What zones does welded joints include?

Structure of the heat affected zone during welding – Osvarke.Net

Different parts of the welded joint have different microstructures. Conventionally, it can be divided into three parts:

1. base metal;
2. heat affected zone;
3. weld seam

The heat-affected zone is the part of the base metal adjacent to the weld that has not melted, but its structure and properties change under the influence of heat during welding.

Rice. 1. Structure and areas of the thermally affected zone

According to the degree of influence of high temperatures on the metal, the thermally affected zone is divided into sections: a section of incomplete melting, a section of overheating, a section of normalization, a section of incomplete crystallization, a section of recrystallization and a section of blue brittleness.

The area of ​​incomplete melting is transitional from the weld metal to the base metal. This area is heated above the melting point and is in a solid-liquid state. In this area, the crystals of the weld metal fuse with the base metal, so the quality of the welded joint largely depends on the properties of this area. For connections made by arc welding, this zone is 0.1-0.5 mm.

The overheating area is a zone of significantly overheated base metal (1100-1500 °C) with a coarse-grained structure. This area is characterized by a decrease in the physical properties of plasticity and impact strength. In compounds with a high carbon content, hardening structures can form in this zone. The area size can reach 3-4 mm. To reduce this size, you should increase the welding speed or perform the connection in several passes.

The normalization section is the base metal heated from 930 to 1100 °C. The metal does not remain heated to this temperature for long and during the process of recrystallization it forms a fine-grained structure of the metal. The mechanical properties of the section are increased in comparison with the state before welding. The length of the section is from 0.2 to 4-5 mm

The area of ​​incomplete recrystallization is an area heated to 720-850 °C. This area is characterized by an incomplete change in the structure of the metal. Around the ferrite grains in this area there are small grains of ferrite and pearlite formed during the recrystallization process. As the name suggests, in this area the metal has not undergone complete recrystallization. The size of the area is from 0.1 to 0.5 mm, depending on the modes and type of welding.

The recrystallization area is the area of ​​metal heated to 450-720 °C. This area can be observed when welding steels subjected to plastic deformation (when welding rolled products). In this area, restoration of grains destroyed during deformation is observed. The area size is from 0.1 to 1.5 mm.

The last section of blue brittleness lies in the temperature range from 200 to 450 °C. Blue tarnish colors can be seen in the area. There are no structural changes in this area, but it is characterized by a decrease in plastic deformations.

Heat affected zone dimensions

The width of the heat-affected zone depends on the selected welding method and parameters of the welding mode:

• for manual arc welding - 3-6 mm;
• when welding under submerged arcs - 2-4 mm;
• when welding in shielding gases - 1-3 mm;
• for gas welding - 8-28 mm;
• for electroslag welding - 11-14 mm.

Increasing the welding speed and decreasing the current leads to a decrease in the size of the heat-affected zone.

Improving the properties and structure of the heat-affected zone

To improve the structure and properties of the weld metal and heat-affected zone, hot weld forging, general heat treatment and slow cooling are used.

To prevent the formation of hardening structures when welding medium- and high-carbon steels, preliminary and accompanying heating is used, and after welding they are slowly cooled.

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Source: http://osvarke.net/soedineniya/zona-termicheskogo-vliyaniya/

Classification of welding joints

Welding of metals is used in cases where it is necessary to obtain the most durable, tight and reliable permanent connections. Welding technologies provided the impetus for the rapid development of technological progress.

Many designs simply could not be created without their use. There are various welding technologies based on the use of electric or gas welding processes, as well as types of welding joints, the classification of which is quite simple.

Welding types

Almost the entire variety of welding technologies used can be classified into one of the following types:

Electric arc welding involves applying a voltage between the workpiece to be joined and the welding electrode, causing the ignition of an electric arc.

The high temperature that occurs when the arc burns leads to the melting of areas of the parts being connected that are directly adjacent to the place of future mating.

At the site of the future seam, a so-called weld pool is formed, that is, molten metal, after crystallization of which a reliable and durable weld is formed.

According to the level of use of process automation, electric arc welding can be manual, using a piece replaceable welding electrode, semi-automatic, with an endless, continuously supplied wire electrode, and automatic, carried out without the participation of a welder.

In addition to the above, the electric arc welding process can be atmospheric or in an environment of protective gases, which prevent the oxidation of the molten metal and contribute to the formation of a higher-quality weld.

In the case of electric resistance welding, the elements being connected are compressed with great force, and a significant electric current is passed through the contact point.

As a result, the metal parts being connected are heated to a plastic state, and under the influence of compressive force they are welded together.

The gas welding process occurs due to the melting of the metals being joined, as well as the filler material when burning gas using special gas-flame equipment.

How the parts are connected

The technology for the production of welding joints, including their types, dimensions of the main elements and their symbols in the drawings, is established by GOST 5264-80. To correctly read a drawing intended for welding a structure, you need to familiarize yourself with this standard.

