Nondestructive testing of welded pipe fittings: NDT

Source: Cangzhou shenlong pipe manufature co.,ltd.

Nondestructive testing of welded pipe fittings: NDT refers to the implementation of a material or workpiece in a way that does not damage or affect its future performance

Nondestructive testing of welded pipe fittings: NDT refers to the testing of materials or workpieces in a way that does not impair or affect their future performance or use.

NDT can find the defects in the interior and surface of the material or workpiece, measure the geometric characteristics and dimensions of the workpiece, and determine the internal composition, structure, physical properties and state of the material or workpiece.

NDT can be used in product design, material selection, processing and manufacturing, finished product inspection, in-service inspection (maintenance) and other aspects, and can play an optimal role between quality control and cost reduction. The NDT also helps ensure the safe operation and/or effective use of the product.

Types of nondestructive testing methods

The NDT contains a number of approaches that have been effectively applied. According to the physical principle or different detection objects and purposes, NDT can be roughly divided into the following methods:

A) Radiation method:

-- X-ray and gamma-ray radiographic testing;

-- Radioscopic testing;

-- Computed tomographic testing;

-- Neutron radiographic testing.

B) Acoustic method:

-- ultrasonic testing;

-- Acoustic emission testing;

-- Electromagnetic acoustic testing.

C) Electromagnetic method:

-- Eddy current testing;

- Flux Leakage Testing.

D) Surface method:

-- Magnetic particle testing;

-- Liquid Penetrant testing (PENETRant Testing);

-- Visual Testing.

E) Leakage method:

-- Leak testing.

F) Infrared method:

C. Infrared thermographic testing. D. Thermographic testing.

Conventional NDT methods refer to the widely used and mature NDT methods, which are: radiographic detection (RT), ultrasonic detection (UT), eddy current detection (ET), magnetic particle detection (MT), penetration detection (PT).

Some NDT methods can produce or produce incidental substances such as radioactive radiation, electromagnetic radiation, ultraviolet radiation, toxic materials, flammable or volatile materials, dust and other substances, which have varying degrees of damage to the human body. Therefore, in the application of NDT, necessary protection and monitoring should be carried out according to the types of harmful substances that may be produced and the requirements of relevant regulations or standards, and necessary labor protection measures should be taken for the relevant NDT personnel.

Each NDT method has its own range of capabilities and limitations, and the odds of detecting defects are neither 100 percent nor the same for each method. For example, radiographic and ultrasonic tests may not give exactly the same results for the same subject.

In the conventional NDT method, radiographic detection and ultrasonic detection are mainly used to detect the defects inside the detected object. Eddy current detection and magnetic particle detection are used to detect the defects on the surface and near surface of the object. Osmotic testing is only used to detect defects in the surface openings of the subject.

Radiographic detection is suitable for detecting the bulk defects inside the object, such as porosity, slag inclusion, shrinkage, porosity, etc. Ultrasonic testing is suitable for detecting area defects in the object under examination, such as cracks, white spots, delamination and non-fusion in welds.

Radiographic testing is often used to detect metal castings and welds, and ultrasonic testing is often used to detect metal forgings, profiles and welds. Ultrasonic testing is usually superior to radiography in detecting defects in welds.

Radiographic Detection (RT)

Scope of ability:

A) Defects such as incomplete welding, porosity and slag inclusion in the weld can be detected;

B) It can detect shrinkage cavity, slag inclusion, porosity, porosity, hot cracking and other defects in the casting;

C) Can determine the plane projection position and size of detected defects, as well as the type of defects.

Note: The radiographic thickness is mainly determined by the ray energy. For iron and steel materials, the thickness of 400 kV X-ray is up to about 85 mm, cobalt 60 gamma-ray is up to about 200 mm, and 9 MeV high-energy X-ray is up to about 400 mm.

Limitations:

A) It is difficult to detect defects in forgings and profiles;

B) It is difficult to detect fine cracks and non-fusion in the weld.

Ultrasonic Testing (UT)

Scope of ability:

A) It can detect defects such as cracks, white spots, layers, large or dense slag inclusion in forgings;

Note 1: Internal defects or defects parallel to the surface can be detected by direct injection technology. For iron and steel materials, the maximum effective detection depth can reach about 1 m.

Note 2: Defects or surface imperfections that are not parallel to the surface can be detected by oblique or surface wave techniques.

B) Defects such as cracks, lack of penetration, lack of fusion, slag inclusion and porosity can be detected in welds;

Note: Oblique shooting technology is usually used. If the steel weld is detected by 2.5mhz ultrasonic wave, the maximum effective detection depth is about 200 mm.

C) It can detect defects such as cracks, folding, lamination and slag-sandwich in profiles (including plates, pipes, bars and other profiles);

Note: The liquid immersion technique is usually used, and the focusing slanting technique can also be used for pipes or bars.

D) Can detect the hot crack, cold crack, loose, slag, shrinkage and other defects in castings (such as steel castings with simple shape, flat surface or processed and renovated ductile iron);

E) It can determine the coordinate position and relative size of detected defects, but it is difficult to determine the type of defects.

Limitations:

A) It is difficult to detect defects in coarse-grained materials (such as castings and welds of austenitic steels);

B) It is difficult to detect the defects in the workpiece with complex shape or rough surface.

Eddy current testing (ET)

Scope of ability:

A) It can detect cracks, folds, pits, inclusions, loose and other defects on and/or near the surface of conductive materials (including ferromagnetic and non-ferromagnetic metal materials, graphite, etc.);

B) It can determine the coordinate position and relative size of detected defects, but it is difficult to determine the type of defects.

Limitations:

A) Not applicable to non-conductive materials;

B) Internal defects on the far surface of conductive materials cannot be detected;

C) It is difficult to detect the defects on the surface or near the surface of the workpiece with complex shape.

Magnetic particle detection (MT)

Scope of ability:

A) It can detect cracks, folds, interlayers, inclusions, pores and other defects on the surface and/or near the surface of ferromagnetic materials (including forgings, castings, welds, profiles and other workpieces);

B) It can determine the location, size and shape of the detected defect on the surface of the detected object, but it is difficult to determine the depth of the defect.

Limitations:

A) Not applicable to non-ferromagnetic materials, such as austenitic steel, copper, aluminum, etc.;

B) Internal defects on the far surface of ferromagnetic materials cannot be detected.

Penetrant Testing (PT)

Scope of ability:

A) Defects such as open cracks, folds, looseness and pinholes can be detected on the surface of metallic materials and compact non-metallic materials;

B) It can determine the location, size and shape of the detected defect on the surface of the detected object, but it is difficult to determine the depth of the defect.

Detection limitations:

A) Not suitable for loose porous materials;

B) Failure to detect defects that exist inside and/or near the material without an open surface