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June/July/August 2015 : Excessive magnetic particles concentration

Written by Laurence
Friday, 15 May 2015 08:39

June 2015
(updated in September 2015)

We got an interesting question, a very practical and “basic” one. Basic, yes, at first sight. In fact, not that basic at all!!!

The question came as: what is a good indicator that the magnetic particle concentration in a magnetic ink was excessive?

Note, by the way that in the ISO 9934-2 standard, there is no testing requirement for the magnetic particle concentration while the paragraph Determination of Wet Particle Concentration in the ASTM E1444/E1444M – 12 standard states it.

The truth is that an inspector may think he (she) has the right magnetic ink, at the right concentration...but, when the magnetic ink is applied, some concern arises, because, for instance, a known discontinuity is not, or only poorly, detected.

An obvious answer is: the indication is fuzzy; background level is too high, contrast too low.

This occurs with black, coloured or fluorescent magnetic particles.

Some situations may lead to too many magnetic particles on a part.

The magnetic ink is prepared on site. There is a mistake in calculating the quantity to add to the liquid carrier (vehicle carrier in the USA), be it water (if a water-based magnetic ink concentrate is used) or oil. Alternatively, there is no available analytical balance (scale). Some people, not accustomed to this non-destructive testing (NDT) method, may think that “more is better than too few”, and pour a bit too much into the liquid carrier.

The magnetic ink is in a spray can. It is always recommended to shake thoroughly the spray can before spraying, to prevent all the magnetic particles settled at the bottom to be sprayed almost immediately, thus providing too many magnetic particles to the part surface.

Again with spray cans: many technicians want to saturate the area to be inspected; not only, it is messy, but it can lead to excessive backgrounds. It is much like with non-aqueous wet developers (NAWD), when performing Penetrant Testing: you do not want a quarter inch (circa 0.5 cm) crust.

A further source of unusually high concentrations. In magnetic benches that are often turned off for long times, or, even, turned off every day, but used only for a short time, when the magnetic bench is switched on, the pump takes a good bit of time to thoroughly mix the magnetic ink bath. If the inspection begins too shortly after turning the equipment on, the inspector may think that the concentration of magnetic particles is too low, and then may add magnetic particles. However, within ten to twenty minutes, when all the magnetic particles are circulating, then, the concentration may be too high. It is a good idea to pull up the grates, or the reservoir lid, and to manually stir the bath of the magnetic ink with a stirring stick or a plastic-made spoon to get the settled magnetic particles up into the magnetic ink.

We must emphasize the reaction of a magnetic benches manufacturer. Some of its recent machines come with an advanced system for stirring the magnetic ink with the pump. After an extended shutdown, a five-minute stirring time is required. When used in automatic mode, this time blocks all the other functions. It is displayed as an information-only in manual operation, or in the entry-level machines.

Thus, as you see, from a very “basic” question, we have been able to draw your attention to the root-causes of too many magnetic particles on the part.

Just to answer the question itself: fuzziness, excessive background, a poor indication visibility/background ratio shall immediately make the inspector worry about the quality of the detection medium ("medium" is the singular of "media", and shall be used when talking about one specific product, as in this sentence), he (she) is applying.

Normative references

ISO 3452-2:2006, Non-destructive testing -- Penetrant testing -- Part 2: Testing of penetrant materials, International Organization for Standardization, Geneva, Switzerland, 2006.

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.

Last Updated ( Friday, 11 September 2015 23:45 )