December 2014 - January 2015 : Doubt removal magnetic particle testing
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December 2014 - January 2015 : Doubt removal magnetic particle testing

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Written by Dubosc
Thursday, 20 November 2014 14:48

December 2014

We dealt with the topic of doubt removal in penetrant testing, which is called the “wipe-off technique”(1), now described in the paragraph 8.7.3 of the ISO 3452-1 standard.

As for magnetic particle testing, in the chapter 12 Interpretation and recording of indications of the ISO 9934-1 standard, strictly speaking, no doubt removal technique is stated.

We did not find anything about it in the ASTM standards.

However, MT operators or inspectors may view either weak indications corresponding to just-below-surface discontinuities or misleading indications due, for example, to the presence of foreign substances (small debris, clothes’ fibres, hair, etc.) on the surface of the part under inspection.

In such situations, they tell us they wipe-off these indications with a finger or with a rag, and that they magnetize again the part to make indications, if any, reappear; they call this action “doubt removal.”

The question we do have, then: does the operator reapply, or not, any detection medium?

When seeing such a technique, an auditor, incredulous and surprised, may ask: "show me where it is written". The operator should be able to show him the specification or the procedure in which this process is stated, approved by a Level III.

 

What do we read in the specifications about this topic?

After having read various specifications, we find that many of them state to reprocess the parts from the very beginning. Others go so far as to require the part be demagnetized and cleaned prior to applying the procedure again.

Interpreting the indications may be difficult in some cases, especially on surfaces with poor surface condition. This is why the ISO 9934-1 standard states in its Chapter 12: "Light surface dressing may be of value where permitted.”

This is what some people call "metal bleaching”, the process that involves removing a very thin layer of metal.

We can consider the next step of the procedure:

  • Then, a local inspection using a hand-held electromagnet, in the AC mode, followed by DC mode, if the part shape is suitable. Quite often, if the discontinuity is just under the surface, this light surface dressing will open it, and it will render the detection far easier. And if it is deeper, deleting even only some micrometres from the surface will greatly improve its detectability.

This is this latter technique that one of us has used many times, with good success. Of course, only if removing a very small metal thickness is allowed in the suspect area. The last time that one of us used it, it was on the wing clamps of a medium-haul aircraft. Five indications were detected, four of which were found to be superficial and non-unacceptable, while the fifth was that of a true defect. According to the relevant specification, this part must have a given minimum thickness. There was "plenty of food for thought". Removing the metal bit by bit, followed by successive electromagnet actions, it was possible to remove the defect, while abiding by the minimum part thickness.

Should this "metal bleaching" not be stated, contrast paint may be applied, which will ease the migration of the magnetic particles along the magnetic flux leakages produced by the discontinuities.

Our aim on this topic is to show you that everything cannot be planned; that knowing some things, practices, etc., may help.

We think it advisable that a paragraph dealing with this doubt removal be included in a future revision of the ISO 9934-1 standard.

References

(1)Pierre CHEMIN and Patrick DUBOSC, The wipe-off technique, DPCNews N° 020, January 2010.

Normative references

ISO 3452-1:2013, Non-destructive testing -- Penetrant testing -- Part 1: General principles, International Organization for Standardization, Geneva, Switzerland, 2013.

ISO 9934-1:2001, Non-destructive testing -- Magnetic particle testing -- Part 1: General principles, International Organization for Standardization, Geneva, Switzerland, 2001.

Last Updated ( Thursday, 20 November 2014 15:01 )