Where and when welded joints are used. Welded joints are general information and applications. Insufficient solder joints

WeldingIs the process of obtaining an all-in-one joint by establishing interatomic bonds between the welded parts by local or general heating or by the method of plastic deformation.

Welded joints are currently the main type of permanent joints. They are formed by local heating of the parts in the zone of their connection. In modern mechanical engineering, various welding methods are used, of which the most widely used is electric welding. Technological processes different ways   welding and the field of their application are considered in the course "Technology of metals and structural materials."

The main types of electric welding are arc and contact. There are three types of arc welding:

1. Automatic welding under a layer of flux. This welding method is highly efficient and economical, giving a good weld quality. It is used in large-scale and mass production for structures with long seams.

2. Semi-automatic welding under a layer of flux. It is used for designs with short intermittent seams.

3. Manual welding. Applicable in cases where other methods of arc welding are irrational. This welding method is not very efficient. The quality of the seam depends on the qualification of the welder.

For arc welding, electrodes with various coatings are used, which are labeled according to GOST 9467-60. Electrodes are recommended for welding structural steels: E34, E42, E42A, E46, E46A, 350, E50A, E55, etc. The number after the letter E indicates the minimum guaranteed strength of the weld metal in kg / mm2. The letter A stands for the guaranteed receipt of increased plastic properties of the weld metal.

Contact welding is used in serial and mass production when lapping thin sheet metal (spot, roller welding) or when joining a butt of circular and strip metal (butt welding).

ADVANTAGES AND DISADVANTAGES OF WELDING COMPOUNDS COMPARED TO THE MOLDINGS

Pluses:

1. Low cost of connection due to low labor input of the welding process   simplicity of design welded seam.

2. Relatively small weight of the structure (20/25% less mass riveted), as:

a) since there are no holes for rivets, less cross-sectional area of ​​welded parts is required;

b) joining parts can be done butt-end without linings (Fig. 2.1.b)

c) there are no protruding massive heads of rivets (compare Figs 2.1a and 2.1b).

3. Tightness and tightness of the joint.

4. The possibility of automation of the welding process,

5. The possibility of welding thick profiles.

Disadvantages:

1. The strength of the welded joint depends on the qualification of the welder. The use of automatic welding eliminates this drawback.

2. Fragmentation of parts due to uneven heating in the welding process.

3. Insufficient reliability with significant vibration and shock loads. As the welding process improves, this deficiency manifests itself to a lesser degree.

Fig. 2.1. Structural design of riveted and welded joints.

APPLICATION OF WELDED CONNECTIONS

In modern mechanical engineering, in building structures and in other industries, welded joints have replaced riveted, except in special cases, specified above. At present, welding is widely used as a method of obtaining billets of parts from rolled products in small-scale and single production, as well as in the repair business. Welded frames are made of frames, frames, gearbox housings, pulleys, gear wheels (Figure 2.2), crankshafts and other parts.

Fig. 2.2. Example of a welded cogwheel

In mass production, press-welded parts obtained by arc or automatic welding are used.

Welded connection

a part of a structure or article on which the elements constituting them, made of homogeneous or dissimilar materials, are welded together.

Classification С. с. and seams.   By the mutual arrangement of the elements to be joined, the butt, t-shaped, lapping and angular joints are distinguished. Each of them has specific features depending on the chosen welding method - arc ( fig. 1 ), electroslag ( fig. 2 ), contact ( fig. 3 ), etc. The section of the SS, which directly connects the welded elements, is called a welded seam. Sutures of all types are distinguished: according to the technique of overlapping, they are performed "naprokhod", from the middle to the ends, in a reverse-step manner; by position in space during welding - vertical, horizontal, lower, ceiling; on the technique of formation of the section - single-layered and multilayered, etc. The main types of suture, the structural elements of the edges and seams, the maximum deviations and rational ranges of the thickness of the joined elements for seams of all types are regulated by state standards and industry normals.

