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Common defect analysis and nondestructive testing method of seamless steel pipe

The quality of seamless steel pipe is becoming more and more prominent, and with the continuous development and advancement of tube rolling production technology, the manifestations and types of quality problems have also changed significantly, therefore, this paper provides a detailed analysis of the common defects of seamless steel pipe, and describes the online nondestructive testing method and its application.

20220328232451 34789 - Common defect analysis and nondestructive testing method of seamless steel pipe

1. Common defects of seamless steel pipe

1.1 External surface defects

  • (1) folding defects are not regularly distributed. If the surface of the continuous casting billet has a local residue of protective slag, the outer surface of the rolled pipe will appear deeper folding defects, and is longitudinally distributed, the surface of some locations will also appear “off the block” phenomenon. (The folding depth of the rolled tube is about 0.5-1mm) large folding defects longitudinal distribution. The surface of the continuous casting billet crack defects and large folding defects, and is longitudinally distributed. Most of the surface of the seamless steel pipe folding depth of about 1-10mm.
  • (2) small crack defects. Seamless steel pipe for flaw detection when the outer wall of the tube body there are surface defects that can not be observed by the naked eye. Seamless steel pipe surface there are multiple small folding defects, the deepest depth of about 0.15mm, the surface of seamless steel pipe covered with a layer of iron oxide, decarburization layer below the iron oxide, the depth of about 0.2mm.
  • (3) linear defects. The outer surface of seamless steel pipe has linear defects, specifically characterized by shallow depth, wide openings, visible bottom, and a certain width. Seamless steel pipe cross-section of the outer wall of the visible depth <1mm scratches, groove-shaped, heat treatment of which the tube groove edge of the presence of oxidative decarburization.
  • (4) scarring defects. The outer surface of the seamless steel pipe part of the location of shallow crater defects, varying in size and area. No oxidation decarburization around the crater, the aggregation of inclusions; in the high temperature state crater around the tissue extrusion by force, there will be plastic rheological characteristics generated.
  • (5) quenching cracks. Tempering heat treatment of seamless steel pipe, its outer surface appears longitudinal fine cracks, distributed in strips of a certain width.

1.2 Inner surface defects

  • (1) convex package defects. Macroscopic features: the inner wall of seamless steel pipe has a random distribution of small longitudinal convex package defects appear, the height of these small convex package defects of about 0.2mm-1mm. microscopic features: seamless steel pipe cross-section of the inner wall convex package on both sides of the tail, the middle and around the presence of chain-like black and gray inclusions. This type of dark gray chain contains calcium aluminate and a small amount of composite oxides (iron oxide, silicon oxide, magnesium oxide).
  • (2) straight channel type defects. Macroscopic features: seamless steel pipe straight defects, depth and width of a certain, similar to scratches. Microscopic characteristics: the scratch on the inner wall of the seamless steel tube cross-section is 1-2cm deep groove shape, the groove edge oxidation decarburization phenomenon does not appear, the groove around the organization of metal rheology and deformation extrusion characteristics, the bottom of the groove usually appears in the sizing process due to sizing extrusion micro-cracking.

2. Common defects analysis of seamless steel pipe

2.1 Folding defects irregular distribution generated by

Protective slag viscosity is not appropriate, the steel vortex appears significantly abnormal or crystallizer vibration frequency is abnormal will cause the roll slag. Most of the protection slag exists under the skin of the continuous casting billet local or surface area, so a local area of the rolled pipe may appear cracks or folding defects.

2.2 Longitudinal distribution of large folding defects generated by the cause

The casting temperature of the open casting furnace is high and the liquid level of the crystallizer shows large fluctuations, which can cause longitudinal cracks on the surface of the continuous casting billet. Due to the longitudinal defects of cracks on the surface of the cast billet, the cast billet is heated for two hours in the ring heating furnace (1280℃), and the cracked part is seriously oxidized and decarburized when heated at high temperature, and the rolling process cannot be rolled together, which will cause deeper folding defects on the surface of the rolled pipe.

2.3 Small crack defects arising from

The heating temperature of the seamless steel tube in the ring furnace is higher and the holding time is longer, which leads to the existence of heavy oxidative decarburization on the surface of the seamless steel tube, and the matrix grain is relatively coarse. As the decarburized layer tissue composition is lower strength ferrite, when the rolling tube run easily lead to shelving or scratching, which will make the tube surface part of the weak parts of serious decarburization in the subsequent rolling process of small crack defects.

