Magnetic Particle Testing Issues
There is a general lack of understanding of the basic MT principles.
This method has been proven to be one of the most reliable NDT methods for the detection of surface and near-surface discontinuities, as mentioned in an article “Key Elements of Magnetic Particle Testing” in the August 2015 issue of Quality. And even though it is a time-proven and accepted method, there are still cases of misuse and a general lack of understanding of the basic MT principles.
This article addresses some of these issues with the hope that future abuses and improper application of this method will be minimized.
The Five Basic Elements to Achieve Successful MT
Effective magnetic particle testing requires the following five basic elements:
- A thorough understanding of the requirements as established in the applicable codes, standards, specifications as stipulated in the contract or customer directives.
- Development of a detailed procedure based on those requirements.
- Qualification of the procedure through a validation process which should include an actual demonstration using a qualified person, calibrated MT equipment, and controlled test specimens with known discontinuities.
- The use of qualified and certified MT personnel to perform the examinations.
- Completion of detailed and legible test reports.
There are many variables in MT; all of which should be optimized to provide the best possible test results. These variables include but are not limited to the following:
1. Continuous v. Residual Techniques
Many codes and standards require the continuous technique where the particles (dry or in a suspension) are applied simultaneously with the application of the current. The benefit of this technique is that the flux density is the greatest during the current flow. However, caution should be exercised to prevent the particles attracted to the discontinuities from being inadvertently removed by the flow of the suspension after the current stops or when the excess dry particles are being removed from the test surface which should be done while the current is still being applied.
So what are the benefits of the residual technique? It is effective for those metals with high retentivity such as hardened tool steels, since once they are magnetized, the magnetic field remains but at a slightly lower flux density than was achieved with the continuous technique. It’s great for confirming that discontinuities are at the surface. Surface discontinuities can be confirmed by just applying the particles after the part has been magnetized and after the current stops. Typical subsurface discontinuities will not provide indications using residual techniques.
2. Visible Particles v. Fluorescent
It is generally understood that the major difference between visible and fluorescent particles is that of “sensitivity” or “see-ability.” The contrast ratio between indications of dark visible particles on the surface of the part (typically gray), is small compared to fluorescent particle indications being observed with a black light. The fluorescent particles actually fluoresce and behave like small light emitters. Therefore, the contrast ratio is much greater, increasing the “see-ability.” Of course there’s the black light to deal with, limiting the use of fluorescent particles where white light can be excluded such as in field applications. In cases of automatic or semi-automatic MT examinations, it is far superior to the visible particles. One issue with the use of fluorescent particles is the fluorescent “contamination” of the test surface, especially when the surface is not smooth. These fine particles tend to cling to rough conditions resulting in an overall fluorescent background which reduces the contrast with the discontinuity indications. This is more apparent when dry fluorescent articles are used. The fluorescent pigment tends to become transferred to the test surface, again affecting the contrast. For this reason, dry fluorescent particles are rarely used.
3. Dry v. Wet Suspension Particles
The choice of particle format is generally based on whether the examination will be conducted in the field using portable equipment or in a shop environment where stationary equipment is used. Of course, either type of particles can be used at either location. Regardless, sensitivity should be the main objective. The dry particles tend to be coarser but are available in a number of different colors and the particles chosen should provide for the best contrast with the test surface. The wet suspension particles are appropriately finer to provide for ease of movement as they are transported with the liquid carrier. These fine particles provide better resolution and detail when attracted to discontinuities—especially when examining very smooth surfaces.
4. Alternating v. Direct Current, Surface v. Subsurface Discontinuities
It is generally believed that if AC is used as the magnetizing current, only those discontinuities that are surface-breaking can be detected and that DC should be used to also detect subsurface discontinuities. In actuality, it is possible to detect discontinuities in some materials that are just slightly under the surface with AC. One of the benefits of alternating current is the normal vibratory effects that accompany its use. As a result, it enhances particle mobility which aids in indication formation.
And what about using DC to detect subsurface discontinuities? It is the opinion of the author that due to the variables involved with MT, it should be considered an effective method for the detection of surface, and in some unique cases, also subsurface discontinuities. These variables include the permeability of the test material, the dimension and orientation of the discontinuity, just how far under the surface it is, and the current type and amount being used. One major concern when using DC, especially with prods, is the possibility of arc strikes to the test surface. The prod tips are usually small and the contact area with the test surface also small. To prevent arcing, the prod tips can be fitted with an attachment which will provide a larger contact area with the surface. And there must be metal to metal contact since the current is actually passing into the test object. Other precautions should be exercised including: (a) applying a firm pressure with the prods and activating the current after the prods are in place, (b) removing the prods from the surface after the current is stopped, (c) preventing the prods from moving once they are in place and the current is flowing, and (d), removing any contamination from the surface and keeping the prod tips clean. Arc strikes cause metallurgical damage to the test surface and should be removed. This is especially true when examining smooth machined surfaces.
5. Surface Conditions and Coatings
Even though surface conditions are not as critical to MT as compared to Penetrant Testing (PT), it is still must be addressed. Rougher surfaces can result in particle accumulation (false indications) which can cause confusion during the evaluation process. Just as in PT, the general rule is that the smoother the surface, the better the evaluation. A basic visual examination of the test surface is essential prior to performing MT. Irregular and rough surface conditions should be addressed and corrective action taken prior to starting the examination. It is more cost effective to address and correct these conditions upfront rather than wait until after the examination is complete and the surface is conditioned which will require a re-examination.
