The welding process requires a lot of 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 preparation of the edges for welding.
Surface preparation before welding
Before welding critical structures, surfaces are always treated. This achieves several goals: removal of dirt, oxide film, rust in places of 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. Liquids 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 seam, preparation before cutting the edge for weldingtakes place in several stages:
- Markup. Using templates or rulers, drawing dimensions are transferred to a sheet of metal. For this, scribers or building markers are used that can apply a stroke on any surface.
- Open. Roller or guillotine shears are used to cut thin metal. Thick steels, as well as carbon steels, are cut with propane torches and plasma cutters.
- Flange bending. This operation is performed before welding a sheet material of small thickness, which makes it possible to increase the amount of melted material and prevent burnout of the near-weld zones. The edges are bent in benders or manually using a hammer and a mandrel for tin work.
- Rolling with rollers. The joints of sheet material with a thickness of 3 mm or more are given the correct shape. This is achieved by the mechanical action of the rollers or by means of a press. Rolling also eliminates metal deformations that occurred during storage and transportation.
Methods of cutting edges
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 to cut the edges for welding:
- Abrasive processing. It is produced in hard-to-reach places, as well as when preparing small surfaces. It is carried out manually using an angle grinder with an abrasive grinding wheel. In addition, such processing is used as a finishing operation for aluminum alloys,because they form a high strength oxide film that must be removed before welding.
- Milling. It is used in the preparation of long edges, as well as those with an uneven surface. Often this method is used when chamfering the same type of parts. The template cutter removes excess metal from the edge, moving along a curved path. For manual processing by milling, a mobile beveler is used.
- Planing. This method is used in industrial production to prepare straight weld surfaces. With a reciprocating motion in several passes, the high-strength cutter removes the necessary layer, forming a welding edge.
- Chalking. To cut the pipe edge for welding, mobile bevelers are used. This operation is similar to planing. The cutter also acts here, only it forms the edge not along, but across. As a result, the bevel is uneven and must be finished manually with an angle grinder.
- Chamfering with a gas cutter. To do this, the edge is heated with propane, and excess material is blown out with a jet 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 ensure deeper penetration, as well as for more convenient access of the electrode to the weld root. Edge preparation allows weldinga 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 - blunting. It protects the part from burning through and does not allow molten metal to flow out of the weld pool. The shapes and dimensions of the chamfers of butt joints are described in the rules for cutting edges for welding GOST 5264-80. For pipe connections, the standards are described in GOST 16037-80.
V-cut
The most popular bevelling method is V-shaped. It is used in a wide range of thicknesses of welded parts from 3 to 26 mm. It happens both one-sided and two-sided. The angle of cutting edges for welding is 60 degrees. In this way, butt, corner, tee joints are made.
X-cut
This type is designed for welding thick parts where other preparation methods are inapplicable. The chamfer angle is also 60 degrees. Such connections are welded in several passes on each side. This method allows to reduce the consumption of electrodes by 1.6-1.7 times, and also reduces residual deformations resulting from heating.
U-cut
This option is used less often than other types of edge preparation for welding due to the complexity of forming such a profile. It is used when it is necessary to obtain a connection of very high quality. In addition, the method reduces the cost of consumables. Savings are achieved due to the optimal shape of the weld pool. Soparts with a thickness of 20 to 60 mm are cooked in this way.
Crack preparation for welding
Sometimes, in the process of restoring 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 along its entire length for effective access of the electrode to the surfacing zone. Crack widening is done with a hammer and chisel or with a propane torch. The edge can be prepared on one or both sides. It depends on the thickness of the part. Holes are drilled along the edges of the crack to relieve the stresses in the metal that caused the crack.
Preparation of joints of round joints
A large percentage of work falls on obtaining round hermetic joints: welding of pipelines, tanks, pipes. These connections are regulated by GOST 16037-80. In various cases, it provides for welding both with cutting edges and without it. It depends on the type of connection, which comes in three forms:
- butt;
- overlapping;
- angular.
Before welding, the edges are cleaned from dirt and rust.
When assembling pipes, the distance between joints should not exceed 2-3 mm, and the difference in thickness should not exceed 10%. Pipe sections are precisely centered relative to each other. Before starting welding, tacks are made along the perimeter so that the stresses that arise during the cooling of the weld do not violate alignment.
Welding of bends is necessary to separate the liquid or gases insidemain pipe. Angle welded elbows do not require chamfering. If the connection is butt, then the shape of the groove for welding takes an angle of 45 degrees.
Reservoirs and round containers are often used in chemical production and are storage for aggressive substances, therefore, increased requirements are placed on the weld. To meet them, an X-shaped or V-shaped bevel is made for a wall with a thickness of up to 26 mm, and a U-shaped groove is used for a thickness of up to 60 mm.
