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Magnetic Particle Testing Techniques and Methods for Welds in Pressure Piping
Release Date:
2022/12/03
The fundamental principle of magnetic particle testing is the magnetic interaction between the leakage magnetic field at a defect and magnetic particles. Specifically, when a ferromagnetic material or workpiece is magnetized, any surface or near-surface discontinuities cause the magnetic flux lines to depart from the workpiece and penetrate its surface, resulting in localized flux distortion and the formation of magnetic poles. This generates a detectable leakage magnetic field that attracts the magnetic particles applied to the workpiece surface, forming visible magnetic indications under appropriate lighting. These indications reveal the location, shape, and size of the discontinuity.
I. Scope of Application for Magnetic Particle Testing
Magnetic particle inspection is suitable for detecting surface and near-surface defects in ferromagnetic materials; therefore, it cannot be used on non-magnetic materials such as austenitic stainless steels, iron and titanium alloys, aluminum and aluminum alloys, and copper.
Magnetic particle inspection can be performed on martensitic stainless steels and precipitation-hardening stainless steels because they are magnetic.
Magnetic particle testing can detect defects such as cracks, inclusions, porosity, lack of fusion, and incomplete penetration; however, it is less effective at identifying shallow, wide surface pits, deeply buried defects, and laminar discontinuities that occur at very small angles to the workpiece surface.
II. Magnetic Particle Testing of Pressure Piping and Selection of Magnetizing Current
The inspection and testing of pressure pipelines include:
Visual inspection, thickness measurement, nondestructive testing, hardness testing, metallographic examination, pressure testing, and other methods are employed; among these, magnetic particle inspection is a commonly used nondestructive testing technique.
III. Magnetization Methods
The effectiveness of magnetic particle inspection depends not only on the magnitude of the applied magnetic field but also on the orientation of defects, their aspect ratio (depth-to-width), their shape, the geometry and dimensions of the workpiece, its surface condition, and the specific locations where defects are likely to occur. Consequently, a variety of magnetization methods have been developed.
Classification of magnetization methods:
For boilers, pressure vessels, and pressure pipelines, the commonly used magnetization methods are the yoke method and the contact method.
3.1 Magnetic Choke Method
The magnetic yoke method involves using a portable electromagnetic yoke whose two poles are brought into contact with the workpiece surface to achieve localized magnetization, thereby detecting defects that are perpendicular to the line connecting the two poles.
The effective magnetization region of the magnetic yoke method is generally defined as the area enclosed by an ellipse whose major axis is the line connecting the two poles (L) and whose minor axis extends 114L on each side from the midpoint of that line.
The effective magnetization region of the magnetic yoke method is generally defined as the area enclosed by an ellipse whose major axis is the line connecting the two poles (L) and whose minor axis extends 114L on each side from the midpoint of that line.
When operating at the maximum magnetic yoke spacing, the AC magnetic yoke shall provide a lifting force of at least 44 N, and the DC magnetic yoke shall provide a lifting force of at least 177 N.
Experimental results demonstrate that DC electromagnetic yokes are unsuitable for magnetic particle inspection of thick-walled welds; although the lifting force of a DC electromagnetic yoke may meet the requirements, the magnetic field intensity at the weld surface fails to satisfy the specified criteria.
The advantages of the magnetic choke method are:
a. There is no electrical contact, so the workpiece will not be burned.
b. By changing the orientation of the magnetic yoke method, defects in any direction can be detected.
c. The portable magnetic yoke can be taken to the inspection site for flaw detection, making it convenient to use.
d. High detection sensitivity.
Its drawbacks are:
a. Inspection of workpieces with complex geometrical shapes is more difficult.
b. To detect defects in all directions, the electromagnetic yoke often needs to be rotated by 90° before inspection.
c. For large-area inspections, block-by-block accumulation is required, which is time-consuming.
The rotating magnetic field flaw detector consists of two Type-II magnets arranged in a crossed, stacked configuration, with phase-shifted currents passed through the electromagnetic yoke.
This type of electromagnetic choke can generate a time-varying circular or elliptical rotating magnetic field.
Because the magnetization direction on the workpiece continuously changes and rotates, a single magnetization is sufficient to detect defects in all directions.
3.2 Contact Method
The contact-probe method is also known as the support-rod method, the probe-stick method, the handheld-electrode method, or the pointed-cone method.
It employs two contacts that make surface contact with the workpiece, applying electromagnetic magnetization to generate a distorted circumferential magnetic field, which is used to detect defects parallel to the line connecting the two contacts.
If the contact gap is too large, the area through which the magnetizing current flows increases, thereby weakening the magnetic field; if the contact gap is too small, the magnetizing current density near the electrodes becomes excessively high, which can easily lead to unrelated indications.
Therefore, the contact gap is typically set at 75–200 mm. During operation, ensure that the contacts make good surface contact with the workpiece before applying electromagnetic magnetization; otherwise, the workpiece may be scorched. After de-energizing the power supply, remove the contacts.
The advantages of the contact method are:
a. Portable devices can be brought on-site for use.
b. High detection sensitivity.
Its drawbacks are:
a. Only a small area can be inspected at one time; when inspecting large areas, it is necessary to perform segmented, cumulative inspections, which is very time-consuming.
b. Poor contact can cause workpiece burnout.
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