We study the types of welded joints. Classification of welds and joints What types of welds are there?
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 compounds found in in various ways welding They are preferred because they are characterized by the lowest intrinsic stresses and deformations. As a rule, butt joints are used to weld structures made of sheet metal.
Rice. 78. Species 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-shape), 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. When the thickness of the parts or sheets to be joined by arc welding is significant, 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. 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 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.
Table 44
Selecting a butt joint depending on the thickness of the metal
5. Slotted, which is 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 signs 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 used 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 the ceiling seam is easier to make gas welding than arc.
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 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 polystyrene 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, i.e. 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 welds
The symbols of a weld also include additional symbols (Fig. 92).
For various types welding letter designations are accepted:
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), before the designation of a particular variety, place letter designation welding method used:
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).
Welding seams are zones of welded joints that are formed by initially molten metal that then crystallizes upon cooling.
The service life of the entire welding structure depends on the quality of the welds. Welding quality is characterized by the following geometric parameters of the weld:
- Width – the distance between its edges;
- The root is the inner part opposite its outer surface;
- Convexity - the largest protrusion from the surface of the metal being joined;
- Concavity - the greatest deflection from the surface of the metal being connected;
- A leg is one of the equal sides of a triangle inscribed in the cross section of two connected elements.
What are the types of welds and connections, classification
Table 1 shows the main types of welding joints, grouped by cross-sectional shape.
| Welded joints and seams | Location Features | Main Application | Note | |
| 1 | Butt |
The connected parts and elements are in the same plane. | Welding of sheet metal structures, tanks and pipelines. | Saving consumables and welding time, joint strength. Careful preparation of the metal and selection of electrodes. |
| 2 | Corner
|
The connected parts and elements are located at any angle relative to each other. | Welding of containers and reservoirs. | Maximum metal thickness 3 mm. |
| 3 | Overlapping
|
Parallel arrangement of parts. | Welding of sheet metal structures up to 12 mm. | Large consumption of material without careful processing. |
| 4 | T-bar (letter T)  |
The end of one element and the side of the other are at an angle | Welding of load-bearing structures. | Careful processing of vertical sheet. |
| 5 | Face
|
The side surfaces of the parts are adjacent to each other | Welding of vessels without pressure | Material savings and ease of execution |
By way of execution:
- Double-sided - welding from two opposite sides with removal of the root of the first side;
- Single-layer – performed in one “pass”, with one weld bead;
- Multilayer – the number of layers is equal to the number of “passes”. Used for large metal thicknesses.
By degree of convexity:
- Convex – reinforced;
- Concave – weakened;
- Normal - flat.
The convexity of the seam is influenced by the welding materials used, welding modes and speed, and the width of the edges.
By position in space:
- Bottom – welding is carried out at an angle of 0° – the most optimal option, high productivity and quality;
- Horizontal - welding is carried out at an angle from 0 to 60° require increased
- Vertical - welding is carried out at an angle from 60 to 120° to the welder’s qualifications;
- Ceiling - welding is carried out at an angle from 120 to 180° - the most labor-intensive, unsafe, welders undergo special training.
By length:
- Solid - the most common;
- Intermittent – leaking structure.
Types of welded joints and seams by relative position:
- Located in a straight line;
- Located along a curved line;
- Located in a circle.
In the direction of the acting force and the vector of action of external forces:
- flank - along the axis of the welded joint;
- frontal - across the axis of the welded joint;
- combined - a combination of flank and frontal;
- oblique - at a certain angle to the axis of the welded joint.
Types of welds according to the shape of the products being welded:
- on flat surfaces;
- on spherical ones.
The types of seams also depend on the thickness of the working material and the length of the joint itself:
- short – not > 25 cm, and welding is carried out using the “one pass” method;
- medium - long< 100 см – используется обратно-ступенчатый способ сварки, при этом строчка разбивается на малые отрезки длиной в 100-300 мм;
All extended seams are processed in a reverse-step manner, from the center to the edges.
