Welding seams and connections - types and classifications. What types of welds are there? Welding seams classification and characteristics
To learn how to cook well, it is not enough to master holding an electric arc. It is necessary to understand what types of welded joints and seams there are. The problem for novice welders is unwelded areas and poor fracture resistance of finished parts. The reason lies in the wrong choice of the type of welded joint, as well as the wrong technique for making it. The drawings always indicate everything a welder needs to know for a quality result. But insufficient knowledge of the designations of welded joints can also lead to defective work. Therefore, it is very important to study other articles about symbols well. The same article discusses in detail the types of welding seams and all sorts of nuances regarding the differences and techniques for their implementation.
Types of welds by type of surface connection
Depending on the thickness of the metal, the required tightness, and the geometric shape of the parts being connected, different types of welds are used. They are divided into:
- butt;
- overlap;
- corner;
- T-bar.
Each has its own purpose, well suited to the specific needs of the finished product. The technique for making a welded joint also varies.
Joint
The most common type of welded joint is the butt. This is applicable when welding the ends of pipes, steel sheets, or other geometric shapes joined side to side. The main types of welded joints and seams include many types of end-to-end joining of parts, differing in the side of the seam and the thickness of the product. They are classified into the following subspecies:
- unilateral normal;
- one-sided with edge processing at 45º and V-shape;
- one-sided with processing one edge at 45º with a grinding machine, or using a cutter to select a semicircle equal in the amount of metal removed from the oblique cut;
- one-sided removal of the edge with a cutter on both attached parts (U-shaped groove);
- double-sided, implying edge cutting at 45º on each side (X-shaped cutting).
In the description of the work, they may be designated “C1”, or have another number after the letter, depending on the technique of execution. A regular one-sided seam is used when joining two plates no more than 4 mm thick. If the parts have up to 8 mm of metal thickness, then the seam is applied on both sides, which is a two-sided type of welded joint. To increase the fracture resistance coefficient, a greater depth of filling with molten metal is achieved, for which a gap of up to 2 mm is set between the two parts.
When working with products whose thickness exceeds 5 mm, and only one side of the seam is required, but high strength is expected, cutting the edges is necessary. It is carried out with a “grinder” or a file. A bevel of 45º is sufficient. To prevent the molten metal from burning through the bottom side and causing an overflow from the back of the joined surfaces, the edges are not beveled completely, leaving a slight dullness of 2-3 mm. Similar cutting can be done on a milling machine, which takes more time and resources. This is only used on very critical projects.
Angular
The main types of welded joints include several fillet weld options:
- one-sided, without cutting;
- single-sided with preliminary cutting;
- double-sided, regular;
- double-sided with cutting.
A corner seam allows you to attach two sheets to each other at an angle of 90º or any other. In this case, one seam will be internal (between two plates), and the second, external (at the end of the joined plates). Welding of this type is widely used in the manufacture of:
- gazebo frames;
- visors;
- awnings;
- truck bodies.
Such a welded joint is designated “U1”, or other related numbers, depending on the nuances of the seam. If two plates have different thicknesses, then it is recommended to place the thicker one at the bottom, and place the thinner one “edge” on it. The electrode or torch is aimed primarily at the thick part. This will allow high-quality welding of parts, without the formation of undercuts and burns.
The optimal way to perform a fillet welded joint is the “boat” position, where the two surfaces, after tack welding, are positioned in such a way that it resembles the equal convergence of the hull of a floating vessel. In this case, the molten metal falls evenly on both sides, minimizing the appearance of defects.
When passing the seam from the reverse side, it is necessary to reduce the current strength so as not to melt the corner. Thanks to this, strong rounding will not appear on the outside of such welded joints.
overlap
Two plates can be welded together not end-to-end, but by slightly stretching one over the surface of the other. Such welds are used where greater tensile strength is needed. The seam must be placed on each side of the contacting surfaces. This not only increases strength, but also prevents moisture from accumulating inside the product.
In the drawings, such a seam will have the sign “H1”. There are only two types. The creation of this welded joint does not require oscillatory movements. The electrode is directed to the lower surface.
Tavrovoe
It is similar to the corner one, but the plate attached “edge” is not placed at the edge of the lower base, but at a certain distance. They are used in the installation of bases of various metal structures. If the steel thickness exceeds 4 mm, then a double-sided seam is recommended. When the dimensions of the product allow it to be turned over and installed “in a boat,” then this should be done at critical nodes. The remaining seams can be made in the usual position, using the recommendations for corner joints.
By spatial position
Subsequent classification of seams and joints is carried out according to the location of application in space. They are divided into:
- Lower. Often found in factories and large industries. Provides uniform distribution of molten metal, with a minimum amount of drips and sagging. To weld large products in the lower position, rotating jigs are used. The electrode or burner is always directed from top to bottom. This way you can make all types of joints according to the method of contact with each other (angle, overlap, etc.).
- Vertical. It is highly complex and requires certain skills. It is used when welding pipes (passing seams on the sides) or fastening large structures, due to the impossibility of turning them over to the lower position. Requires more welding time, less current, and an intermittent arc to prevent drips. The electrode is directed from bottom to top. Welding is also carried out.
- Horizontal. Used when connecting vertical pipes or sheets of metal. It is fraught with drips when running the seam slowly, or unwelded places when passing quickly. For convenience, the sides are set with an offset of 1 mm to form a “step” to delay the applied metal. After applying the seam, a 1 mm difference in surface protrusion is not visible.
