The construction of long pipelines requires numerous welded joints of individual pipes. The quality of welding can have a significant impact on the performance of the entire highway. Methods of control of welded joints allow with a sufficient degree of reliability to speak about their quality.
Assembly scheme of pipe joints by welding.
Government standards strictly regulate the need to verify the reliability of pipelines. Such verification is necessary to ensure the safety of their services. Non-destructive testing of welded joints of pipes is included in the system of objective quality control of highways and is a mandatory and effective way to assess their reliability and safety.
When conducting welding of pipelines and the corresponding thermal effects in the weld and areas adjacent to it, defects (fracture) may form.
Defects of the weld
These defects during operation can lead to a decrease in the strength characteristics of the metal, a decrease in the operational reliability and durability of the pipeline, changes in transport characteristics, insufficient dimensional accuracy and degrade the appearance. The main causes of defects are: violation of the welding technology, the use of poor quality materials or an insufficient qualification of the worker. A number of defects in welded joints are noticeable by visual inspection, but most of them are hidden and can only be detected by special methods. In other words, defects can be internal and surface (external).Back to table of contents
Types of damage and defects
It is possible to identify the main forms of defects in the welded zones: mass influx, undercut, uneven penetration, cracks and pores (both external and internal), foreign inclusions.
Defects can be divided and because of their occurrence. There are two main groups: defects caused by metallurgical characteristics and thermal effects, and defects caused by human factors, violation of welding conditions. The first can be attributed in the crystal structure - cracks (cold and hot) in the weld and the seam section, pores, slag, structural changes in the metal. From the second group, such defects as unnormalized weld dimensions, uneven penetration, undercuts, burn-throughs, nodules, craters, unfilled metal and some others stand out.
Scheme of pipe edge preparation for welding.
Violations of the seam sizes can affect the reliability of pipelines, so if such deviations are more than normalized by standards, they are considered to be defects. Such defects indirectly indicate the presence of internal defects in the weld. The main defects of this type are: a sharp unevenness of the width and height of the weld along its length, a steep transition from the pre-weld zone to the weld, a noticeable roughness of the weld metal, large saddles and waistings.Back to table of contents
Pipelines: analysis of various defects
Defect of the welded joint in the form of overflow appears when the melt is strongly leaking onto the cold near-suture zone. The sags may be in the form of individual droplets, and may extend over a considerable distance along the welding seam. Causes of nodules:
- excess welding current;
- wrong tilt;
- electrode movement during welding;
- ignoring the angle of the pipes when they are connected.
The sagging is often accompanied by the appearance of uneven and poor-quality penetration of the weld metal, as well as the appearance of external and internal cracks.
Schemes for the formation of nodules.
Undercuts are grooves in metal that appear at the border with a weld. Such a defect reduces the actual cross-section of the seam and leads to the appearance of excessive stresses, which can lead to their growth into cracks with the subsequent destruction of the welded joint.
Defects in the form of burn-throughs appear as holes through which the melt has flowed from the weld pool. The reason for the formation of such a defect can be a low welding speed, an overestimated gap between the ends of the pipes when they are welded, and an excess of the welding current. Inadequate or uneven penetration of the seams is caused by the lack of a reliable connection of edges in small areas. Such a defect reduces the real cross-section of the weld and leads to residual stresses, which can cause subsequent cracking and destruction of the metal.
Cracks can be attributed to the most dangerous types of defects. They can appear at any point of the welding zone (including the near-hewn metal area) and have any direction (longitudinal and transverse). By their size, they are divided into microcracks and cracks. Such a defect is caused both by improper crystallization of the melt, and by exceeding the concentration of carbon, sulfur and phosphorus in the weld pool. Cracks significantly affect all the main parameters of welded joints of pipelines.
Table of allowable values of undercut during welding.
Foreign inclusions weaken the strength characteristics. The most dangerous is the presence of slag inclusions formed when there is insufficient cleaning of slag from the surface of the welds after welding. These inclusions greatly accelerate the corrosion of the metal.
Gas or air pores are usually formed inside the weld. They can have a single character, and they can originate in groups and even form chains of voids. Pores can settle on the surface of the metal, forming indentations (fistula). Pores significantly reduce the strength of seams, and the formation of chains of voids can cause depressurization of the pipeline.
Violations in the structure of the metal of the seam or the non-seamless zone can manifest themselves through an increase in the concentration of oxides, micropores and microcracks, large grain size. Thermal regime in the formation of the metal structure plays a crucial role. Excessive heating leads to the formation of large grains in the structure. When metal is burned, grains with oxidized surfaces may appear. All this leads to the fragility of the metal.Back to table of contents
General theory of non-destructive testing
Under the method of non-destructive testing lies a set of methods that allow you to determine the required parameters without compromising the integrity of the welded joint of the pipeline. Welded joints must be monitored at all stages of preparation, manufacture and preparation for use, and periodically during operation.
Factors affecting the quality of welds.
Non-destructive control methods combine control by external examination to detect external defects, study the tightness of welds of welded joints, and control methods to establish hidden defects using special devices. Non-destructive testing is allocated as a separate stage of the technological process of manufacturing the entire pipeline.
