Magnetic Particles Testing (MT) Technique

Transcription

1 Research Group Magnetic Particles Testing (MT) Technique Professor Pedro Vilaça * * Contacts: Address: Puumiehenkuja 3 (room 202), Espoo, Finland pedro.vilaca@aalto.fi October 2017 Contents Historical scope of magnetic particles testing (MT) technique Fundaments of the MT technique Sequence of phases in the application of the MT technique Fundaments of electromagnetism supporting the MT technique Detailed characterization of each of the application phases Classification of the magnetization methods Classification of the electric current in induced magnetic fields Classification of the magnetization equipment Classification of the application of the magnetic particles Method of demagnetization 2 1

2 Learning Outcomes At the end of the seminar the student should be able to: To establish the fundaments To characterize each of the application phases To classify and select the magnetization methods To select the electric current in induced magnetic fields To classify and select the magnetization equipment To classify and select the application method and condition of the magnetic particles To characterize the method of demagnetization 3 Introduction What is the historical scope of the Magnetic Particles Testing (MT) technique? The method appeared in the USA around It was only after World War II ( ) that it expanded as a method of NDT What are the main target application focus and materials of the Magnetic Particles Testing (MT) technique? Detection of superficial and subsuperficial defects Ferromagnetic materials (e.g. steels, alloys of nickel and cobalt) 4 2

3 Fundaments of the Technique What are the fundaments of the Magnetic Particles Testing (MT) technique? The detection of defects is based on the fact that the magnetic particles are attracted to the areas with magnetic flux leakage resulting from defects in the component being subjected to a magnetic field Magnetic particles cumulated due to magnetic flux leakage 5 Fundaments of the Technique What are the main phases in the application of the Magnetic Particles Testing (MT) technique? 1. Surface preparation (cleaning) 2. Magnetization of the component 3. Application of the magnetic particles 4. Inspection 5. Demagnetization 6. Cleaning of residues from the MT 6 3

4 Fundaments from the Electromagnetism The magnetic Flux Density (B) [Tesla] is defined as: The magnetic Flux Density (B) is the number of flux lines per unit of area produced by the material upon the application of a magnetic Field Strength (H) [A/m], i.e. external applied magnetic field that does not depend on the medium. B B H H Absolute magnetic Permeability (µ), measures the resistance of the material to be magnetized, and is defined as: µ = µ 0 x µ r Permeability of iron Where: Constant magnetic permeability: µ 0 = 4π 10 7 H m H m 1 or N A 2 ) corresponds to the permeability of free space (i.e. is a measure of the amount of resistance encountered when forming a magnetic field in a classical vacuum) Relative magnetic permeability: µ r - Unlike µ 0, µ r is not constant and changes with magnetic flux density B. Also, if the temperature is increased then typical there is a drop in permeability 7 Fundaments from the Electromagnetism Ferromagnetic Materials: Permeability (μ) > 1 Materials such as iron, nickel or cobalt, which are strongly attracted to magnets and can be magnetized. Some of the atoms or ions in material have a net magnetic moment due to unpaired electrons in partially filled orbitals. However, the individual magnetic moments do not interact magnetically, and magnetization is zero when the field is removed. In the presence of a field, there is now a partial alignment of the atomic magnetic moments in the direction of the field, resulting in a net positive magnetization and positive susceptibility Paramagnetic Materials : Permeability (μ) = 1 Materials such as austenitic steel, aluminum, tin, which are little attracted by magnetic forces, and can not be magnetized Diamagnetic Materials: Permeability (μ) < 1 Materials such as silver, zinc, lead, which are slightly repelled by magnetic forces, and can not be magnetized. It is due to the non-cooperative behavior of orbiting electrons when exposed to an applied magnetic field. These substances are composed of atoms which have no net magnetic moments (all the orbital shells are filled and there are no unpaired electrons). However, when exposed to a field, a negative magnetization is produced and thus susceptibility is negative 8 4

