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Written by Administrator
Saturday, 15 June 2013 08:18

Magnetic particle testing is a very forgiving test method

July 2013

One of our readers, an auditor for a company that manufactures mechanical components, lets us know he has audited a sub-contractor who uses MT to check its fully machined and finished parts after the ASTM E709 and DIN 54130 standards and after the acceptance criteria supplied by the prime.
This inspection is performed on magnetic bench with a water-based fluorescent magnetic ink.

The auditor is very surprised by the inspection process, though endorsed by a Level 3: after longitudinal and transverse simultaneous magnetizations, the parts are demagnetized, and, only then, inspected.

The auditor cannot believe it. For him, this process is absolutely unacceptable, and he tells so to the auditee. The auditee says that the magnetic bench’s manufacturer assures that inspecting prior to, or after demagnetization, leads to the same results. The auditee shows the auditor a test on a defective part: the defect is detected, for sure. However, the auditor does not accept this process, as non compliant to the applicable standards.

The auditor asks our opinion.

Therefore, we gave him our answer:

As for the simultaneous longitudinal and transverse magnetizations, we have already written about(1):

The “combined magnetization” wording is used when the part is sprayed with the detection medium while being magnetized by a transverse magnetization followed without a break by a longitudinal magnetization without any inspection between. Doing so, a risk exists that, during the longitudinal magnetization, the detection medium washes off any indication given off by the transverse magnetization, dramatically impairing the quality of the inspection.
On the other hand, on a magnetic bench the MT swinging field technique comes from the using of simultaneous, but out-of-phase longitudinal field through magnetic heads and a transverse field produced by current flow, resulting in a swinging magnetization vector. The entire piece is then magnetized in all directions in a single operation. The inspection’s reliability is high along an acceptable background level.

Further, we are surprised by an inspection performed after demagnetization.

In fact, this is not compliant with the ISO 9934-1 and ASTM E1444/E1444M standards, which state demagnetisation after inspection.

Note that the ASTM E709 – 08 standard does not clearly state that inspection shall be performed before demagnetization, but the chapter 16 Interpretation of indications is placed before the chapter 18 Demagnetization.

We do not know the DIN 54130 standard, a very old one, published in April 1974. In fact, all the countries of the European Union have cancelled their own standards, and replaced them with the European (EN) or international (ISO) standards.

An American expert, who happens to be a friend of ours, let us know that he is aware of a very similar case. This was a German-manufactured magnetic bench, used in the USA, to check crankshafts. The multidirectional magnetization technique was used along with a water-based fluorescent magnetic ink. Parts were demagnetized, then, inspected.

The American expert draw the attention of the operators on this bad practice: inspect after demagnetization. The operators told him that the magnetic bench’s manufacturer said it was acceptable. For that, they showed him a defective crankshaft, with linear indications.

The expert answered:
“Magnetic particle inspection is a very forgiving test method. An inspector can violate the process parameters and still find cracks. Large cracks can be found; however small cracks will not be found. Obviously, when using such a practice, the indication is being held mechanically, not magnetically.
If the part was truly demagnetized, how can it hold an indication?  That does not follow. A better indicator of inspection validity would be to machine artificial flaws into the same part undergoing inspection. These flaws would need to mimic the smallest rejectable flaw size in accordance with the acceptance criteria. If the subcontractor did this, I do not believe the inspection would be validated.”

We completely agree with the expert, whose opinion supports ours.

However, we do know that this process is still used in the automotive industry. Right, there are risks, but, if the suitable precautions are taken, particularly, if parts are dried prior demagnetization, results may be satisfactorily, though the detection ability is impacted by the type and dimensions of the discontinuities to detect, as well as by the part’s surface roughness.

We may add, as our reader pointed out that auditing a nondestructive testing, or any other activity, is not only to check the compliance of the application to the written documents  it is also worth to check the technical quality of the written documents, especially by taking into account physics and chemistry.

One of the first versions of the ISO 9000 series standards came with this “defect”: if the procedure was such that only parts, which did not meet the customer’s expectations, could be manufactured, the day when manufactured parts met the customer’s expectations, this meant that the procedure had not been followed…hence, that Quality Assurance had not been applied! Several years, many incidents, and many papers by involved people (auditors, auditees, customers) later, the customers’ expectations and the quality improvement management were finally considered.


(1) Pierre CHEMIN and Patrick DUBOSC, Magnetic particle testing and the swinging field technique: it works! DPCNewsletter N° 033, February 2011: on our Website.

Normative references

• DIN 54130, Zerstörungsfreie Prüfung; Magnetische Streufluß-Verfahren, Allgemeines (Editor’s note : Non-destructive testing; magnetic leakage flux testing, general), Deutsches Institut Fur Normung e.V. Am DIN-Platz, Burggrafenstraße 6, D-10787 Berlin, Germany. April 1974.

• ASTM E1444/E1444M-12 Standard Practice for Magnetic Particle Testing, ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA, 19428-2959, USA, 2012.

• ASTM E709 – 08 Standard Guide for Magnetic Particle Testing, ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA, 19428-2959, USA, 2008.

Last Updated ( Saturday, 15 June 2013 08:33 )