Manual arc welding of a pipe - fixed joint when the pipe is vertical
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Welding a pipe in this position is equivalent to welding horizontal seams, but differs in that when the electrode moves forward, it is necessary to constantly change the angle of inclination of the electrode in relation to the seam around the perimeter of the pipe.
Welding the root bead on a pipe joint
If high-quality formation of a reverse bead is required without sampling and back-welding, then welding is performed with an electrode with a diameter of 3 mm. The welding current is selected depending on the thickness of the base metal, the gap between the edges and the thickness of the blunting and lies in the minimum or medium ranges. The inclination of the electrode is 80°-90° to the vertical plane of the lower pipe (Fig. 64a). Maintain the inclination of the electrode in relation to the welding direction depending on the penetration of the reverse side of the weld root.
When welding “backwards”, maximum penetration is achieved. “Angle forward” is the minimum.
The arc length must be kept short for insufficient penetration or medium for normal penetration.
The welding speed is determined by the volume of the weld pool. In Fig. Figure 64b shows defects in the root of the seam on the reverse side when the bead is too full.
The greater the fullness of the root bead, the longer the weld metal remains in a liquid state and the larger the size of the defects.
The welding speed must be selected so that both edges are well fused and the bead is “normal.” If the thickness of the metal allows and the back side of the root of the weld is to be sampled and welded, it is recommended to use an electrode with a diameter of 4 mm.
The second root bead is made across the entire width, melting through the first root bead and gripping the edges of both pipes. The diameter of the electrode depends on the width of the first bead; the welder determines it by measuring the end of the electrode.
The end of the electrode with the coating should touch the surface of the first root bead. Welding current is in the medium range. The inclination of the electrode relative to the vertical plane is the same as when welding a root bead. Welding should be done with a backward angle.
The speed is such that the roller is “normal”.
Filling the cut
Filling the groove begins with the bottom edge, which is a platform, which allows for more productive welding methods.
The third roller of the horizontal seam (Fig. 65 a and b) is recommended to be performed at high settings. Welding current in the average or maximum range. The inclination of the electrode is 70°-80° to the surface of the edge of the lower pipe. Welding should be done “at a right angle” or “backward angle”, depending on the location of the slag.
Select the welding speed so that the bead is reinforced (“hump”), as when welding in the lower position, which creates a shelf that will allow the next bead to be welded at higher speeds. The center of the arc (electrode) must move along the lower edge of the second root roller.
Before welding the third bead, it is necessary for the welder to choose how full the third bead should be so that the width of the unfilled groove (between the upper edge of the groove and the completed seam) does not turn out to be very wide for one (4th bead) or very narrow for two beads.
The width from the top edge of the third roller to the top edge should remain minimal: the diameter of the coated electrode or slightly larger.
In Fig. 65a and b show the third layer, made in two rollers (3rd and 4th). Welding of the 4th bead is performed in the same modes, although the bead is purely horizontal. The surface tension force prevents the liquid metal from flowing down.
The inclination of the electrode when welding the 4th bead is 80°-90° to the vertical surface of the lower pipe. The bulk of the slag flows to the bottom of the seam, which allows welding to be done “at a right angle” or a slight “angle back”. The welding speed must be chosen such that the weld pool fuses the upper edge of the groove with the top of the 3rd bead, melting the 2nd bead in the center.
The 4th roller should be “normal”.
The last layer when filling the groove is performed as surfacing and is described in section 3.2 (features of welding horizontal seams and surfacing on a horizontal plane).
Making “locks” (beginning and end of weld beads)
The end of welding of each bead must be done after the beginning has been fused and the seam has been approached at a distance of 20-30 mm. If the pipe joint is performed by one welder, each bead (layer) must be completed around the entire perimeter without additional interruption. Each “lock” (the beginning of each layer) must be offset from one another by at least 50 mm.
When multi-pass and multi-layer welding of pipes of large thickness, it is recommended to weld in a spiral, because at the same time, defects are reduced by reducing the number of “locks” (the beginning and end of welding). In Fig. Figure 66 shows spiral welding.
When approaching the beginning of each bead, it is recommended to stop welding at a distance of at least 20 mm before the beginning of the bead and thoroughly clean it (if necessary, trim and grind off the high beginning of the bead). Approaching the beginning of the roller, it is necessary to delay the arc, melting the beginning.
Go to the roller and, without extinguishing the arc, proceed to perform the next roller on the previous one and so on until the layer is finished. This method reduces the number of “locks” and allows for productive and high-quality welding - slag welding.
Welding on slag requires high skill and is possible mainly with electrodes with a basic coating. It is more difficult to achieve high quality with electrodes with rutile coating, because a large amount of more fluid slag can lead to slagging in some areas. In conventional welding and when welding on slag, the last upper bead of each layer must be performed after thoroughly cleaning the cutting from slag, splashes and a layer of soot (burnt-out elements).
Welding the face layer
It is recommended to weld the face layer using electrodes of the same diameter as the filling of the groove, but no more than a diameter of 4 mm. Welding current is in the medium range. Conventional horizontal surfacing with narrow beads 1.5 times the diameter of the coated electrode (Fig. 67). The last upper roller, for a smooth transition to the base metal, should be performed at a higher speed to obtain a narrow and flat roller.
You can switch to a smaller electrode diameter with the appropriate selection of welding current.
If the pipe welding is carried out by several welders, then the pipe must be divided into equal sections according to the number of welders. The optimal welding option that excludes “locks” is interception welding.
Welding is carried out by all welders of the same bead (layer). Each welder finishes the end of each roller in his area with an offset of 20-30 mm (not reaching the end of the previous roller). Continuing the end of the adjacent welder's bead, each welder melts the beginning of the completed bead and makes a spiral approach to the next bead. The next layer is performed with an offset of at least 50 mm.
Tudvasev V.A. "Recommendations for welders."
See also:
- Manufacturing of welded pipes
- Welding joints of pipes and pipelines
Source: https://www.autowelding.ru/publ/1/1/ruchnaja_dugovaja_svarka_truby_styk_nepovorotnyj_pri_vertikalnom_raspolozhenii_truby/5-1-0-489
Welding the root of a pipe seam under the clearance
Not every welder, even with solid experience, knows how to weld pipes through the gap. For beginners, this work seems so difficult that they don’t even try to take it on. In fact, transmission welding is not as difficult as it seems from the outside. You just need to know its nuances and features.
What does "transmission welding" mean?
This method got its name because this pipe connection is checked using radiographic testing methods. When it was carried out, the seam was previously examined with x-rays to identify flaws. Ultrasonic devices are now used for monitoring. However, more often the term “transmission welding” refers to the connection of pipes with a gap between them. The connected sections of pipes are called coils. This method is also called under-gap welding.
Preparatory work
Before welding the pipe, it is necessary to prepare the metal in the joint area. The ends must be cut straight, otherwise it will be difficult to apply a reliable seam. The edges are chamfered so that the angle between them is 65 - 70˚.
