The requirements to guarantee certain mechanical properties - e.g. strength, etc., assume that the weld seam, when bending longitudinally welded tubes, is located within the neutral phase or drillings are not placed within the weld seam.
In addition, for example at press lines, the coil material to be processed is increasingly welded automatically at the ends in order to avoid loss of time and material. However, this requires knowledge of the position of the weld seams related to the tooling to protect it from damage.
ROLAND ELECTRONIC has, for many years, been helping find solutions for identifying the location of welds during modern manufacturing processes. Three non-destructive methods used are:
- Flux leakage method (magnetic flux)
- Eddy current method
- The combination of leakage flux and eddy current method.
ROLAND ELECTRONIC offers the combination of the leakage flux and the eddy current method.
The foundations of these processes are the different metallurgical micro- structures of the weld seam and the base material. It is then of secondary importance to discover whether it is iron (ferrous) or non-ferrous metal.
In simple terms, a penetration into the material to be processed with the aid of a process-specific electromagnetic field is the secret. In a comparative measurement, a structural deviation (inhomogeneity) to the basic material is then sought and recognized.
In general, the following prerequisites must be ensured for performing a successful measurement:
- The sensor must be placed on the material to be processed and also remain there during a measuring cycle.
- There must be a constant relative movement between sensor and material.
During the magnetic leakage flux method, a magnetic circuit is created or closed by placing the sensor. The magnetic field (field lines) is guided through the material. If inhomogeneities (structural changes) do occur, they force a "leakage of the field (field lines)" out of the material. Using a metaphor of a river makes this process better understandable. Within a river bed, the water flows undisturbed. When a large stone is thrown into the water, the water has to flow along the stone on the left and the right, possibly overflowing the bank. This leakage is reliably detected by the sensor, graphically displayed as the resultant and provided in waveform (weld seam detected) at the electrical interface. Another important element of the SND40 is the eddy current method. Hereby, in simple terms, the electrical conductivity is checked. The conductivity is a material constant and the reciprocal of the resistivity. Accordingly, it is assumed that the weld seam and the base material have different electrical conductance. The AC field generated at the sensor induces currents into the material to be processed, which in turn builds up an electric field. This field strength is measured by the sensor. The special feature of this type of weld seam detection is the parallel measurement at two defined points. For the same material (structure), the difference between the two field strengths must be zero.
If the absolute value is > 0 the materials (structures) are different. The distance between the two measuring points is determined by the design of the sensor.
The eddy current method is applied to austenitic materials.
When using ferritic material (FE), the penetration of eddy currents is difficult to achieve without support. In order to enable and to control this, a magnetic field is created hereby. This reduces the inhibiting effect of the relative permeability of the FE material and allows penetration of the eddy currents in deeper layers.
In many cases, a deeper knowledge of the evaluation of the weld seam detection is necessary to find the optimal settings for the device depending on the material to be processed. In particular, when using the eddy current method, it can be expensive to find the best settings.
Depending on properties of the material such as wall thickness or sheet thickness, the welding process, etc., various options in selecting the measuring method or its resulting program parameters are possible. The best measurement result can then only be achieved through tests or with large experience.
ROLAND ELECTRONIC has taken account of this problems with its operating software. Simple menu navigation makes it possible to find the best parameters for the localization of weld seams depending on the product – without any deeper knowledge of the measuring device or measuring method.