2.4 Linear defects arising from

The low temperature during rolling, the misalignment of the hole pattern between the size reduction machine and the sizing machine, the surface of the rolls sticking to the steel, and the excessive wear of the roll surface of the continuous rolling machine can lead to the surface of the seamless steel pipe being scratched in the process of traveling at high temperature, causing linear defects.

2.5 Scarring defects caused by

When using high-pressure water for phosphorus removal operation, not completely remove the seamless steel tube surface iron oxide, and these residual iron oxide will be pressed into the tube surface in the subsequent process of rolling, thus forming scars, after its heat treatment, most of the scars will fall off, resulting in surface crater defects. There are foreign objects, such as metal, etc. attached to the surface of the hole pattern, resulting in the formation of bumps on the surface of the hole pattern, resulting in craters on the surface of the pipe.

2.6 Quenching crack generation causes

Because the protection slag contains more carbon powder content, the crystallization vibration appears abnormal and roll slag, resulting in local carbon increase in the continuous casting billet. In the heat treatment of seamless steel pipe, tube body carbonization parts will have high carbon martensite generation, while the normal part is generated low carbon martensite, due to tissue transformation with different characteristics, there will be a large tissue stress generated, once the stress exceeds the limit of cracking of seamless steel pipe, there will be quenching cracks generated in the high carbon martensite parts.

3. What are the nondestructive testing methods?

The main methods of nondestructive testing of seamless steel pipe are: ultrasonic flaw detection method, eddy current flaw detection method, leakage magnetic flaw detection method and magnetic particle flaw detection method, of which the latter three methods have can be combined as electromagnetic flaw detection method.

A seamless steel pipe ultrasonic flaw detection method features and applications

Seamless steel pipe due to its shape and geometry of the characteristics of the general use of transverse wave reflection method of flaw detection. According to the probe and the pipe surface coupling can be divided into contact method and water immersion method two basic ways. The following are briefly.
(A) contact method of flaw detection
Contact method refers to the probe and the pipe surface in direct contact with the detection method, the general use of manual operation, this method of detection is slower, but the equipment is simple, easy to operate, easy to change the specifications of the debugging, so in small batches, specifications are still often used.
(B) water immersion method of flaw detection
Water immersion method refers to the probe and the pipe surface is not in contact, between the two water as a medium for flaw detection method. Water immersion method of flaw detection has the following advantages: it can be used to focus the sound beam, thus significantly improving the sensitivity, resolution and the ability to detect small diameter pipe, easy to achieve mechanization and automation; due to solve the problem of probe wear and the use of inexpensive coupling medium, it can reduce production costs. Therefore, the water immersion method in the inspection of high-volume pipe has been widely used.

The characteristics and application of eddy current flaw detection method for seamless steel pipes

Eddy current flaw detection for steel pipe quality inspection has been decades of history, after continuous improvement and development, eddy current flaw detection has become a recognized alternative to hydrostatic testing at home and abroad, one of the inspection methods, wide application, is applied to conductive materials, almost all steel pipes, whether ferromagnetic or non-ferromagnetic, general diameter greater than 2mm, wall thickness of not less than 0.1mm, can use eddy current flaw detection, the current Especially applied to the pipe and bar online or offline automatic flaw detection, has the characteristics of high detection speed. According to the requirements of the variety of steel pipe and other aspects, the general steel pipe eddy current flaw detection can have the following forms.
(A) through the eddy current flaw detection
Through the eddy current flaw detection is a steel pipe eddy current flaw detection in the application of more hair, the general diameter of less than 180mm steel pipe mostly using this detection method, which is characterized by fast detection speed, the ability to achieve the entire steel pipe surface inspection, equipment adjustment and operation are relatively simple.
Commonly used through the eddy current flaw detection equipment by the detection part, including eddy current detector, detection coils and magnetic saturation device; auxiliary parts, including mechanical parts, such as loading and unloading table, transmission roller conveyor, detection host, control system and demagnetization and marking devices and other components.
(B) rotating point probe is eddy current flaw detection
Rotating point probe type eddy current flaw detection is the point probe network rotating around the steel pipe while the steel pipe straight ahead of the automated flaw detection method, point probe placed in the rotating head, probe rotation to complete the sweep of the steel pipe surface.
Point probe detection sensitivity is high, so generally used when there are high quality requirements for steel pipe, rotating point probe hair often used in combination with through type, this is because the rotating point probe is more sensitive to cracks along the axial distribution of steel pipe, and through type coil is more sensitive to defects or cracks distributed around the steel pipe, both of which can achieve a mutually complementary role.
Rotating point probe method is more suitable for the detection of medium and small diameter steel pipe, while the point probe is more sensitive, so it is suitable for the detection of steel pipe with a more polished surface, for hot-rolled steel pipe and other surface roughness and with iron oxide steel pipe, should not use the rotating point probe type eddy current flaw detection.
Rotating point probe is eddy current flaw detection equipment composition and through the eddy current flaw detection equipment is basically the same, the difference only lies in the use of like to install the head instead of magnetic saturation device installed on the detection body.

