Mail Inbox - May 2013

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Written by Administrator
Friday, 12 April 2013 08:11

Ht and B figures

May 2013

Mr Leau, a French NDT technician, sent us a message:

First of all, thank you for all your articles on your Website, which are always very interesting.
I submit to you a "topic":
In many MT procedures (and standards) the suitable high and low values of the tangential magnetic field to perform the inspection are stated, for example, between 2,000 and 6,000 A/m. Some operators think this shall be followed blindly, and dare in no case exceed the high value (i.e. 6,000 A/m in the above quoted example.)
Be it an oversight or not in the standards, it seems that they mention no constraining high figures.

• ISO 9934-1:2001 standard: “The minimum flux density in the component surface shall be 1 T. This flux density is achieved in low alloy and low carbon steels with high relative permeability with a tangential field strength of 2 kA/m.”
The general standard does not state any upper limit, but only a minimum value.

Let us see some commonly used “Product” standards:

•  EN 10228-1 standard: “A flux density of 1,0 T is generally achieved with a tangential field strength from 2 kA/m to 6 kA/m, dependent upon the magnetic permeability of the material being tested.
If we make the link with what is said in the ISO 9934-1:2001 standard, we get the minimum magnetic flux density for a tangential magnetic field between 2 kA/m and 6 kA/m ... We logically may exceed these 6 kA/m ... in reasonable proportion, of course (ideally around 1.2 to 1.5 T). When reading thoroughly, one could, without any ulterior motive, write in an inspection report, COMPLYING WITH THIS STANDARD, that a tangential magnetic field of 7 kA/m has been used.

• ISO 17638:2003 standard: “A tangential magnetic field strength of 2 kA/m to 6 kA/m (r.m.s.) is recommended.[Editor’s note: the expert, in charge of the validation of this standard, forgot that, in accordance with the ISO 9934-1 standard, (rms) instead of (r.m.s.) shall be written.]

As a general matter, for welds, figures between 2 kA/m and 6 kA/m for the tangential magnetic field are right; however, nothing prevents us going beyond.

• EN 1369:2012 standard “An iron or steel casting is considered to be ferro-magnetic if the magnetic induction is greater than 1 T (Tesla) for a magnetic field strength of 2,4 kA/m,” and in the ISO 4986:2010 standard: “A steel casting is considered to be ferro-magnetic if the magnetic induction is greater than 1 T (Tesla) for a magnetic field strength of 2,4 kA/m.
Here, it is clear ... just a minimum threshold limit is stated, no likely ambiguity.

These remarks highlight a thing we shall put aside: we look for defects. Often, trainees ask, when on the verge to set the unit: "I have 6,000 A/m but my flux indicator gives no indication, what do I do?" ... If we go up to 7,000 A/m, the flux indicator gives indications, and the discontinuities to detect are detected. Everything goes smoothly.

Our answer:

Thank you very much for your e-mail and the interest you have shown to our Website.

The value of the tangential magnetic field may exceed 6,000 A/m; the goal is to detect the discontinuities, WITHOUT MAGNETICALLY SATURATING THE MATERIAL. The value of the tangential magnetic field is only a means to check the magnetization conditions, which depend on many parameters, difficult to assess. The new wording of the “clause 8.1 General requirements” of the draft of the forthcoming revision of the ISO 9934-1 standard will be much clearer about it.

As per what is written in the standards ... "It is "recommended" to work between 2 and 6 kA/m", nothing prevents us to be below (when we look only for large defects), nothing prevents to be over, because we know that tiny defects detection, especially on difficult-to-magnetize materials, REQUIRE higher tangential magnetic fields to obtain the magnetic flux induction of 1 T.
In any event, the minimum value is much more important, as it is a major factor for the detectability threshold.

The purpose of the maximum figure is only to limit the influence of background and, therefore, to increase, at least theoretically, the signal/background ratio of the indication. The maximum value is especially taken into account in the aerospace industries.

If the tangential magnetic field is too high, disturbances may occur. For example, the "furring" phenomenon, the definition of which is given in the ASTM E1316 - 11b standard, should be avoided.
In our MT Lexicon(1),we have translated "furring" by “empâtement (indication fallacieuse due à une trop forte aimantation).”

Mr Leau’s answer :

People often make "shortcuts" in the instructions and procedures by stating “tangential magnetic field values between 2,000 and 6,000 A/m”, as written in my e-mail. So formulated, the operator has no choice, he may not go beyond 6,000 A/m.

In the same way, "shortcuts" are made by the writers when they specify a trademark for the equipment or for the MT detection media. It happened that a person was “trapped”, as not having available the detection media mentioned in the procedure. (In my opinion, this occurs, due more to laziness than for promoting a brand. It is easier to name a product and a trademark than writing “products shall comply with the ISO 9934-2 standard"). The person, not having these materials available, was somewhat confused.
Another "shortcut" example is the extent of inspections: a document required a 100% inspection of a large quantity of parts (such as bars) on a magnetic bench, which involved a provision for an inspection of the part ends (surfaces in contact with contact pads); in this specific example, this was not at all justified (this area not being subject to load), but this increased the inspection time. Writing a procedure and an instruction requires that the writer thinks twice before freezing it on paper.

Our comment:

What we may always recommend is that the procedure writer works closely with the operators to ensure that ... at least it is achievable with the available equipment and products ... and that everything that is written is useful: useless to record parameters that cannot be checked, for example, when they are not requested by the prime! A kind of shooting yourself in the foot, when auditors are looking after your procedures!


Reference

(1) Pierre CHEMIN and Patrick DUBOSC, MT Lexicon (English/French), March 2010 (document updated in March 2013): on our Website.


Normative references

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

• ISO 9934-2:2002, Non-destructive testing -- Magnetic particle testing -- Part 2: Detection media, International Organization for Standardization, Geneva, Switzerland, 2002.

• EN 10228-1, Non-destructive testing of steel forgings – Part 1: Magnetic particle inspection, European Committee for Standardization, Brussels, Belgium, 1999.

ISO 17638:2003, Non-destructive testing of welds -- Magnetic particle testing, International Organization for Standardization, Geneva, Switzerland, 2003.

• EN 1369:2012, Founding - Magnetic particle testing, European Committee for Standardization, Brussels, Belgium, 2012.

• ISO 4986:2010, Steel castings – Magnetic particles inspection, Organization for Standardization, Geneva, Switzerland, 2010.

• ASTM E1316-11b, Standard Terminology for Nondestructive Examinations, ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA, 19428-2959, USA, 2011.

• ISO 9934-2:2002, Non-destructive testing -- Magnetic particle testing -- Part 2: Detection media, International Organization for Standardization, Geneva, Switzerland, 2002.

Last Updated ( Friday, 12 April 2013 08:48 )