In accordance with GOST, when performing welding work, the following types of connections can be used:

• butt;
• corner;
• T-bars;
• overlap

It is worth talking about each of them separately, since the choice of welding joint and preparation of the edges of the workpieces directly affect the quality of the seam.

Butt

This welding joint is characterized by the adjacency of the side surfaces of the parts being welded, located in the same plane.

There are varieties of this welding operation. The work can be performed without preparing the surfaces to be joined. When welding relatively thin sheet material, its edges can be pre-flanged, that is, bent at an angle of 90 °C.

For thicker workpieces, to ensure complete penetration of the material through thickness, bevel the edges on one or both sides. The bevel shape of the edges can be straight or curved.

To hold the weld pool in the seam area, a flat lining is sometimes placed under the sheet blanks being welded, which is removed after the work is completed.

The seam itself when making a butt joint can be one-sided or two-sided.

Corner

This connection is applied to parts that are not in the same plane, the edges of which are located at a certain angle relative to each other. Such connections are also made with or without preliminary surface preparation.

Preparation consists of beveling the edges to be joined in different ways; one of the planes can be beaded. Welds, depending on design requirements, are single-sided or double-sided.

T-bar

With a T-joint, the edge of one of the parts being welded is attached to the surface of another part at an angle of 90°. Thus, the cross-section of the connection is shaped like the letter “T”.

To improve the quality of the connection, one-sided or two-sided bevels of the edges of the attached element are used. Those parts are processed whose ends are welded to the planes of other parts.

For better welding of the metal, when fastening parts, usually ensure that there is a small gap between them. The gap size is 2 – 3 mm.

In general, for each specific procedure, a flow chart of the operation must be drawn up, taking into account all the requirements of the design of the assembled structure.

overlap

Parts connected with an overlap are superimposed on one another, while being in parallel planes. Seams are made on one or both sides.

In this case, it makes no sense to bevel the edges, since the end sections and planes of the parts to be joined form a concave angle that holds the weld pool well and makes it possible to make a strong seam.

Other classifications

Depending on the length of the seam, welding joints can be intermittent (stitches) or continuous. The latter are used to obtain a sealed structure (pipes, various containers).

By changing the arc length, welding speed, and groove depth, you can obtain convex and concave welding joints. There is also a normal option, when the connection is almost flat, and the seam does not protrude above the surface, but we also do not form a recess.

Welding seams are obtained at different positions of the workpiece. Depending on this, they can be bottom (the simplest connection), horizontal, vertical and ceiling (the most complex version of welding work).

Calculations

When designing various structures, all technical parameters, including the use of a certain type of welding, as well as the choice of method for connecting parts, are carried out on the basis of preliminary calculations. First of all, the structure is calculated for strength .

The initial data for this is the modeling of the loads to which the structure will be subjected during operation. Based on this, the material and method of connecting individual elements are selected. Calculation methods for each type of welded joint are standardized and unified.

When carrying out calculations for the strength of welds, the nature, directions and magnitudes of loads on the joint areas are determined. The obtained values ​​are compared with the maximum permissible values ​​for the materials used.

Based on their comparison, a conclusion is made about the safety margin of the structure. This calculation is made many times, separately for each connection option, type of welding used and structural material. Only with correct calculation can a reliable connection be obtained.

Source: https://svaring.com/welding/teorija/svarochnye-soedinenija

Welded joints - types and elements | Web Mechanic

Good afternoon dear friends. Today we will look at the topic “Welded joints - main types and elements.” Welded connections, unlike threaded ones, are permanent connections between sections of structures or products.

Welded joints include three zones that are formed during welding:

• weld zone,
• fusion zone,
• heat affected zone

In addition, welded joints also include that part of the metal that is adjacent to the heat-affected zone.

This is a short digression :), and now:

Welded joints - main types and elements

Depending on the relative position of the elements being connected, the following types of welded joints are used.

Butt welded joints

The simplest and most reliable of all welded joints, they are recommended in structures exposed to alternating stresses. In Fig. 1. a-d show various options for butt welds made by manual arc welding with different thicknesses of the elements being connected. During automatic welding, deeper penetration of the metal occurs; the seam is formed mainly by the base metal, and not by the metal of the electrode as in manual welding.

Rice. 1 Butt welded joints: a - single-sided without beveled edges: b - single-sided with beveled edges: c - double-sided with two symmetrical bevels of one edge; g - double-sided with two symmetrical bevels of two edges

https://www.youtube.com/watch?v=qBf24cIxYuU

The elevation of the butt weld above the base metal is a stress concentrator. Therefore, in critical connections it is removed mechanically.

Lap welded joints

Lap welded joints are shown in Fig. 2.1 a-c.