- Ceiling. The most difficult for welders, but accessible after a specialist masters the vertical method. The seam is applied with an intermittent arc, using a lower current. Used when welding pipes when there is no possibility to turn the product. It is actively used on construction sites in the installation of ceiling channels and beams.
According to the seam shape and technology
The types of welding joints also differ in the shape of the seam itself. It could be:
- Smooth - achieved with optimal device settings and a comfortable spatial position.
- Convex - possible due to low current strength and passage through several layers. Often requires subsequent mechanical processing.
- Concave - achieved by increased current strength. It has good penetration and does not require grinding.
- Continuous - is carried out continuously and has a “lock” that prevents the appearance of fistulas.
- Intermittent - used on products made of thin sheets and with light load.
All types of seams can be performed in one pass or several. This is determined by the thickness of the parts being welded and the required strength. The first seam is called the root seam. It has narrow boundaries and is made at a lower current. Subsequent seams are multi-pass. They allow you to fill the space between the edges of the plates. They are performed at high currents and with contact with the base metal.
Knowing the main types of joints and their fundamental differences, you can correctly select the required type of seam that will satisfy the key requirements for tightness and strength in each specific case.
Welded joints and seams are classified according to the following main characteristics:
- type of connection;
- the position in which welding is performed;
- configuration and length;
- type of welding used;
- method of holding molten weld metal;
- number of layers;
- material used for welding;
- the location of the parts to be welded relative to each other;
- force acting on the seam;
- volume of deposited metal;
- the shape of the welded structure;
- shape of prepared edges for welding
Depending on the type of connection, welds can be butt and corner welds. Based on their location in space, the seams of welded joints are divided into bottom, vertical, horizontal and ceiling. The exit of the seam from the ceiling position to the vertical position when welding cylindrical products is called the semi-ceiling position.
According to the configuration, the seams of welded joints can be straight, circular, vertical and horizontal. According to their length, seams are divided into continuous and intermittent. Solid seams, in turn, are divided into short, medium and long.
According to the type of welding, the seams of welded joints are divided into:
- arc welding seams
- automatic and semi-automatic submerged arc welding seams
- gas shielded arc welding seams
- electroslag welding seams
- electric riveted seams
- contact electric welding seams
- solder seams
According to the method of holding molten metal, the seams of welded joints are divided into seams made without linings and pillows; on removable and remaining steel linings: copper, flux-copper. ceramic and asbestos linings, as well as flux and gas cushions. Depending on which side the suture is applied on, one-sided and two-sided sutures are distinguished.
According to the material used for welding, the seams of welded joints are divided into joints of carbon and alloy steels; welds connecting non-ferrous metals; bimetal connection seams; seams connecting vinyl plastic and polyethylene.
According to the location of the parts being welded relative to each other, the seams of welded joints can be at an acute or obtuse angle, at a right angle, and also located in the same plane.
Based on the volume of deposited metal, normal, weakened and reinforced welds are distinguished.
According to the shape of the structure being welded, the seams of welded joints are made on flat and spherical structures, and according to the location on the product, the seams are longitudinal and transverse.
Welded connections are permanent connections made by welding. They can be butt, corner, lap, tee and end (Fig. 1).
Butt joint is the connection of two parts with their ends located in the same plane or on the same surface. The thickness of the welded surfaces may be the same or different from one another. In practice, butt joints are most often used when welding pipelines and various tanks.
Corner - a welded connection of two elements located at an angle relative to each other and welded at the junction of their edges. Such welded joints are widely used in construction practice.
An overlap welded connection involves the superposition of one element on another in the same plane with partial overlap of each other. Such connections are most often found in construction and installation work, during the construction of farms, tanks, etc.
A T-joint is a joint in which the end of another joint is attached to the plane of one element at a certain angle.
Welding seams
The section of a welded joint formed as a result of crystallization of molten metal is called a weld seam. Unlike joints, welds are butt and corner welds (Fig. 2).
A butt weld is a weld in a butt joint. Fillet is a weld of corner, lap and T-joints.
Welding seams are distinguished by the number of overlay layers, their orientation in space, length, etc. So, if the seam completely covers the joint, then it is called continuous. If a seam breaks within one joint, it is called intermittent. A type of intermittent weld is a tack weld, which is used to fix elements relative to each other before welding. If welding seams are placed one on top of the other, then such seams are called multilayer.
Depending on the shape of the outer surface, welding seams can be flat, concave or convex. The shape of the weld affects its physical and mechanical properties and the consumption of electrode metal associated with its formation. The most economical are flat and concave welds, which, moreover, work better under dynamic loads, since there is no sharp transition from the base metal to the weld. Excessive overflow of convex welds leads to excessive consumption of electrode metal, and a sharp transition from the base metal to the weld under concentrated stresses can cause joint failure. Therefore, in the manufacture of critical structures, the convexity on the seams is removed mechanically (cutters, abrasive wheels, etc.).
Welding seams are distinguished by their position in space. These are bottom, horizontal, vertical and ceiling seams.
Elements of the geometric shape of preparing edges for welding
The elements of the geometric shape of preparing edges for welding (Fig. 3, a) are: edge cutting angle α; the gap between the joined edges a; blunting of edges S; sheet bevel length L in the presence of a difference in metal thickness; displacement of the edges relative to each other δ.
