Weld quality
The term weld seam quality refers to the quality of welded joints that have been made using fusion welding processes.
The aim of welding is to connect components to one another in such a way that they can meet the requirements placed on them over an expected service life. For this purpose, the weld seams must meet the quality criteria or quality features that meet the requirements. General specifications for the quality characteristics cannot be made, because these are always closely linked to the respective component requirements.
Requirements for adequate weld seam quality
The required quality can only be achieved if the weldability of the component is guaranteed. To this end, a quality assurance system must be integrated into production:
- The construction must be suitable for welding.
- Devices and tools must meet the requirements of quality manufacturing.
- The materials to be welded must be sufficiently suitable for welding .
- The manufacturing conditions and the personnel employed must meet the requirements of the welding process.
The welded product is created in the interaction of these factors. The quality of the product is verified by quality tests and operational controls. If poor quality can be ruled out, the product will be forwarded, otherwise reworked or rejected. Corrective measures are taken that can affect all four influencing factors.
Quality criteria
Depending on the loads and conditions of use of the welded component and the type of weld joint, verifiable quality criteria are defined for each weld seam. This involves stipulating permissible tolerances for defined irregularities in the weld seam (see DIN EN ISO 5817: 2014-6). If the irregularities exceed the prescribed tolerance, they are referred to as weld seam defects.
The reference standard for the quality assessment of welds (steel, nickel, titanium) is DIN EN ISO 5817: 2014-6. This applies to the steel , the pressure vessel , the road and rail bridge , the rail car - and pipeline construction . The test certificates for welder qualifications and procedure qualifications are also regulated by this standard. The standard applies to all fusion welding processes (not for beam welding ) regardless of process execution (manual, mechanized, automated), welding position and types of welding joint , for all welds made of steel , nickel , titanium and their alloys with workpiece thicknesses greater than 0.5 mm. The DIN EN ISO 10042: 2005 standard applies to aluminum .
There are three assessment groups of manufacturing quality defined. These do not relate to the usability of the manufactured product, but to the quality of the weld seam based on the type, size and number of selected irregularities.
Values for the dimensions of imperfections are defined for each group .
| Assessment group | conditions |
|---|---|
| B. | high |
| C. | medium |
| D. | low |
In steel construction, there is also evaluation group B + (requirements from evaluation group B and the additional requirements from Table 17 of DIN EN 1090-2).
The classification of a weld into an assessment group is carried out by the designer together with the manufacturers and / or the monitoring bodies in accordance with the applicable application standards. Criteria such as:
- Strength requirements
- Consequences of component failure
- visual appearance of the weld seam
- Manufacturing conditions.
Welds of different assessment groups can occur on a component. For example, the butt welds for welding a boiler can fall into evaluation group B, the fillet welds for connecting the feet to group C. Different evaluation groups can be defined for different types of irregularities on the same weld seam.
The essential geometric dimension of a fillet weld is the so-called a dimension. It is the height of an isosceles triangle that can be drawn in the seam cross-section DIN EN ISO 2553.
The geometric dimension of a butt weld is the butt weld thickness "s" (with partial penetration the target penetration, with a welded seam the smallest distance between the top and bottom of the part).
Test procedure
Compliance with the quality values is determined by destructive or non-destructive testing methods. The tests are carried out by specially trained personnel. The non-destructive testing methods include a .:
Non-destructive testing methods (NDT method)
Due to the non-destructive testing method (NDT method), the actual quantitative dimensions of the irregularities often cannot be measured directly. Therefore, the selection of NDT processes for weld seam testing, the test standards to be used, the associated test classes, the assessment standards and the selection of the permissibility limits depending on the required welding quality, the material and the welding arrangement (T or butt joint) are specified in the DIN EN ISO standard 17635: 2010-08 regulated. This standard can be seen as a link between the required welding quality and the associated permissibility limits.
Visual inspection
On the basis of experience, certain conclusions can be drawn about the quality of a welded joint from the external weld seam appearance. Such an evaluation is made by visual inspection. The DIN EN ISO 17637: 2011-05 standard regulates the visual inspection of fusion welds.
After that, sufficient illuminance (at least 350 lux , better 500 lux) and accessibility are required (the eye must be within a distance of 600 mm from the seam to be tested and be able to view it at an angle of no less than 30 °).
The inspection of the weld seam geometry is carried out by means of various weld seam gauges.
