Ambiguities due to using the Ketos Ring in Europe

November 2010

Dear Readers,
We have the pleasure to publish on our Website a contribution by a French MT expert, Stéphane GRAVELEAU, from the R and D department of SREM TECHNOLOGIES (FRANCE).

The Editors:
Patrick DUBOSC and Pierre CHEMIN

Many are the standards, codes, dealing with Magnetic Testing (MT) further to many manufacturers’ specifications in aerospace, railways, etc industries. In these documents magnetization conditions are checked by measuring the applied electric current and the magnetic field. They should state the minimum and maximum figures to use, be they either RMS, mean or peak.
Unfortunately, some of these documents display quite difficult-to-understand explanations, leading to misunderstanding by users and auditors.

Let us have a look to the ASTM E1444 standard which is the basis for NADCAP audits.

The writing is quite clear in paragraph X4.1, titled “Measurement of tangential field strength’’. It even goes to describing the geometrical characteristics of the sensor and to require the probe be put at 90° on the surface, etc. It also requires that ‘‘if alternating current or half-wave rectified alternating current is being used, the gaussmeter should be set to read the peak value during the shot.’’

On the other hand, it is somewhat ambiguous about the measurement of the current’s intensity as written in paragraph 6.3.3 ‘‘Magnetization current levels’’.

Indeed, one reads that ‘‘the current values given in 6.3.4 and 6.3.5 are average current values and are applied to full-wave rectified current. For other types of current, the operator’s manual, the equipment manufacturer or the CEO should be consulted.’’

In Table XI.2 amperage and requirements about the number of holes to be detected on Ketos 01 tool steel rings are given.

Be it half-wave or full-wave rectified current is no wonder for American users as three-phase full wave rectified DC is almost “standard” in the US. Then, the current waveform has a very important DC part while the AC part is very small. Therefore, peak values are roughly equal to the mean and RMS values. The exact ratios in purely sinusoidal mode are respectively: mean value/peak value ratio equal to 3/π i.e. 0.955, RMS value/peak value ratio equal to 0.956 and RMS value/mean value ratio equal to 1.001.

In Europe the situation is a bit different: many equipments produce a single phase half-wave rectified DC. Results then come with very different figures: in purely sinusoidal mode, mean value/peak value ratio equal to 1/π i.e. 0.318; RMS value/peak value ratio equal to 0.5 and RMS value/mean value equal to π/2 i.e. 1.57.

This is one reason why in Europe it is sometimes difficult to detect, either on the Ketos Ring or on the SAE-AS5282 ring, the specified minimum number of holes in relation with the specified current intensities. However, as “detection” is subjective, some people say “I saw it” while others say “No”.  This ambiguity sometimes gives rise to non-conformance reports (NCR) during NADCAP (or other) audits of the system performance.

What can we do to overcome this ambiguity?

When using half-wave rectified DC (HWDC), SREM TECHNOLOGIES, as a manufacturer of equipment, recommends to carry out the test using an RMS current of respectively 1,400 A, 2,400 A or 3,500 A in the copper central conductor, and not peak current of the same figures. Additionally keep in mind that using AC with a Ketos Ring is meaningless: the skin effect (due to the 50Hz frequency of the current – 60 Hz in the USA and Canada: the skin effect is even stronger) makes the magnetic field stay very close to the surface and does not allow for detecting subsurface defects (only the first hole may be seen).

It is a given that these tests are carried out to ensure that:
• performance of the inspection means is in accordance with the needs to assure the user of detecting discontinuities on the parts under inspection.
• performance does not drift over time.

NADCAP in no way requires using only the Ketos Ring or the SAE-AS5282 ring.

Indeed, in the NADCAP questionnaire regarding the system performance, the question in § 6.2.1 comes as:
‘‘Is a device used to monitor the system performance? (Identify the applicable method used).
• AISI 01 Ketos Ring
• AS 5282 Ring
• TP 1-4
• Known Defect Part
• Other approved standard

Compliance Assessment Guidance. One or more of the above devices shall be used to monitor the system performance. Any other standard approved by the customer shall have documentary evidence of approval.’’

Further, when a Ketos Ring or an SAE-AS5282 ring is used to check a unit with a small power capability (such as a current generator able to deliver 2,000 A RMS max), the test is carried out with the intensity (ies) that the unit is (are) able to deliver: for instance, the 1,400 A RMS figure will be the only test for such a unit.

Reference

• ASTM E1444–05: Standard Practice for Magnetic Particle Testing. American Society for Testing and Materials, West Conshohocken, Pennsylvania, USA, 2005.

• SAE-AS5282: Tool Steel Ring for Magnetic Particle Inspection, Society of Automotive Engineers (SAE), 400 Commonwealth Drive, Warrendale, Pennsylvania 15096, USA, 1998, reaffirmed in 2007.

• NADCAP AC7114/2 REV.A, Audit Criteria for Nondestructive Testing Facility Magnetic Testing Survey, Nadcap, 161 Thorn Hill Road, Warrendale, Pennsylvania 15096-7527, USA, 2006.

We, Pierre CHEMIN and Patrick DUBOSC, welcome any comment, any idea. If you have some examples you would like to see discussed here, please give us all the useful indications. If you require confidentially, we would modify locations, names and some parameters to prevent any traceability.
Nevertheless, we are convinced that our site may be a kind of surge-valve: the topic is NOT to target this company, or that auditor; but it is always to make users think, to make them ask themselves, or others, the right questions.
We may also give advice, once again on a confidential basis if needed: please, feel free to ask questions, to document our data basis: about Material Safety Data Sheets (MSDS), about environment, a chemical name you don't understand, a Penetrant process you have heard about, etc.
We have plenty of examples, some being out of all the specifications/standards, which led to the discontinuities detection, when the "current, normal, processes" prevented discontinuity finding.