The cutting angle of the edges is carried out when the metal thickness is more than 3 mm, since its absence (cutting the edges) can lead to lack of penetration along the cross-section of the welded joint, as well as to overheating and burnout of the metal; In the absence of cutting edges to ensure penetration, the electric welder always tries to increase the welding current.
Grooving the edges allows welding to be carried out in separate layers of small cross-sections, which improves the structure of the welded joint and reduces the occurrence of welding stresses and deformations.
The gap, correctly set before welding, allows for complete penetration along the cross-section of the joint when applying the first (root) layer of the seam, if the appropriate welding mode is selected.
The length of the sheet bevel regulates a smooth transition from a thick welded part to a thinner one, eliminating stress concentrators in welded structures.
Blunting of edges is carried out to ensure stable conduction of the welding process when performing the root layer of the weld. The lack of blunting contributes to the formation of burns during welding.
The displacement of the edges worsens the strength properties of the welded joint and contributes to the formation of lack of fusion and stress concentrations. GOST 5264-69 allows displacement of the welded edges relative to each other up to 10% of the metal thickness, but not more than 3 mm.
Geometry and classification of welds
The elements of the geometric shape of the weld are: for butt joints - seam width “b”, seam height “h”, for T-joints, corner and lap joints - seam width “b”, seam height “h” and seam leg “K” (Fig. . 3, b).
Welds are classified according to the number of deposited beads - single-layer and multi-layer (Fig. 4, a); by location in space - lower, horizontal, vertical and ceiling (Fig. 4, b); in relation to the current forces on the seams - flank, frontal (end) (Fig. 4, c); in direction - rectilinear, circular, vertical and horizontal (Fig. 4, d).
Weld properties
The quality indicators of welded joints are influenced by many factors, which include the weldability of metals, their sensitivity to thermal influences, oxidation, etc. Therefore, to ensure that welded joints comply with certain operating conditions, these criteria should be taken into account.
The weldability of metals determines the ability of individual metals or their alloys to form, with appropriate technological processing, compounds that meet specified parameters. This indicator is influenced by the physical and chemical properties of metals, the structure of their crystal lattice, the presence of impurities, the degree of alloying, etc. Weldability can be physical and technological.
Physical weldability is understood as the property of a material or its compositions to create a monolithic compound with a stable chemical bond. Almost all pure metals, their technical alloys and a number of combinations of metals with non-metals have physical weldability.
The technological weldability of a material includes its reaction to the welding process and the ability to create a connection that satisfies the specified parameters.
Welds and connections
A permanent connection that was made by welding is called welded. It consists of several zones (Fig. 77):
Weld seam;
Fusion;
Rice. 77. Welded joint zones: 1 – weld; 2 – fusion; 3 – thermal influence; 4 – base metal
Thermal influence;
Base metal.
According to their length, welded joints are:
Short (250–300 mm);
Medium (300–1000 mm);
Long (more than 1000 mm). Depending on the length of the weld, the method of its execution is chosen. For short joints, the seam runs in one direction from beginning to end; for the middle sections, it is typical to apply a seam in separate sections, and its length should be such that a whole number of electrodes (two, three) are enough to complete it; long joints are welded using the reverse-step method discussed above.
By type, welded joints (Fig. 78) are divided into:
1. Butt. These are the most common joints used in various welding methods. They are preferred because they are characterized by the lowest intrinsic stresses and deformations. As a rule, sheet metal structures are welded using butt joints.
Rice. 78. Types of welded joints: a – butt; b – tee; c – angular; g – overlap
Rice. 78 (end). d – slotted; e – end; g – with overlays; 1–3 – base metal; 2 – cover: 3 – electric rivets; h – with electric rivets
The main advantages of this connection, which can be counted on subject to careful preparation and adjustment of the edges (due to the blunting of the edges, burn-through and leakage of metal during the welding process are prevented, and maintaining their parallelism ensures a high-quality, uniform seam), are the following:
Minimum consumption of base and deposited metal;
The shortest time period required for welding;
The completed connection can be as strong as the base metal.
Depending on the thickness of the metal, the edges during arc welding can be cut at different angles to the surface:
At a right angle, if connecting steel sheets with a thickness of 4–8 mm. In this case, a gap of 1–2 mm is left between them, which makes it easier to weld the lower parts of the edges;
At a right angle, if metal with a thickness of up to 3 and up to 8 mm is connected using one- or two-sided welding, respectively;
With one-sided bevel of edges (V-shaped), if the metal thickness is from 4 to 26 mm;
With a double-sided bevel (X-shaped), if the sheets have a thickness of 12–40 mm, and this method is more economical than the previous one, since the amount of deposited metal is reduced by almost 2 times. This means saving electrodes and energy. In addition, double-sided bevels are less susceptible to deformation and stress during welding;
The bevel angle can be reduced from 60° to 45° if you weld sheets with a thickness of more than 20 mm, which will reduce the volume of deposited metal and save electrodes. The presence of a gap of 4 mm between the edges will ensure the necessary penetration of the metal.
When welding metal of different thicknesses, the edge of the thicker material is beveled more strongly. If the parts or sheets to be joined by arc welding are of significant thickness, cup-shaped edge preparation is used, and with a thickness of 20–50 mm, one-sided preparation is carried out, and with a thickness of more than 50 mm, two-sided preparation is carried out.