Cutting edges for welding
To create a strong and high-quality weld, the edges of the joined products pass necessary preparation and they are given a certain shape (V, X, U, I, K, J, Y – shaped). To avoid burn-through, edge preparation can be done with a metal thickness of at least 3 mm.
Edge preparation procedure:
- Cleaning metal edges from rust and dirt;
- Chamfering a certain size - depending on the welding method;
- The size of the gap depends on the type of welded joints.
Edge preparation options:
Table 2 shows the features of edge preparation depending on the thickness of the metal.
Table 2
| No., no. | Metal thickness, mm | Edge cutting | Angle, α | Gap b,mm | Blunting of edges c, mm |
| 1 | 3-25 | One-sided V-shaped |
50 | – | – |
| 2 | 12-60 |
Double sided X-shaped |
60 | – | – |
| 3 | 20-60 |
Single sided, double sided U-shaped |
– | 2 | 1-2 |
| 4 | >60 | I-shape | – | – | – |
A weld is a line of molten metal at the edges of two joining structures, resulting from the action of an electric arc on the steel. The type and configuration of seams is selected individually for each case; its choice depends on factors such as the power of the equipment used, thickness and chemical composition welded alloys. This type of seam also occurs during welding. polypropylene pipes soldering iron
This article discusses the types of welds and the technology for their implementation. We will study vertical, horizontal and ceiling seams, and also learn how to clean them and check for defects.
1 Classification of welds
Classification of seams into varieties is carried out according to many factors, the main of which is the type of connection. According to this parameter, seams are divided into:
- butt seam;
- overlap seam;
- tee seam.
Let's consider each of the presented options in more detail.
1.1 Butt connection
This connection method is used when welding the end parts of pipes, square profiles and sheet metal. The connecting parts are placed so that there is a gap of 1.5-2 mm between their edges (it is advisable to fix the parts with clamps). When working with sheet metal whose thickness does not exceed 4 mm, the seam is laid only on one side; in sheets of 4-12 mm it can be either double or single, with a thickness of 12 mm or more - only double.
If the wall thickness of the parts is 4-12 mm, mechanical cleaning of the edges and sealing of the edges using one of the following methods is necessary. It is recommended to join particularly thick metal (from 12 mm) using X-shaped stripping; other options are unprofitable here due to the need for a large amount of metal to fill the resulting seam, which increases the consumption of electrodes.
However, in some cases, the welder may decide to weld thick metal in one seam, which requires filling it in several passes. Seams of this configuration are called multilayer; the technology for welding multilayer seams is shown in the image.
1.2
The lap joint is used exclusively when welding sheet metal with a thickness of 4-8 mm, while the plate is welded on both sides, which eliminates the possibility of moisture getting between the sheets and their subsequent corrosion.
The technology for making such a seam is extremely demanding in maintaining the correct angle of inclination of the electrode, which should vary in the range of 15-40 degrees. In case of deviation from the norm, the metal filling the seam will move from the joint line, which will significantly reduce the strength of the connection.
1.3 T-seam
The T-joint is made in the shape of the letter “T”; it can be made on both sides and on one side. The number of seams and the need for cutting the end part of the part depends on its thickness:
- up to 4 mm - one-sided seam without cutting the ends;
- 4-8 mm - double, without cutting;
- 4-12 mm - single with one-sided cutting;
- more than 12 mm - double-sided, double cut.
One type of T-joint is a fillet weld, used to connect two sheets of metal perpendicular or inclined towards each other.
2 Types of seams according to spatial position
In addition to classification according to the type of connection, seams are divided into varieties depending on the position in space according to which they occur:
- vertical;
- horizontal;
- ceiling
The problem with making vertical seams is the sliding of the molten metal downwards, which occurs due to gravity. Here it is necessary to use a short arc - keep the end of the electrode as close to the metal as possible. Welding vertical seams requires preliminary work - stripping and cutting, which are selected based on the type of connection and the thickness of the metal. After preparation, the parts are fixed in the required position and a rough connection is made with transverse “clamps” that prevent the workpieces from moving.
