The welding process requires the implementation of many preliminary operations on which the final result depends. One of them is the preparation of joints. Beginners often neglect this process, but with experience comes an understanding of how much the quality of the weld depends on the cutting of the edges for welding.
Surface preparation before welding
Before welding critical structures, surfaces are always machined. This achieves several goals: the removal of dirt, oxide film, and rust in future mates. The following methods are used for this:
- Mechanical cleaning with metal brushes, abrasive wheels.
- Chemical treatment with solvents that remove grease and oxides from the welding surface. Fluids based on xylene, white spirit, gasoline are used. Acids are used to remove oxide films.
Depending on the thickness of the metal and the configuration of the weld, preparation before cutting the weld edge takes place in several stages:
- Markup. Using templates or rulers, drawing dimensions are transferred onto a metal sheet. For this, scrippers or construction markers are used, capable of applying a finishing touch to any surface.
- Cut it. For cutting metal of small thickness, roller or guillotine shears are used. Steel of large thickness, as well as carbon steel, are cut using propane cutters and plasma cutters.
- Bending flanging. This operation is performed before welding sheet material of small thickness, which allows to increase the amount of molten material and to prevent burning of heat-affected zones. The edges are bent in a sheet bending machine or manually using a hammer and a mandrel for tin works.
- Rolling. Joints of sheet material with a thickness of 3 mm are given the correct shape. This is achieved by mechanical action of the rollers or by means of a press. Also, rolling eliminates the deformation of the metal that occurred during storage and transportation.
Edging methods
Welding works are used not only for welding simple workpieces with a straight surface, but also for structures of complex shapes. Therefore, there are many different ways of cutting edges for welding:
- Abrasive treatment. It is produced in hard-to-reach places, as well as in the preparation of small surfaces. It is carried out manually using an angle grinder with an abrasive grinding wheel. In addition, this treatment is used as a finishing operation for aluminum alloys, since they form a high-strength oxide film, which must be removed before welding.
- Milling. It is used in the preparation of long edges, as well as having an uneven surface. Often this method is used when chamfering the same parts. The cutter according to the template removes excess metal from the edge, moving along a curved path. For manual machining using the milling method, a mobile edge cutter is used.
- Planing. This method is used in industrial production for the preparation of direct welding surfaces. By reciprocating in several passes, the high-strength cutter removes the required layer, forming a welding edge.
- Chiselling. For cutting pipe edges for welding, mobile edge cleavers are used. This operation is similar to planing. A cutter also acts here, only it forms an edge not along, but across. As a result of this, the chamfer is not removed evenly, and it must be modified manually by an angle grinder.
- Chamfering with a gas torch. To do this, the edge is heated with propane, and excess material is blown out with a stream of oxygen. The edge is uneven and requires further machining with an abrasive wheel.
Cutting technology
Chamfers on the edges of the parts to be welded are needed to provide deeper penetration, as well as for more convenient access of the electrode to the root of the seam. Cutting the edges allows you to weld a large thickness in several passes, obtaining a strong uniform seam.
More often, the chamfer is not removed to the full depth, but a small layer of material is left - dulling. It protects the part from burning through and does not allow molten metal to flow out of the weld pool. The shapes and sizes of bevels of butt joints are described in the rules for cutting edges for welding according to GOST 5264-80. For pipe connections, the standards are described in GOST 16037-80.
V-shaped cutting
The most popular way of cutting edges is V-shaped. It is used in a wide range of thicknesses of welded parts from 3 to 26 mm. It happens both unilateral and bilateral. The angle of cutting edges for welding is 60 degrees. In this way, butt, corner, tee joints are made.
X-shaped cut
This type is intended for welding parts of large thickness, where other cutting methods are not applicable. The angle of the chamfers is also 60 degrees. Such joints are boiled in several passes on each side. This method allows to reduce the consumption of electrodes in 1.6-1.7 times, and also reduces the residual deformation resulting from heating.
U-shaped cut
This option is used less often than other types of cutting edges for welding due to the complexity of forming such a profile. It is used when it is necessary to obtain a very high quality compound. In addition, the method reduces the cost of consumables. Savings are achieved due to the optimal shape of the weld pool. In this way, parts from 20 to 60 mm thick are cooked.
Weld crack preparation
Sometimes in the process of repairing parts you have to weld a crack. In this case, cutting edges for welding is also required. The essence of the operation is to deepen the defect over its entire length for effective access of the electrode to the surfacing zone. The expansion of the crack is done with a hammer and a chisel or with a propane cutter. The edge can be prepared from both one and two sides. It depends on the thickness of the part. To remove the stress of the metal, as a result of which a crack has occurred, holes are drilled along its edges.
Joint preparation for round joints
A large percentage of the work is to obtain round tight joints: welding of pipelines, tanks, pipes. These compounds are regulated by GOST 16037-80. In various cases, it provides for welding with and without cutting edges. It depends on the type of connection, which is of three types:
Before welding, the edges are cleaned from dirt and rust.
When assembling pipes, the distance between the joints should not exceed 2-3 mm, and the difference in thickness is not more than 10%. Pipe sections are precisely centered relative to each other. Before the start of welding, potholders are made around the perimeter so that the stresses that arise when the weld cools down do not disturb alignment.
Welding of the bends is necessary to separate the liquid or gases inside the main pipe. Bends welded in an angular manner do not require chamfering. If the connection is end-to-end, the shape of the cutting edges for welding takes an angle of 45 degrees.
Tanks and round tanks are often used in chemical production and are a storage for aggressive substances, therefore, increased demands are placed on the weld. To match them, for a wall up to 26 mm thick, make an X-shaped or V-shaped bevel, and for a thickness up to 60 mm U-shaped cutting of edges is used.