Characteristic of S. p.   For S. with. a set of zones formed in the material of welded elements is characteristic. Zones differ from the basic materials and among themselves in terms of chemical composition, structure, physical and mechanical properties, micro- and macro-stresses. To the SS, made by welding melting, include zones ( fig. 4 , a) the seam material (weld seam), fusion, thermal influence, adjacent basic material retaining its properties and structure. The pressure welded joints do not have zones of weld material and alloying and consists of ( fig. 4 , b) from the joint zone, in which the interatomic bonds of the connected elements, the zones of mechanical influence, the zones of the adjacent base material were formed. In the weld, the material is an alloy formed by melted base materials and additional electrode and filler materials or only by melted base materials. In the zone of thermal influence, the basic material does not undergo melting, but in certain areas, as a result of the action of heating and cooling, changes the properties and structure differently. In the most general case of welding by melting of low-carbon steel, the zone of thermal influence. consists of the sections shown in fig. 5 . Overheating area I   adjoins directly to the fusion zone. The material in this section of superheating is heated above 1100 ° C and acquires a coarse-grained structure, which causes a decrease in its viscosity. At the site of recrystallization (normalization) II   the material is heated in the temperature range from 900 to 1100 ° C, which causes a significant grain refinement and increase in viscosity. On the partial recrystallization site III   metal is heated in the temperature range from 700 to 900 ° C and is characterized by uneven structure or partial grinding of grain. In the area of ​​recrystallization IVwhen the material is heated from 500 ° C to the temperature corresponding to the critical point A 1 ,   there is a decrease in strength, in some cases - a decrease in plasticity. On the aging site V   when heated from 100 to 500 ° C, the material has no visible structural changes, but differs from the initial base material by a reduced viscosity, most pronounced in the range of 100-300 ° C. The width of the zone of thermal influence in the welding of steel depends on the welding method, technological process, the thermal regime of welding, the thermophysical properties of the base metal.

Properties of S. p.   Quality C. with. is determined by their performance, resistance to brittle and fatigue failure. Working capacity of s. is characterized by a complex set of properties of alternating zones - interlayers that differ from the base material and strength properties between them. Interlayers with higher strength properties are conventionally called hard, and adjacent to them interlayers with lower strength properties - soft. Depending on the properties of the base material, welding materials. Welding materials), the method and mode of welding and heat treatment, as well as the temperature-velocity conditions of loading by soft interlayers, can be a weld seam, a fusion zone, a weakened section of the heat-affected zone, intermediate inserts of other (heterogeneous materials). Soft interlayers are deformation localizers: at a very low relative thickness they do not reduce the load-bearing capacity of the SS, while at a relatively large thickness their properties limit the load-bearing capacity of the SS. When calculating, designing and manufacturing welded structures, the degree of influence of the stress-strain state on the operability of the sys- tem, the accuracy of their dimensions and shape, and also the stability of these properties during operation are taken into account. In this case, the zone of plastic deformations, the zone of elastic deformations, own residual stresses (stretching and compressing) are distinguished. Diagrams showing temporal and residual longitudinal deformations and stresses in a joint of a plate made of carbon steel are shown in fig. 6th .

Resistance of C. with. brittle and fatigue failure depends on the properties of the material and the presence of stress concentrators and deformations in them. Concentrators are of constructive origin (a section of a sharp change in the cross section of a sintered body, for example, a transition from a seam to a base metal in tie and lap joints), technological origin (non-melting transitions with incoming corners in the seam strengthening area, non-penetration, non-melting and undercutting) chemical origin (pores, slag inclusions, cracks in the joints and the zone of thermal influence).

The education of S. p. The thermoplastic process of deformation of the basic material, which is most pronounced for steel welded joints, accompanies. This process causes fragility in some areas of the thermal effect zone. The metal becomes most brittle due to aging, which occurs during the deformation of the metal at temperatures of 150-300 ° C. In these areas, with. have limited resistance to brittle fractures.

Education С. с. is accompanied by a reduction in the dimensions of the joined elements in the longitudinal and transverse directions, i.e., longitudinal and transverse shrinkage, which is taken into account in the design and manufacture of products.

