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DPCNews 031 - Ketos Ring: Discrepancies

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Written by Administrator
Wednesday, 01 December 2010 17:11

Ketos Ring: Discrepancies between auditors and users. Why?

December 2010

1 - Introduction

Some papers already published on our Website deal with the Ketos Ring. This one comes as a follow-up of the paper referenced in Note 1.
Ketos Ring has been described for dozens of years.
In 2008, SREM TECHNOLOGIES, a manufacturer of MT equipment, carried out comparative tests on the Ketos Ring.

Over the years, and taking into account the discrepancies of results from one company to another or from various magnetic benches of the same company, our experience leads us to the following remarks:

• Ring demagnetization is of the utmost importance; a non-demagnetized Ketos Ring may lead to absurd results (several holes detected even without any use of electric current). Nevertheless, demagnetizing a circular part is far from easy!

• The material used for the ring manufacture is also of the utmost importance. Rings manufactured from AISI 1035 (C35E, as per the EN 10083-1 standard; XC 38, as per French documents) gave poor results.
Even if certified AISI 01 steels are used, variations between manufacturers are impressive. Chemical analysis and mechanical data are needed to certify a ring, but the magnetic characteristics of the ring must also be considered.

• Thyristor-controlled magnetic benches so modify the waveform that it is no longer sinusoidal. Thyristors, as adjustable switches, “chops” the sinusoidal wave to adjust the power in the bench. This makes it impossible to link the PEAK figure of the current with the RMS figure: a high peak current may be due to a low RMS current. That is why, after many tests, it is obvious that MT indications are poorer if peak values are used. Then the question comes: are the figures given in ASTM E1444–05 standard peak values, mean values or RMS values?

• Thyristors or not, it seems wiser to use RMS values rather than peak values. This is what was recommended in a paper already published on our Website(1). A further explanation is in paragraph 5 underneath.

• Adding material differences to using a thyristor-controlled magnetic bench displaying peak values leads only to dramatically bad results.

2 – Ketos Ring: The basic parameters

Discrepancy of results is not a “typically French problem”. This problem goes back to the ‘80s-‘90s; but it has been left aside!
And yet, it has been the topic of many papers.

Just as an example, we may have a look at Evaluation of Steel Ring Standards for Magnetic Particle Inspection(2), which obviously shows that some Americans, who happened to be MT experts, had really understood the problem!

Thus, two Ketos Rings with similar hardness and spectrographic analysis, may have different magnetic permeabilities, and, then, give different results for holes detection.

Several parameters are to be considered for the quality of Ketos Rings:
• Steel grade, i.e. its chemical composition which may be determined by a spectrographic analysis.
• The magnetic permeability of the steel, which itself is a consequence of the heat treatment (annealing) which modifies the magnetic properties of the material.

Along the years, annealing parameters have been improved. Their optimization is a major factor in manufacturing more easily reproducible Ketos Rings.

Ketos Rings and SAE-AS5282 tool steel rings shall have a Rockwell hardness between 90 and 95 HRB(3).

It seems that the optimal hardness value is in the 95 HRB range while the SAE-AS5282 specification states 95 HRB as the maximum in its paragraph 3.5.
A 95 HRB hardness is got when following the process: heat to 760°C – 780°C (1,400-1,440°F), hold heat for at least 3 hours, and then cool at a rate no faster than 10+/-3 C (50+/-5°F) per hour down to 650°C (1,200°F) followed by air cooling to room temperature.

An improperly annealed Ketos Ring will give inconsistent results due to erratic magnetic flux density leakages above holes.

3 – Quality control

Here are services by two suppliers. We think there are right; nevertheless, keep in mind that we can supply only information we were supplied with.

Supplier number 1

The quality control steps are as follows:

• Each ring is magnetized by a 3-phase FWDC, using the threading conductor technique. The copper conductor dimensions are: 40 to 60 cm (16 to 24’’) long and 2.5 to 2.8 cm (1.0 to 1.1’’) diameter.