State standards strictly regulate the control of welded joints of pipelines. So, GOST 3242-79 defines six basic types of control, and GOST 30242-97 classifies types of defects in welded joints of pipelines. The main purpose of using special methods is to determine the location and size of hidden defects, so all methods belong to the flaw detection group. The system of non-destructive testing includes the following methods: capillary, radiation, acoustic, magnetic, ultrasonic.Back to table of contents
External visual inspection
The first method of non-destructive testing of welded joints of pipelines is an external visual inspection and measurement, which is carried out continuously at all stages of the manufacture and operation of pipelines. First of all, by visual inspection of the welded joint of pipes, the presence of external defects, such as overflows, undercuts, pore craters, burn-throughs, external cracks and others, is determined. With this inspection, it is recommended to use a magnifying glass with a tenfold increase.
The generalized scheme of substantiation of the norms of admissibility of defects.
The next step is to measure the dimensions of the welded elements and fix the location of defects. When measuring, the following dimensions of the weld are established: its width and height, the size of the convex portions and the corners on the border with the near-suture zone. For control of the sizes special templates are used. The results of measurements of the weld are compared with the normalized values established by state standards for these types of welding.Back to table of contents
Welded pipe joints must be leakproof for those substances (liquids or gases) that are transported through this pipeline. Tightness (impermeability) control is carried out after the pipeline is assembled. It includes the following basic methods of testing: capillary, chemical, bubble, as well as by vacuuming and using a leak finder.
Checking welded joints using the capillary method is based on the property of kerosene to use capillaries to penetrate through internal voids (pores, cracks). To check the tightness of the weld, an aqueous solution of chalk is applied and dried. From the side of the seam opposite the chalk painted surface, the surface is poured abundantly with kerosene. If a leak occurs, then traces of kerosene will appear on the chalky surface. When using kerosene, it will be possible to determine the presence of internal defects with a size of less than 0.1 mm.
Diagram of rejection level selection with ultrasonic testing of butt seams.
The tightness control using ammonia is based on the dyeing of the indicator in contact with alkali. The indicator is a solution of phenolphthalein or nitric acid mercury, the reagent is ammonia in a gaseous state.
Bubble control method includes checking air pressure. Compressed air is fed into the tube and the tightness of the weld is checked by bubbles when the pipeline section is immersed in a water bath. The test may be based on the detection of water bubbles when creating a hydraulic pressure inside the pipe. Before testing, the pipe surface is dried, and during testing, the internal pressure of water is provided that exceeds the working pressure in the pipeline by 1.5 times.
When conducting control of welded joints of critical pipelines, the control with a gas-electric leak detector is used. For testing, helium gas is used, which has a high permeability. A special leak finder probe detects the appearance of gas, and the electronic unit analyzes its quantity and the degree of tightness of the weld.Back to table of contents
Magnetic defect installation method
Diagram of the magnetic method of quality control of the weld.
Non-destructive testing methods for welded joints of pipelines, taking into account the magnetic properties of materials, make it possible to determine the location of defects by studying magnetic scattering in inhomogeneous structures during magnetization of the metal of the welded zone. Part of the pipe is magnetized using a solenoid placed in the internal cavity, or by winding a winding wire over the welding seam. Testing is carried out by the method of powder, induction or magnetographic method, which differ in the method of measuring the dispersion of magnetic flux.
Powder control method involves the use of magnetic powder (iron filings) deposited on the surface of the welded area. When a magnetic field is created in a powder, the particles are oriented and the picture of the magnetic spectrum is clearly formed. So you can detect cracks and pores at considerable depths. To control using the induction method, seekers are used in which an electromotive force is created under the influence of a scattering magnetic flux. In this case, a sound signal is emitted or a light signal appears. The magnetographic control method allows recording the dispersion flux on a magnetic tape placed on the surface of a welded joint. Using a magnetic flaw detector, the presence of defects is determined by comparing the results obtained with the standard.Back to table of contents
Non-destructive radiation methods of control of welded joints of pipelines are based on the properties of x-rays and gamma rays. The metal absorbs radiation differently in the presence of defects or structural changes, which takes into account this method of verification. Welds are penetrated by rays using special radiation sources. The rays are fixed on a special film, where areas of darkening indicate the presence of defects. The location and their sizes are easily distinguishable. The sources of radiation are widely used devices RUP 150-1 and RUE 120-5-1.
Sources of gamma rays are radioactive substances and their isotopes, for example, cobalt-60 and indium-192.
The method of verification is similar to x-ray inspection. The penetrating ability of gamma rays is higher than that of x-rays, which increases the possibilities of the radiation method of control of welded joints.Back to table of contents
Ultrasonic inspection of welds.
The non-destructive ultrasonic method of control of welded joints is widely used for fixing defects in the bulk of metals. This method uses the property of ultrasonic waves to penetrate the material to a considerable depth and disperse at the boundaries of defects. The source of waves is a piezocrystal placed in a special plate (probe).
Ultrasound is applied at a different angle from 40 to 73º C to ensure a complete picture. Another plate of the device captures the reflected waves. As an ultrasonic method, the most widely used method is the echo method, in which the device records the direct reflection of a pulse from a defect (echo). During the inspection of welded joints of pipelines, ultrasonic flaw detectors of the TUD-320 or TUD-310 type are used. In addition, shadow and mirror-shadow methods of ultrasonic flaw detection are used.
Instruments and devices recommended for non-destructive testing of pipeline welds:
- depth indicator;
- weld patterns;
- X-ray, radiation, ultrasound and magnetic flaw detectors;
- leak indicator;
- gas analyzer.
Non-destructive analysis of welded joints of pipelines is an important element to ensure reliability and safety. Its conduct is governed by standards and necessarily in the manufacture and operation of pipelines.