May 2010

Among the many questions sent us every month we have chosen two which will be of some
interest for you- at least we hope so!

Why a dry developer must be used only with a fluorescent penetrant?

Our answer comes as follows:

On our Website as in many PT training courses/documents, there is no clear explanation why a dry powder developer can be used only with fluorescent penetrants.

Whatever the developer “forms”, as they are called in the SAE-AMS 2644 specification and in the ISO 3452 series standards, the first basic function of any developer is to draw the entrapped penetrant out of open to the surface discontinuities which it came in due to the capillary effect. A developer shows a capillary effect far stronger than the capillary effect of the discontinuity. This is the only way to take the penetrant out of the discontinuity.

To answer our reader’s question needs to remind our readers of some things about contrast and viewing conditions.

Contrast may be defined in PT as the ratio between the illuminance given by a discontinuity indication and that by the surrounding background.

Under white light as well as under UV-A radiation the dry developer thin layer displays a “background” which is either the surface’s colour in white light or black/purple under UV-A radiation.

Under white light, should a (non-aqueous wet) solvent-based developer be used, the developer thin layer displays a white background, while this background is black/purple under UV-A radiation.

Therefore:

• Should a dry developer be used with a colour contrast penetrant, when viewed under white light, indications of discontinuities would be faintly, or not at all, visible. Then we may say they would come as undetected.

• Should a (non-aqueous wet) solvent-based developer be used with a colour contrast penetrant, when viewed under white light, indications of discontinuities will give a very good contrast (dark red against a matt white background). Then they will be detected with a high probability of detection (POD).

• Should a dry developer be used with a fluorescent penetrant, when viewed under (UV-A) ultraviolet radiation, indications of discontinuities will be seen as brilliant yellow/green against a black/purple background with an excellent contrast. Then they will be detected with a very high POD.

• Should a (non-aqueous wet) solvent-based developer be used with a fluorescent penetrant, when viewed under (UV-A) ultraviolet radiation, indications of discontinuities will be seen as brilliant yellow/green against a black/purple background with an excellent contrast. Then they will be detected with a very high POD.

Therefore:

Keep in mind that for ANY APPLICATION A FINE AND EVEN LAYER OF DEVELOPER SHALL BE APPLIED ON THE ENTIRE SURFACE UNDER INSPECTION.

Permanent magnet and magnetic particle inspection

A French reader wrote us about weld inspection in a fuel tank in a refinery.

MT, for safety reasons, cannot be carried out with a portable electromagnet.

Our reader wants to write a procedure based on two permanent magnets linked by a cable. He wants also to use strip-type flux indicators (as written in the ISO 9934-1:2001 standard) and spray cans of black magnetic ink.

He wonders whether he could have his procedure accepted as ISO 9934-1:2201 and ISO 9934-3: 2002 standards do not apply to magnetic testing using permanent magnets; this is the wording in the French version of the ISO 9934-1:2001 standard, while in the English version, wording is about residual magnetization method. A “slight” technical mistake in translation!

Further questions :

• May he use the reference block type 2 of the ISO 9934-2:2002 to check his magnetic ink on site?

• Where can he find out the magnetic field figures required for this type of inspection: should he need to use those stated in ISO 17638:2003 for the current flow technique?

Our answer:

Right, due to the specific safety requirements, the magnetic flow technique from permanent magnets is the only option. He must prevent any spark due to friction or impact of the magnets on the surface under inspection for instance.

To check magnetization conditions he must put the flux indicator strip on the surface, perpendicular to the “line” between the poles and halfway from the poles. When applying the black magnetic ink on the strip, one should see three lines if the right magnetization conditions are met. Nevertheless, using a true DC field leads to a “dilution” of the magnetizing energy in the entire part. It is then almost impossible to get the figures stated in standards. Poles shall be at a short distance of each other. A good idea is to use 3 pairs of magnets and to put them side by side every 10 cm (4 in). Doing so for welds inspection allows for a more homogeneous magnetic field for a more reliable result.

The flux indicator strip allows for checking the performance of the magnetic ink at the same time. Keep in mind that, as a general matter, inspectors using true DC magnetization on a large scale preferably use dry magnetic powders.

As a further inspection check the permanent magnets set at the required pole spacing shall be able to lift a steel sheet or a bar weighing at least 18 kg (40 lbs), this being a lifting force of 176 N.

With these parameters, the inspection procedure may be accepted.

When a contrast paint on the flux indicator strip is useless, we think that using it on the weld seam and on the HAZ (Heat Affected Zone) is more than helpful.

ISO 9934-1:2001 standard does not apply to MT with permanent magnets. To counterbalance that we will publish within the coming months a paper dealing with this topic on our Website.

As a final point, we emphasize the ISO 9934-1:2001 paragraph 8.3.2.4: "permanent magnets may only be used by agreement at the time of enquiry and order."

References

• SAE-AMS 2644E: Inspection Material, Penetrant, Society of Automotive Engineers (SAE), 400 Commonwealth Drive, Warrendale, Pennsylvania 15096, 2006.

• ISO 3452 comprises the following standards:

- ISO 3452-1:2008 Non-destructive testing - Penetrant testing - Part 1: General principles, International Organization for Standardization, Geneva, Switzerland, 2008.

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

- ISO 3452-3:1998 Non-destructive testing - Penetrant testing - Part 3: Reference test blocks, International Organization for Standardization, Geneva, Switzerland, 1998.

- ISO 3452-4:1998 Non-destructive testing - Penetrant testing - Part 4: Equipment, International Organization for Standardization, Geneva, Switzerland, 1998.

- ISO 3452-5:2008 Non-destructive testing - Penetrant testing - Part 5: Penetrant testing at temperatures higher than 50 degrees C, International Organization for Standardization, Geneva, Switzerland, 2008.

- ISO 3452-6:2008 Non-destructive testing - Penetrant testing - Part 6: Penetrant testing at temperatures lower than 10 degrees C, International Organization for Standardization, Geneva, Switzerland, 2008.

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

- ISO 9934-3:2002 Non-destructive testing - Magnetic particle testing - Part 3: Equipment, International Organization for Standardization, Geneva, Switzerland, 2002.

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