The above is clearly shown in table. 44.
2. Lap welds, most often used in arc welding of structures whose metal thickness is 10–12 mm. What distinguishes this option from the previous connection is that there is no need to prepare the edges in a special way - just cut them off. Although the assembly and preparation of metal for lap joints is not so burdensome, it should be taken into account that the consumption of base and deposited metal increases compared to butt joints. For reliability and to avoid corrosion due to moisture getting between the sheets, such joints are welded on both sides. There are types of welding where this option is used exclusively, in particular with spot contact and roller welding.
3. T-bars, widely used in arc welding. For them, the edges are beveled on one or both sides or are dispensed with without a bevel at all. Special requirements apply only to the preparation of a vertical sheet, which must have an equally trimmed edge. For one- and two-sided bevels, the edges of a vertical sheet provide a gap of 2–3 mm between the vertical and horizontal planes in order to weld the vertical sheet to its full thickness. A one-sided bevel is performed when the design of the product is such that it is impossible to weld it on both sides.
Table 44
Selecting a butt joint depending on the thickness of the metal
5. Slotted, which are used in cases where an overlap seam of normal length does not provide the necessary strength. There are two types of such connections - open and closed. The slot is made using oxygen cutting.
6. End (side) in which the sheets are placed one on top of the other and welded at the ends.
7. With overlays. To make such a connection, the sheets are joined and the joint is covered with an overlay, which, naturally, entails additional metal consumption. Therefore, this method is used in cases where it is not possible to make a butt or overlap weld.
8. With electric rivets. This connection is strong, but not tight enough. For this, the top sheet is drilled and the resulting hole is welded in such a way as to capture the bottom sheet as well.
If the metal is not too thick, then drilling is not required. For example, with automatic submerged arc welding, the top sheet is simply melted by the welding arc.
The structural element of a welded joint, which during its execution is formed due to the crystallization of molten metal along the line of movement of the heating source, is called a weld. The elements of its geometric shape (Fig. 79) are:
Width(b);
Height(h);
Leg size (K) for corner, lap and T-joints.
The classification of welds is based on various characteristics, which are presented below.
Rice. 79. Elements of the geometric shape of the weld (width, height, leg size)
1. By connection type:
Butt;
Angular (Fig. 80).
Rice. 80. Corner seam
Fillet welds are practiced for some types of welded joints, in particular lap, butt, corner and overlay joints.
The sides of such a seam are called legs (k), zone ABCD in Fig. 80 shows the degree of convexity of the seam and is not taken into account when calculating the strength of the welded joint. When performing it, it is necessary that the legs are equal, and the angle between the sides OD and BD is 45°.
2. By type of welding:
Arc welding seams;
Automatic and semi-automatic submerged arc welding seams;
Gas-shielded arc welding seams;
Electroslag welding seams;
Resistance welding seams;
Gas welding seams.
3. According to the spatial position (Fig. 81) in which welding is performed:
Rice. 81. Welds depending on their spatial position: a – bottom; b – horizontal; c – vertical; g – ceiling
Horizontal;
Vertical;
Ceiling.
The easiest seam to make is the bottom seam, the most difficult is the ceiling seam.
In the latter case, welders undergo special training, and it is easier to make a ceiling seam using gas welding than arc welding.
4. By length:
Continuous;
Intermittent (Fig. 82).
Rice. 82. Intermittent weld
Intermittent seams are practiced quite widely, especially in cases where there is no need (strength calculations do not involve making a continuous seam) to tightly connect products.
The length (l) of the joined sections is 50–150 mm, the gap between them is approximately 1.5–2.5 times larger than the welding zone, and together they form the seam pitch (t).
5. According to the degree of convexity, i.e., the shape of the outer surface (Fig. 83):
Normal;
Convex;
Concave.
The type of electrode used determines the convexity of the weld (a‘). The greatest convexity is characteristic of thinly coated electrodes, while thickly coated electrodes produce normal seams, since they are characterized by greater fluidity of the molten metal.
Rice. 83. Welds that differ in the shape of the outer surface: a – normal; b – convex c – concave
It was experimentally established that the strength of the seam does not increase with increasing convexity, especially if the connection “operates” under variable loads and vibration. This situation is explained as follows: when making a seam with a large convexity, it is impossible to achieve a smooth transition from the seam bead to the base metal, so at this point the edge of the seam is, as it were, cut, and stresses are mainly concentrated here.
Under conditions of variable and vibration loads in this place, the welded joint may be subject to destruction. In addition, convex welds require increased consumption of electrode metal, energy and time, i.e. it is an uneconomical option.
6. According to configuration (Fig. 84):
Straight-line;
Ring;
Rice. 84. Welds of various configurations: a – straight; b – ring
Vertical;
Horizontal.
7. In relation to the acting forces (Fig. 85):
Flanking;
Face;
Combined;
Oblique. The vector of action of external forces can be parallel to the axis of the seam (typical for flank forces), perpendicular to the axis of the seam (for end forces), pass at an angle to the axis (for oblique ones) or combine the direction of flank and end forces (for combined ones).
8. According to the method of holding molten weld metal:
Without linings and pillows;
On removable and remaining steel pads;
Rice. 85. Welds in relation to the acting forces: a – flank; b – end; c – combined; g – oblique
On copper, flux-copper, ceramic and asbestos linings, flux and gas cushions.