Radiographic examination
A suitable radiator ( X-ray tube , gamma-emitting radionuclide ) is used to generate the density of a component as a projection image. The different material thickness or density can be recognized by the degree of blackening. The thicker or denser a component, the less radiation it can penetrate and the brighter this place in the picture is.
Eddy current testing
For eddy current testing , the effect is used that most of the impurities and damage in an electrically conductive material change the electrical conductivity or permeability. Eddy currents are induced in the material by an alternating magnetic field. A sensor measures the amplitude of the eddy current and the phase shift to the excitation signal.
Magnetic particle inspection
The workpiece is magnetized for the test. Field lines are created that run parallel to the surface. Cracks and flaws close to the surface generate a magnetic stray field, so that field lines emerge from the ferromagnetic material at the flaws. Iron powder collects on imperfections and makes them visible.
Penetrant testing
In the penetrant test , the capillary forces of fine surface cracks and pores are used to make them visible.
Ultrasonic testing
Sound waves propagate at different speeds in different media and are partly reflected at interfaces with different wave impedances and partly they propagate further when the direction changes. These changes in the acoustic properties at interfaces (e.g. pores, inclusions, cracks or other separations in the structure) inside the part to be tested are used for testing.
Process monitoring
Devastating exam
Methods of destructive material testing are used to quantitatively determine the material properties of parts or entire welded workpieces . Reactions to external stresses are measured. This involves measuring mechanical properties such as
Destructive test methods are also used to determine what are known as technological material properties, such as:
Tensile test
The tensile test on fusion welds is used to determine the strength and deformation behavior across the seam. The tensile strength , the breaking position and the type of break are determined. If the break is in the weld metal, irregularities in the break surface can be assessed. The implementation is regulated by the DIN EN ISO 4136: 2013-02 standard .
Bending or folding test
The bending test is used to determine the bending or deformation behavior of welded joints. The implementation is regulated by the standard DIN EN ISO 5173: 2012-02.
Notched bar impact test
The notched bar impact test is carried out to assess the fracture behavior of weld seams under specified conditions. Characteristic values for the strength calculation are not determined. The implementation is regulated by the standard DIN EN ISO 9016: 2013-02.
Hardness test
The hardness test is used to determine the hardness of the weld seam or heat-affected zone at selected test locations. Depending on the test objective, the test can extend to the determination of inadmissibly high or too low hardness values. The procedure for the hardness test on arc welded joints is regulated by the DIN EN ISO 9015-1: 2011-05 standard, the micro-hardener test by the DIN EN ISO 9015-2: 2011-05 standard.
Metallographic examinations
The metallography provides important methods, material structure macroscopically and microscopically to make visible, to analyze and classify. Structures are described qualitatively and quantitatively. The structural components are determined according to:
- Art
- amount
- size
- shape
- local distribution
- Orientation relationships.
In this way, connections between chemical composition, the influence of welding and the structure formation can be clarified.
Individual evidence
- ↑ a b c DIN EN ISO 5817: 2014-06 Welding - Fusion welded joints on steel, nickel, titanium and their alloys (without beam welding) - Assessment groups for irregularities. 2014.
- ↑ DIN EN ISO 10042: 2005 Welding - Arc welded joints on aluminum and its alloys - Assessment groups for irregularities. 2005.
- ↑ DIN EN ISO 2553 - Welding and related processes Symbolic representation in drawings - Welded connections. 2014.
- ↑ DIN EN ISO 17635: 2010-08 Non- destructive testing of welded joints - General rules for metallic materials. 2010.
- ↑ DIN EN ISO 17637: 2011-05 Non- destructive testing of welded joints - Visual inspection of fusion welded joints. 2011.
- ↑ DIN EN ISO 4136: 2013-02: Destructive testing of welded joints on metallic materials - transverse tensile test .
- ↑ DIN EN ISO 5173: 2012-02: Destructive tests of welds on metallic materials - bending tests .
- ↑ DIN EN ISO 9016: 2013-02: Destructive testing of welded joints on metallic materials - notched impact test - specimen position, notch direction and assessment .
- ↑ DIN EN ISO 9015-1: 2011-05: Destructive testing of welded joints on metallic materials - Hardness test - Part 1: Hardness test for arc welded joints .
- ↑ DIN EN ISO 9015-2: 2011-05: Destructive testing of welded joints on metallic materials - Hardness test - Part 2: Micro hardness test on welded joints .