Rice. 2.1 Lap welded joints using fillet welds

They are made using fillet welds with different cross-sectional shapes:

• normal (Fig. 2.2.a), the profile of which is an isosceles triangle;
• concave (Fig. 2.2. b) are used in critical structures under variable loads, since the concavity ensures a smooth transition, as a result of which stress concentration is reduced. A concave profile is obtained by subsequent mechanical processing of the seam, which increases the cost of the connection;
• convex (Fig. 2.2. c) - irrational, as they cause increased stress concentration;
• special ones (Fig. 2.2. d), the profile of which is an unequal right triangle, are used for variable loads. The leg of the seam k- is taken to be the leg of an isosceles triangle inscribed in the seam section (see Fig. 2.2 b). In most cases, the value of k is taken equal to the thickness of 5 parts being welded, but not less than 3 mm.

Rice. 2.2 Fillet welds

Corner seams are:

• frontal, located perpendicular to the line of action of force F (see Fig. 2.1. a):
• flank, located parallel to the line of action of force F (see Fig. 2.1. b);
•  combined, consisting of a combination of frontal and flank seams (see Fig. 2.1. c).

In lap joints, bending coin M = Fδ occurs (see Fig. 2.1. a) from the eccentric action of tensile or compressive forces, which is a disadvantage of joints.

T-weld joints

In them, the elements to be welded are located in mutually perpendicular planes. The connection can be made with corner (Fig. 3. a) or butt (Fig. 3. b) seams.

Rice. 3. T-welded joints

Source: http://web-mechanic.ru/tehnich-svedeniya/konstruktivnye-elementy/svarnye-soedineniya-tipy-elementy.html

Types and classifications of welded joints and seams

A weld is a section where two parts are joined into a single whole due to the melting of the metal under the influence of high temperature and its further crystallization. Today, more than 100 types of connections are distinguished. They are all divided according to special parameters and divided into various groups and subgroups, and therefore there are many classifications of welds.

According to the type of welded joint

The classification of welds according to the type of welded joint is divided into butt and corner. The master decides which connection to make in a given situation, based on the position of the parts in space.

• Corner welds are made when the workpieces are at an angle to each other.
• Welding of butt joints is formed as a result of the adjacency of two parts or parts with their ends facing each other, which are located on the same plane. The track itself can be of three types - concave, convex or flat. The latter is used most often, since it does not have a particularly pronounced transition at the junction of parts, which looks more natural in comparison with the other two types. This method is most often used when electric arc welding at low currents, so as not to scorch the workpiece. For example, sheet steel is an ideal material for butt welding applications.
• Slotted (electric rivet) is made in the hole that is on the part and is made in the form of spot rivets. That is, in this case, a weld pool and a seam are not formed as a result, and the parts are soldered in small sections through grooves in the workpiece.

At the place of welding

The classification of welded joints and seams in this category depends on the position of the welded parts in space. For example, if you need to repair a part of some kind of structure that cannot be removed and put down, but it is located at some distance from the floor, then the master will carry out the work using a ceiling, bottom, horizontal or vertical connection, starting from the placement of this part.

• Horizontal are welds that extend from left to right (or vice versa) on a vertical part. To prevent the mass of metal from flowing down, it is necessary to correctly select the speed of movement of the electrode or torch and the current strength (this is selected for each case individually, based on the type of welding, the characteristics of the parts and the skill of the specialist).
• The vertical method of producing butt welds is carried out on vertically positioned workpieces, with the seams being made from top to bottom (or vice versa). The complexity of this process lies in the fact that the force of gravity of the Earth is triggered and the molten metal mass flows down all the time, which spoils both the quality and appearance of the part. Such connections are recommended to be carried out in extreme cases and only to those craftsmen who already have a certain theoretical and practical knowledge for working with such paths. More information about vertical seam technology can be found here.
• Ceiling is a position in which the part is located above the master’s head, which greatly complicates the process. When making ceiling welds, you must strictly follow safety rules and welding technology, because in this case the danger lies in the flow of molten metal.
• Lower welding methods are performed when the part is located below in relation to the master. This is the most convenient connection method, since the metal does not spread to the sides or down, but flows into the crater. In addition, gases and slags freely escape to the surface. A butt welded joint in the lower position is performed by forming beads throughout the entire joint of the parts. At the same time, the welding technology is simple - it is enough to move the electrode or torch straight or in a zigzag to create a reliable and aesthetically attractive path.

By configuration

This category of butt welds is used in manual arc welding with an electrode. This includes three types of welds - straight, curved and annular (spiral). They are produced regardless of the position of the work product. All types of seams of this classification require both butt and lap welded joints.

By length

Classification of welds by length is of two types: continuous or intermittent.

• Intermittent is a seam that is made of a certain length with a synchronous interval. It, in turn, is divided into two types - chain track and checkerboard seam. For example, double-sided intermittent joints on one side of the wall are located opposite the welded sections of the seam on the other side. These types of coupling can be either one-way or two-way. That is, the part is soldered on both sides. The distance between these welded sections is called the “welding pitch”.
• Continuous welding methods are also divided into short and long tracks, and are performed along the entire workpiece.
• The spot method of butt welds differs significantly from others, due to the fact that there is no weld pool and track. In this case, the workpieces are connected at points using an overlap weld. This method is often used for soldering thin metal or batteries.