Seamless steel pipe magnetic particle / leakage magnetic flaw detection method features and applications

Magnetic particle flaw detection and leakage magnetic flaw detection is collectively referred to as leakage magnetic flaw detection. In terms of physical nature, they belong to the same detection method for ferromagnetic materials, both through the detection of leakage magnetic field to find defects in the material.
Magnetic particle flaw detection room leakage magnetic flaw detection method, in practice is widely used, and the use of detection elements to determine the presence of leakage magnetic field, is another leakage magnetic field flaw detection method, in recent years, in the metallurgical industry, pipe, bar flaw detection is increasingly used, often this flaw detection method commonly known as leakage magnetic flaw detection.
(A) magnetic particle flaw detection
Seamless steel pipe magnetic particle flaw detection is generally used for automatic flaw detection equipment to detect the presence of non-detectable area of the tube end, that is, the end of the tube flaw detection, generally using fluorescent magnetic particle flaw detection method, longitudinal and axial magnetization within about 300mm of the tube end, generally using magnetization coils for steel pipe longitudinal magnetization, mandrel energized steel pipe axial magnetization, and the formation of a composite magnetic field, check the inner and outer surface and near the surface of defects in all directions. Equipment magnetization and demagnetization using automatic control technology, manual observation of the inner and outer surface magnetic trace display under the irradiation of ultraviolet light source. The method uses a fluorescent magnetic particle flaw detection method, with high detection sensitivity, simple operation, convenient, intuitive test results, no blind spot in the detection range, high degree of automation, fast inspection speed, generally can do multi-way steel pipe inspection at the same time, improve the inspection capacity, can match the capacity of automatic flaw detection equipment with the former channel, but because of the need to observe the magnetic marks through the human eye, so the quality and reliability of the inspection depends entirely on the human factors.
(B) leakage magnetic flaw detection
Seamless steel pipe leakage magnetic flaw detection through the use of sensors to detect defects leakage magnetic field to achieve the detection of defects, the use of sensors and the relative motion of the detected steel pipe, cutting magnetic lines of force, through the processing of electronic instruments to obtain the defect signal.
Leakage magnetic flaw detection equipment is generally composed of magnetization power supply, magnetization coil (magnetic yoke), detection probe, probe rotation device (or steel pipe rotation device), demagnetization (lateral leakage requires demagnetization), marking and other devices.
Leakage magnetic flaw detection is generally divided into longitudinal magnetic flaw detection equipment and transverse magnetic flaw detection equipment, longitudinal magnetic flaw detection equipment on the steel pipe to apply circumferential magnetization, detection of longitudinal defects on the inner and outer surface of the steel pipe and near the surface; transverse magnetic flaw detection equipment on the steel pipe to apply transverse magnetization, detection of transverse defects on the inner and outer surface of the steel pipe and near the surface.
Leakage magnetic flaw detection has the following advantages.

  • Easy to achieve automation;
  • Has a high reliability of detection;
  • Can achieve the initial assessment of defects;
  • Highly efficient and non-polluting.

At the same time, leakage magnetic flaw detection also exists electromagnetic flaw detection to improve the effect, affecting the sensitivity of flaw detection, using the contact method of flaw detection, probe wear will affect the detection sensitivity and other deficiencies.

4. Summary

Steel pipe industry product quality defects is an important measure to evaluate whether the quality of the product is up to standard. China’s current seamless steel pipe mainly exists on the external surface defects and internal surface defects of two types, and the development of the international steel pipe industry still exists a large gap, in order to promote the improvement of steel pipe testing technology inspection accuracy, steel pipe enterprises to vigorously invest in human and material resources to narrow the development gap.

Source: Network Arrangement – China Steel Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at [email protected]

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