Principles of calculation.   In the USSR, two methods are used to calculate the s. on the strength at static loading: on a limiting condition (in building designs) and on supposed voltages (in mechanical engineering). For S. with. of steels of different strength, tensile strengths R   p cv ,   compression R   c cc, cut in butt welds R   cp cc, cut in corner joints R   y over,   a also allowable tensile and compressive stresses [σc] and slice [τ sv] are established by industry rules and design design standards. Calculation of fatigue C. with. machine-building metal structures is performed in accordance with the generally accepted methods of calculating the fatigue of machine parts. The effect of low temperatures on the operability of the connection can be taken into account in the design and manufacture of the SS. choice of basic and welding materials, constructive and technological solutions, methods of quality control of materials, etc. In the calculations of S. p. on the strength under static load, the influence of stress concentrators and temperature for ordinary carbon and low-alloy steels is not taken into account. In the calculations of S. p. For fatigue strength, the influence of concentrators and residual stresses is taken into account when determining the permissible stresses. C. with. spanning structures of bridges and steel structures of industrial structures count on endurance for the limiting state.

LIT.: Nikolayev GA, Welded constructions, 3rd ed., Moscow, 1962; Okerblom N. O., Structural and technological design of welded structures, M. - L., 1964; Nikolaev GA, Kurkin SA, Vinokurov VA, Calculation, design and manufacturing of welded structures, M., 1971; Trufyakov VI, Fatigue of welded joints, K., 1973.

  A. A. Kazimirov.

Welded joints   - non-detachable joints formed by establishing between the parts of interatomic bonds, by melting the joined edges, their plastic deformation, or the combined action of both.

Welded joints have found the widest application in the industry. Without the use of welding at the present time, almost no machine is produced. Many cars have welded frames, rear axle housing, wheel disks, bodywork.

Wide distribution of welded joints was facilitated by the presence of a large number of advantages over riveted joints.

Advantages of welded joints:

1. high processability of welding, which determines the low cost of the welded joint;

2. reduction of the mass of welded parts in comparison with cast and riveted parts by 25 ... 30%;

3. The possibility of obtaining a weld seam, equally strong base metal (with the correct design and manufacture);

4. the possibility of obtaining details of complex shape from simple workpieces;

5. the possibility of obtaining sealed connections;

6. high maintainability of welded products.

Disadvantages of welded joints:

1. Warping (spontaneous deformation) of products during welding and aging;

2. the possibility of creating strong stress concentrators in the welding process;

3. The complexity of quality control of welded joints without their destruction;

4. The complexity of ensuring high reliability under the impact of shock and cyclic, including vibration, loads.

By the method of formation of a welded joint, welded joints can be divided into formed with melting of joined edges(fusion welding) and without meltingedges of the connected parts. Among the most common methods for fusion welding are compounds made electric arc   welding with its various modifications ( manual arcmelting and non-consumable electrode, welding under a layer of flux, welding in a protective gas environment, etc.) gas welding(when the welded edges are heated by the heat of a gas flame), electroslag welding, laser beam welding, an electron beam, and some other types of welded joints.

The group of joints without melting edges includes connections made by forge welding, all kinds of contact welding (butt, spot, suture), welding by plastic cold deformation, explosion welding, diffusion welding   in vacuum, friction welding and other types of joints.

At present, the majority of welded joints made by electric arc welding are standardized. By mutual arrangement of parts of the welded joint, the latter can be divided into 5 main types: butt, angled, t-shaped, lapping   and face.

The metal, solidified after melting and joining the welded joints, is called welding seam. The formation of the weld is accompanied by a partial reflow of the surfaces of the parts involved in the formation of the welded joint. The surfaces of the welded parts that undergo partial reflow during the formation of the weld and the joints involved in the formation are called welded edges.

By analogy with riveted seams, welded seams for functional purposes are divided into strong, which do not require the provision of tightness, dense, the main requirement for which is tightness, and tight-knit, in which the requirement of strength is combined with the requirement of tightness of shared spaces.

According to the shape of the cross-section, weld seams are divided into butt   (Figure 35, I) and angular (Figure 35, II). In addition, the cross-section of the seam depends on the shape of the preparation of the edges for welding. So, for example, seams with flanging   edges, without bevel   (Figure 35, Ia), with V -shaped cutting of the edges (Figure 35, Ib) with K -shaped cutting of the edges (Figure 35, Ic) X -shaped cutting of the edges (Figure 35, I r). Seams with edge cutting are also used in other types of joints. The shape of the cutting of the edges depends on the thickness of the metal to be welded, on the type of welding (manual or automatic), on the method of protecting the molten metal from oxidation (welding under a flux layer, welding in shielding gases, etc.) and some other factors. For the most common types of welding (manual melting electrode, semi-automatic and automatic under a layer of flux, etc.), the cutting of the edges is standardized.

