Welding of metals appeared with the emergence of forging. The emergence of increasingly complex mechanisms required the improvement of the processes of forging and welding. The connection of individual parts into solid forging units is a complex and time-consuming process, but before the advent of welding using an electric arc was the only possible one.
Welding is the easiest and most durable way to connect various metal parts.
Currently, more than 150 methods of arc welding have already been developed, and the developments of new technologies continue.
Main types of welds
A segment of a joint formed as a result of the crystallization process of a molten metal is called a weld. One of the main characteristics of welding joints is the weld leg.
There are two types of welds (not to be confused with the welding joint):
The main types of welded joints.
- Butt welded: used for butt joint, i.e. parts are connected end edges. Butt-joint is carried out without cutting edges, with cutting and flared parts. Edges can have a curvilinear shape, V-shaped and X-shaped. Sheets up to 8 mm can be welded without cutting edges, but sheets must be laid with a gap of up to 2 mm. In practice, butt variants are more often used for connecting pipelines and in the manufacture of sheet metal structures. Such compounds are the most economical and less energy consuming.
- Corner: there are actually angular, T-shaped and overlapped. Edging can be one- and two-sided, depending on the thickness of the metal. The cutting angle is selectable from 20 ° to 60 °. However, it should be borne in mind that a larger cutting angle requires more metal to fill, which means that productivity and quality are reduced.
- Electrofusion welding is used for covering large structures with thin sheet metal. Used, for example, in the manufacture of passenger cars, when the use of solid joints is difficult and unprofitable. Electro-riveted joints are quite strong, but not dense.
Usually, welding is performed at one time, but if the thickness of the metal being welded does not allow the material to be boiled, it is carried out in several passes. This method is called multilayer. In this case, each previous layer is subsequently annealed, as a result of this heat treatment, the properties and structure of the seam are significantly improved.
It is necessary to choose the type of connection depending on the configuration of the element being executed. The final product must be operational, transfer the design load and not succumb to fatigue damage.
Advantages of welding joints:
The influence of welding mode on the shape of the seam.
- Low complexity and ease of connection.
- Small, compared with other types of connections, the noise of the process.
- You can easily automate the process.
The disadvantages include the possibility of residual stresses and unreliability in work during vibrations and shock loads.Back to table of contents
Properties and geometry of welds
In all designs there are working seams that perceive the main loads. Strength calculations of working seams are carried out at full load plus 25%. Binding seams are used to connect individual elements - the requirements for them are not so rigid, since in the event of their destruction, the operation of the structure will not be disturbed.
The quality of welding joints is influenced by many factors: the ability of a material to create a monolithic seam, additives and fluxes, the oxidizability of the metal, the position of the seam: horizontal, vertical, inclined or ceiling.
The properties of a weld are determined mainly by its geometric dimensions.
General geometric parameters:
Classification and designation of welds.
- Width - the distance between the fusion boundaries.
- Concavity (convexity) - the distance between the line and the base metal, and the surface, visually passing along the line of maximum concavity (convexity).
- The root is the lowest part.
For angular joints, the following values are also characteristic: the presence of a weld leg, thickness, convexity and design height.
The leg of the corner seam is the leg of the largest isosceles triangle inscribed in the cross section. When welding blanks of the same thickness, the leg can be set along the edge; if different, it is set according to the thickness of the thinner material. The size of the leg should ensure the strength of the connection, but an excessive increase in it can cause deformation of the product.
The shape of the surface of the weld is important: convex, concave or flat. Seams with a convex surface - reinforced - work better under static loads. Concave surfaces - weakened - better withstand dynamic loads. In practice, seams with a flat surface are more often used as more universal.Back to table of contents
The shape of the surfaces of the seams is directly dependent on the electrodes used.
In addition to the general requirements for all manufactured electrodes (steady arc burning, a certain chemical composition of the weld metal, no splashing, etc.), special requirements are also imposed. These include obtaining a seam of a given shape. Electrodes, the melt of which gives a thick and viscous mass easily form a convex seam.
Liquid spreading melt forms a concave surface.
The choice of electrodes is carried out according to the technical characteristics indicated on each package in accordance with the specifications specified in the project.
Welding conditions affect the properties and geometry of the weld.
With an increase in the strength of the current, the penetration depth increases with a constant weld width. With increasing voltage, the width of the seam increases sharply with a subsequent decrease in the penetration depth. With an increase in the speed of the electrode movement up to 50 m / h the width of the seam decreases, and the penetration depth increases. Increasing the speed of more than 50 m / h is irrational, because undercuts may appear due to poor heating of the base material.
The quality of the compounds is carried out in two ways: destructive and non-destructive testing.
Non-destructive testing allows you to identify external defects using special welding patterns, internal - using ultrasound, x-ray transmission and gamma-ray emitter.
Destructive testing is carried out by drilling, testing the tensile strength, bending, impact strength with the destruction of samples.