What is the meaning of welding process?
Welding operation means the process of joining two metal parts together to give a strong joint.
What is fusion welding?
Fusion welding is defined as melting together materials using heat. Filler metals, which are metals are added to the weld area during welding, also may be used. Lack of fusion between the materials is a major drawback in this process.
What is an autogenous weld?
When filler metals are not used in fusion welds then the process is known as autogenous welds.
What is oxyfuel-gas welding?
- Any welding process which uses a fuel gas combined with oxygen to produce a flame is called a oxyfuel-gas welding.
- Melting of metals at the joint takes place due to the flame which acts as a source of heat.
- The most common gas welding process uses acetylene, the process is known as oxyacetylene-gas welding and is used for structural metal fabrication and repair work.
Which is the combustion process occurring in the inner core of the flame?
- The primary combustion process which occurs in the inner core of the flame involves the following reaction.
- This reaction dissociates the acetylene into carbon monoxide and hydrogen and produces about one-thirds of the total heat generated in the flame. The secondary combustion process is
- This reaction further consists of the further burning of both the hydrogen and the carbon monoxide and produces about two-third of the total heat.
What are the equipment needed for Gas welding?
The details of welding equipments are listed below:
- High-pressure oxyacetylene welding equipment consists of two large steel cylinders.
- One is conventionally painted black and is a long thin cylinder that contains oxygen-filled in at a high pressure of 125-140 kg/ sq.cm.
- The other cylinder which is painted maroon and is shorter but of slightly larger diameter contains acetylene gas dissolved in acetone at a pressure of 16-21 kg/ sq.cm.
- Both these cylinders are fitted with valves which are normally kept in a closed position.
- To draw gas from the cylinder, each cylinder is fitted with a pressure regulator with two gauges.
- The function of the pressure regulator is to reduce the pressure of the gas before delivering it.
- The two gauges indicate the pressure inside the cylinder and the reduced pressure of gas after the pressure regulator stage.
- The gases are carried from the pressure regulator to the welding torch (also called blow pipe) using rubber hose pipes.
- A welding torch connected to welding machine consists of different passages for oxygen and acetylene gases.
- Supply of these gases is controlled by pin valves.
- These two gases are then allowed to mix in a mixing chamber before being driven out through the orifice of the blow pipe.
- These orifices are of different sizes and can be screwed on the blow pipe.
What are the different flame types in the welding process?
In oxyfuel gas welding, the proporation of acetylene and oxygen plays a vital role.
- Neutral flame:
At a ratio of 1:1 (i.e. when there is no excess oxygen), the flame is said to be neutral.
- Oxidizing flame:
The flame can be harmful when the content of oxygen is excess (especially for steels), because it oxidizes the metal. For this reason, a flame with excess oxygen is known as an oxidizing flame.
- Reducing or carburizing flame:
If the oxygen is insufficient for fuel combustion, the flame is known as reducing or carburizing flame.
What is Filler metal?
The main function of filler metals is to supply additional metal during welding. They are available as filler rods or wire and maybe bare or coated with flux. The purpose flux is to retard oxidation of the surfaces of the parts being welded by generating a gaseous shield around the weld zone. The flux also helps to dissolve and remove oxides and other substances from the weld zone.
What is an oxyacetylene cutting?
- The oxyacetylene flame can also be used for cutting plate.
- This is done through a special “cutting torch”, which has an extra passage for high-pressure oxygen in addition to the usual passage for oxygen and acetylene gas.
- The process of flame cutting is an oxidation process.
- The place, where a cut is to be made is heated with the welding flame and when it is red hot, high-pressure oxygen is allowed to impinge on it.
- The cut has to start either from the edge of the steel plate or a pilot hole has to be drilled in the plate from which the cut can start is the only limitation of this process.
What is the principle of arc welding process?
- In arc welding, the source of heat is an electric arc.
- The temperatures reached in an electric arc may be as high as 5500°C.
- A spark is produced in an electric circuit carrying current if the circuit is broken accidentally.
- An electric arc is a spark created intentionally by a gap between the welding electrode and the workpiece.
- Because of larger output and less oxidation, the quality of weld produced by an electric arc is much better than a gas weld.
- Either A.C. or D.C. power supply may be used for arc welding.
- For A.C., an open-circuit voltage of about 75-80 V is required.
- The current requirement is however very heavy and the welding machine should be capable of delivering 100-300 Amperes.
- D.C. supply is characterized by positive and negative terminals.
- With D.C., a slightly lesser open circuit voltage 70-75 volts will be adequate to strike the arc.
- Normally, the electrode is connected to a negative terminal and the workpiece to a positive terminal.
- Such an arrangement is called D.C. straight polarity (DCSP).
What is a Heat affected zone?
- In the arc welding process, a great amount of heat output takes place resulting in the formation of a molten pool in the arc area.
- The heat is also conducted into the vicinity of the joint on either side.
- The temperature of the material on both sides of the weld bead may not be as high as the melting point of the metal, but, is very close to it.
- As we move away from the joint or weld bead, the metal may be heated to lesser temperatures.
- As the electrode travels over the joint and moves away, the heated metal cools as quickly as it was heated.
- If steel is being welded, this heating and quick cooling may result in the formation of martensitic and other structure which may be prone to cracking and hardness.
- Due to heat affected zone created near the weld portion, testing of weld strength is of utmost importance.
Mention the various welding positions from the point of view of the welder?
There are 4 types of welding position techniques which might affect the quality of the weld.
a) Downhand welding position: This is the most comfortable position for the welder to use and he can produce welds of good quality.
b) Horizontal welding position (on a vertical surface).
c) Vertical welding position (on a vertical surface).
d) Overhead welding process (say on the ceiling of the room): This is the most difficult welding position. Not only the operator has to turn his neck upwards and raise his arm to maintain an arc.
Note: The direction of movement of hand is also specified in the Fig. 5.
What is Electric Resistance welding?
- In electric resistance welding (ERW) methods, a high current is passed through the metal pieces to be joined together and the heat is produced due to resistance in the electric current.
- The electric resistance welding process is a pressure welding process and not a fusion welding process.
The following are the types of electric resistance welding processes which are described with diagrams:
- Spot welding process.
- Seam welding process.
- Butt welding process.
- Flash butt welding process.
1. Spot welding process:
Spot welding consists of joining of two pieces by placing them between two electrodes and passing a heavy current through them for a very short duration.
- Usually, spot welding is automatic and work on the following weld cycle:
- Squeeze the two metal pieces together with light pressure.
- Pass heavy electric current for a very brief time.
- Apply pressure and hold for some time.
- Remove the pressure.
2. Seam welding process:
- A seam is produced by overlapping spot welds.
- The seam welding machine is similar to a spot welding machine.
- In a seam welding machine, the electrodes are in the form of copper rollers.
- The two work pieces which are to be joined pass between the rollers.
- The rollers exert pressure on the workpiece and also rotate at the same time.
- The seam welding process results in a successive series of spot welds being made.
3. Butt welding process:
- Welding two pieces of metal together, end to end, is called butt welding.
- In butt welding, the ends are cleaned and made square so that the two places touch each other over the entire cross-section.
4. Flash butt welding process:
- In this process, the end preparation is not as detailed as in the butt welding process and the ends need not be dead square.
- In this case, the current is switched on before bringing the two ends to be welded, close together.
- This result in flashing as the two ends almost touch each other but have a little gap between them.
- This flashing or arcing generates heat and the two metal end heat up to coalescence temperature.