According to the shape of the outer surface, the seams can be flat (Figure 35, IIa), concave (Figure 35, IIb), convex (Figure 25, IIb). Sometimes convex seams are unreasonably called strengthened, and concave - weakened. However, the reinforcement of the welding seam promotes stress concentration in the weld zone of the metal, which adversely affects the performance of the joint under varying loads, and the concavity reduces the working cross-section of the seam, thereby increasing the stresses therein.

According to the location of the joints relative to the acting load, weld seams are divided into: frontal   (Figure 36, a), the longitudinal axis of which is perpendicular to the acting forces, flank   (Figure 36, b) or lateral, longitudinal axis of which in the direction coincides with the direction of the acting forces, and slanting   (Figure 36, c), the longitudinal axis of which is directed at some angle to the direction of the acting load. Sutures, the areas of which have a different direction in relation to the acting forces, are called combined   (Figure 36, d).

For welded structures, the most important is the difference in joints in terms of working conditions. By this feature, all the seams can be divided into workersintended for the perception of basic loads, and connecting   or binders, the purpose of which is only the binding of individual elements of the structure into a single whole.

There are some other signs of division welding seams, not presented in this lecture.

The criterion for the efficiency of most welded joints can be considered the strength of the seam and the weld zone with the loads acting in the connection, which can have a very different character.

6.3. Soldered joints

Soldered joints are compounds formed due to chemical or physical (adhesion, dissolution, eutectic formation) of the interaction of the melted material - solder with the edges of the parts being joined.

The use of melted solder causes heating of the parts to be joined. Nevertheless, the essential difference between soldering is the absence of reflow of the surfaces to be joined.

Solder joints are widely used in transport engineering (brazed radiators of cooling systems), in instrumentation and electronics (assembly of printed circuit boards and hinged elements), as well as in some other industries. Some types of solder joints are shown in Fig. 37.

Advantages of soldered joints:

1. the possibility of combining dissimilar materials;

2. the possibility of joining thin-walled parts;

3. the possibility of obtaining a compound in hard-to-reach places;

4. corrosion resistance;

5. Low concentration of stresses due to plasticity of solder;

6. tightness of the soldered seam.

Drawbacks of soldered joints:

1. reduced strength of the joint in comparison with the base metal;

2. Requirements for high accuracy of surface treatment, assembly and fixing of parts for soldering.

As solder for soldering joints, various metals and some alloys are used, the melting point of which is much lower, and the melting point of the material of the parts to be joined.

6.4. Adhesive joints

Adhesive joints   are formed by means of adhesion forces arising during the solidification or polymerization of the adhesive layer applied to the surfaces to be bonded.

The difference between the glue joint and the soldered joint is that the glues are not metals, while the solders are either metals or their alloys. Depending on the composition and properties of the glues, their polarization can occur both at room temperature and during heating.

All adhesives can be divided into structural   - those that are capable of withstanding, after solidification, the load on tearing and shearing, and nonconstructive   - connections with the use of which are not able to withstand the loads for a long time.

BF, epoxy, and cyacrine glues can be classified as structural. Non-structural glues include 88H, sometimes rubber, etc.

Most adhesives require holding the glue joint under load before setting up and then drying to a free state. Some adhesives require heating for evaporation of the solvent and subsequent polymerization. Adhesive compounds are often used as a backing for threaded connections. As a rule, glue joints work better on shear, than on separation.

Calculation of soldered and glued joints is carried out on a shear or on a detachment - depending on their design.

In conclusion, it should be noted that the list of fixed connections used in industry is far from being limited to those presented in this lecture. In addition, technical thought does not stand still, and, therefore, new methods of joining parts, and therefore new types of connections, are constantly emerging.

In addition to fixed connections, which can not be disassembled, there is a large class of detachable connections. The latter will be considered in subsequent lectures.

