Non destructive welding tests are a set of techniques aimed at evaluating the integrity of a structure, without damaging its state. Very important in modern industry as they allow to maintain high quality standards, there are various types. Let’s see what they are and how they are regulated.
Non destructive tests, reference standards
Non-destructive controls are regulated by different standards, such as UNI EN 12062, where methods of verification are recommended according to the material, the thickness of the metal and the type of joint.
The various types of tests, and any welding defects, are linked to the acceptability criteria in two different standards:
- UNI EN ISO 5817, which regulates the quality levels of imperfections
- UNI EN ISO 3834, which lists the requirements for high quality welding
When the welded structure has a strong impact on everyday social life, for example an industrial air cleaning plant, the controls must be tightened and comply with EN 1090-2: according to NTC (Technical Standards for Construction), verification of joints must be carried out by highly qualified personnel and certified UNI EN ISO 9712.
Types of non destructive tests
Non destructive tests are divided into two groups:
- surface tests: visual inspection of welds, magnetoscopic control, control with penetrating liquids; allow to investigate surface defects.
- volumetric tests: radiographic and ultrasound tests; they are useful for checking the presence of defects inside the weld.
Choose which type of control to apply it is necessary to have a deep knowledge of it, to know which rules regulate them and to have great experience to identify which is the most appropriate. Relying only on qualified experts is the only possible solution.
Let’s analyze in more detail each type of control, what it consists of and how it takes place.
Tips for truly effective electrode welding
Regulated by the ISO 17637 standard, the visual inspection of welds is the most common and basic verification.
It is very economical and is used to check for cracks, shape and colour defects on both individual welds and finished structures.
The magnetoscopic control exploits the magnetic properties of ferrous materials to detect a discontinuity in welding, using a normal magnet.
The result of the lack of continuity is a distortion of the magnetic field, formed by the two poles: sprinkling the surface with ferromagnetic particles, they will have the tendency to agglomerate at the inhomogeneous perimeter.
It is regulated by ISO 17638 and has great precision, detecting defects that are invisible to the naked eye. It is limited to application on ferromagnetic metals.
Non destructive control with penetrating liquids
The non destructive control using penetrating liquids detects, like the magnetoscopic control, the presence of discontinuities on the welded joint.
It is based on the capacity of the liquid used to penetrate into the smallest cracks, impossible to verify with the human eye: perfect for detecting welding cracks, porosity and folds.
The reference standard is ISO 3452-1.
Radiographic control of welds
Following the ISO 17636-1 and ISO 17636-2 standards, it is possible to carry out a radiographic control of welds: the use of X-rays is a very important verification method, as it does not alter the chemical-physical structure of the metal, nor its mechanical properties, providing a detailed record of the result of the examination.
It can be applied at any stage of the production process, ensuring a very high quality of the finished structure at a relatively low cost, going to examine the inside of the weld to see density variations due to inclusions or voids.
Non destructive ultrasonic checks
Applicable to all ferrous materials, non destructive ultrasonic checks have the great advantage of being able to penetrate very high thicknesses, identifying discontinuities due to cracks, slag, porosity or missing castings.
Regulated by the ISO 17640 standard, they are based on the propagation of the molecules that make up the sound inside the materials. If there are inhomogeneities, the sound waves alter their path, dispersing.
Check of welds: conclusions
The choice of the type of verification of the weldings is very much based on the experience and competence of the operator: based on the defect, an experienced welder will know which control to choose.
Not only: costs and feasibility play a very important role in the choice, especially in a complex process and at risk of defects such as welding.