Surfaces at a distance of at least 3 cm from the ends are cleaned with a grinding machine or a metal brush until shiny inside and out. Areas that have changed color after treatment with a sander are removed. Burrs are removed with a file.
The sharp edges of the edges are dulled to 2 mm, otherwise they will quickly melt. Then the surfaces are degreased with acetone.
The preparation of the electrodes is no less important. Before starting work, they are calcined in an oven at a temperature of 380 - 400˚C for two hours. For long-term storage, the electrodes are placed in a special case in which the temperature is maintained at 80˚C.
A container for this purpose can be made independently from a polyurethane foam pipe. When long-term calcination is not possible, the electrodes are dried with a burner for one and a half minutes with a low flame. This method is prohibited by the rules, but in an emergency there is no choice.
Since the coating of the electrodes dries quickly and picks up moisture again, you should not dry more than two pieces at the same time.
Pipe joining
To prevent the workpieces from moving relative to each other, the coils are laid on a corner or channel. Ideally, the discrepancy should be zero, since even a discrepancy of 1 mm, allowed by the rules, leads to lack of penetration. The gap, depending on the wall thickness, is set within 2 - 3 mm.
In everyday practice, for this purpose an electrode of a suitable diameter is inserted between the ends of the pipes. For wall thicknesses of 10 mm or more, the gap is set to 3 mm. When joining, you also need to take into account the error that occurs during tack welding due to the heating of the metal at its points.
The resulting thermal stresses tighten the pipes, so the gap is increased by a few tenths of a millimeter.
Correct preparation and joining of pipes
The docking process is simplified if you use a centralizer. It will ensure precise alignment of the pipe axes and constant position during welding. Usually an external centralizer is used, but it is better to choose an internal one, as it corrects irregularities in shape, for example, ovality of pipe ends.
Setting the welding machine mode
To weld a pipe under the clearance, it is recommended to use a DC welding machine. The work is carried out in direct polarity mode, when the electrode is connected to the plus and the pipe to the minus.
The amount of welding current is set depending on the thickness of the pipe walls and the diameter of the electrodes. Its exact value in each case is selected experimentally.
However, regardless of the size of the pipes, the creation of the root of the weld is carried out using electrodes with a diameter of 2.5 mm at the lowest possible current. Attempts to use the troika to speed up the process end in failure.
Source: https://svarkaprosto.ru/tehnologii/svarka-kornya-shva-truby-pod-prosvet
Welding of pipelines. Type, technology and weld defect
The use of electric arc welding is found everywhere in our lives; it is characterized by the reliable connection of metal pipes to each other.
Therefore, in our specificity, it is widely used in the heating system, because where there is high pressure and extreme temperatures, no other material will compete with this material.
Such a system provides for the use of seamless pipes, and welding them together requires a special technology that must be strictly followed. It consists in welding the root of the seam.
When welding pipes and equipment elements on a regular water supply, or, say, sewerage system, everything is much simpler. What I describe below directly concerns the steam system, and the process of installation on a high-pressure gas pipeline is similar to it. Are you, dear visitor, interested in such information? Then I invite you to familiarize yourself, I tried to present everything in simple language.
Type of weld
So, we have two pipes of the same or different diameters, let’s consider both options. In the first case, we connect the pipe to the outlet, and in the second, we need to cut a pipe with a diameter of 76 millimeters into a pipe with a diameter of 133 millimeters.
In order for us to achieve absolute (I’m not afraid of this word) tightness of the welded joint, the weld will be double. First, we boil the so-called root of the seam, and then we will cover it with the second one.
Let’s divide the whole process into several stages, each of them is important and is carried out without “jambs”; if something goes wrong, it is better to achieve “ideality” at the initial stage. In our case, the cheerful saying: “Maybe it won’t flow” will not work.
1. Preparation of surfaces to be welded
Includes a thorough adjustment of them to each other. A chamfer is removed on both mating surfaces, ideally at 45 degrees to the pipe axis. It must be removed with the condition that the end is left flat, 2-3 millimeters wide.
Here and further, in order not to particularly search for suitable words, for clarity, I provide the post with photographs and a video.
In this photograph there is a marking on the end of the pipe, following the line of which we need to cut out an element, we call it “making a mustache”, or “cutting out a fish”.
And here is a picture of what the cut and processed surface of the workpiece looks like.
2. Joining of welded surfaces
This photo shows a joint on tacks. Both planes of the surfaces have chamfers and do not touch each other, having a gap of 2-3 mm, this gap is necessary for penetration of the root of the seam.
It is important that the elements being welded are coaxial; there can be no talk of any displacement relative to each other, otherwise the joint will be rejected.
We put them out and grab them together. In case of unequal gap between the surfaces, which occurs when the pipe is cut unevenly, we modify it using a grinder with a cutting disc, achieving uniformity around the entire perimeter.
Pipes with a diameter of up to 50 millimeters are tacked in two places, but for larger diameters - at least three tacks, and you begin to cook from the logical location of the fourth. Already during the welding process, when reaching the next tack, it must be cleaned off.
Root welding
Everything is ready for penetration for the first time, which is what the welder does next.
Upon completion of running in the joint, it is the installer’s turn to work.
3. Selecting the root of the seam
It consists of treating the welding area around the entire circumference to a metallic shine. Literally - there should not be a hint of unevenness of the groove, much less slag residues.
All edges and irregularities are smoothed out, and the surfaces on both sides of the seam are also sanded to a metallic shine, about twenty millimeters in each direction. I do this using the same grinder, with a cleaning disk installed on it.
While performing this stage, I noticed a defect - it’s time, it formed at the site of the welding “lock”.
Here is a visual result:
If the installer notices this “jamb”, it is necessary to clean this place, until a gap appears, while providing for slopes of the chamfers of the surfaces being welded.
Point out the presence of a pore to the welder, he will “throw it in.” Then select the cooked area until it has a metallic shine. As I wrote above, bring everything to perfection.
After this, the welder can proceed to the next stage.
4. Overlapping the root seam
Observing what he wrote above in the box, in paragraph 2, he rolls the joint in a circle. Without saving electrodes, the seam turns out “greasy”. Then the installer processes the seam using the same grinder with a grinding wheel.
In general it looks like this:
Another important point here: when cleaning, “undercutting” of the welded surfaces is unacceptable; to prevent this, grind in one given direction - from the pipe to the seam.
Here the places where “undercuts” may appear are marked in red.
If this “jamb” is present, the joint is rejected.
I'll add a couple more photos to the description. They have a welded joint between a pipe and a flange. On the outside of the flange, penetration is carried out in accordance with the entire process described above, that is, welding the root of the seam, removing it, covering it and cleaning it.
In addition to this, we also weld the joint from the inside of the flange:
Weld defect
Porina, and how to remove it, we looked at it, and now let’s explain the points due to which there is a high probability of its appearance.
Every welder should be aware of the first probability, because they undergo special training. The second and third are characterized by a conscientious attitude towards the performance of their direct duties.