When applying the first layer of a weld, the main thing is to be able to hold the liquid metal in the weld pool.
To prevent it from leaking, use:
Steel, copper, asbestos and ceramic linings, which are placed under the root seam. Thanks to them, it is possible to increase the welding current, which ensures through penetration of edges and guarantees 100% penetration of parts. In addition, the linings hold the molten metal in the weld pool, preventing the formation of burns;
Inserts between welded edges, which perform the same functions as gaskets;
Hemming and welding the root of the seam from the opposite side, without attempting to achieve through penetration;
Flux, flux-copper (for submerged arc welding) and gas (for manual arc, automatic and argon-arc welding) pads that are brought or fed under the first layer of the seam. Their goal is to prevent metal from flowing out of the weld pool;
Lock joints when making butt seams, which prevent burns in the root layer of the seam;
Special electrodes, the coating of which contains special components that increase the surface tension of the metal and do not allow it to flow out of the weld pool when making vertical seams from top to bottom;
A pulsed arc, due to which a short-term melting of the metal occurs, which contributes to faster cooling and crystallization of the weld metal.
9. On the side on which the seam is applied (Fig. 86):
One-sided;
Double-sided.
10. For welded materials:
On carbon and alloy steels;
Rice. 86. Welds, differing in their location: a - one-sided; b – double-sided
On non-ferrous metals;
On bimetal;
On foam and polyethylene.
11. According to the location of the parts to be connected:
At an acute or obtuse angle;
At right angles;
In one plane.
12. By volume of deposited metal (Fig. 87):
Normal;
Weakened;
Reinforced.
13. By location on the product:
Longitudinal;
Transverse.
14. According to the shape of the structures being welded:
On flat surfaces;
On spherical surfaces.
15. By the number of deposited beads (Fig. 88):
Single layer;
Multilayer;
Multi-pass.
Before welding, the edges of the products, structures or parts to be joined must be properly prepared, since the strength of the seam depends on their geometric shape
Rice. 87. Welds that differ in the volume of deposited metal: a – weakened; b – normal; c – reinforced
Rice. 88. Welds that differ in the number of welded beads: a – single-layer; b – multilayer; c – multilayer multipass
The elements of form preparation are (Fig. 89):
Edge cutting angle (?), which must be made if the metal thickness is more than 3 mm. If you skip this operation, then such negative consequences as lack of penetration along the cross-section of the welded joint, overheating and burnout of the metal are possible. Cutting the edges makes it possible to weld in several layers of small cross-section, due to which the structure of the welded joint is improved, and internal stresses and deformations are reduced;
Rice. 89. Elements of preparing cromo
Gap between edges to be joined (a). The correctness of the established gap and the selected welding mode determines how complete the penetration will be across the cross section of the joint when forming the first (root) layer of the weld;
The blunting of the edges (S) is necessary in order to give the root suture process a certain stability. Ignoring this requirement leads to burnout of the metal during welding;
The bevel length of the sheet if there is a difference in thickness (L). This element allows for a smooth and gradual transition from a thicker part to a thin one, which reduces or eliminates the risk of stress concentration in welded structures;
Offset of edges relative to each other (?). Since this reduces the strength characteristics of the connection, and also contributes to lack of penetration of the metal and the formation of stress spots, GOST 5264–80 establishes acceptable standards, in particular, the displacement should be no more than 10% of the metal thickness (maximum 3 mm).
Thus, when preparing for welding, the following requirements must be met:
Clean the edges from dirt and corrosion;
Remove chamfers of the appropriate size (according to GOST);
Set the gap in accordance with GOST developed for a particular type of connection.
Some types of edges have already been discussed earlier (although they were considered in a different aspect) when describing butt joints, but nevertheless it is necessary to once again focus on this (Fig. 90).
The choice of one type of edge or another is determined by a number of factors:
Welding method;
Metal thickness;
The method of connecting products, parts, etc.
For each welding method, a separate standard has been developed, which specifies the form of edge preparation, the size of the seam and the permissible deviations. For example, manual arc welding is carried out in accordance with GOST 5264–80, contact – in accordance with GOST 15878–79, electroslag welding – in accordance with GOST 15164–68, etc.
Rice. 90. Types of edges prepared for welding: a – with bevel of both edges; b – with a bevel of one edge; c – with two symmetrical bevels of one edge; d – with two symmetrical bevels of two edges; d – with a curved bevel of two edges; e – with two symmetrical curved bevels of two edges; g – with a bevel of one edge; h – with two symmetrical bevels of one edge
In addition, there is a standard for the graphic designation of a weld, in particular GOST 2.312–72. To do this, use an inclined line with a one-way arrow (Fig. 91), which indicates the seam area.
The weld characteristics, recommended welding method and other information are presented above or below the horizontal shelf connected to the inclined arrow line. If the seam is visible, that is, it is on the front side, then the characteristics of the seam are given above the shelf, if invisible - below it.
Rice. 91. Graphic designation of welds
The symbols of a weld also include additional symbols (Fig. 92).
For various types of welding, letter designations are adopted:
Arc welding - E, but since this type is the most common, the letter may not be indicated in the drawings;
Gas welding – G;
Electroslag welding – Ш;
Welding in an inert gas environment – I;
Explosion welding – Vz;
Plasma welding – Pl;
Resistance welding – Kt;
Welding in carbon dioxide – U;
Friction welding – Tr;
Cold welding - X.