5 Fundaments from the Electromagnetism Magnetic Retentivity - the ability of a substance to retain or resist magnetization, frequently measured as the strength of the magnetic field that remains in a sample after removal of an inducing field "iron is easily magnetized but has low retentivity Lorentz Force, (F) [N] The magnetic Flux Density B, is related to the Lorentz force F, on a moving charge q (in Coulombs, C), with speed v (in meters per second, m/s): m V. s C.( J / C) F q( E v B) q( v B) C.. 2 s m m J m N 9 Fundaments from the Electromagnetism Magnetic Flux leakage - magnetic lines of force that flow out of the intended path in a magnetic circuit, due to a imperfection Biot-Savart law - relates the magnetic field generated due to the magnitude, direction, length, and proximity of the electric current. The law is valid in magnetostatic conditions db P I 4 r 2 ds grad P r 10 5

6 Phase 1: Surface Preparation The components to be inspected shall be cleaned in order not to jeopardize the efficiency of the PM inspection The same cleaning procedures applied to die Penetrant testing (PT) are normally used in MT although the result of MT are less sensitive to this phase, than the results of PT. WHY? Cleaning methods: - Alkaline: to remove organic materials. Typically by emersion - Acid: to remove oxides, calamines, and corrosion products. Careful to avoid the attack of the base materials. - With detergents: adhere to the dirty particles facilitating their removal - With solvents: to remove oils, masses, waxes and paints. A post-cleaning operation with detergent should be made - With vapor: typically in degassing tanks with solvent vapor - Ultrasonic: combining mechanical stirring with solvents or detergent. Suitable for removing contaminants inside defects - With bath salts: e.g. caustic soda (sodium hydroxide (NaOH)) to remove oxides and calamines. The exothermic reaction facilitates the oxide removal - Mechanical: sand blasting, blasting, wire brushes, etc. This method is not so critical to be applied in MT as in Die Penetrant Testing (PT). WHY? 11 Classification It consists of subjecting the component to a magnetic field of known intensity and direction, according to the following different methods: 12 6

7 Portability: typically applied when there is no power source available Field inspection or when there is a fire hazard Impossibility to magnetize large surfaces Impossibility to adjust the intensity of the magnetic field Permanent magnets 13 Electromagnets Made of a nucleus of plain steel in U shape with one coil of current carrying wire With YOKES It is possible to vary the intensity of the magnetic field Can be switched on and off, which facilitates aplication and removal 14 7

8 Electromagnets Application of YOKES with adjustable legs Portable Power Supplies for YOKES 15 By direct contact Only imperfections aligned with the current are possible to detect Circular magnetization induced by electric current In this case the current flows through the components 16 8

9 Only imperfections aligned with the current are possible to detect Circular magnetization induced by electric current In this case the current flows a copper shaft Good to inspect hollow components 17 Magnetization by single prod contacts: Circular magnetization induced by electric current 18 9

10 Only imperfections aligned with the current are possible to detect Longitudinal magnetization induced by electric current 19 Only imperfections aligned with the current are possible to detect Multidirectional magnetization induced by electric current 20 10

11 What kind of electric current is used? Direct Current (DC) Generates magnetic fields with higher penetration Better for subsuperficial defects Demands the magnetic particles to be applied via wet suspension The application of magnetic particles via dry do not have enough mobility to be sensitive to eventual magnetic leak flux induced by DC Magnetization induced by electric current typical value of electric currents : A 21 What kind of electric current is used? Alternating Current (AC) Range of frequencies: Hz Magnetization induced by electric current Generates magnetic fields with less penetration, due to the skin effect of AC. This effect is more relevant as the frequency increases Generates alternating magnetic fields that promote the agitation of the magnetic particles and consequently their mobility Better for superficial defects Enables the application of dry magnetic particles typical value of electric currents : A 22 11

12 Half wave rectified AC High penetration compared with AC, enabling to detect subsuperficial defects Promote similar mobility of magnetic particles as in AC Couples advantages of DC + AC Magnetization induced by electric current Full wave rectified AC Even higher penetration Less mobility of the magnetic particles 23 Selection of equipment depends on: Type of Component (namely: material, dimension, shape) Local of Inspection Portable equipment: Magnetization equipments 24 12