Welding a vertical seam can be done either top-down or bottom-up; in terms of ease of operation, the latter option is preferable. The electrode must be held perpendicular to the parts being joined; it is permissible to rest it on the edges of the weld crater. The movement of the electrode is selected based on the required thickness of the seam; the strongest joint is achieved when the electrode is moved transversely from side to side and with loop-shaped oscillation.
On vertical planes, horizontal type seams are laid out from left to right or from right to left. Welding horizontal seams is complicated by the pool flowing down, which requires maintaining a significant angle of inclination of the electrode - from 80 to 90 0. To prevent an influx of metal in such positions, it is necessary to move the electrode without transverse vibrations, using narrow rollers.
The speed of movement of the electrode is selected so that the center of the arc passes along the upper boundary of the seam, and the lower contour of the molten pool does not reach the upper end of the previous roller. Special attention Here it is necessary to pay attention to the upper edge, which is most susceptible to the formation of various defects. Before starting welding of the last bead, it is necessary to clean the formed seam from slag and carbon deposits.
The most difficult to perform are the ceiling seams. Because in such spatial position the molten pool is held solely by the surface tension of the metal; the seam itself must be made as narrow as possible. The standard width of the roller is no more than twice the width of the electrodes used, and in this case it is necessary to use electrodes with a diameter of up to 4 mm.
When laying a seam, the electrode must be held at an angle of 90 to 130 0 to the planes being connected. The roller is formed by oscillatory movements of the electrode from edge to edge, while in the extreme lateral position the electrode is delayed, which avoids undercuts. Please note that welders without experience are not recommended to tackle ceiling seams.
2.1 Technology for welding ceiling seams (video)
2.2 Cleaning and defect control
After the formation of a seam, slag, drops of molten steel and scale remain on the surface of the connected parts, while the seam itself may have a convex shape and protrude above the plane of the metal. These shortcomings can be eliminated by cleaning, which is carried out in stages.
Initially, you need to remove scale and slag using a hammer and chisel, then use a grinder equipped with an abrasive disc or a grinder to level the connected planes. The grain size of the abrasive wheel is selected based on the required smoothness of the surface.
Weld defects, often encountered by inexperienced specialists, are usually the result of uneven movement of the electrode or incorrectly selected strength and current. Some defects are critical, some can be corrected - in any case, monitoring the seam for their presence is mandatory.
Let's look at what defects exist and how they are checked:
Defects can also form in the form of cracks that appear during the cooling stage of the metal. Cracks come in two configurations - directed across or along the seam. Depending on the time of formation, cracks are classified into hot and cold, the latter appearing after the joint has hardened due to excessive loads that a particular type of seam cannot withstand.
Cold cracks are a critical defect that can lead to complete failure of the joint. If they form, it is necessary to re-weld the damaged areas; if there are too many of them, the seam must be cut off and re-made.
Welding still remains one of the most popular methods for producing permanent structures from metals and polymers. This popularity also determines the variety of welded joints, which are similar in some ways, but fundamentally different in others. In this article we will look at all the main types of thermal welding joints.
So, what are the types of welded joints? The types of welding joints are as follows:
Butt
The most widely used variety, which can be single- or double-sided, with a removable or non-removable lining or without it at all. A butt welding joint can be used to connect parts with a flange, with a locking edge, as well as with a variety of bevels: two- and one-sided, symmetrical and asymmetrical, broken and curved.
Angular
As the name itself makes clear, this connection welds corner structures. Besides, Using corner joints, it welds structural elements in hard-to-reach places. This type of connection is used in the following cases:
- Bevels (one-sided or two-sided) are available at the edges of the two parts being connected;
- The edges of the parts being connected do not have bevels;
- There is a flange at one edge.
In other cases, a corner connection cannot be used, since due to the complexity of the edges, the quality of the connection deteriorates sharply.