Methods of extended seams: a) continuous b) intermittent, c) point, d) intermittent checkerboard, e) intermittent continuous (chain)

According to execution technology

Depending on the technology used for fastening, there are four main types:

• The underwelding, where - the smaller part of the double-sided seam, is performed in advance to prevent burn-throughs during subsequent welding;
• a tack weld allows you to fix parts that are already positioned for welding;
• a temporary seam is necessary to hold the workpieces together for a while, and upon completion of the work it is removed.
• installation weld, used during installation of various structures.

In relation to the direction of current efforts

Butt welding contains another important classification, depending on the relationship to the direction of force:

• Longitudinal method of creating a joint (flank), in which the force acts parallel to the axis of the track;
• Transverse method (frontal) of the weld, in which its axis is perpendicular (90 degrees) to the axis of force;
• A combined welding connection includes both flank and transverse types;
• Oblique, in which the axis of the seam is located at an angle to the direction of the acting forces.

According to the shape of the outer surface

According to the shape of the clutch surface, they are divided into three main types:

• Convex (reinforced) are multi-layer seams used in clutches under static loads, but increased influx leads to excessive consumption of electrode metal and therefore an economic justification is required for its use.
• Concave (loose) methods are used to hold thin metal together.
• Normal or flat are relevant for dynamic loads, since they do not have a special difference between the track and the base metal.

By type of welding

The classification of welds by type of welding is divided depending on the type of impact of the welding machine. For example, when working in an environment of argon or other protective gas, the connection will be nothing other than “gas”; when working with an electrode, it will be “electric arc”. The most basic types are the following seams:

• manual arc welding - butt or lap joints are made manually using an electrode. Thus, it is possible to fasten almost any metal with a thickness from 0.1 to 100 mm in any position;
• automatic welding, which is carried out when working with a device - a transformer, rectifier or inverter;
• welding in inert gas. Such butt, corner and lap joints are considered the most durable, since welding occurs in an environment of inert gases, which protect it from oxidation. The big advantage of such fastening is the aesthetic appearance and the absence of waste and slag;
• gas welding - the track is formed under the influence of temperature, which is created due to the combustion of the working gas emanating from the torch;
• solder connections that are made using a soldering iron.

In addition to those described, there are many more methods for connecting parts, both conventional and non-standard, which are used for welding parts in hard-to-reach places. For example, seams can be single-layer (a) or multi-layer (b, c), in which several rollers are applied, located at the same level of the cross-section of the seam.

Source: https://svarkaed.ru/svarka/shvy-i-soedineniya/vidy-i-klassifikatsii-svarnyh-soedinenij-i-shvov.html

Heat Affected Zone

The heat-affected zone (HAZ) is the area in the weld area. During the welding process, the metal in this place experiences different thermal loads, which affects the change in the structure of the alloy.

In the heat-affected area, the influence of heating is manifested by internal stresses and cracks. The strength of the connection decreases. Although the metal in the HAZ does not completely melt, it is heated to critical temperatures.

The structure and physical properties of the alloy change in the heating region. This affects the strength of the welded joint.

Properties

Throughout the heat-affected zone, the properties of the metal change. They are determined by the thermoplastic cycle and depend on the locality of heating. Under the influence of temperature, graininess is formed. The longer the alloy is heated to the phase transition temperature, the larger the grains. The indicators of impact strength and ductility change. These are the basic physical properties of metal products.

How does the width of the heat-affected zone change with increasing welding speed?

The faster the part heats up and cools down, the smaller the HAZ. As the current decreases, the effect of temperature is reduced and the size of the HAZ decreases.

Structure and dimensions of the thermally affected zone

Based on the concept of a heat-affected zone (this is a heated area), it is easy to assume that the part heats up at different distances from the seam. For clarity, let’s imagine a section of the heat-affected welding zone of low-carbon steel.

Structure of the thermally affected zone

The scheme of structural changes in the thermally affected zone is divided into several sections:

1 – incomplete melt. It is transitional, the metal is in a state of diffusion of the surfacing and the base alloy, two phases are combined - liquid and solid. The length of the area is small, from 100 to 500 microns. At a temperature of 1500°C, the formation of large grains begins.

2 – overheating (length 3–4 mm), large grains are formed in the alloy, characteristic of the quenching process, s-iron transforms into y-iron. The impact strength and ductility of steel are reduced. The temperature gradually drops from 1500 °C to 1100 °C.

Source: https://svarkaprosto.ru/tehnologii/zona-termicheskogo-vliyaniya

Welded joints: all types, detailed description

To make a competent and good connection of metals, it is necessary to use welding work. This can only be done by a trained professional who knows all the nuances of cooking. Thanks to the welding seam, you can join not only metals, but also other materials. All elements that have been joined into an integral unit represent a connection that can be delimited into several zones.

