Non-destructive testing of welded pipe fittings: NDT refers to the implementation of a material or workpiece that does not damage or affect its future performance or use.

NDT can find the defects inside and on the 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 applied to 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. NDT also helps to ensure the safe operation and/or effective use of the product.

Types of nondestructive testing methods

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

a) Radiation method:

(X and gamma-ray radiographic testing);

- radioscopic real-time imaging 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;

-- visual testing.

e) Leakage method:

-- leak testing.

f) Infrared method:

-- infrared thermographic testing.

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

Some NDT methods can produce or incidental produce 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, when applying NDT, necessary protection and monitoring should be carried out according to the types of harmful substances that may be produced in accordance with relevant regulations or standards, and necessary labor protection measures should be taken for relevant NDT personnel.

Each NDT method has its range of capabilities and limitations, and the detection probability of defects by each method is neither 100% nor exactly the same. For example, radiographic testing and ultrasonic testing, the test results of the same object will not be exactly the same.

In the conventional NDT methods, radiographic detection and ultrasonic detection are mainly used to detect the internal defects of the tested object. Eddy current testing and magnetic particle testing are used to detect surface and near-surface defects. Penetration testing is only used to detect defects in the surface openings of the subject.

Radiographic detection is suitable for detecting the volume defects inside the object, such as pores, slag inclusion, shrinkage holes, porosity, etc. Ultrasonic inspection is suitable for detecting area defects inside the object, such as cracks, white spots, delamination and non-fusion in the weld.

Radiographic inspection is often used to detect metal castings and welds, and ultrasonic inspection is often used to detect metal forgings, profiles and welds. Ultrasonic inspection is usually superior to radiographic inspection in the ability to detect defects in welds.

Radiographic Inspection (RT)

Scope of competence:

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

b) Can detect the shrinkage hole, slag inclusion, porosity, porosity, hot cracking and other defects in the casting;

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

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

Limitations:

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

b) It is difficult to detect small cracks and non-fusion in the weld.

Ultrasonic Inspection (UT)

Scope of competence:

a) Can detect cracks, white spots, delamination, large or dense slag inclusion and other defects in forging;

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

Note 2: Defects or surface defects that are not parallel to the surface can be detected by oblique beam technology or surface wave technology.

b) Defects such as cracks, incomplete penetration, non-fusion, slag inclusion and porosity can be detected in the weld;

Note: Usually using oblique beam technology, if the steel weld is tested with 2.5MHz ultrasonic wave, its maximum effective detection depth is about 200 mm.

c) Can detect cracks, folding, delamination, flake slag inclusion and other defects in profiles (including sheet, pipe, bar and other profiles);

Note: Liquid immersion technology is usually used, focusing oblique technology can also be used for pipes or rods.

d) Can detect the hot cracking, cold cracking, porosity, slag inclusion, shrinkage and other defects in castings (such as steel castings with simple shape, flat surface or processed and renovated);

e) The coordinate position and relative size of detected defects can be determined, but it is difficult to determine the type of defects.

Limitations:

a) It is difficult to detect defects in coarse crystalline materials (such as austenitic steel castings and welds);

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

Eddy Current Testing (ET)

Scope of competence:

a) Can detect cracks, folds, pits, inclusions, porosity and other defects on and (or) near the surface of conductive materials (including ferromagnetic and non-ferromagnetic metal materials, graphite, etc.);

b) The coordinate position and relative size of the detected defect can be determined, but it is difficult to determine the type of defect.

Limitations:

a) Not applicable to non-conductive materials;

b) The internal defects existing on the far surface of the conductive material cannot be detected;

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

Magnetic Particle Detection (MT)

Scope of competence:

a) 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) The location, size and shape of the detected defect on the surface of the detected object can be determined, 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 and other materials;

b) The internal defects present on the far surface of ferromagnetic materials cannot be detected.

Penetration Testing (PT)

Scope of competence:

a) Can detect the surface of metal materials and dense non-metallic materials open cracks, folding, loose, pinholes and other defects;

b) 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 loose porous materials;

b) Defects that exist inside and/or near the surface of the material without surface openings cannot be detected