If necessary (if several welding methods are implemented), the letter designation of the welding method used is placed before the designation of one or another type:
Rice. 92. Additional designations of a weld: a – intermittent weld with a chain sequence of sections; b – intermittent seam with a checkerboard sequence of sections; c – seam along a closed contour; d – seam along an open contour; d – installation seam; e – seam with the reinforcement removed; g – seam with a smooth transition to the base metal
Manual – P;
Semi-automatic – P;
Automatic - A.
Submerged arc – F;
Welding in active gas with a consumable electrode - UP;
Welding in inert gas with a consumable electrode - IP;
Welding in inert gas with a non-consumable electrode - IN.
There are also special letter designations for welded joints:
Butt – C;
Tavrovoe – T;
Lap – N;
Angular - U. Using the numbers after the letters, the number of the welded joint is determined according to GOST for welding.
Summarizing the above, we can state that the symbols of welds develop into a certain structure (Fig. 93).
A welded joint is a connection of two or more elements obtained by welding.
The welded joint (Fig. 2) consists of: a weld that has a cast structure and is formed as a result of crystallization of the weld pool, a fusion zone, a heat-affected zone - a section of the base metal that has not been subjected to melting, where structural and phase changes and parts have occurred as a result of heating base metal.
Rice. 2. Welded joint diagram
1 – weld, 2 – fusion zone, 3 – heat-affected zone, 4 – base metal
There are 5 types of welded joints (Fig. 3) - butt, corner, T, lap and end.
Rice. 3. Types of welded joints: a – butt, b – lap, c – end, d – corner,
d – tee
The most typical and preferred butt joints, but the heat of the energy source is not always enough to melt the entire thickness, therefore, for parts with a thickness of more than 4 mm, special edge preparation is used (Fig. 4)
Rice. 4. Examples (a-g) of edge preparation
The structural elements of edge preparation are as follows:
S – thickness of connected elements;
C – blunting of edges (usually 1-2mm);
β – for bevel from 25 to 45˚;
α – cutting angle, equal to 2 β;
B – gap, depends on the thickness and welding method;
R – Curvature radius for shaped edge cutting.
Classification of welds.
All welds are classified according to the following criteria:
1. According to the type of welded joint: butt joints, formed by butt joints and corner joints - T-joints, lap joints, corner joints.
2. By position relative to the current force - flank, frontal and oblique.
3. By position in space (Fig. 5)
Rice. 5. Basic spatial positions of welding:
1 – bottom, 2 – vertical or horizontal, 3 – ceiling
4. According to the external shape (Fig. 6) - convex, normal and concave.
Fig.6. Shape of welds: a – normal, b – convex, c – concave
5. By length (Fig. 7) – into continuous and intermittent, which are divided into chain and chess.
Rice. 7. Classification of welds by length.
A permanent connection that was made by welding is called welded. It consists of several zones:
Welded joint zones: 1 - welded seam; 2 - fusion; 3 - thermal influence; 4 - base metal
- weld seam;
— fusion;
— thermal influence;
- base metal.
According to their length, welded joints are:
— short (250-300 mm);
- medium (300-1000 mm);
— long (more than 1000 mm).
Depending on the length of the weld, the method of its execution is chosen. For short connections, the seam is carried out in one direction from beginning to end; for the middle sections, it is typical to apply a seam in separate sections, and its length should be such that a whole number of electrodes (two, three) are enough to complete it; long joints are welded using the reverse-step method discussed above.
By type, welded joints are divided into:
1. Butt. These are the most common joints used in various welding methods. They are preferred because they are characterized by the lowest intrinsic stresses and deformations. As a rule, sheet metal structures are welded using butt joints.
The main advantages of this connection, which can be counted on subject to careful preparation and adjustment of the edges (due to the blunting of the edges, burn-through and leakage of metal during the welding process are prevented, and maintaining their parallelism ensures a high-quality, uniform seam), are the following:
— minimum consumption of base and deposited metal;
— the shortest time period required for welding;
— the completed connection can be as strong as the base metal.
Depending on the thickness of the metal, the edges during arc welding can be cut at different angles to the surface:
- at a right angle, if connecting steel sheets with a thickness of 4-8 mm. At the same time, a gap of 1-2 mm is left between them, which makes it easier to weld the lower parts of the edges;
- at a right angle, if metal with a thickness of up to 3 and up to 8 mm is connected using one- or two-sided welding, respectively;
— with one-sided bevel of edges (V-shaped), if the metal thickness is from 4 to 26 mm;
- with a double-sided bevel (X-shaped), if the sheets have a thickness of 12-40 mm, and this method is more economical than the previous one, since the amount of deposited metal is reduced by almost 2 times. This means saving electrodes and energy. In addition, double-sided bevels are less susceptible to deformation and stress during welding;
— the bevel angle can be reduced from 60° to 45° if you weld sheets with a thickness of more than 20 mm, which will reduce the volume of deposited metal and save electrodes. The presence of a gap of 4 mm between the edges will ensure the necessary penetration of the metal.
When welding metal of different thicknesses, the edge of the thicker material is beveled more strongly. For significant thicknesses of parts or sheets connected by arc welding, cup-shaped edge preparation is used, and for a thickness of 20-50 mm, one-sided preparation is carried out, and for a thickness of more than 50 mm, two-sided preparation is carried out.