But on the last point a little more detail:
I have had to deal with this problem: the welder is welding, I select the root, there is a porina there - I clean it, he welds again, I select - porina, I clean it, cooks - porina. Then they realized that the pipeline was long and on one side had a connection with the atmosphere, in general, until this connection was plugged with cotton pants, they did not come to a positive result.
Well, that seems to be all I wanted to tell you, I’ll wrap it up. If you follow everything described, you will end up with a perfect joint. No “luminary” will find reasons to reject it, and an x-ray will show compliance with the standards.
Here is the video promised at the beginning of the article, which I edited as clearly as I could:
I don’t stroke my pride, and I don’t consider myself a “master of the pen” at all; I only shared what I know from my experience. For those interested in deeper knowledge in this area, I can recommend studying the book, which can be downloaded by clicking on the picture below.
The book is not free, but the price is not high, only 84 rubles; moreover, you have the opportunity to read a fragment for reference, and make a purchase only if you are interested. The book consists of 510 pages and has 234 illustrations.
You can rest assured that the distributor is trustworthy; there will be no scam when transferring money, I checked it personally.
I recommend that blog guests subscribe to receive new blog articles; to do this, you just need to enter your email address in the form that will open when you scroll to the bottom of the page.
Useful links:
Dimensions of the depth of the “whiskers” for pipes of various diameters.
Sealed thread on high pressure steam line.
Installation of polypropylene water pipes.
If you have any questions or have anything to add to the article, you are welcome to add it to the comments section.
Good luck to everyone in the installation, best regards Andrey.
Click on the icon if you think this information will be useful to your friends.
Source: http://santehskript.ru/svarka-truboprovodov-vid-texnologiya-defekt-svarnogo-shva/
Rb 089-14 safety guidelines for the use of atomic energy, unified methods for monitoring basic materials (semi-finished products), welded joints and surfacing of equipment and pipelines of nuclear power plants. visual and measuring control
FEDERAL SERVICE FOR ECOLOGICAL,
TECHNOLOGICAL AND ATOMIC SUPERVISION
| APPROVED by order of the Federal Service for Environmental, Technological and Nuclear Supervision dated June 6, 2014 No. 247 |
SAFETY GUIDE FOR THE USE OF NUCLEAR ENERGY “UNIFIED METHODS FOR CONTROL OF BASIC MATERIALS (SEMI-FINISHED PRODUCTS), WELDED JOINTS AND SUPPLIANCE OF EQUIPMENT AND PIPELINES OF NUCLEAR ENERGY INSTALLATIONS.
VISUAL AND MEASURING CONTROL"
(RB-089-14)
| Effective June 6, 2014 |
Moscow 2014
This safety guide for the use of atomic energy “Unified methods for monitoring basic materials (semi-finished products), welded joints and surfacing of equipment and pipelines of nuclear power plants.
Visual and measuring control" (RB-089-14) (hereinafter referred to as the Safety Guide) was developed in accordance with Article 6 of the Federal Law of November 21, 1995.
No. 170-FZ “On the Use of Atomic Energy” in order to facilitate compliance with the requirements of federal standards and regulations in the field of the use of atomic energy in terms of capillary control.
This Safety Guide contains recommendations from the Federal Service for Environmental, Technological and Nuclear Supervision on carrying out visual and measurement inspection of materials (semi-finished products), welded joints (surfacing) of equipment and pipelines of nuclear power plants.
This Guide also defines recommendations for equipment, technological sequence of operations, recording control results and personnel qualifications.
This Guide has been developed to replace the regulatory document “Unified methods for monitoring basic materials (semi-finished products), welded joints and surfacing of equipment and pipelines of nuclear power plants. Visual and measuring control. PNAE G-7-016-89".
CONTENT
I. General provisions
1. This safety manual for the use of atomic energy “Unified methods for monitoring basic materials (semi-finished products), welded joints and surfacing of equipment and pipelines of nuclear power plants.
Visual and measuring control" (RB-089-14) (hereinafter referred to as the Safety Guide) was developed in accordance with Article 6 of the Federal Law of November 21, 1995.
No. 170-FZ “On the Use of Atomic Energy” in order to facilitate compliance with relevant federal standards and regulations in the field of the use of atomic energy in terms of visual and measurement control.
2. This Safety Guide contains recommendations from the Federal Service for Environmental, Technological and Nuclear Supervision on carrying out visual and measurement control, recording control results, which are subject to the requirements of the relevant federal norms and rules in the field of atomic energy use (hereinafter referred to as the Rules).
3. The Safety Guide has been developed for organizations engaged in the design, design, manufacture, installation, repair, operation of equipment and pipelines of nuclear power plants (hereinafter referred to as NPP), as well as for Rostechnadzor specialists who carry out supervision and licensing in the design, construction, manufacture, installation, repair, operation of equipment and pipelines of nuclear power plants.
4. This Safety Manual uses the terms and definitions given in Appendix No. 1 and other concepts defined by the Rules.
5. Visual inspection of base metal, semi-finished products, parts, assembly units, welded joints and surfacing (hereinafter referred to as control objects) is carried out in order to identify surface cracks, delaminations, sunsets, nicks, cavities, caps, lack of fusion, peelings, burns, fistulas, sagging , shrinkage cavities, metal splashes, undercuts, inclusions, accumulations and other discontinuities located on the surface of the test objects.
6.
Measurement control of controlled objects (hereinafter referred to as MC) is carried out to check the conformity of their shapes and geometric dimensions, the size of gaps, displacements, and bluntness of the controlled surfaces; geometric position of axes or surfaces; recesses between the rollers and scaliness, width and convexity (concavity) of the surface (root) of the weld of welded joints, as well as compliance of the dimensions, location and number of discontinuities identified during visual inspection of the base metal and welded joints with the requirements of the relevant Rules, regulatory technical documentation (hereinafter - RD) , design documentation (hereinafter - CD), standards, technical specifications (hereinafter - TU), production control documentation (hereinafter - PKD), production and technological documentation (hereinafter - PDD).
7. Visual and measuring control of QA is carried out at the following stages:
input control;
operational control;
defect corrections;
assessment of the technical condition of the equipment during operation.
8. At the stage of incoming control, the following is subjected to visual and measurement control:
all accessible surfaces outside and inside;
edges of elements to be welded;
existing welded joints;
overall and other structural dimensions established in the relevant design documentation, standards, specifications, and technical specifications.
