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NDT for Inspecting Performance and Quality of Valves

  • Oct 09, 2021
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In the casting process of a valve, sometimes there are defects such as gas holes, porosity, impurities, etc., which affect the normal use of the castings. Welding repairs are usually performed. After welding repairs, non-destructive testing should be carried out to verify the quality of welding repairs.
 
The overview of non-destructive testing
  • NDT refers to the implementation of a detection method that does not damage or affect the future performance or purpose of materials or workpieces.
  • NDT can find defects in the interior and outer surface of materials or workpieces, measure the geometric features and dimensions of the workpieces, and determine the internal composition, structure, physical properties and status of the materials or workpieces.
  • NDT can be used for product design, material selection, processing and manufacturing, inspection of finished products, 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 the effective use of the product.
 
The range of capabilities and limitations of conventional non-destructive testing
(1) Radiographic testing (RT)
Ranges of capabilities
  • RT can detect defects such as incomplete welding, gas holes, and slag inclusion in the welding seam.
  • RT can detect the defects such as shrinkage cavity, slag inclusion, gas holes, porosity and thermal cracking in the casting part.
  • The plane projection position and size of the detected defects can be determined, as well as the types of defects.
Please note that the transillumination thickness detected by radiography is mainly determined by the radiation energy. For steels, the transillumination thickness of 400 kV X-rays can reach about 85 mm. The transillumination thickness of cobalt 60 gamma rays can reach about 200 mm, and the transillumination thickness of 9 MeV high-energy X-rays can reach about 400 mm.
 
Limitations
  • It is difficult to detect defects in forgings and profiles.
  • It is difficult to detect small cracks and lack of fusion in the welding seam.
(2) Ultrasonic Testing (UT)
Ranges of capabilities
a) UT can detect defects such as cracks, white spots, delamination, large or dense slag inclusions in the forgings. Please note that direct radiation technology can be used to detect internal defects or defects parallel to the surface. For steels, the maximum effective detection depth can reach about 1 m. Oblique beam technology or surface wave technology can be used to detect defects that are not parallel to the surface or defects on surfaces.
b) UT can detect defects such as cracks, lack of welding and fusion, slag inclusions, and gas holes in the welding seams. Please note that oblique beam technology is usually used. If ultrasonic with 2.5 MHz is used to detect welding seams of steel, the maximum effective detection depth is about 200 mm.
c) UT can detect defects such as cracks, folds, and slag inclusions in profiles including plates, pipes and bars. Please note that liquid immersion technology is usually used, and focused oblique beam technology can also be used for pipes or bars.
d) UT can detect defects such as hot cracks, cold cracks, slag inclusions, shrinkage holes and other defects in castings in steel castings or ductile irons with simple shapes and flat surfaces.
e) The coordinate position and relative size of the detected defects can be measured, but it is difficult to determine the types of defects.
 
Limitations
  • It is difficult to detect defects in castings and welding seams in coarse-grained materials, for example, austenitic steel.
  • It is difficult to detect defects in workpieces with complex shapes or rough surfaces.
(3) Eddy current testing (ET)
Ranges of capabilities
  • ET can detect cracks, folds, pits, inclusions and other defects on and near the surface of conductive materials, including ferromagnetic and non-ferromagnetic metals and graphite.
  • ET can determine the coordinate position and relative size of the detected defect, but it is difficult to determine the type of defect.
Limitations
  • ET is not applicable to non-conductive materials.
  • It is impossible to detect internal defects in the conductive material that exists on the far surface.
  • It is difficult to detect defects on or near the surface of a workpiece with a complex shape.
(4) Magnetic particle testing (MT)
Ranges of capabilities
  • MT can detect defects such as cracks, folds, inclusions and gas holes on and near the surface of ferromagnetic materials, including forgings, castings, welding seams and profiles.
  • MT can determine the position, size and shape of the detected defect on the surface of the object to be inspected, but it is difficult to determine the depth of the defect.
Limitations
  • MT is applicable to non-ferromagnetic materials, such as austenitic steel, copper, aluminum and other materials.
  • MT can't detect internal defects in the far surface of ferromagnetic materials.
(5) Penetration Testing (PT)
Ranges of capabilities
  • PT can detect open cracks, folds, porosity, pinholes and other defects on the surface of metallic materials and dense non-metallic materials.
  • PT can determine the location, size and shape of the detected defect on the surface of the object to be inspected, but it is difficult to determine the depth of the defect.
Limitations
  • PT is not suitable for materials with loose holes.
  • PT can't detect defects existing in the material and near the surface without openings on the surface.

The ASME B16.34 and ASTM A217/A217M-2007 standards stipulate that if the casting is non-magnetic like a nickel-based alloy, the liquid penetrant test can be used on the basis of meeting the requirements of the standard. For example, the manufacturing of castings meets the supplementary requirements of S4 (MT). Repair welding shall be inspected by MT which inspects castings with the same quality standards. If the casting leaks during the hydraulic test, the casting meets the supplementary requirements for radiographic inspection or the depth of any pit to be repaired exceeds 20% of the wall thickness or 25 mm; the area of any pit to be repaired is approximately greater than 65cm2 must be inspected by radiographic inspection. For defects in important parts and major repair welding, effective non-destructive inspection must be carried out, and they can be used only after being qualified. RT should be conducted for serious defects done by repair welding. UT can also be used in some cases.
 
How to evaluate the results of non-destructive testing?
If the test results are evaluated according to GB/T5677-1985, Grade 3 or above is required for non-destructive testing methods for valve grooves and repair welding parts of steel casting globe valves used in the power station. JB/T644-2008 also provides clear regulations for the simultaneous existence of two different grades of defects in castings. When there are two or more types of defects with different grades in the assessment area at the same time, the lowest grade is taken as the comprehensive assessment grade. When there are two or more types of defects with the same level at the same time, the comprehensive level should be reduced by one level. Both the manufacturer and user should pay sufficient attention to strengthen the non-destructive testing of the valve body of the welded repair castings to ensure the safety of the valve.