The above is clearly shown in table.
2. Overlapping, most often used in arc welding of structures whose metal thickness is 10-12 mm. What distinguishes this option from the previous connection is that there is no need to prepare the edges in a special way - just cut them off. Although the assembly and preparation of metal for an overlap joint is not so burdensome, it should be taken into account that the consumption of base and deposited metal increases compared to butt joints. For reliability and to avoid corrosion due to moisture getting between the sheets, such joints are welded on both sides. There are types of welding where this option is used exclusively, in particular with spot contact and roller welding.
3. T-bars, widely used in arc welding. For them, the edges are beveled on one or both sides or are dispensed with without bevel at all. Special requirements are imposed only on the preparation of a vertical sheet, which must have an equally trimmed edge. For one- and two-sided bevels, the edges of a vertical sheet provide a gap of 2-3 mm between the vertical and horizontal planes in order to weld the vertical sheet to its full thickness. A one-sided bevel is performed when the design of the product is such that it is impossible to weld it on both sides.
4. Angular, in which structural elements or parts are combined at one angle or another and welded along the edges, which must be pre-prepared. Similar connections are found in the manufacture of containers for liquids or gases, which are contained in them under low internal pressure. Corner joints can also be welded from the inside to increase strength.
5. Slotted, which are used in cases where a lap seam of normal length does not provide the necessary strength. There are two types of such connections - open and closed. The slot is made using oxygen cutting.
6. End (side) in which the sheets are placed one on top of the other and welded at the ends.
7. With overlays. To make such a connection, the sheets are joined and the joint is covered with an overlay, which, naturally, entails additional metal consumption. Therefore, this method is used in cases where it is not possible to make a butt or overlap weld.
8. With electric rivets. This connection is strong, but not tight enough. For this, the top sheet is drilled and the resulting hole is welded in such a way as to capture the bottom sheet as well. If the metal is not too thick, then drilling is not required. For example, with automatic submerged arc welding, the top sheet is simply melted by the welding arc.
The structural element of a welded joint, which during its execution is formed due to the crystallization of molten metal along the line of movement of the heating source, is called a weld. The elements of its geometric shape are:
— width (b);
— height (n);
— leg size (K) for corner, lap and T-joints.
The classification of welds is based on various characteristics, which are presented below. 1. By connection type:
- butt;
- angular.
Fillet welds are practiced for some types of welded joints, in particular lap, butt, corner and overlay joints. The sides of such a seam are called legs (k), zone ABCD in Fig. 33 shows the degree of convexity of the seam and is not taken into account when calculating the strength of the welded joint. When performing it, it is necessary that the legs are equal, and the angle between the sides OD and BD is 45°.
2. By type of welding:
— arc welding seams;
— seams of automatic and semi-automatic submerged arc welding;
— gas-shielded arc welding seams;
— electroslag welding seams;
— contact welding seams;
- gas welded seams.
3. According to the spatial position in which welding is performed:
- lower;
— horizontal;
— vertical;
- ceiling.
The easiest seam to make is the bottom seam, the most difficult is the ceiling seam. In the latter case, welders undergo special training, and it is easier to make a ceiling seam using gas welding than arc welding.
4. By length:
- continuous;
- intermittent.
Intermittent seams are practiced quite widely, especially in cases where there is no need (strength calculations do not involve making a continuous seam) to tightly connect products. The length (I) of the joined sections is 50-150 mm, the gap between them is approximately 1.5-2.5 times larger than the welding zone, and together they form the seam pitch (t).
5. According to the degree of convexity, i.e. outer surface shape:
- normal;
- convex;
- concave.
The type of electrode used determines the convexity of the seam (a"). The greatest convexity is characteristic of thinly coated electrodes, and thickly coated electrodes produce normal seams, since they are characterized by greater fluidity of the molten metal.
It was experimentally established that the strength of the seam does not increase with increasing convexity, especially if the connection “operates” under variable loads and vibration. This situation is explained as follows: when making a seam with a large convexity, it is impossible to achieve a smooth transition from the seam bead to the base metal, so at this point the edge of the seam is, as it were, cut, and stresses are mainly concentrated here. Under conditions of variable and vibration loads in this place, the welded joint may be subject to destruction. In addition, convex welds require increased consumption of electrode metal, energy and time, i.e. is not an economical option.
6. By configuration:
- straight;
— ring;
— vertical;
— horizontal.
7. In relation to the acting forces:
— flank;
— end;
- combined;
- oblique.
The vector of action of external forces can be parallel to the axis of the seam (typical for flank forces), perpendicular to the axis of the seam (for end forces), pass at an angle to the axis (for oblique ones) or combine the direction of flank and end forces (for combined ones).
8. According to the method of holding molten weld metal:
— without linings and pillows;
— on removable and remaining steel linings;
- on copper, flux-copper, ceramic and asbestos linings, flux and gas cushions.