• Recording the residual magnetic flux density leakages at hole N°1 up to hole N°9 levels, measurement being performed manually or automatically(4).

• The values are plotted as a graph according to holes numbers. An average curve is drawn for the entire batch of Ketos Rings. On the graph below, the average figure for the hole N°1 is 2.01 mT. A value equal to a 10% deviation of the value for this hole is considered as the acceptable maximum for all the holes: 0.2 mT is then this acceptable maximum. The lower and upper acceptance limit curves for the batch of Ketos Rings are drawn at ±0.2 mT for all the holes from the average curve. On the graph below, we see that the Ketos Rings N°2, N°3 and N°4 are acceptable while the Ketos N°1 is unacceptable(4).

A certificate of conformity to the SAE-AS5282 specification is supplied with each ring, together with the graph giving the results for the different holes.

Supplier number 2

This supplier does as per the following procedure:

• His steel supplier gives him a material analysis certificate.

• The dimension control is carried out by a subcontractor.

• Hardness is measured in the nbr 2 supplier’s laboratory, using the method of the "ratio between the rebound speed of a tungsten carbide impact body and its impact speed". Being calibrated, the hardness tester displays reliable results.
Hardness measurements prevent using “too magnetically permeable" rings ... on which it would be much easier to find the defects; we would then have no guarantee of the quality control on the magnetic bench.

• Each Ketos Ring is tested “for real” on a magnetic bench with half wave rectified alternating current single phase (HWAC), often referred to as half wave DC (HWDC) or pulsating DC, and fluorescent magnetic ink. The ASTM E1444-05 standard “recipes” are understood as peak value, this leading to rather unfavourable conditions.

• Check the number of holes detected against the ASTM E1444-05 requirements.

This supplier is ready to let us know that, despite right dimensional figures and right hardness values, many are the Ketos Rings which do not pass the detection test.

That’s why this supplier is prone to think that the figures in the ASTM recipes should be considered as RMS values. 

The Ketos Ring’s supplier supplies a certificate of calibration with all the pertaining data.

A not-to-be-followed example

We knew a third supplier who had a quite peculiar method to certify Ketos Rings. He measured the magnetic field right to the holes only on a single Ketos Ring out of a batch of one hundred. The certificate was “valid” for N°1 to N°99 Ketos Rings, another one for N°100 to N°199 Ketos Rings, etc.

4 – Ketos Ring and SAE-AS5282 ring demagetization

ANY MT REFERENCE TEST BLOCK shall be not only cleaned but also demagnetized before re-use ... it is a given, as is a thorough cleaning of PT reference tests blocks before re-use! However, few people do it!
It is THE ONE point that an auditor should check, without warning the user: "Process the Ketos Ring, please". And just have a look! Very easy indeed to issue a non-conformance report!
Further, demagnetizing a ring is a complex task. Another reason for auditors to thoroughly check this process!

5 - Corrective action

The easiest “corrective action” to solve many discrepancies is to require that only RMS current intensities be specified, measured, displayed and recorded when testing the Ketos Ring on a magnetic bench. The RMS value of a current (therefore that of the magnetic field which it gives birth to), as a general matter, seems to be the most suitable to get reproducible results when using different technologies, such as thyristor/no thyristor, magnetic bench/portable generator, etc.
This value is bound to the area of the surface included between the abscissa axis and the curve of the current. This area is exactly related to THE ENERGY given to the magnetic particles, while a peak value, even if very high, but during a short period of time (as a peak), will not give sufficient energy to particles; so they risk not to be put in place ... and we shall not see what we should see.

Peak values are stated in many specifications/procedures, especially in the aerospace industry. Many documents take it for granted that the peak value is 3.14 times the mean value. This is true for purely half-wave rectified alternating current (HWAC) single phase. When using thyristors to set the electric current, unless the unit is used at 100% of its power (then, the thyristor has almost no influence on the curve’s shape), the curve has nothing to do with a sinusoidal one; hence, the 3.14 ratio between the peak value and the mean value (which is NOT the RMS value!) is wrong.