These are the connections that are made during the welding process. They are divided into several zones:

1. Rafting place. This is the name given to the boundary between the base material and the metal of the resulting weld. It is in this place that there will be grains that will differ in their structure from the state of the main type of material. This occurs because there is partial melting of the material during welding.
2. Area of ​​thermal influence. This is the name of the zone of the base material that does not undergo melting, although the heating process has occurred and the structure has changed.
3. Weld. This is the area that will form during the crystallization process. All this happens when the metal begins to cool.

Types of welds and joints

Differences in welded joints are explained by the fact that the worker uses non-identical arrangements of the joined parts relative to each other.

By location

1. Butt. The joining of elements will be carried out on the same plane with their ends facing each other. Materials can have different thicknesses, and therefore the ends being connected relative to each other can move vertically.
2. Corner connection. In this option, the ends will shift at a certain angle. The welding process is carried out on all edges of the parts that will be adjacent to each other.
3. Overlapping connection.

   The parts to be welded are arranged parallel and partially overlap each other.

4. End connection. Several parts of the elements that need to be welded will be combined parallel to each other, and then they will be joined at the ends.
5. T-joint. With this option, the end of the part adjoins the side of another element at a certain angle.

   The types of welding joints will also depend on the type of welding seams, which are qualified according to some basic characteristics.

By method of execution

1. One-sided seam. It can be performed by completely melting the metal along the entire length of the structure.
2. Bilateral. First, you need to perform one-sided welding, remove the root, and only then proceed to welding work on the other side of the material being processed.
3. Single layer.

   This type is usually performed using one pass welding, resulting in one weld bead.

4. Multilayer. The use of this type is usually determined by the large thickness of the metal, when welding in one pass is impossible for various reasons. The seam layer consists of several rolls or passes.

   Thus, it is possible to limit the spread of thermal effects. The result is a very high-quality and durable welded joint.

By spatial position

There are several welding positions:

1. Bottom position. The seam will be in the lower horizontal plane, this is an angle of 0 degrees relative to the earth's surface. Horizontal position. The roller will be guided horizontally, and the part can be positioned at an angle from 0 to 60 degrees.
2. Vertical. In such a situation, the surface that is being welded will be located in a plane from 60 to 120 degrees, and the welding itself will be carried out in the vertical direction.
3. Ceiling position. All work will take place at an angle of 120 or 180 degrees. This means that the weld is positioned above the welder.
4. Boat position. This situation is explained by the fact that it is necessary to weld a corner or T-shaped surface. The parts will be placed at a certain angle, and welding will take place in a corner.

By length

It is possible to produce a continuous seam . Typically, these are used in production when a high-quality and strong connection is needed. But there are also exceptions.

The second option is an intermittent seam, which is usually used in corner joints. This type of seam can be used if it is necessary to stagger some parts to each other. This type of connection is also made if a chain welding procedure is required.

Weld seam index

There are several basic parameters that characterize all the resulting seams:

1. Width. This is the size that is set between the seam boundaries, which are drawn with visible fusion lines.
2. Root. This will be the second side, located away from the front part of the structure.
3. Convex. You can notice it in the most convex part of the seam. This parameter indicates the distance from the boundary of the largest protrusion to the plane of the base metal.
4. Leg. This parameter is observed only in T-joints or corner joints. This indicator can be measured by the smallest distance from the surface on the side of one of the parts to the boundary lines that are on the surface of the second part.

This design feature will be used in situations where the metal thickness is more than 7 mm . Edging means removing pieces of metal from an edge in a specific shape. This process must be performed when welding single-pass butt seams.

This is necessary in order to get the correct connection. If there is thick material, then cutting must be carried out in order to melt the root passage, and then use guide rollers to evenly fill the cavity. This way the metal will be welded throughout its entire thickness.

Edge cutting is also performed if the metal thickness is more than 3 mm. If the value is lower, then you can burn through the metal.

The cutting is characterized by several design parameters:

• gap;
• cutting angle;
• dullness

To see all these parameters, you need to study the drawing. If you cut edges, the amount of consumables will increase. That is why they try to minimize this value as efficiently as possible.

It will be divided into several types of design:

1. V-shaped.
2. X-shaped.
3. Y-shaped.
4. U-shaped.
5. Slit.

Peculiarities

1. If there is a small thickness of the material, which ranges from 3 to 25 mm, then it is necessary to use a one-sided V-shaped groove. The bevel can be made on 2 ends or only on one.
2. If the metal has a thickness of 12-60 mm, then it is best to weld with a double-sided X-shaped groove.
3. For a thickness of 20−60 mm, it is advisable to use the metal consumption for U-shaped cutting.

   This will be much more economical. The bevel can be made along two or one ends. Then the blunting will be 1 or 2 mm, and the gap value will be two millimeters.

