
2. The intermediate zone 
3. The outer zone where the secondary combustion of the mixture takes place. 

Types of Gas Welding flames

Following are the three types of Gas Welding Flames:

1. Neutral or balanced flame
2. Reducing or carburizing flame
3. Oxidizing flame. 

Neutral Flame or Balanced Flame

• A neutral flame is produced by the burning of approximately equal amounts of oxygen and acetylene. 
• The oxygen to acetylene ratio is around 1.1 to 1.0. 
• A neutral flame is a well-defined flame that is most widely used for welding purposes. 
• The inner luminous cone slightly rounds off at the ends. The temperature of the inner luminous cone is about 3250^oC. 
• It is difficult to distinguish between the inner zone and the intermediate zone. The intermediate zone has a temperature of about 2100^oC. 
• The outer zone is neutral in nature and is non-hazardous. The temperature of the outer zone is nearly 1300^oC. 
• The amount of smoke produced is very low with a soft hissing noise. 
• The quality of the weld produced by neutral flame is excellent and the chemistry of the workpiece is also not distributed. 
• Materials for which a neutral flame can be used are copper, cast iron, aluminum, mild steel, and stainless steel. 

Reducing Flame or Carburizing Flame

• The oxyacetylene flame is obtained when excess acetylene is burnt which is called reducing or carburizing flame. 
• Oxygen to acetylene ratio in case of reducing flame varies from 0.85 to 0.95.
• The acetylene is burnt with atmospheric oxygen which results in a bigger flame size. 
• The inner luminous cone is not well defined, and same is the case with the intermediate zone. The flame temperatures of the innermost and intermediate zones are 2900^oC and 2500^oC respectively. 
• The outermost zone may carry some soot due to improper burning of acetylene. The temperature of the outermost zone is 1275^oC. 
• If more amount of acetylene is supplied then it  turns the flame to carburizing from reducing,  and this contains carbon deposits. 
• These flames are used for welding where reducing the nature of the flame is taken as an advantage. 
• These flames are capable of welding nickel, alloys of steel, and various nonferrous metals. 
• Reducing the flames may increase the welding rate but, as soon as the carbon percentage increases, the rate of welding decreases due to a decrease in the temperature of the flame. 
• The amount of smoke produced is high with the absence of noise. 
• Materials for which a reducing flame can be used are carbon steel, aluminum alloys, lead, and oxygen-free copper. 

Oxidizing flame

• An Oxidizing oxyacetylene flame is produced when more oxygen is burnt with less amount of acetylene. 
• The oxygen to acetylene ratio in the case of Oxidizing flame is 1.15 to 1.5.
• The inner luminous cone becomes small and sharp having a temperature of about 3500^oC. The intermediate zone is not defined properly but has a temperature of 2100^oC. 
• The intermediate zone also contains some remains of free oxygen traveling to the envelope. 
• High level of oxidation results in a short envelope or the outermost zone having a temperature of about 1275^oC. 
• Oxidizing flame is the hottest among all oxyacetylene flames and is used in places where very high temperature is of great concern. 
• It is not only used for simple cutting but can be used for cutting operations by flame. This is known as oxyacetylene cutting. 
• Oxidizing flame is used for welding nonferrous metals like brass and copper. 
• Welds made can withstand high temperatures after being settled. 
• The flame production is noisy but no smoke is produced during the process.
• Oxidizing flame can be used for materials such as brass, bronze, cast iron, zinc, and manganese steel. 

Comparison between Neutral flame, Reducing Flame & Oxidizing Flame

Advantages of Oxyacetylene flames

The following are the Advantages of Oxyacetylene flames:

• Wide Range of Metals : Almost all the commercial metals can be welded using oxyacetylene flames. The temperature obtained by burning oxyacetylene gas is high enough to melt any commercial metal and form a weld. 

• Cost-efficient: The cost of equipment used for producing oxyacetylene flame is low. The cost of running is also quite low. 

• High welding speed: The speed of welding in the case of oxyacetylene welding is high. High-speed results in an increased production rate in many industries. 

• Easy to learn: Handling oxyacetylene flame is very easy. The operator can easily control the amount of oxygen and acetylene in the flame. The welding operation performed using oxyacetylene flame is also not complex. 

• Multiple operations: Besides welding operations, cutting operations can also be performed using an oxyacetylene flame. Cutting operations can be achieved by narrowing down the cross-section of the flame. 

• Portable: The equipment used for the production of oxyacetylene flames is more portable than many other welding equipment’s. This provides an advantage to oxyacetylene welding over other welding processes. 

• No electricity required: One of the major advantages of oxyacetylene flame operations is that it does not require any electricity to operate. This makes oxyacetylene flame operations suitable for remote places where there is no electric supply. 

Disadvantages of oxyacetylene welding flames

The following are the disadvantages of Oxyacetylene flames:

• Surface finish: The surface finish obtained after welding is not great. Oxyacetylene welding promotes the generation of rough surfaces after welding. 

• More heat-affected zones: A large amount of heat is produced while performing any operation using an oxyacetylene flame. The heat produced is not concentrated at a single point. This results in the generation of thermal cracks and thermal stress development on the workpiece. 

• Not suitable for thick materials: One of the disadvantages of oxyacetylene welding flame is that it cannot weld very thick materials as it cannot heat very thick cross-sections. 

• Slow cooling rate: Cooling is an important stage after the welding is done. Workpieces are not converted to a finished product until the weld is cooled. In the case of oxyacetylene welding, the cooling rate is quite low. This may affect the production rate of the plant. 

Applications of oxyacetylene welding flames

Following are some applications of oxyacetylene welding flames :

• The major application of oxyacetylene flame is for repair work. Most of the welding done to repair a certain part in any industry is done using an oxyacetylene flame. 
• Oxyacetylene flames are used for welding purposes in many industries such as aircraft, automobile, and marine industries. 
• Due to its ability to join two or more dissimilar metals, oxyacetylene welding is used for the fabrication of various metals. 
• Oxyacetylene flames are also used for silver soldering different metals. 
• Besides joining operations oxyacetylene flames are also used for cutting large plates and large pieces of metals. 

Video on Different Types of Gas Welding Flames

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