9. Operational control is carried out in accordance with the requirements of the relevant ND. In terms of visual and measurement control, operational control includes checking:
when preparing for welding:
cleanliness (absence of contamination, dust, corrosion products, oil, etc.) of the edges and adjacent surfaces to be welded (surfacing), as well as areas of the base metal subject to control;
absence of surface damage caused by deviations in manufacturing technology, transportation and storage conditions;
shapes and sizes of edges; shapes and sizes of pipe boring (distribution);
shapes and sizes of backing rings or meltable inserts;
when assembled for welding:
correct installation of backing rings or fusible inserts;
correct assembly and fastening of parts in assembly fixtures;
quality, size and location of tacks; gap sizes in connections;
the amount of displacement of edges, fracture of axes or planes of connected parts;
dimensions of the assembly assembled for welding; after finishing welding:
absence of defects on the surface of welded joints or surfacing (cracks of all types and directions, peeling, burns, fistulas, sagging, shrinkage cavities, undercuts, lack of penetration1, splashes of molten metal, entrapment between rollers, rough scaliness, as well as places where the welding arc touches the surface of the base material );
__________
1With the exception of structural failures.
sizes of surface discontinuities (pores, inclusions) identified during visual inspection;
width, convexity (concavity) of the weld joint;
the height (depth) of the recesses between the rollers (inter-roller depressions) and the scaliness of the seam surface;
dimensions of the fillet weld leg;
correspondence of the axes of welded cylindrical elements;
quality of metal stripping in places of welding of temporary technological fastenings (inductor combs, mounting bosses of thermoelectric converters (thermocouples), as well as the absence of surface defects in stripping areas;
the quality of cleaning the surface of the welded joint and adjacent areas of the base metal for subsequent control using non-destructive methods (if such control is provided for by the PTD);
the presence of markings (branding) of the seam and the correctness of its execution.
10. Monitoring the correction of defects in terms of visual and measurement control includes inspection.
completeness of defect removal;
smooth transitions at sampling points;
shape, size and surface quality of prepared samples;
the width of the mechanically stripped metal surfaces adjacent to the edges of the sample;
the absence on the surface of both the corrected area and areas adjacent to it of the following defects: cracks, accumulations of pores and inclusions, fistulas, burns, sagging, shrinkage cavities, undercuts, lack of fusion, splashes of molten metal, entrapments between rollers, rough scaliness.
11. When assessing the technical condition during operation, visual and measurement control is carried out with the aim of:
checking the absence of mechanical damage, changes in shape (deformed areas, warping, sagging and other deviations from the original location); if there is a change in shape, their geometric dimensions or parameters or values are determined;
checking the absence of cracks and other surface defects formed or developed during operation;
checking corrosion-erosion wear of surfaces, measuring the depth of corrosion pits, measuring the area of damage.
12. Visual and measuring control is carried out to the extent of 100%, unless otherwise indicated in the relevant ND, CD, PKD or PDD.
13. If welded joints are accessible for visual inspection on both sides, inspection is carried out both from the outside and from the inside.
14. Visual and measuring control is carried out before testing by other methods.
15. Measurements are carried out after visual inspection or simultaneously with it, first of all, in those areas that raise doubts based on the results of visual inspection. Measurements of OK prepared for welding are carried out before their assembly.
II. Preparation for inspection
16. Visual and measurement control during manufacturing, installation, repair and operation is carried out at the work site. In this case, it is necessary to ensure that inspectors can easily approach the work site.
If necessary, fences, scaffolding, scaffolding, cradles, mobile towers or other auxiliary devices are installed to ensure optimal access (ease of operation) for the controller to the OK.
It is also possible to connect local lighting lamps with a voltage of 12 V.
17.
Visual and measurement control during the operation of equipment and pipelines of nuclear power plants operating under pressure is carried out after stopping the operation of the said equipment or pipeline, releasing pressure, cooling, draining, disconnecting from other equipment, unless otherwise provided by the current PDD. If necessary, the internal devices are removed, the insulating coating and lining that impede the inspection of the base metal and welded joints are partially or completely removed in the places specified in the inspection documentation.
18. Visual and measurement inspection at the stage of incoming inspection, if possible, is recommended to be carried out in stationary areas that are equipped with work tables, stands, roller supports and other means that ensure the convenience of performing work. Recommendations for organizing stationary sites are given in Appendix No. 2 to this Safety Guide.
19. The dimensions of the zones within which visual and measurement control are carried out are determined in accordance with the requirements established in the relevant Rules, ND, CD, PKD or PDD.
20. Before carrying out visual and measuring inspection, the surface of the OK is cleaned to bare metal from corrosion products, scale, dirt, paint, oil, moisture, slag, splashes of molten metal and other contaminants that interfere with the inspection (the presence of tarnished colors is allowed on the controlled surfaces, in cases where this is specified in the PDD).
21. Cleaning of the controlled surface is carried out in the manner specified in the relevant ND, PDD.
22. When cleaning materials and welds made of austenitic steels and high-nickel alloys, brushes made of stainless cold-worked wire are used.
23. The illumination of the surface being tested for reliable detection of defects is at least 500 Lux.
24. Preparation of QA is carried out by the department of the enterprise that carries out the manufacture, installation, repair or operation of equipment and pipelines of the nuclear power plant.
25. The preparation of controlled surfaces is not the responsibility of the inspector.
III. Carrying out visual and measuring control
26. Visual and measurement control of the quality control unit is carried out in accordance with the requirements and instructions of standards, specifications, design documentation, design documentation, and technical documentation.
Source: https://files.stroyinf.ru/Data2/1/4293770/4293770840.htm
How to weld a ceiling seam
Ceiling welding is considered one of the most difficult types of welding because the weld pool is located upside down. Welding ceiling seams requires a precise sequence of all actions. If you do not follow the rules and recommendations, you can get a poor-quality connection, which can lead to leaks of hot metal.
When making a ceiling seam, you need to follow safety precautions and protect your face and hands from possible contact with hot drops of metal.
Features of the ceiling seam technique
Note! Preparatory work before welding ceiling seams is accompanied by a number of difficulties. This is due to the fact that during the welding process, molten metal can flow down and accidentally fall on the skin of the person performing the weld. That is why experienced specialists, knowing how to weld such a seam, use special welding techniques that ensure strict compliance with safety rules.
Before connecting ceiling seams, you must clearly understand all the requirements and not neglect to fulfill the mandatory conditions. This information is especially important for novice welders.
- To connect ceiling seams, semi-automatic or manual welding should be used.
- To achieve a strong and high-quality connection, coated electrodes are used.
- Welding the ceiling joint is carried out in short arc mode; the electrodes you use should be no more than 4 mm.
- Arc welding involves carrying out preparatory work: to prevent the metal used from spreading, the electrode must be held slightly to the side.
- The width of the seam should be less than the thickness of the electrode.
- During the welding process, gases can be generated that have a negative impact on the quality of the seam. To prevent this from happening, the electrodes must be completely dry.
- Welding the ceiling seam should be done in the “toward” direction. This way you can choose the appropriate pace of progress and monitor the quality of the seam.
- In order to ultimately obtain the most durable and reliable connection, the edges of the workpieces must be well cleaned before welding.
If your workpieces are more than one centimeter thick and you do not know how to weld a ceiling seam, then know that in such a situation welding must be done in stages. At the very beginning of work, you need to use an electrode whose diameter does not exceed 3 mm; at all other stages, an electrode with a diameter of 4 mm is used.
Note! Welding a ceiling seam involves creating not only horizontal seams, but also connections along a certain slope (from 10 to 80°C). In this case, both manual welding and semi-automatic welding can be used. You can also use arc welding using coated electrodes.