4. If there is a large thickness of metal, then the most effective method is slot cutting.

To produce a high-quality welded joint, it is necessary to choose the right procedure, since all this will influence several factors of the weld:

1. Performance.
2. Strength and quality of connection.
3. Economical.

1. Arc welding. Welded seams and connections in accordance with GOST 5264−80 will include types, design dimensions for welding, which are covered with electrodes in any spatial positions. This will not include pipelines made of steel.
2. Connection of steel pipelines. GOST 16037–80 is used, which will determine the main type, edge cutting, design size for the mechanized joining method.
3. Connection of copper and copper-nickel alloy pipeline. GOST 16038–80 is provided.
4. Aluminum arc welding. GOST 14806–80 applies. Shapes, sizes, edge preparation for welding aluminum and alloys, the process takes place exclusively in a protective environment.
5. Flux. GOST 8713–19. All seams will be performed using automatic or mechanized floating welding using a flux pad. Suitable for metals from 1.5 to 160 mm.
6. Aluminum in inert gases. GOST 27580–88. This is the standard for semi-automatic, manual or automatic welding. It must be done with a non-consumable electrode in inert gases, where there is filler material and all this is distributed if the aluminum has a thickness of 0.8 to 60 mm.

Designation of welding seams

There are special regulatory documents that indicate the name of welding seams in drawings or in general terms.

If the seams are visible, they are indicated by a solid line. And if they are not visible, then with a hatched line. Special callouts with arrows will be drawn from the line.

The designation of the weld will be made on a special shelf for the leader. The inscription must be made exactly above the shelf if the connection will be on the front side of the part. If there is a reverse option, then the designation is located under the shelf. Here you will need to include information about the seam in a certain sequence:

1. Auxiliary symbols.
2. Designation of a seam, structural element and GOST connection.
3. The name of the seam according to a certain standard.
4. Method of connecting parts.
5. If there is a corner connection, then the leg is indicated in this place.
6. Seam discontinuity, if any. Here you need to indicate the location of the welding segment, as well as the pitch.
7. Additional signs that have auxiliary meaning.

Auxiliary signs

Such signs must be applied on top of the shelf, if the seam in the drawing is visible, and below it, if it is invisible:

1. Removing seam reinforcement.
2. When processing parts that provide a smooth transition to the main type of material, it is necessary to eliminate sagging and irregularities.
3. The seam must be made along an open line; such a sign will be used if it is visible in the drawing.
4. Cleanliness of the joint surface.

If each connection is made according to only one GOST, has identical grooves, as well as design dimensions and designations, then the welding standards will be included in the technical requirements.

It is not necessary to indicate all the same seams in the design, but they must be divided into groups and assigned a serial number. The full designation must be indicated on one seam. For the rest, you can only put a serial number.

It is not necessary to indicate in the regulatory document the exact number of groups, as well as the number of seams.

https://www.youtube.com/watch?v=ZbnEIr5ITFc

As you can see , there are a lot of nuances in welding work . A true professional must clearly understand all the features of welded joints, as well as know all the nuances of welding in order to competently carry out his work. All necessary information will be indicated on the drawing, which the welder must also be able to read.

Source: https://tokar.guru/svarka/raznovidnosti-svarnyh-soedineniy.html

Critical welded joint: calculation required

Andrey Alekhin, Vadim Shelofast

About welded joints

Main types of welded joints

Weld joint model

Selection of permissible stresses when calculating the static strength of welded joints

Calculation of welded joints under time-varying loads

Stress concentration in welds

conclusions

About welded joints

Compared to other types of permanent connections, welded connections are currently the most common - this is due to the fact that they are the most durable, technologically advanced and economical. The use of welded structures, for example, instead of foundry ones, makes it possible to reduce their weight by more than 30%.

However, welded joints have a number of significant disadvantages:

• heating the seam during welding changes the mechanical properties of the base metal towards their deterioration;
• non-uniform heating leads to the occurrence of residual stresses and, as a consequence, to residual deformations;
• in the weld there is a significant anisotropy of the properties of the metal;
• local stresses arise in the area of ​​the weld, significantly affecting its strength, especially under conditions of variable loading;
• high stress concentration and other unfavorable factors make welded joints short-lived under variable external load, especially under shock loading conditions;
• quality control of a weld is quite complex and not always economically justified.

Main types of welded joints

Depending on the type of relative position of the parts being welded, welded joints can be butt, lap, tee, corner and spot. This article will discuss the calculation of seam welded joints.

Traditional methods for calculating and designing a welded joint under constant external load depend on the type of joint, welding method and type of seam and are implemented in the calculation and design module of APM Joint of the APM WinMachine system. In addition to the general assumptions characteristic of strength calculations in general, when constructing models of welded joints to perform approximate engineering calculations, some additional assumptions are made that are specific to this type of joint:

• the parts to be welded are considered non-deformable, and the welded seams, on the contrary, are considered pliable;
• stress concentrations, the presence of which is typical for places with a sharp change in shape, are not taken into account, and the calculation is performed only based on nominal stresses;
• the weld material is considered homogeneous and isotropic;
• deformations are considered small and proportional to stress;
• sections that were flat before the onset of deformation retain their shape.