Tavrovoe
It is used for welding T-shaped structures, as well as for parts that are connected at a slight angle to each other. This connection is compatible with the following types of edges:
- There is no bevel;
- The edge can have symmetrical or asymmetrical one- and two-sided bevels;
- The edge has a curved one- or two-sided bevel located in the same plane.
The small number of edges to which a T-joint is applicable is explained by the complex geometry of the parts being connected.
Overlapping
This type of welding connects the ends of parts or structural elements. Welding work overlaps are made only with edges without bevels.
End
A rather rare type of connection, since it involves welding one part to the end of another. Therefore, often the main types of welding joints do not include the end joint as a separate item, but combine it with an overlap joint.
Classifications of seams
Also, the types of welded joints differ in the seam obtained as a result of welding work. Current standards imply several classifications:
By spatial location
According to their location, welds can be:
- Bottom, if their angle relative to the horizontal does not exceed 60 degrees;
- Vertical, if their angle relative to the horizontal is in the range of 60-120 degrees;
- Ceiling, if their angle relative to the horizontal is in the range of 120-180 degrees.
By their continuity
Welds can be continuous (without breaks) or intermittent (with breaks). The latter are most typical for corner and T-joints.
According to the nature of the ruptures, intermittent seams are divided into:
- Chain - uniform breaks, like cells in a chain;
- Chess - tears move small seams relative to each other, like white squares on a chessboard;
- Dotted seams are similar to checkerboard seams, only the seams do not look like lines, but in the form of single dots.
Note that continuous seams are more reliable and more resistant to corrosive destruction, but they are often impossible to use for technological reasons.
By type of welded joint
Welded joints also differ from each other in the resulting seam:
- Butt joint is obtained by joining parts of the same name;
- Corner is formed not only when welding parts with corners, but also during T- and butt welding;
- It is obtained through T-welding and overlapping joints of parts whose thickness does not exceed 1 cm;
- Electric riveting is obtained by welding T-joints and overlaps. The technology for making these seams is as follows. Metal parts whose thickness does not exceed 3 mm are cooked without pre-treatment, since the electric arc pierces them through. If the thickness of the parts being welded exceeds 3 mm, then one part is drilled and the second is tacked through it by welding;
- End welds are obtained by welding parts at their ends.
According to the nature of the profile section
This classification indicates the cross-sectional shape of the weld in section:
- Convex ones protrude in a semicircle above the surface of the connected parts;
- Concave form a small depression relative to the surface of the connected parts;
- Normal are one line with the surface;
- Special. They are formed when parts are joined at an angle or a tee. IN cross section they look like an equilateral triangle.
The internal cross-section determines the performance characteristics of welded joints. For example, a convex section gives good resistance to static loads; such seams are considered reinforced. While concave ones, on the contrary, are considered weakened, they are better able to withstand dynamic and multidirectional loads. The performance characteristics of normal welds are similar to those of concave welds. Special seams cope well with variable loads. They also reduce the stress that occurs in welded parts during their daily use.
According to the technology of welding work
Here, welds are classified according to the path of the electrode during welding:
- Longitudinal is formed when the electrode moves along the joint of the parts being connected;
- Transverse is obtained when the electrode moves across the joint of the parts being connected;
- An oblique is formed when the electrode moves at a certain angle relative to the extreme points of its trajectory;
- Combined is formed by alternately using the three above mentioned seams.
By number of layers
The specified welding work is carried out in one or several layers (passes). With one pass, a bead of molten metal is formed. Rollers can be performed at the same or at different levels. In the first case, one layer will consist of several rollers. The bead farthest from the facing level is called the root of the seam.
Multi-layer and multi-pass welded joints are used when welding thick-walled elements or to avoid thermal deformation in the structure of a steel alloy.
To avoid thermal deformation and burn-through, a weld seam is often used. Facing is used to improve appearance welded joint of structural elements welded to each other.