How to weld a ceiling seam correctly. Recommendations for Beginners
As already noted, the main distinctive feature of such welding is based on an inverted weld pool and holding the hot metal by tension force. It is very important to maintain the minimum size of the bath and sometimes move the electrode to the side so that the metal has time to cool and set. In addition, this welding method will protect your connection from cracks.
Welding methods
- Crescent, this method involves placing the electrode at an angle of 90-140° to the surface. Using oscillatory movements in an arc, it moves from edge to edge.
- Ladder, here the electrode is placed in exactly the same way. It is taken away from the hot metal and returned back.
- Back-and-forth method. In this case, the electrode must be constantly returned to the frozen seam.
The first method is considered the simplest and is well suited for inexperienced welders. Options involving ladder and back-and-forth welding are more complex and time-consuming.
But such a connection will be of better quality.
Stages of work execution
Initially, the places that will be welded must be well treated by cleaning the edges with a grinder or a metal brush. Then the root of the seam is welded with an electrode (3 mm). Through it, a roller of molten metal is formed.
If it is necessary for the roller to be on the reverse side, then the electrode is directed into the gap in small short throws. Monitor and control the uniform arc burning. The resulting roller must be freed from slag. If its shape is too convex, then it is necessary to bring it to pure metal.
As already mentioned, in the following stages an electrode with a large diameter (4 mm) is used.
Safety Recommendations
In order to obtain a reliable and durable seam, all safety regulations must be followed. Not only the quality of the seam, but also your personal safety depends on compliance with the rules.
- Be sure to insulate all wires, and when you pause work, completely turn off the welding machine. This way you will protect yourself from electric shock during the welding process.
- It is recommended to weld ceiling seams in a warm, dry room.
- Under no circumstances should you work in rainy or snowy weather. Only specialists with extensive experience should carry out welding in wet areas, but even then you should be extremely careful.
- Wear special protective clothing, a mask and goggles. The entire body should be tightly covered with clothing.
In conclusion, it is worth noting once again that welding ceiling seams is a rather labor-intensive and energy-consuming process. It is quite difficult for a novice welder to make this type of connection.
In order to do the job efficiently and create a good, strong seam, you need to gain special knowledge, master the technique of working with a welding machine and become familiar with all the nuances of this type of work. It’s better to practice welding some parts first. If you do not have enough knowledge and experience, then it is better to entrust this process to specially trained people.
In any case, both experienced welders and beginners must strictly follow safety rules. Accurate compliance with all requirements and instructions is the key to your personal safety.
Source: https://svarkaed.ru/svarka/shvy-i-soedineniya/kak-svarit-potolochnyj-shov.html
Types and methods of making welding seams
The ultimate goal of any welder is to obtain a high-quality weld. The strength and durability of the connection of parts depends on this. For successful operation, it is important to make the connection correctly; select the current strength, the angle of the electrode; have a good command of the seam technique. The result of proper work will be reliable welding of metal parts.
Electrode tilt
Welding seams are classified according to several criteria. The types and types of welding joints must be considered sequentially, delving into the intricacies of the process. The seam is affected by the location, direction and trajectory of the electrode.
After fixing the selected electrode in the clamp, setting the current, connecting the polarity, the welding process begins.
Each master has his own preferred angle of the electrode. Many consider the optimal value to be 70° from the horizontal surface.
An angle of 20° is formed from the vertical axis. Some work at a maximum angle of 60°. In general, most training guidelines include a range of 30° to 60° from the vertical axis.
In certain situations, when welding in hard-to-reach places, it is necessary to orient the electrode strictly perpendicular to the surface of the material being welded.
You can also move the electrode in different ways, in opposite directions: away from you or towards you.
If the material requires deep heating, then the electrode is directed towards itself. Following him in the direction of the welder is the working area. The resulting slag covers the fusion site.
If the work does not involve strong heating, then the electrode is moved away from you. The welding zone “crawls” behind it. The depth of heating with this type of weld is minimal. The direction is clear.
Trajectory of movement
The trajectory of the electrode has a particular influence on the seam. In any case, it has an oscillatory character. Otherwise, it will not be possible to sew the two surfaces together.
Oscillations can be similar to zigzags with different steps between the sharp corners of the trajectory. They can be smooth, resembling movement in an offset figure eight. The path can be similar to a herringbone or a capital letter Z with monograms at the top and bottom.
An ideal seam has a constant height, width, uniform appearance without defects in the form of craters, undercuts, pores, or lack of penetration. The name of the possible flaws speaks for itself. Having mastered your skills well, you can successfully apply any seam and weld a variety of metal parts.
Standards and the concept of leg
The weld begins to form in the working area when the metals are molten, and is finally formed after solidification.
The existing classification groups seams according to various criteria: the type of connection of parts, the resulting shape of the seam, its length, the number of layers, orientation in space.
The types of possible welded joints are shown in the standard for manual and arc welding GOST 5264. Connections made by arc welding in a shielding gas atmosphere are standardized by the GOST 14771 document.
GOSTs have a designation for each welded joint, as well as a table containing the main characteristics, in particular the values of the leg of the weld.
It’s quite easy to understand what a leg is by looking at the drawing of the parts being connected. This is the side of a speculative isosceles triangle of maximum dimensions that will fit in the cross section of the seam. A correctly calculated leg value guarantees the strength of the connection.
For parts of uneven thickness, the cross-sectional area of the part in its thinnest part is taken as a basis. You should not try to unduly increase the leg. This can lead to deformation of the welded structure. In addition, the consumption of materials will increase.
Checking leg dimensions is carried out using universal reference templates presented in specialized literature.
Types of connections
Depending on the relative position of the parts, welding joints occur:
- end-to-end;
- overlap;
- in an angular way;
- in a tee way.
When butt welding, the ends of two parts located in the same plane are welded. The joint can be made with a flange, without a bevel, or with a bevel. The shape of the bevel may resemble the letters X, K, V.
In some cases, welding is done with an overlap, then one part is partially mounted on another, located parallel. The combined part is an overlap. Welding is done without bevel on both sides.
Often there is a need to make a welded corner. This connection is referred to as the corner type. It is always done on both sides and may not have bevels or have a bevel on one edge.
If the welded parts result in the letter T, then a T-joint has been made. Sometimes parts welded with a T-seam form an acute angle.
In any case, one part is welded to the side of the other. Welding is carried out on both sides without a bevel or with bevels on each side.
Shape and extent
The shape of the seam can be convex, even (flat). Sometimes it becomes necessary to make a concave shape. Convex joints are designed for increased load.
The concave areas of the alloys withstand dynamic loads well. Flat seams, which are made most often, are characterized by versatility.
The length of the seams is continuous, without intervals between the fused joints. Sometimes interrupted stitches are sufficient.
An interesting industrial variation of the intermittent seam is the joint formed by resistance seam welding. It is done on special equipment equipped with rotating disk electrodes.