The calculation of joints in the APM Joint system has already been discussed in more detail on the pages of the CAD and Graphics magazine.

Determination of stress distribution along a weld is based on the principle of superposition, or independent action of forces. Errors arising from the models used should be taken into account by introducing safety factors.

A significant portion of the error is introduced by the assumption of the absolute rigidity of the parts being connected and the compliance of the welds. This especially affects the design of welds when welding thin parts.

Certain assumptions can be abandoned if FEM is used as a tool for performing calculations.

The APM WinMachine computer-aided design system includes all the necessary software for analyzing the strength of a welded joint using the finite element method: the APM Studio 3D graphic editor and the APM Structure3D finite element analysis system.

Weld joint model

The model is prepared in the APM Studio 3D graphic editor. Creating solid parts and assemblies is possible both directly in APM Studio and by importing from third-party graphic editors through the STEP format.

The weld is modeled as a separate solid part within the assembly. This approach makes it possible to take into account the preparation of edges to analyze their influence on the stress-strain state of the connection.

Examples of assemblies of welded joints of the main types are presented in Fig. 1-3.

Rice. 1. Assembly model of a welded assembly of a lap joint between an eyelet and a flat plate

Rice. 2. Assembly model of a pipe T-joint with a flat surface

Rice. 3. Model of a welded joint made with a fillet weld

For finite element analysis of an assembly, you first need to define all matching faces. Subsequently, the calculation will take into account the joint movements of the coinciding faces of the weld and the corresponding mating surfaces. Preparing the assembly model for calculation also includes specifying fastenings and loads.

APM Studio's finite element analysis mode allows you to specify both restraints and loads directly in the editor. To specify the main types of load: uniformly distributed load over a surface (pressure), uniformly distributed load along an edge, variable load along an edge - APM Studio has the necessary tools.

The initial load values ​​can be obtained based on the strength calculation of the entire rod or plate structure model in the APM Structure3D system.

After preparing the assembly model for calculation, it is necessary to generate a finite element mesh. An automatic “improver” is built into the finite element mesh generator, ensuring that equilateral tetrahedra are predominantly used as finite elements, which are considered the most optimal in terms of minimizing the error in calculating the stress-strain state.

It is necessary to select the partition step (side of the tetrahedron) so that it is approximately 3-5 times smaller than the leg of the weld. This division makes it possible to take into account stress concentrators in the weld. It should be noted that the maximum dimension of the problem solved in APM Structure3D is determined mainly by the hardware capabilities of the computer and is approximately 1350 thousand nodes, which is quite enough to simulate almost all possible node connections.

After generating the finite element mesh, the model can be transferred to the APM Structure3D finite element analysis system.

APM Structure3D allows you to analyze mating parts both taking into account the mutual penetration of parts (contact problem), and without taking them into account. To calculate a welded joint, solving the contact problem is not required, since the welded structure perceives the load as a single whole.

Let's consider how the above-described assumptions of traditional calculation methods can be removed when using FEM:

• the compliance of both welds and welded parts is taken into account, which allows for joint calculation of thin-walled welded parts;
• Based on the stress map, it is possible to determine zones of stress concentration in places of sharp changes in shape, as well as numerical values ​​of maximum stresses in these zones. Complex asymmetrical loading significantly affects stress raisers. By making changes to the shape of the mating parts and the weld, it is possible to reduce stress concentrations and obtain a structure that is close to equal strength;
• it is possible to set different physical and mechanical properties for different sections of the weld (taking into account heterogeneity);
• deformation calculation allows you to take into account both the movement of structural elements and the change in shape under the influence of loading.

Selection of permissible stresses when calculating the static strength of welded joints

Calculating the static strength of welded joints is not much different from calculating the strength of parts in general. Its features include the fact that the values ​​of permissible stresses when calculating welded joints using traditional methods are underestimated in comparison with similar values ​​​​accepted when calculating monolithic parts.

The use of FEM allows one to obtain a more accurate solution and apply higher permissible stresses for strength calculations, taking into account only technological defects. This approach ensures a reduction in metal consumption and design costs. Traditionally, allowable stresses for fillet welds are calculated based on the hypothesis of the highest tangential stresses and are approximately half of the normal ones.

Since the APM Structure3D system provides viewing of stress components, such visualization capabilities are very important in this context.

Calculation of welded joints under time-varying loads

The variable nature of the loading of the weld and the presence of a large number of various defects that inevitably arise during welding reduce the durability of this connection. The calculation of strength under variable external loading is based on methods for calculating static strength.

Moreover, such calculations are carried out both based on nominal stresses and taking into account local concentrations that arise at the boundaries of the welded zones. In most cases, FEM remains the only method for calculating local stresses.

Analytical methods can be used to determine the perceived stress values ​​only for the simplest welded joints.

