French (Fr)English (United Kingdom)

DPC NEWS: a website dedicated to Penetrant Testing and Magnetic Testing

DPC

Search

mod_vvisit_countermod_vvisit_countermod_vvisit_countermod_vvisit_countermod_vvisit_countermod_vvisit_countermod_vvisit_countermod_vvisit_counter
visits on site since April 2008

Log in

DPCNews


Receive HTML?

Some of the limits to industrial uses of MT

Print
E-mail
Written by Administrator
Tuesday, 01 February 2011 13:17

Dear Readers,
This paper is the fruit of our long industrial experience; we are well aware that it is far from being complete, and that it may contain some inaccuracies.
This is why we would like to get your comments, your remarks and your suggestions to improve and to complete it.
We deeply thank you in advance for your kind contribution.
The Authors: Pierre CHEMIN and Patrick DUBOSC

February 2011

1 - Introduction

As any of the non destructive testing (NDT) methods, magnetic testing (MT) has its own limitations.

Magnetic Testing (MT) is used to detect open-to-surface or subsurface (generally down to some mm deep) discontinuities, only on ferromagnetic materials.

The following materials are classified as ferromagnetic: iron, carbon steels, low alloy steels, maraging steels, cast iron, nickel, etc.

As more and more non-ferromagnetic materials are used, a major change occurred in Magnetic Testing use.

Many materials now widely used are non-ferromagnetic: aluminium, magnesium, light alloys, copper and its alloys (bronzes and brasses), titanium and its alloys, austenitic stainless steels, Hadfield steels (12-14% manganese), etc.

These latter materials, which cannot be inspected with MT, are more and more used in:

• Aerospace industry: titanium, aluminium and lithium-aluminium alloys, plastics, carbon, composites, ceramics, plasma coatings, parts formed by powder metallurgy.
• Automotive industry: plastics, composites, aluminium alloys.
• Nuclear industry: austenitic stainless steels.
• Railways: Aluminium duplex (two levels) carriages of TGV and AGV (very high speed) trains.

2 – Temperatures

There is no ISO standard dealing with MT at high or low temperatures.

2.1 - High temperature (HT) MT

Water-based or oil-based magnetic inks cannot be used at more than 80°C (ca 175°F). Another organic carrier could be used but probably not above 120°C (ca 250°F).

Colour contrast dry magnetic powders, depending on type and colour, as a general rule can be used at temperatures up to 200-300°C (ca 390-570°F). Indeed, when at high temperature for a while, some dry magnetic particles may be altered: colour fades, they stick to each other, and they may give rise to smoke. Some types may smoulder or spark if above ca 482°C (900°F) or in contact with a flame.

2.2 - Low temperature (LT) MT

Water- based magnetic inks are not designed to be used at temperatures lower than 0°C (32°F).

Oil-based magnetic inks cannot be used at temperatures lower than -40°C (-40°F). However, other organic carriers may be used, such as light petroleum distillates, alcohols, etc. to work at lower temperatures. Nevertheless, keep in mind that some UV-A sources may not work at very low temperatures: better to check this point before going to the North or South Poles!

It is also always possible to use the dry powder technique.

Spray cans propellent is mostly LPG (liquid petroleum gas) or DME (dimethylether), both being liquefied gases. When temperatures are very low, the pressure in spray cans using liquefied gases may dramatically drop, more than in spray cans propelled with compressed gases, such as carbon dioxide CO2. This is why the spray cans shall be kept at a minimal temperature of + 10°C (50°F); quite easy to do: just keep spray cans along the body in underwear, for instance.

3 - Defects depth

MT does not allow for the determining of defects depth.

4 – Limits due to the magnetization equipment

Equipment used to supply the magnetic field in parts have their own limits:

• Permanent magnet yokes supply only a weak, true “DC” magnetic field. Furthermore, magnetic particles tend to pile up near the poles.

• Hand-held electromagnets may not be suitable for use on large parts, as inspection may be time-consuming.

• Portable current generators, if not adequately chosen, may be not powerful enough relative to the parts under inspection.

• Mobile current generators may be heavy, depending on the size/power.

• Mobile current generators may need a 3-phase, 400 volt mains.

• Most of the modern mobile current generators come very often with thyristor-controlled amperage. This leads to non-sinusoidal waveforms, with surges of current; tangential magnetic field meters, as well as ammeters, may not display the right figure of current/magnetic field except if they display TMRS (True Mean Root Square) values.

• Making a coil to use with a mobile current generator may be not that easy.
Parts too large to be processed on a magnetic bench may be inspected only through a coil wrapped around them.
Depending on the size of the part and the required tangential magnetic field, 3000 A mobile current generators fitted with 100-120 mm² cross section cables are used, or even 6000-8000 A mobile current generators fitted with 200 mm² cross section cables.
It is then easy to understand that making coils with such cables, especially the 200 mm² ones, is not an easy task because of their weight and their stiffness.

