Residual field inside a bar generated by a) slow break b) “quick break” transient current. Source: Magnaflux Corp.
In the late 1950s, Magnaflux noted that the sudden breaking of a direct current at the end of the magnetization shot causes a transient current to be induced in the part by a rapid collapse of the external magnetic field. This quick break method increases the residual magnetic field inside the part, which leads to an enhanced ability to find defects in a ferromagnetic part.
The U.S. military realized the importance of quick break in relation to the inspection of its parts, and consequently created Mil-Spec MIL-M-6867 through collaboration with Magnaflux Corp. in 1958 to establish the basic requirements for magnetic particle inspection.
MIL-M-6867 defines quick break and the quality of the break as measured by the voltage induced in a small air coil placed inside the unit-magnetizing coil. This induced voltage was measured with an oscilloscope. The higher the peak voltage induced during the break at the end of the mag shot, the better the quality of the break, and the faster the associated collapse of the external magnetic field.
It should be noted the MIL-M-6867 was the only true reference standard for measuring quick break, and this fact remains true even to this day. In 1991, the governing body of ASTM created the standard ASTM E1444, which also specifies the need for checking quick break regularly on three-phase magnetizing units. According to ASTM, this practice establishes minimum requirements for magnetic particle examination used for the detection of surface or slightly subsurface discontinuities in ferromagnetic material.
So how does the ordinary operator of magnetic particle inspection equipment test for quick break?
In 1970, Magnaflux developed a small handheld device for testing quick break in a qualitative (go/no go) way, using a small inductive coil, with a neon bulb connected to its output. If the mag unit had quick break, the neon bulb would flash at the end of the mag shot. Other manufacturers have followed with their own neon bulb testers, and it was not until two years ago that a digital indicating instrument was developed that could measure the true quantitative quality of the quick break, by indicating on a handheld meter the value of the induced quick break voltage, based on the original MIL-M-6867.
So how did the manufacturers of magnetic inspection units incorporate quick break into their DC magnetizing units?
The early machines from the 1950s through the late 1970s relied on a power contactor to open the DC coil circuit at the end of the mag shot, drawing an arc across the contactor, and assuring in a rapid decay of the DC current flowing through the unit coil. Many machines at this time also incorporated this quick break feature on the heads circuit, although most operators found quick break more relevant for magnetizing coil-based applications.
The drawback of this contactor drop out method was the electrical wear on the contactor. But in the early 1980s, with the advent of higher power low-voltage silicon-controlled rectifiers (SCR), a new method was found for making a quick break using the turn off time of the SCR at the zero current crossing, in conjunction with a reasonably high transformer secondary open circuit voltage. This new method was called secondary SCR mag current control and it is widely in use today for providing quick break in magnetic inspection units.
Is there a quantitative difference in the quick break quality produced by the older contactor drop out method and newer secondary SCR mag current control method? The answer can be found using the digital quick break indicator and recording the induced voltage produced by each quick break method using the same coil circuit. The result shows a much stronger quality break using the older contactor drop out method.
The question is how strong of a quick break is needed to produce quality indications? Part of the answer lies in the fact that the newer SCR-controlled quick break does meet the minimum induced voltage requirements of MIL-M-6867.
Tests also were performed in a laboratory using a Magnaflux test block and AS5282 tool steel ring, and when looking for sensitivity of indications produced with both electronic quick break and contactor drop out method-on both head and coil circuits of the same machine-there was not any practical difference in the brightness of the indications produced by either method, thus confirming that the newer electronic quick break method using secondary SCR control is quite satisfactory.
Induced current method of magnetizing ring-shaped parts to locate circumferential defects. Source: Magnaflux Corp.
Common Applications
When a bar is magnetized longitudinally in a magnetizing unit 5-turn coil, without quick break, the two ends become poles and the flux lines leave and enter the ends of the bar at a 90-degree angle to the surface. The near-end portions of the bar are therefore not truly longitudinally magnetized, and transverse cracks near the ends of the bar will probably not give reliable defect indications.However, when the same bar is magnetized using quick break method, a transient current is generated which flows circularly inside the bar, and generates a field near the surface of the part that is truly longitudinal clear to the ends.
Another application of quick break is induced current magnetization. This method was developed for the magnetizing of ring-shaped parts such as bearing races, without the need to make direct contact with the surface of the part. By making the ring a one-turn short circuited secondary of a DC transformer, a large current flowing circumferentially around the ring can be induced.
For parts that have high retentivity-the property to retain a greater or lesser degree of residual magnetism-DC current with quick break can be used, and the parts are tested with the residual method. When the DC field is caused to collapse suddenly by the abrupt interruption of the magnetizing current, the circular field generated by the resulting transient current leaves the part with a strong residual field. The induction current method may benefit from the extremely strong quick break of the contactor drop out circuit. NDT