Further, measuring a peak value needs a “peak-meter” able to “catch” the very short spike encountered when thyristor “chops” the current. Meters available nowadays have a 10 kHz recurrence frequency, i.e. they are able to take a measure 10,000 times per second. Specifications writers, as well as auditors, should be aware that a magnetic bench is a very tough equipment. That means 40-year old units are not uncommon. If these units still come with “peak ammeters” that old ... these meters are calibrated with the 3.14 ratio ... and DO NOT measure the real peak value.

This does not make MT invalid. Along the years, users have learned to know which figure the meter shall display to get the right magnetizing conditions; but, then, no comparison can be made if the same part is tested on an MT bench with a “modern peak ammeter”. Figures on this ammeter will likely be very different from the ones got with the old meter.
If RMS figures become mandatory, it is today possible to ask for both RMS and true peak values. This may allow users to adjust the figures stated in specifications/procedures which rely upon “peak values” got from the 3.14 ratio.

We understand that probably a decade more will be needed to have both many “old units” decommissioned, or retrofitted with “true peak ammeters”, and specifications writers, users and auditors adapt to the RMS values requirements.
Better if everybody speaks the same language. Electric current is easily defined by the quantity of heat it delivers when going through a wire/cable ... when RMS value is used. It is a kind of “universal wording.”

A comparison with HiFi amplifiers may be useful. A 2x70 W RMS amplifier easily becomes a 2x140 W peak, or even a 200 W+200 W “musical”. Why? Because the “musical watt” does not exist. But some manufacturers have invented it: it is the maximum output on one channel for one second, the mains voltage being set at the maximum allowable (say, 230 volts in France); the balance potentiometer is set to shut off completely the second channel. Then the second channel is tested the same way. But in the “real life”, both channels run together, the power rack is flowing current in both channels ... and is unable to sustain 200+200 watts!

Some 35 years ago, a European manufacturer was known to sell very powerful, though small, quite not too heavy mobile magnetic units. They were able to supply 10,000 A units with the size and weight of competitors’ 3,000 A units. In fact, its units were 10,000 A “peak-to-peak” units, equivalent to ... 1,600 A RMS units!

6 – Conclusion

This paper is not to question the usefulness of the Ketos Ring or similar. Our purpose is to draw the attention of users and auditors to one cause for discrepancies in results, a reason for many "non-conformance reports" on a key point of the Quality Assurance: "no “”grasp”” on the control-process”, verified by "non-conforming results on reference test blocks".


Acknowledgement: The authors gratefully acknowledge Stephane GRAVELEAU from the R and D department of SREM TECHNOLOGIES (France) for his expertise and for editing this paper.


References

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

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

(1) Stéphane GRAVELEAU, ‘‘Ambiguities due to using the Ketos Ring in Europe’’ Dpcnewsletter N° 030, November 2010, on our Website:
http://www.ressuage-magnetoscopie-penetranttesting-magnetictesting-dpc.info

(2) Donald J. HAGEMAIER, Evaluation of Steel Ring Standards for Magnetic Particle Inspection, pages 1046 à 1050 September 1993, Materials Evaluation, 1711 Arlingate Lane, PO Box 28518, Columbus, Ohio, 43228-0518.
Paper presented at the ATA (Air Transport Association) NDT Forum in Cincinnati, Ohio, on August 25-27, 1992.
Also published in the Magnetic Particle paper Summaries book from the 1993 ASNT Spring Conference in Nashville, Tennessee.

(3) Louis CARTZ, Nondestructive testing, page 156, ASM International PO Box 473, Novelty, Ohio 44072-9901.

(4) Bruce BATES, Donald HAGEMAIER, John PETTY and Chuck ARMSTRONG, NDT Solution, Evaluation of Steel Ring Standards for Magnetic Particle Testing, October 1997, Materials Evaluation, 17711 Arlingate Lane, PO Box 28518, Columbus, Ohio, 43228-051.


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.

Last Updated ( Saturday, 21 May 2011 17:38 )