They are often called rollers, and this type of welding is called roller welding. Continuous connections can also be made using such equipment. The resulting seam is very strong and absolutely airtight. The method is used on an industrial scale for the manufacture of pipes, containers, and sealed modules.
Layers and spatial arrangement
A metal seam can consist of a bead made in one pass. In this case it is called single-layer. If the parts being welded are thick, several passes are performed, as a result of which beads are sequentially formed one on top of the other. This welding joint is called multilayer.
Considering the variety of production situations in which welding occurs, it is clear that the seams are oriented differently in each specific case. There are lower, upper (ceiling) seams, vertical and horizontal.
Vertical seams are usually welded from bottom to top. The trajectory of moving the electrode along a crescent, herringbone or zigzag is used. It is more convenient for novice welders to move the crescent.
When welding horizontally, several passes are made from the lower edge of the parts being joined to the upper edge.
In the lower position, butt welding is carried out or by any angular method. A good result is obtained by welding at an angle of 45 °, “in a boat”, which can be symmetrical or asymmetrical. When welding in hard-to-reach places, it is better to use an asymmetrical “boat”.
The most difficult thing to do is weld in the ceiling position. This requires experience. The problem is that the melt tries to drain out of the work area. To prevent this from happening, welding is carried out with a short arc, the current strength is reduced by 15-20% compared to normal values.
If the thickness of the metal at the welding site exceeds 8 mm, then several passes must be performed. The diameter of the first pass should be 4 mm, subsequent ones - 5 mm.
Depending on the orientation of the seam, select the appropriate position of the electrode . To make horizontal, vertical, ceiling connections, and weld non-rotating pipe joints, the electrode is directed at an angle forward.
When welding corner and butt joints, the electrode is directed at an angle backwards. Hard-to-reach places are welded with an electrode at a right angle.
Weld joint processing
When welding, slag is formed. If slag inclusions get into the weld, its quality deteriorates. All slag deposits must be cleaned off.
If welding is performed in several passes, then the seams are cleaned after each welding stage. In this case, any methods are used. First, the welded parts are hammered and cleaned with a stiff brush.
Then a rough cleaning is carried out. Small parts are cleaned with special knives or grinding wheels. Large blanks are cleaned on machines. At the final stage, the welded joint is polished.
Often a fiber wheel of a grinding machine is used for this. There are other ways to polish welded joints.
Welding is constantly evolving. New materials are appearing and technology is improving. It is necessary to follow the news in welding to learn a lot of new and interesting things.
Source: https://svaring.com/welding/teorija/svarochnye-shvy
Welding seams
Welds are sections of a welded joint formed as a result of crystallization (solidification) of molten metal or as a result of plastic deformation during pressure welding or a combination of crystallization and deformation.
The separation of the concepts of welding joint and welding seam is necessary because the latter, as the connecting part of the elements being connected, determines the geometric shape, continuity, strength and other properties of the metal directly at the welding site.
The properties of a welded joint are determined by the properties of the metal of the weld itself and the zone of the base metal adjacent to the weld, with a changed structure and, in many cases, with changed properties of the heat-affected zone.
It is also necessary to take into account some part of the base metal adjacent to the heat-affected zone and determining the stress concentration at the point of transition from the weld metal to the base metal and plastic deformations in the heat-affected zone, which affects the nature and distribution of forces acting in the welded joint.
Criteria for classification
Welding seams are divided depending on:
- on the shape of the section;
- on the nature of the mating of the welded parts;
- in appearance;
— on implementation;
- by the number of layers;
— by the number of passes;
- from the length;
- in the direction of the current force;
— by position in space;
- according to the intended purpose;
— on the operating conditions of the welded product;
- by the method of holding molten metal;
- by type of welding;
- according to the material used for welding.
Classification
Depending on the cross-sectional shape, welding seams can be:
butt; corner; slotted (electric riveted).
Depending on the nature of the mating of the welded parts, the following types of welded joints are distinguished:
— butt connections;
- corner connections;
- T-joints;
— lap joints;
- end connections.
Welding seams are divided into:
- normal (flat)
- convex (reinforced)
- concave (weakened).
Convex weld Normal weld Concave weld
Convex welds work better under static (constant) loads, but they are not economical. Normal and concave welds are better suited for dynamic and alternating loads, since due to a smoother transition from the base metal to the weld, the likelihood of stress concentrations leading to weld failure is reduced.
According to the execution, welding seams can be one-sided or two-sided.
Single-sided seam Double-sided seam
Depending on the number of layers, welding can be single-layer or multi-layer; depending on the number of passes, welding can be single-pass or multi-pass.
Single layer, single pass Multilayer Multipass
A multi-layer weld is used when welding thick metal, and also to reduce the heat-affected zone.
Pass - a single movement of a heat source in one direction during welding or surfacing. A bead is a portion of the weld metal that has been deposited in one pass.
Weld layer - weld metal consisting of one, two or more beads that are placed at the same level of the cross-section of the weld. When welding, each layer of a multilayer butt weld, except for the reinforcement and back weld, is annealed when the next layer is applied. As a result of this thermal effect, the structure and mechanical properties of the weld metal are improved.
Depending on the length, welds can be continuous or intermittent. Butt seams are usually made continuous. Fillet welds can be made:
- continuous;
- one-sided intermittent;
— double-sided chain;
— double-sided chess;
- point.
In the direction of the current force
According to this criterion, welds are divided into:
- longitudinal (flank) - the direction of the acting force is parallel to the axis of the weld;
- transverse (frontal) - the direction of the acting force is perpendicular to the axis of the weld;
- combined - a combination of longitudinal and transverse seams;
- oblique - the direction of the acting force is placed at an angle to the axis of the weld.
According to their position in space, seams are divided into:
According to their intended purpose, welding seams are divided into
- durable;
- dense (sealed);
- durable and dense.
Depending on the operating conditions of the welded product, seams are divided into:
- workers intended directly for loads;
- non-working (binding or connecting), used only for connecting parts of a welded product.
The width of the welds is divided into
- thread welds with a seam width equal to or slightly greater than the diameter of the electrode, are performed without transverse oscillatory movements of the welding electrode;
- normal with width
- widened, which are performed with transverse oscillatory movements of the electrode.
According to the method of holding molten metal, the seams of welded joints are divided:
— on seams made without linings and pillows;
— on removable and remaining steel pads;
— on copper, flux-copper, ceramic and asbestos linings;
- on flux and gas cushions.
According to the weld configuration:
— straight;
— ring;
— vertical;
— horizontal;
According to the type of welding, the seams of welded joints are divided into:
— arc welding seams (GOST 5264-80);
— seams of automatic and semi-automatic submerged arc welding (GOST 8713-79);
— gas-shielded arc welding seams (GOST 14771-76);
— electroslag welding seams (GOST 15164 - 78);
- electric riveted seams (GOST 14776 - 79);
- contact electric welding seams (GOST 15878 - 79);
— gas welding seams;
- seams of soldered joints.