Stress concentration in welds

The main reasons for the concentration are a sharp change in geometric shape and uneven temperature deformations, which cause the appearance of residual stresses.

As already noted, welds are a serious source of local stresses, since they are characterized by heterogeneity of the weld material, its properties, the presence of defects and stresses caused by temperature deformations, etc.

The static strength of the weld depends little on the presence of local concentration, but the latter has a significant effect under variable loading conditions, since a fatigue crack may appear at the location of the concentrator, which will lead to destruction.

Rice. 4. Map of equivalent voltages (for the model in Fig. 1)

Let us consider several specific examples of concentration for various types of welded joints. In Fig. Figure 4 shows a map of equivalent stresses acting in the lap connection of the eyelet with the plate. The figure shows the complex nature of equivalent stresses in the parts being connected and in the weld. This picture allows us to draw a conclusion regarding the strength of the weld both under constant external loading and under the condition of a variable nature of the change in the external load.

The distribution of stresses in the T-joint of a pipe with a flat surface, shown in Fig. 5.

Rice. 5. Map of equivalent voltages (for the model in Fig. 2)

Often, complex welded profiles are used in metal structures (Fig. 6), when using which it is necessary to take into account the presence of stress concentrators in the welds. Analysis of the stress-strain state by the finite element method makes it possible to obtain stress maps that make it possible to determine the theoretical value of the concentration coefficient, which can then be used in fatigue strength calculations.

Rice. 6. Map of equivalent stresses acting in the corner joint of the model from Fig. 3

conclusions

FEM is used when performing verification calculations, for which it is necessary to know the geometry of the connection and at least approximate linear dimensions. For this purpose, as a first approximation for performing calculations, you can use traditional calculation methods implemented in the Joint automated workplace system, and then move on to FEM. This approach can significantly reduce the time required to implement a complex calculation of a welded joint.

The use of the finite element method at the second stage for strength analysis makes it possible to increase the reliability of critical connections, reduce metal consumption and improve the manufacturability of the welded structure.

CAD and graphics 4`2007

Source: https://sapr.ru/article/17556

Characteristic zones of welded joints

WELDING AND WELDED MATERIALS

Fig 5 1 Characteristic zones of welded joints / - weld, 2 - heat-affected zone, 3 - base metal, 4 - heat-affected zone zone, 5 - fusion zone, Тl, Гс and Гп - temperatures of liquidus solidus and the beginning of phase and structural transformations

Welded joints made by fusion welding can be divided into several zones that differ in macro- and microstructure, chemical composition, mechanical properties and other characteristics: weld, fusion zone, heat-affected zone and base metal (Fig. 5.1). The characteristic features of the zones are associated with the phase and structural transformations that the metal undergoes during welding in each zone.

The weld is characterized by a cast metal macrostructure. It is characterized by a primary microstructure of crystallization, the type of which depends on the composition of the weld and the conditions of the phase transition from the liquid to the solid state.

Heat-affected zone (HAZ) is a section of the base metal adjacent to the weld, within which, due to the thermal effect of the welding heat source, phase and structural transformations occur in the solid metal. As a result, the HAZ has a different grain size and secondary microstructure from the base metal. The heat-affected zone of the HAZ or the near-seam zone (HSZ) is often identified.

It is located directly at the weld and includes several rows of large grains. The weld metal, which has a cast macrostructure, and the HAZ in the base metal, which has the macrostructure of a rolled product or a recrystallized macrostructure of a cast or forged workpiece, are separated from each other by the fusion surface.

On the surface of sections cut from a welded joint and etched with reagents, it is observed at low magnifications as a line or fusion boundary.

The fusion zone (FZ) is the zone of the welded joint where the fusion of the deposited and base metal occurs.

It includes a narrow section of the seam located at the fusion line, as well as a melted section of the OSHZ. The first section is formed due to insufficiently efficient transfer of the molten base metal to the central parts of the weld pool. Here, mixing of the deposited and base metals takes place in commensurate proportions. In the melted section of the OSHZ, liquid layers of a similar composition may appear between the melted grains.

In the case of using dissimilar deposited and base metals (for example, austenitic and pearlitic), the GS is clearly observed in the form of a transition layer. It often has a chemical composition, secondary microstructure and properties that are significantly different from the weld and HAZ metal.

The distribution of elements across the width of the zone is complex, which is determined by the processes of mixing of the directed and base metal, diffusion redistribution of elements between the solid and liquid phases and in the solid phase during the cooling stage.

The base metal is located outside the HAZ and does not undergo changes during welding. It can influence transformations in the HAZ depending on its macro- and microstructure, determined by the method of primary metal processing (rolling, casting, forging, cold deformation) and subsequent heat treatment (annealing, normalization, hardening and tempering, hardening with aging, etc.). P.).

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Source: https://msd.com.ua/svarka-i-svarivaemye-materialy/xarakternye-zony-svarnyx-soedinenij/