Non-Destructive Test (NDT)

Destructive (Mechanical) and Non-destructive (NDT)

 

Non-destructive Test (NDT)

There are a number of NDT methods, all have advantages and disadvantages. All require operator skill and knowledge and up to date qualifications. It is another step in the quality control which started with the steel making through to consumable manufacture, weld procedure qualification, welder qualification, NDT procedure and operator qualification.

  • Visual inspection

Visual Inspection is a form of NDT often used as the sole means of quality control. It is not simply inspecting the final weld appearance.

The inspection begins with a review of the welding procedures, do they comply with the standard requirements. Has the welder followed the procedure? Inspection of production welds includes random inspections of Joint design, consumables electrical parameters and weld dimensions.

A weld can be sent out the workshop for painting or galvanising then assembled on site, to discover welds are missing, or are undersized or does not comply in some other way. This can be a major cost, destroying the coating, welding it then re-coating. All standards mandate a visual inspection.

A detailed inspection report ends with the compliance statement; confirmation the weld was made in accordance with the accepted weld procedure, made by a qualified welder and it visually complies with the acceptance criteria plus confirmation it has been non-destructive tested as per specification.

  • Radiography

The term radiography refers to both Gamma and X rays. A source of radiation on one side and a film on the other and the subject being of reasonable consistent thickness, without this a successful radiograph is not possible. (Multiple films of different speeds to match the different thickness have had some success).

Advantage:

  • Availability of permanent record

Disadvantage:

  • Dangerous radiation; the area must be cleared of people.
  • There many government regulations controlling the use of radiation equipment

 

  • Ultrasonic Testing

High frequency sound penetrates material and bounces off interfaces such as areas of non-fusion, and inclusions.

Advantage:

  • Safe to work in the same area.

Disadvantage:

  • The only record is the operator report
  • It is limited to thickness's greater than 8 mm.
  • Magnetic Particle Testing

A magnetic field is created in the steel, then spraying an ink of fine iron powder in a liquid such as paraffin. Defects that break the surface such will have a north and south pole and the iron power is attracted to the break. To make it stand out either white background paint is used or a fluorescent dye and a black light is used.

  • Dye Penetrant Testing

The Principle is a viscous liquid dye sprayed on the surface and allowed time to sink into defects that have broken the surface, the dye is than removed from the surface and sprayed with a talcum powder type substance, this draws out the dye. The Dye is red and the powder is white.

Both Magnetic Particle and Dye Penetrant are known as 'surface methods'.