According to the material used for welding, the seams of welded joints are divided into welding seams:
— carbon and alloy steels (GOST 5264-80; 14771-76; 15164-78; 8713 - 79, etc.);
— compounds of non-ferrous metals (GOST 16038 - 70; 14806 - 69);
— bimetal compounds (GOST 16098 - 70);
— connections of vinyl plastic and polyethylene (GOST 16310-70).
Source: https://blog.svarcom.net/technologia-svarki/svarnye-shvy.html
How to weld non-rotating pipe joints - a step-by-step guide
:
The technology for welding fixed pipe joints is selected in accordance with how the pipes are located and with what slope.
In this regard, there are several types of welded joints:
- Vertical.
- Horizontal.
- With a slope of 450.
In addition, the welding method largely depends on the wall thickness of the pipe products. For example, the connection of pipe products with walls 12 millimeters thick is carried out by applying three layers. In this case, the thickness of each layer should not exceed 4 mm. Welding fixed pipe joints has many features that determine the inclination of the electrodes that you should know.
Safety regulations
When starting to connect rotary joints of pipes or non-rotary analogues, you need to know that work of this type has a high level of danger. Therefore, certain requirements must be met.
Butt jointing of pipe products by gas or electric welding must be performed on specially equipped sites that have special equipment, including various means of protection against electric arcs. These elements are distributed so that people present nearby are completely isolated.
To connect tubular products with a large diametrical cross-section and a weight of over 20 kg, it is recommended to use special lifts. The entrance to the site must be cleared; its width cannot be less than 1 m. Operating temperature values in the room must be maintained within +160C. A prerequisite is the presence of a ventilation system and free space.
According to the technology for carrying out work involving the use of welding equipment, all metal parts and elements must be grounded (read also: “Types of pipe welding technologies - advantages and disadvantages of methods”). Similar requirements apply to the transformer housing and workplace. The use of a welding device is allowed only with insulated wires and cables.
As mentioned above, the connection of non-rotating pipe joints is carried out in several ways, which directly depend on how the pipe is located.
Vertical welding of fixed joints
Applying seams vertically to the non-rotating ends of the pipes being welded is performed similarly to horizontal welding with one difference: a constant change in the inclination of the electrode relative to the perimeter of the seam.
The welding process involves the following steps:
- A joint is created, obtained during the pipe welding process, which relates to the root bead.
- Three rollers are formed that should fill the cut.
- A lock is created connecting the beginning and end of the roller.
- A decorative seam is made.
The first step is considered the most important, since this is the time when the joint is created, forming the base of the seam. The range of welding current is determined by the thickness of the metal and the gap between the joining parts. At the first stage, two main rollers are created.
To create a joint on the pipe, grab the base of each edge being connected, at the same time the second root layer is formed and the first layer is corrected.
Forming a reverse bead using electrodes with a diameter of 3 mm is carried out only in cases where the welded joint must be of high quality.
To perform the job, select the average or minimum current range, taking into account the following:
- Thickness of the metal blank.
- The distance between the edges of products.
- Blunt thickness.
The inclination of the electrode is determined by the direction of welding and depends on the penetration of the first layer of the seam.
The length of the arc also depends on the degree of penetration:
- A short arc is used when the main shaft is not sufficiently fused.
- Medium arc - with good penetration.
Welding speed performance largely depends on the volume of the weld pool. A roller of great height at the joints of metal parts leads to the fact that it does not harden for a long time. This can cause the formation of various defects. When selecting the welding speed, it is necessary to remember that only high-quality edge fusion ensures the normal condition of the bead.
Processing of metal of a certain thickness, as well as sampling and welding, is recommended to be done with electrodes with a diameter of 4 mm. In this case, the inclination of the electrode must be different from the inclination angle when working with the root roller. Here you should use a method called “back angle”. The speed in this case should be such that the roller remains normal.
Rules for filling pipe seals
You need to start filling the seal from the bottom of the edge, which is the platform. This is necessary to select the optimal welding method. The horizontal roller should be performed in high mode. Also, the welding method is determined by the location of the slag, “backward angle” or “right angle”.
To obtain a bead, reinforcement or a “hump” is necessary, which is formed when welding in a position from below to create a shelf, thanks to which the next bead is welded at an increased mode. The second treatment should be carried out with gentle movements, adhering to the lower edge.
Before starting welding of the third bead, its level of completeness is determined. It is important that the gap left between the connecting seam and the top edge is not very large for the fourth roller and not very narrow for the two rollers. The third roller along the top edge should have the smallest width to the top edge. The optimal size may coincide with the diameter of the electrode.
Filling the groove is carried out by forming 3 more beads, which make it possible to fill the base of the weld and strengthen the joint. When carrying out work, it is important to maintain a right angle and fill the groove at high welding speed. This is the only way to achieve strong bonding of the layers to each other.
Making locks
The stage of making the locks involves the final work on the formation of the rollers. In this case, the welding of each bead is accompanied by a 2 mm offset on the main seam. The finished lock represents the starting point of the roller, offset by 5 mm relative to the previous layer.
A decorative seam completes the welding of pipes in a fixed position. When surfacing is carried out in a horizontal position, narrow beads are formed. The last one should be completely flat. Welding is performed in high speed mode.
When fully welding a joint, the entire perimeter without breaking must be taken into account. The displacement of the locks relative to each other is allowed no more than 50 mm.
The multi-pass type of welding of rotary and fixed pipes with thick walls involves spiral guidance. In this case, the number of locks is reduced and, as a result, the number of defects is reduced. Welding should be stopped at a distance of about 20 mm from the beginning of the bead in order to align the weld seams in height. The larger roller can be hemmed and also ground off.
Hemming the bead can be done in a practical way, allowing you to reduce the number of locks and make a better connection. This method starts from the edge of the roller, which is melted by holding the arc. Then they approach the roller with an active electric arc and move on to the next layer, taking into account the previous one. As a result, the end of one layer becomes a continuation of another roller.
Horizontal surfacing
Welding of horizontally positioned butt pipes is considered a rather complex technology. Only a professional welder with certain skills and experience can perform such work. The most difficult thing is the constant adjustment of the electrode to change the angle of inclination.
Welding is performed in three consecutive positions:
- Ceiling.
- Vertical.
- Bottom.
Each seam is made with an individual current value. The ceiling position allows welding at a high power level. All stages involve continuous welding; at the beginning it is best to use the “backward angle” method, and complete the work with the “forward angle”.
Welding pipes at an angle of 45 degrees
Welding pipe products located at an angle of 450 has some peculiarities. In particular, we are talking about the spatial position of the seam, taking into account a certain angle. This type of work can be performed by general craftsmen with a variety of welding skills. The first roller is created using an electrode at a right angle.
The seam is formed by continuously filling the second layer. After this, they immediately proceed to melting the first layer. After welding with the constant use of an electrode, it is necessary to fix the pipe to create horizontal and vertical seams. In this case, the welding layer on the front side is not flat when compared with the rest of the beads.