Results of violation of welding technology
If the welding technology is violated at the joint, the following may occur:
- Burns (undercuts) are zones of critical heating of the metal, in which, under the influence of high temperatures, various chemical reactions(crystalline corrosion, etc.);
- Lack of penetration - zones in which the temperature was insufficient for mutual penetration of the edges into each other and the formation of a single monolithic structure;
- Non-fusion - the edges being joined have not reached the melting temperature and have not fused to each other;
- Slag clogging - points of concentration of slag substances that have penetrated in a liquid state from low-quality electrodes into the weld pool and, upon solidification, formed foreign crystalline inclusions;
- Pores appear due to spattering metal due to sudden peak temperatures in the weld pool;
- Cracks appear due to poor-quality joining of two types of steel that have different melting points;
- Microcavities arise due to uneven heating and cooling of the metal.
Quality Control Technologies
All types of welded joints must be checked. Depending on the requirements for the quality of work, the following quality control technologies are performed:
- Visual inspection allows you to determine only visible quality defects (slag inclusions, cracks, burns, etc.);
- Length and width measurements indicate consistency of the result obtained technical specifications and GOST;
- Checking tightness using crimp testing. Used in the manufacture of various containers;
- Special instrumentation establishes the characteristics of the internal structure of the resulting welded joint;
- Laboratory studies make it possible to determine the behavior of a welded structure under the influence of various loads and chemicals.
Main types of welded joints. A welded connection is a permanent connection of parts made by welding. IN metal structures The following main types of welded joints are found:
- butt;
- overlap;
- T-bars;
- corner;
- end
A butt joint is a welded connection of two elements adjacent to each other with their end surfaces.
Lap joint - a welded joint in which the welded elements are located parallel and partially overlap each other.
Tee - a welded connection in which the end of one element adjoins at an angle and is welded to the side surface of another element.
Corner - a welded connection of two elements located at an angle and welded at the junction of their edges.
End - a welded joint in which the side surfaces of the welded elements are adjacent to each other.
Classification and designation of welds. A weld is a section of a weld joint formed as a result of crystallization of molten metal or as a result of plastic deformation during pressure welding or a combination of crystallization and deformation. Welds can be butt and fillet.
A butt weld is a weld in a butt joint. Fillet is a weld of corner, lap or T-joints (GOST 2601-84).
Welds are also divided according to their position in space (GOST 11969-79):
- lower - in a boat - L;
- semi-horizontal - Pg;
- horizontal - G;
- semi-vertical - Pv;
- vertical - B;
- semi-ceiling - Pp;
- ceiling - P.
Based on their length, the seams are divided into continuous and intermittent. Intermittent seams can be chain or staggered. In relation to the direction of the acting forces, seams are divided into:
- longitudinal;
- transverse;
- combined;
- oblique.
Depending on the shape of the outer surface, butt seams can be made normal (flat), convex or concave. Connections formed by convex seams work better under static loads. However, excessive overflow leads to unnecessary consumption of electrode metal and therefore convex welds are uneconomical. Flat and concave welds work better under dynamic and alternating loads, since there is no sharp transition from the base metal to the weld. Otherwise, a stress concentration is created, which can lead to the destruction of the welded joint.
According to the operating conditions of the welded unit during the operation of the product, welds are divided into working welds, which directly bear the load, and connecting (binders), intended only for fastening parts or parts of the product. Bonding seams are more often called non-working seams. When manufacturing critical products, the bulge on the working seams is removed with electric sanders, special cutters or the flame of an argon arc burner (smoothing).
The main types, structural elements, dimensions and conditions for designating seams of welded joints for manual electric arc welding of carbon and low-alloy steels are regulated by GOST 5264-80.
Structural elements of welded joints. The shape of the edges and their assembly for welding is characterized by three main structural elements: the gap, the blunting of the edges, and the bevel angle of the edge.