Coatings such as plating, paint and other surface treatments are generally not a problem for MT examinations providing they’re thin and uniform. Some codes (such as the ASME Section V) require a qualification process to verify that the coating will not prevent the detection and evaluation of discontinuities.
Evaluation - Indications, Defects, and Discontinuities
In MT and in fact, in NDT in general, there still exists a misuse of the various terms used in the evaluation and interpretation of test results. The most serious is the improper use of the term “defect.” The most appropriate definitions for use in NDT follows:
- Indication - a response or evidence of a response disclosed through NDT that requires further evaluation to determine its full significance. When particles form on the test surface while performing an MT examination, it is an “indication.” It requires further evaluation to determine whether the indication is “false,” “nonrelevant,” or “relevant,” the result of an actual discontinuity.
- False Indication - A false indication is the presence of particles on the surface that are not held by a magnetic leakage field. These particles are held by gravity or some type of contaminant on the test surface. To confirm that it is “false,” the indication should be removed and the surface reexamined after addressing the reason for the particle accumulation. False indications will not reappear once the corrective steps have been taken. So, it can be said that a false indication is one which is not predictable and not repeatable.
- Nonrelevant Indication - A nonrelevant indication is due to magnetic flux leakage from known conditions other than actual discontinuities. Many times they are the result of the design or configuration of the test object. This may be the result of constriction that can be caused by sharp corners or abrupt changes in the geometry of the test object. Other nonrelevant indications may be caused by changes in permeability within the test object. A forging with smooth machined or ground surfaces will reveal “flow lines,” a pattern due to grain alignment. In some materials, a change in permeability may occur at the fusion line in a weld and the resultant linear indication may incorrectly be evaluated as a crack or lack of fusion.
- Relevant Indications - Relevant indications are caused by actual discontinuities. They are defined as flaws, imperfections or other conditions that are not part of the normal structure of the test object. There are many different types of discontinuities that can be classified as inherent (formed during the original making of the material), primary processing (caused during a rough shaping process - forging, casting, etc.), secondary processing (occurring during the finishing steps – machining, surface grinding, heat treating, etc.), service (after the part is subjected to service – fatigue cracks, corrosion, erosion, etc.), and those that occur during the welding process (porosity, cracks, lack of fusion/penetration, inclusions, etc.). MT is most responsive to those discontinuities that are generally linear and at or very close to the surface. Rounded discontinuities such as porosity are not always detected since their shape does not always provide flux leakage.
Magnetic Field Verification
Since the magnetic field cannot be seen, several devices are available to confirm the presence, strength, and/or direction of the flux lines. They include but are not limited to the following:
- Pie Gage – These devices are positioned on the surface of the area of interest and particles are applied when the part is being magnetized revealing linear indications on the gage that indicates the direction of the flux lines.
- Residual Field Indicator – Indicates the amount of residual magnetism in the part. The meter reading can indicate the residual field on a relative scale or one that indicates it in units of gauss.
- Flux Indicator Strips – Sometimes referred to as Burma Castrol strips, they contain small highly permeable ferromagnetic strips of different widths and sandwiched between two brass shims. They are designed for repeated use in establishing magnetic field direction and relative strength when using wet or dry particles.
- Quantitative Quality Indicators (QQI’s) -The Quantitative Quality Indicator (QQI) assures proper field direction and adequate field strength. They are used with the wet suspension, continuous technique only.
- Gauss Meters or Hall Effect Gage - A Gauss meter with a Hall Effect probe is generally used to measure the tangential field strength on the surface of the magnetized part.
Dragging the AC Yoke Technique – A major pipeline company issued a contract to a relatively new NDT lab to perform MT examinations to detect stress corrosion cracks on the outer surface. The technique developed for this examination required the technician to “drag” the energized the AC yoke along the length of the pipe while a “helper” sprinkled dry particles behind the yoke. Not only was a region between the legs of the yoke being magnetized but as the yoke was moved along the length, the AC field was also demagnetizing the area! In addition, the expected cracks were oriented in a circumferential direction—the same direction as the flux lines being created! Surprisingly, no cracks were detected!
AC Yoke with One Leg – Cracks in critical structural welds were observed when an oversight examination was performed. These welds had originally been examined by a “certified” Level II, but after investigation, it was determined that the AC yoke that was used only had one leg! Apparently it was the only one available for the project at that time. Needless to say, the resultant field was not effective and did not detect the cracks.
Demagnetization of Long Bars – A lesson from the past. If difficulty is experienced in demagnetizing long bars, check the direction in which the bars are passing through the box demagnetizer with respect to the Earth’s terrestrial field. If the direction happens to be north to south or south to north, try changing the direct to east to west or west to east. This usually results in a much lower residual field.
As with any inspection, the key to successful and meaningful results depends primarily on the qualifications of the examiners. Following qualified and approved procedures, the use of calibrated equipment, and having a thorough understanding of the test material and the discontinuities related to that material are all essential elements of successful MT.