The vertical connection of metal pipes by means of manual arc welding is carried out similarly to welding in a horizontal position. A distinctive feature of the first method is the use of a method, the implementation of which involves the use of translational movement of the electrodes. Consequently, it is necessary to constantly adjust the angle of inclination of the electrodes relative to the seam, which runs along the entire perimeter of the welded tubular product.
Source: https://trubaspec.com/soedinenie-trub/kak-vypolnyaetsya-svarka-nepovorotnykh-stykov-trub-poshagovoe-rukovodstvo.html
Important facts about the structure of the roller
Roller structure
In Fig. The roller is shown in cross section, where you can find three main dimensions that characterize the roller:
- Width
- height
- and the so-called melting depth.
the structure of the seam is clearly visible in the figure
weld
In normal welding cases, the bead width ranges from 6 to 15 mm, the height from 3 to 8 mm and the fusion depth from 2 to 5 mm.
If we cut a metal plate with a roller across the axis of the roller and grind the cut surface, then in the resulting section we will see that the metal of the roller is sharply separated from the base metal of the plate.
The metal that forms the bead is called weld metal and has the properties of cast metal, i.e., one that is obtained as a result of solidification of liquid molten metal without subsequent forging, rolling, or similar processing.
Welding progress
examples of welds
seams
Chemical composition
Weld metal is an alloy of base and electrode metal, forming something in between.
At the same time, the chemical composition of the deposited metal changes significantly due to the action of high temperature and ambient air.
Ordinary carbon steel, as is known, is an alloy containing, in addition to iron, small quantities of:
- carbon,
- manganese,
- silicon,
- sulfur
- phosphorus
- (special steels may contain other chemical elements).
Even a small change in the content of these elements significantly changes the properties of steel—its hardness, toughness, hardening ability, etc.
In the deposited metal, the amount of carbon, manganese and silicon decreases sharply, partly due to evaporation under the influence of high temperature and mainly from the burning of these elements by atmospheric oxygen having access to the bath. The amount of sulfur and phosphorus (impurities harmful to steel) does not change during the welding process.
The deposited metal absorbs a significant amount of oxygen and nitrogen from the air, which are almost absent in the base and electrode metal. The presence of oxygen and nitrogen makes the weld metal brittle and thereby reduces its strength.
Metal structure
The welding process deteriorates not only the chemical composition, but also the structure of the metal, its structure.
Upon simple examination, the metal appears to be continuous and homogeneous, but it is enough to look at a well-cleaned surface of the metal under a microscope to make sure that in fact the metal consists of individual crystal grains fused together and has the same structure as, for example, granite or a piece of sugar, individual crystals of which are visible to the naked eye.
Individual metal crystals are so small that they are invisible to the naked eye and become noticeable only at high magnification under a microscope. The structure of the metal, i.e. the size, shape and location of these crystals, has a strong influence on the hardness, strength and other properties of the metal. Changing the structure of the metal by hardening, annealing, forging, etc.
, we can significantly change its properties without changing the chemical composition. When a liquid metal solidifies, individual crystals form, just like ice crystals when water freezes, with crystals of a purer metal first formed, and then (between them) crystals containing a large amount of harmful impurities.
- Large sizes, heterogeneity and irregular shape of crystals (formed during the solidification of liquid metal) give any cast metal, including those deposited by arc welding, reduced properties compared to metal that has undergone subsequent pressure treatment, for example, forging, rolling, etc.
- A rolled sheet of metal is much better in its properties than the cast billet from which it is made.
bead welding
the structure of the roller is visible on the seam
- Processing cast metals by pressure in combination with heat treatment makes the crystals smaller, more uniform, compacts the metal, and distributes harmful impurities in it more evenly.
Source: http://svarak.ru/osnovyi-svarki/stroenie-valika/
Reverse step method of welding long seams - Metals, equipment, instructions
One of the reasons for dividing welding seams into types is their length (extension). This indicator is very important, since determining the order of their implementation depends on it. Based on this basis, seams can be divided into three groups: seams of short, medium and long length.
Types of seams depending on the length (extent)
Short seams have a length of up to 300 mm. The average seam length ranges from 300 to 1000 mm. Seams longer than 1000 mm are called long or long seams. Each type has its own characteristics that you need to know about during the welding process.
Short welds are welded in one direction. The middle seams are divided into several zones, each of which is welded in the direction opposite to the previous one.
In this case, you need to choose a zone length such that two to four electrodes can be used on it. To weld medium-length seams, a reverse-stage welding method can be used.
The use of long seams occurs in tank construction and shipbuilding. In this case, reverse-stage welding is also used.
Reverse-stage welding is used to minimize welding deformations and stresses during welding work with medium and long seams, as well as to avoid warping of parts.
Stress and Strain
To know for what purposes it is necessary to minimize the occurrence of stresses and deformations, you need to understand what these concepts mean. It is known that all metals expand when heated and contract when cooled. Stresses are forces that are applied to one unit area of a part (both surface and cross-section). Deformation is a change in the shape and/or dimensions of a product under the influence of temperature changes and/or mechanical and other influences.
Stresses inside the product during welding arise as a result of uneven heating, cooling or casting shrinkage of the weld pool in the liquid state. This process is typical for both ferrous and non-ferrous metals. Casting shrinkage of the weld pool leads to residual stresses and deformations in those parts of the metal that are adjacent to the seam.
This can happen due to the fact that when the weld pool cools, it becomes smaller, narrows in volume, and begins to stretch the nearby layers of metal. In this case, the product may be deformed and subsequently become of poor quality. That is, deformation is a consequence of improper work of the welder and a large amount of internal stress.
If the work is carried out correctly, internal stresses will be present, however, their indicators will not go beyond the established norm and this will not cause deformation of the product.
Deformations are divided into several types: elastic and residual (plastic). Elastic deformation appears when heated and a certain amount of force is applied to the product, and disappears when the part either cools down or the force stops. In case of residual deformation, the part does not return to its original shape. Deformations increase on long seams and large cross-sections.
The main way to eliminate deformation is to cook the product in jigs. A jig is a special device for fixing a product. This is called the pre-bending method. It is widely used for deformation caused by fillet welding and lap welding. If metal sheets act as parts for welding, they are bent in the direction opposite to the expected deformation.
Reverse step welding is used for single-layer and multi-layer seams. When working with multi-layer seams, the beginning and end of each step in the passage must be shifted relative to the previous ones by 20-40 mm. The seam is divided into separate parts 100-300 mm long.
Reverse-stage welding requires the use of large-diameter electrodes and work with increased values and indicators of electric current. The operating scheme is such that each new section must be welded with a new electrode and in the direction opposite to the previous one.
Depending on this, the size of the area into which the weld will be divided is determined.
Reverse-stage welding comes in several varieties: from the middle to the edges and randomly.
As with any welding process, the reverse-stage welding method requires compliance with electrical safety rules. It is important to know what can be used as a return wire. The return wire is the wire connecting the workpiece being welded to the welding machine. You can use flexible wires or steel bars as it.
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Source: https://spb-metalloobrabotka.com/obratnostupenchatyy-sposob-svarki-dlinnyh-shvov/