Welded connection - one-piece. It is formed by welding materials of parts in the joint area and does not require any auxiliary elements. The strength of the joint depends on the uniformity and continuity of the weld material and the surrounding zone.

The types of welding used in modern engineering are very diverse. Each of them has its own specific applications. Of all the types of welding, electric is the most widely used. There are two main types of electric welding: arc and contact.

Arc welding   based on the use of the heat of the electric arc to melt the metal. To protect the molten metal from the harmful effects of ambient air, a thick protective coating is applied to the electrode surface, which releases a large amount of slag and gas, forming an insulating medium. This provides an improvement in the quality of weld metal, the mechanical properties of which can deteriorate sharply under the influence of oxygen and nitrogen of air.

With the same purpose produce   submerged arc welding.   This type of welding is currently the main type of automatic welding. The productivity of automatic submerged arc welding is 10 ... 20 times or more above manual. Increase in productivity is achieved by applying a current of 1000 ... 3000 A instead of 200 ... 500 A for manual welding. This provides a more rational formation of the weld and increases the welding speed.

While in manual welding, the formation of the seam is achieved mainly due to the metal of the electrode (Figure 3.1, a), in automatic welding, the seam is formed to a large extent due to the molten base metal (Figure 3.1, b), which not only shortens the time , but also significantly reduces the consumption of electrode material. Automatic welding under a layer of flux provides high and, most importantly, homogeneous mechanical properties of joints independent of the individual qualities of the welder.

In electroslag welding   The source of heating is the heat released when the current passes from the electrode to the product through the slag bath. Electroslag welding is designed to connect parts of large thickness. The thickness of the welded parts is almost unlimited. Electroslag welding allows you to replace complex and heavy solid-cast and all-welded structures with welded parts from individual simple castings, forgings and sheets, which greatly facilitates and reduces the cost of production. This welding is also applicable for cast iron castings.

contact welding   is based on the use of high ohmic resistance in the joint of parts and is carried out in several ways.

When   butt-welding   Through the details, a current is passed, the force of which reaches several thousand amperes. The basic amount of heat is released at the junction where the greatest
  resistance; the metal in this zone is heated to a plastic state or even to surface reflow. Then the current is switched off, and the heated parts are squeezed with some force - the metal of the parts is welded all over the interface. This type of welding is recommended for use for butt joints of parts, the cross-sectional area of ​​which is relatively small.

When   spot welding   the connection is not formed over the whole surface of the joint, but only at separate points, to which the electrodes of the welding machine are fed.

When   seam welding   A narrow continuous or intermittent seam is located along the joint of the parts. This welding is performed with the help of electrodes having the shape of discs which roll in the welding direction. Point and seam welding is used in overlap joints mainly for sheet metal parts with a thickness of not more than 3 ... 4 mm and thin rods of reinforcing mesh. Unlike point welding, seam welding forms a sealed connection.

All considered types of contact welding are highly efficient, they are widely used in mass production for welding pipes, fittings, car bodies, metal sheathing of railway wagons, aircraft hulls, thin-walled tanks, etc.

Welded joint is the most perfect of all-in-one joints, since it is better than others to approximate composite components to integral ones.   In the case of a welded joint, it is easier to ensure the conditions of equal strength, weight reduction and product cost.

Welding is used not only as a way of joining parts, but also as a technological way of manufacturing the parts themselves.   Welded parts in many cases successfully replace cast and forged (Figure 3.2, where   a   - cogwheel;   b   - bracket;   at   - housing). For the production of welded parts, models, molds or dies are not required. This significantly reduces their cost in single and small-scale production. Welding of products such as cog wheels or crankshafts makes it possible to produce more

Fig. 3.1

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The stems (crenellated, cervical) of high-strength steels, and the less important ones (the disc and hub of the wheel, the cheek of the crankshaft) are made from less durable and cheaper materials. Compared to cast parts, welded ones allow a smaller wall thickness, which allows to reduce the weight of parts and reduce material consumption. Widely used were stamped welded structures (Figure 3.2,   at),   Replacement molding, riveted and other products. The use of welded and stamped welded structures allows in many cases to reduce the material consumption or the weight of the structure by 30 ... 50%, reduce the cost of products in 1,5 ... 2 times.