When applying the first layer of a weld, the main thing is to be able to hold the liquid metal in the weld pool. To prevent it from leaking, use:
- steel, copper, asbestos and ceramic linings, which are placed under the root seam. Thanks to them, it is possible to increase the welding current, which ensures through penetration of edges and guarantees 100% penetration of parts. In addition, the linings hold the molten metal in the weld pool, preventing the formation of burns;
— inserts between the welded edges, which perform the same functions as gaskets;
- hemming and welding of the root of the seam from the opposite side, while not striving for through penetration;
- flux, flux-copper (for submerged arc welding) and gas (for manual arc, automatic and argon-arc welding) pads, which are brought or fed under the first layer of the seam. Their goal is to prevent metal from flowing out of the weld pool;
— lock joints when making butt seams, which prevent burns in the root layer of the seam;
- special electrodes, the coating of which contains special components that increase the surface tension of the metal and do not allow it to flow out of the weld pool when making vertical seams from top to bottom;
- a pulsed arc, due to which a short-term melting of the metal occurs, which contributes to faster cooling and crystallization of the weld metal.
9. On the side on which the seam is applied:
- one-sided;
- bilateral.
10. For welded materials:
— on carbon and alloy steels;
- on non-ferrous metals;
- on bimetal;
- on foam plastic and polyethylene.
11. According to the location of the parts to be connected:
- at an acute or obtuse angle;
- at right angles;
- in one plane.
12. By volume of deposited metal:
- normal;
— weakened;
- reinforced.
13. By location on the product:
— longitudinal;
- transverse.
14. According to the shape of the structures being welded:
- on flat surfaces;
- on spherical surfaces.
15. By the number of deposited beads:
- single-layer;
- multilayer;
- multi-pass.
Before carrying out welding work, the edges of the connected products, structures or parts must be properly prepared, since the strength of the seam depends on their geometric shape. The elements of form preparation are:
- edge cutting angle (a), which must be made if the metal thickness is more than 3 mm. If you skip this operation, then such negative consequences as lack of penetration along the cross-section of the welded joint, overheating and burnout of the metal are possible. Cutting the edges makes it possible to weld in several layers of small cross-section, due to which the structure of the welded joint is improved, and internal stresses and deformations are reduced;
- the gap between the joined edges (a). The correctness of the established gap and the selected welding mode determines how complete the penetration will be across the cross section of the joint when forming the first (root) layer of the weld;
- blunting of the edges (S), necessary in order to give the process of applying the root seam a certain stability. Ignoring this requirement leads to burnout of the metal during welding;
- length of the sheet bevel if there is a difference in thickness (L). This element allows for a smooth and gradual transition from a thicker part to a thin one, which reduces or eliminates the risk of stress concentration in welded structures;
— displacement of the edges relative to each other (5). Since this reduces the strength characteristics of the connection, and also contributes to lack of penetration of the metal and the formation of stress spots, GOST 5264-80 establishes acceptable standards, in particular, the displacement should be no more than 10% of the metal thickness (maximum 3 mm).
Thus, when preparing for welding, the following requirements must be met:
— clean the edges from dirt and corrosion;
— remove chamfers of the appropriate size (according to GOST);
- set the gap in accordance with GOST developed for a particular type of connection.
Some types of edges have already been mentioned earlier (although they were considered in a different aspect) when describing butt joints, but nevertheless it is necessary to once again focus on this.
The choice of one type of edge or another is determined by a number of factors:
— welding method;
— metal thickness;
- the method of connecting products, parts, etc.
For each welding method, a separate standard has been developed, which specifies the form of edge preparation, the size of the seam and the permissible deviations. For example, manual arc welding is carried out in accordance with GOST 5264-80, contact welding in accordance with GOST 15878-79, electroslag welding in accordance with GOST 1516468, etc.
In addition, there is a standard for the graphic designation of a weld, in particular GOST 2.312-72. To do this, use an inclined line with a one-way arrow, which indicates the seam area.
The weld characteristics, recommended welding method and other information are presented above or below the horizontal shelf connected to the inclined arrow line. If the seam is visible, i.e. is on the front side, then the characteristics of the seam are given above the shelf, if invisible - below it.
The symbols of a weld also include additional symbols.
- arc welding - E, but since this type is the most common, the letter may not be indicated in the drawings;
— gas welding — G;
— electroslag welding — Ш;
- welding in an inert gas environment - I;
— explosion welding — Вз;
— plasma welding — Pl;
— resistance welding — Kt;
- friction welding - T;
- cold welding - X.
If necessary (if several welding methods are implemented), the letter designation of the welding method used is placed before the designation of one or another type:
- manual - P;
— semi-automatic — P;
- automatic - A.
— submerged arc — F;
— welding in active gas with a consumable electrode — UP;
- welding in inert gas with a consumable electrode - IP;
— welding in inert gas with a non-consumable electrode —
IN.
There are also special letter designations for welded joints:
- butt - C;
— tee — T;
- overlap - N;
- corner - U.
The numbers placed after the letters determine the number of the welded joint in accordance with GOST for welding.
Summarizing the above, we can state that the symbols of welds develop into a certain structure.
seam - seam length, sign / or Z, step; 6— for a spot weld—the size of the point; 7 - for resistance welding - point diameter,
sign / or ~Z. , step; 8—for seam welding—seam length;
9 - width and length of the seam, sign or, step; 10 - sign and leg according to the standard; 11 — conventional representation of the welding method; 12 — type of seam; 13 - connection standard
As an example, let's decipher the notation:
- the seam is located on the invisible side - the designation is under the shelf;
— T-joint, seam No. 4 according to GOST 1477176 — T4;
— welding in carbon dioxide — U;
— semi-automatic welding — P;
— leg length 6 mm — Г\ 6:
- interrupted seam with staggered sections - 50 ~Z_ 150.