13 Fixed equipment: Tank with magnetic particles Coil for longitudinal magnetization Copper shat for longitudinal magnetization Control panel and power source with variable current (e.g A) Adjustable for components of different dimension Magnetization equipments 25 Phase 3: Application of the magnetic particles Classification 26 13

14 Phase 3: Application of the magnetic particles The magnetic particles are made of ferromagnetic materials: Iron and /or iron oxide Characteristics of the magnetic particles The main properties of the magnetic particles (MPs) are: Dimension (from 10 to 30 m) Strong influence on their behavior (mobility) A weak magnetic field attracts and retains better smaller MPs But small MPs can adhere to rough surfaces, reducing the mobility For this reason the MPs consist of a set of several granulometries: o The smaller ones increase the sensitivity and mobility of the set o The larger ones overcome the trend of the set to adhere to the surface 27 Phase 3: Application of the magnetic particles Characteristics of the magnetic particles The main properties of the magnetic particles (MPs) are: (cont.) Shape (Spherical or Elongated, e.g. needle) Elongated MPs are more likely to line up along force lines The spherical shape have better mobility Permeability Should be high as possible Retentivity Should be low as possible 28 14

15 Phase 3: Application of the magnetic particles The main properties of the magnetic particles (MPs) are: (cont.) Mobility When applying the MPs in Dry conditions the mobility should be increased by: o Vibrating the component o Using AC or Rectified AC o When applying the MPs via Wet suspension the viscosity of the suspension should be carefully considered Visibility (e.g. contrast) Colors for best contrast to the piece: red, yellow, black or white. Fluorescent pigments (demand the application of black light) Characteristics of the magnetic particles 29 Phase 3: Application of the magnetic particles In this case the MPs are supplied in powder form and have different colors The application can be done by sprayers or gun projectors On vertical surfaces or overhead position the leak flux must overcome gravity Advantages: Easy to apply on large parts; Portable equipment; Cleaner Disadvantages: More difficult to apply to complex geometries Difficult to automatize Magnetic particles Dry type of application 30 15

16 Phase 3: Application of the magnetic particles In this case the MPs are in suspension in water or Magnetic particles petroleum products The suspension needs to be stirred to keep the particles Wet Suspension uniformly distributed, but the stirring may also remove the particles attracted by small intensity magnetic fields type of application (e.g. subsuperficial defects and small defects) The suspension includes additives (e.g. corrosion inhibitors, wetting agents) The concentration of the MPs suspension ranges from 0.1 to 2 ml / 100 ml The application can be made: By pouring the suspension into the part By immersing the part in tanks with automated circulation systems Through pressurized packaging Advantages: Good mobility of the MPs in suspension Easy automatization and application to complex shapes Disadvantages: May demand more complex equipment 31 Phase 3: Application of the magnetic particles Continuous method The MPs are applied during the flow of magnetization current: The magnetic field is of high intensity More sensitive in the detection of subsuperficial and small superficial defects 32 16

17 Phase 3: Application of the magnetic particles Residual method The MPs are applied after ending the flow of magnetization current: It is only possible to apply to materials with some retention In these conditions the magnetic field is of small intensity It is not used to detect subsuperficial defects When using a central conductor for induction of the circular magnetic field, the accessibility for inspection increases significantly 33 Phase 4: Inspection, Analysis and Registration Inspector should have knowledge on the production engineering: Inspector should: Distinguish from relevant and non-relevant indications e.g. the ones produced by distortions of the magnetic field Proceed with the registration of the relevant indications, using: o transparent vernish or adhesive tape, photographs 34 17

18 Phase 5: Demagnetization It becomes necessary to demagnetize the component if: It interferes with in-service behavior in general May affect instrumentation or interfere with welding operations It may not be necessary to demagnetize if: The material has low retentivity The parts are subsequently subjected to temperatures above the Curie point 35 Phase 5: Demagnetization Most common method: Subjecting the magnetized part to a magnetic flux that continually reverses direction and decreases in intensity Typical Hysteresis cycles produced during the demagnetization operation: To verify the residual magnetic fields, it is typically applied: Hall effect probe 36 18

19 Phase 5: Demagnetization F m rv db B dx 0 37 Phase 5: Demagnetization 38 19