• Magnetic benches may be used only on parts, the length and/or the diameter of which do not exceed the dimensions for which the magnetic bench is designed.

• Magnetization by the central conductor technique requires that the conductor be able to withstand high electric current intensities. On thick parts, the sensitivity of detection on the external surface may be impaired.

5 – Detectable dis discontinuities

As a common rule, MT is the right tool to detect open-to-surface tight and deep discontinuities. Sensitivity is then at its maximum as the magnetic gradient is very high.
On the other hand, detecting discontinuities whose depth is small compared to width may be difficult.

When using a contrast aid paint or a non ferromagnetic coating, the sensitivity of detection decreases with the coating thickness.

Even when using the best parameters for magnetization, a lower sensitivity is seen for:
• Quite wide discontinuities.
• Discontinuities with round edges.
• Discontinuities running in a flat way (such as forging laps).
• Discontinuities filled with ferromagnetic oxides.

6 - Others limits

As in any other NDT method, an important problem is due to non-significant indications, which may come from: part surface condition, sharp shape changes and magnetic permeability changes. They may even make true indications difficult or impossible to detect.

Note for our English-speaking as well as French-speaking readers:

Sometimes, “non-significant” and “”non-relevant” wordings are used as if synonyms. They are not:
• “non-relevant” means that an indication shall not be recorded, due to its dimension, shape, position, which make it acceptable, within the acceptance criteria.
• “non-significant” means an indication which cannot be “connected” to a discontinuity or a flaw. French speaking people use the term “indication fallacieuse”...but also the word “artefact”...which, in English, does not have the same meaning! Be aware of would-be problems of understanding!!

Three “classic” examples among many others:

6.1 - Chromium-based steels

When the carbon percentage increases, this element can no longer be dissolved in iron, but combines with iron to become cementite, Fe3C, less ferromagnetic than iron. As a general rule, increasing the carbon content leads to a decrease of the magnetic flux density at saturation and to a lower magnetic permeability. This is a current drawback when inspecting high chromium steels, such as Z12C13 or 30 CD4 (AISI 4130).

Furthermore, some non-conductive chromium oxides may appear on the surface of the part. If the current flow technique is to be used, a stainless steel brush shall be used to remove the thin layer of oxide on the area of contact with pads or prods.

Fake defects due to some metallurgical parameters may be seen as magnetic indications where the magnetic powder is attracted due to differences of magnetic permeability.

On 13% chrome martensitic steels, these defects-like are due to structure changes:
• Either some residual austenite.
• Or ferrite δ areas.
• Or an area where martensite settled down in residual austenite (in this case, this martensite may be removed by a heat treatment: double tempering).

On 13% chromium steels, MT sometimes displays lines parallel to the lamination direction; these lines are ferrite δ areas with chromium carbide. These lines in no way are dangerous for the part as they cannot lead to stress concentration. Except for very specific applications, such as manufacturing of turbine blades, these lines are not a cause for rejection. In any doubt about open flaws, a Penetrant Testing inspection may be performed.

However, keep in mind that any ferromagnetic part may be checked using Magnetic Testing. Very high magnetic fields may be required for this purpose.

6.2 - Friction welding of different ferromagnetic materials

Indications may come from the different magnetic permeabilities of the two materials and not from discontinuities.

6.3 - Detection of forging laps at the bottom of screw thread

This is almost impossible because of the shape (very small radius angles). The shape induces a spike of magnetic field which attracts magnetic particles all along the thread.

Two ways to overcome this last problem:

• Residual method may be used with “standard” fluorescent magnetic inks, provided that the steel is able to “keep” some magnetisation.

• A fluorescent magnetic ink containing only 0.1g/L of fluorescent magnetic particles, or even less, used along a simultaneous magnetization. This concentration is out of any standard or specification ... but it works very well!!! If such a highly diluted magnetic ink is used in a magnetic bench, it shall be replaced quite often as the magnetic particles taken off by every part are a significant proportion of these particles in the magnetic ink: its magnetic particles content falls quite quickly.

Conclusion

Magnetic Testing is very simple … yes … as Penetrant Testing is!!! That means a thorough knowledge of the BASICS of the method as well as of the pitfalls that a user may face daily is a given. The above examples are only for our readers to think to ask the “right questions” BEFORE performing an MT.

Should you have other examples, solutions to problems, or questions, please feel free to mail them.


Note for our readers

The above-mentioned pieces of information are based on our current knowledge and on the results of our long experience. However, our readers’ attention is drawn to the fact that they are given for information only, and they do not come as a specification/recommendation: we cannot be held responsible in any way.

Last Updated ( Sunday, 22 May 2011 11:46 )