The type and angle of the edge groove determine the amount of electrode metal required to fill the groove, and therefore the welding performance. X-shaped cutting of edges, compared to V-shaped, makes it possible to reduce the volume of deposited metal by 1.6-1.7 times. In addition, such cutting provides less deformation after welding. For X-shaped and V-shaped grooves, the edges are blunted to ensure proper formation of the seam and prevent the formation of burns.
The gap during assembly for welding is determined by the thickness of the metals being welded, the grade of material, the welding method, the form of edge preparation, etc. For example, the minimum gap value is prescribed when welding without filler metal of small thicknesses (up to 2 mm) or when arc welding with a non-consumable electrode aluminum alloys. When welding with a consumable electrode, the gap is usually 0-5 mm; increasing the gap promotes deeper penetration of the metal.
The seam of a welded joint is characterized by the main structural elements in accordance with GOST 2601-84: width; convexity; depth of penetration (for a butt weld) and leg for a fillet weld; thickness of the part.
The main elements of the weld are shown in Fig. 1.
Rice. 1. : a - fillet weld; b - butt seam
Technological strength of the weld. The term “Technological strength” is used to characterize the strength of a structure during its manufacturing process. In welded structures, technological strength is limited mainly by the strength of the welds. This is one of the important indicators of steel weldability.
Technological strength is assessed by the formation of hot and cold cracks.
Hot cracks are brittle intercrystalline fractures of the weld metal and heat-affected zone. They appear in a solid-liquid state at the final stage of primary crystallization, as well as in a solid state at high temperatures at the stage of predominant development of intergranular deformation.
The presence of a temperature-time interval of brittleness is the first reason for the formation of hot cracks. The temperature-time interval is determined by the formation of liquid and semi-liquid layers that violate the metallic continuity of the weld. These layers are formed in the presence of low-melting, sulfur compounds (sulfides) FeS with a melting point of 1189 °C and NiS with a melting point of 810 °C. At the peak moment of development of welding stresses, metal shifts along these liquid layers and develops into brittle cracks.
The second reason for the formation of hot cracks is high-temperature deformation. They develop as a result of difficult shrinkage of the weld metal, changes in the shape of the workpieces being welded, as well as during relaxation of welding stresses under nonequilibrium welding conditions and during post-weld heat treatment, structural and mechanical strain concentration.
Cold cracks. Cold cracks are those that form during the cooling process after welding at a temperature of 150 °C or over the next few days. They have a shiny crystalline fracture without traces of high temperature oxidation.
The main factors causing the appearance of cold cracks:
- the formation of hardening structures (martensite and bainite) leads to the appearance of additional stresses due to the volumetric effect;
- exposure to welding tensile stresses;
- concentration of diffusion hydrogen.
Hydrogen moves easily in unquenched structures. In martensite, the diffusion capacity of hydrogen decreases, it accumulates in the microvoids of martensite, transforms into a molecular form and gradually develops high pressure, which promotes the formation of cold cracks. In addition, hydrogen adsorbed on the metal surface and in microvoids causes embrittlement of the metal.
Weldability- the property of a metal and a combination of metals to form, with the established welding technology, a connection that meets the requirements determined by the design and operation of the product. The complexity of the concept of weldability of materials is explained by the fact that when assessing weldability, the relationship of welding materials, metals and product design with welding technologies must be taken into account.
There are many indicators of weldability. An indicator of the weldability of alloy steels, intended, for example, for the manufacture of chemical equipment, is the ability to obtain a welding joint that provides special properties - corrosion resistance, strength at high or low temperatures.
When welding dissimilar metals, an indicator of weldability is the possibility of formation of interatomic bonds in the joint. Homogeneous metals are joined by welding without difficulty, while some pairs of dissimilar metals do not form interatomic bonds at all in the connection, for example, copper cannot be welded with lead, or titanium with carbon steel.
An important indicator of the weldability of metals is the absence of hardened areas, cracks and other defects in welded joints that negatively affect the operation of the welded joint.
There is no single indicator of the weldability of metals yet.
