MT Module 2: Fundamentals of MT PDF

MT Module 2, Fundamentals of MT Section Agenda Right Hand Rule Magnetiza...

MT Module 2, Fundamentals of MT Section Agenda Right Hand Rule Magnetization Methods Fundamentals of MT Magnetic Field Theory – Magnetizing an Object – Domains – Hysteresis/Hysteresis Loops – Permeability versus other properties 1 2 1 2 Right Hand Rule Magnetization Methods Direction of lines of force is always at right angles to the magnetizing current. Direct magnetization – passing Defects must be perpendicular to the lines of force. current through the part Indirect magnetization – bringing the part under the influence of a magnetic field 3 4 3 4 ©NDT Consulting Group Inc. 1 MT Module 2, Fundamentals of MT Magnetizing an Object Domain Materials contain Smallest permanent electrons are like little magnet magnets with a north and Composed of about 1 south pole. quadrillion atoms Material are magnetized Occupy an area about by aligning these little the size of the head of magnets. a pin 5 6 5 6 Domains Domains In unmagnetized materials, domains are randomly oriented and neutralize each other. Domains align in Applying a magnetizing force causes the domains to align adding the direction of the their magnetic moments together. magnetic field. Longitudinally magnetized. Video 7 8 7 8 ©NDT Consulting Group Inc. 2 MT Module 2, Fundamentals of MT Hysteresis Hysteresis Loop The lagging of the A hysteresis loop magnetic effect when shows the relationship the magnetizing force between the induced changed. magnetic flux density B and the magnetizing force H. 9 10 9 10 Hysteresis Loop Permeability – vs – Other Properties H – magnetizing force Low Permeability High Permeability B – Flux Density – High Reluctance – Low Reluctance Coercive Force – The reverse magnetizing force needed to reduce – High Retentivity – Low Retentivity the residual field to zero. – High residual field – Low residual field Residual Field – the field remaining – High coercive – Low coercive force in a ferromagnetic material after the force for removal for removal magnetizing force is reduced to zero. 11 12 11 12 ©NDT Consulting Group Inc. 3 MT Module 2, Fundamentals of MT Permeability Section Review Right Hand Rule Magnetization Methods Magnetic Field Theory – Magnetizing an Object – Domains – Hysteresis/Hysteresis Loops – Permeability versus other properties 13 14 13 14 Section Agenda Magnetic Poles Magnetic Poles Region at each end of a magnet where Permanent Magnets the external magnetic field is strongest. Material Classification North Pole and South Pole – Ferromagnetic Lines of force travel from north pole to – Paramagnetic south pole – Diamagnetic 15 16 15 16 ©NDT Consulting Group Inc. 4 MT Module 2, Fundamentals of MT Like Poles REPEL Unlike poles ATTRACT 17 18 17 18 Permanent Magnets Material Classification Permanent magnets are made Ferromagnetic from "hard" ferromagnetic materials such as alnico and ferrite Paramagnetic Subjected to a powerful magnetic Diamagnetic field during manufacture to align their internal microcrystalline You Tube Video structure, making them very hard to demagnetize. 20 19 19 20 ©NDT Consulting Group Inc. 5 MT Module 2, Fundamentals of MT Ferromagnetic Paramagnetic Permeability much greater than 1 A form of magnetism which occurs only in the presence of an externally applied magnetic field. Strongly attracted by a magnet. Permeability slightly above 1 – Iron Slightly attracted by a magnetic field. – Nickel Can not be magnetized. – Aluminum = 1.000021 – Cobalt 21 22 21 22 Diamagnetic Permeability Permeability less than 1. Magnetic field created is opposite of the magnetizing force. Slightly repelled by a magnet. – Gold – Silver – Copper 23 24 23 24 ©NDT Consulting Group Inc. 6 MT Module 2, Fundamentals of MT Section Review Section Agenda Magnetic Poles Magnetic Fields Permanent Magnets – Circular – Longitudinal Material Classification – Transverse – Ferromagnetic – Paramagnetic Defect Orientation – Diamagnetic Field Distribution 25 26 25 26 Circular Field MT Bench Unit The magnetic field surrounding any electrical conductor or part resulting from a current being passed through the part or conductor from one end to another. Not detectable with normal instruments. 27 28 27 28 ©NDT Consulting Group Inc. 7 MT Module 2, Fundamentals of MT Circular Field Longitudinal Field Created by passing current Magnetic field wherein the through a part (direct flux lines traverse the magnetism) or by placing a component in a direction conductor through a part essentially parallel with its opening (indirect magnetism). longitudinal axis. Magnetic field is contained North and south poles entirely within the part. created. 29 30 29 30 Longitudinal Field Circular and Longitudinal Created by passing current through a coil into which the part has been inserted (indirect magnetism). North and south pole established and field flows between Easily detected with instruments. 31 32 31 32 ©NDT Consulting Group Inc. 8 MT Module 2, Fundamentals of MT Caterpiller Crank Coil MT 33 34 33 34 Defect Orientation Defect Orientation Must be oriented 1 - magnetic field perpendicular to the flux direction lines. 2 - optimum sensitivity Defect parallel to flux 3 - reducing sensitivity may be missed. 4 - insufficient Between perpendicular sensitivity and parallel show up with varying results. 35 36 35 36 ©NDT Consulting Group Inc. 9 MT Module 2, Fundamentals of MT Magnetic Field Distribution DC Solid Nonmagnetic Varies from zero at the center of the component to a maximum The magnetic field at the surface. distribution in and Surface field strength decreases as the radius of the conductor around a solid increases when the current strength is held constant. conductor of a Field strength outside the conductor is directly proportional to the nonmagnetic current strength. Field strength outside the conductor decreases with distance material carrying from the conductor. direct current. 37 38 37 38 Magnet Solid DC Magnetic Solid AC The magnetic field The magnetic field distribution in and distribution in and around a solid around a solid conductor of a conductor of a magnetic material magnetic material carrying alternating carrying direct current. current. 39 40 39 40 ©NDT Consulting Group Inc. 10 MT Module 2, Fundamentals of MT Nonmagnetic Hollow DC Magnetic Hollow DC The magnetic field The magnetic field distribution in and distribution in and around a hollow around a hollow conductor of a conductor of a nonmagnetic magnetic material material carrying carrying direct direct current. current. 41 42 41 42 Magnetic Hollow AC CBC DC The magnetic field The magnetic field distribution in and distribution in and around a hollow around a conductor of a nonmagnetic central magnetic material conductor carrying carrying alternating DC inside a hollow current. magnetic material. 43 44 43 44 ©NDT Consulting Group Inc. 11 MT Module 2, Fundamentals of MT Yoke Field Distribution Coil Field Distribution One-fourth (1/4) of Use a QQI to the leg separation ensure the field is on either side of a adequate in a centerline joining given location. the two yoke poles. 45 46 45 46 Transverse Section Review Transverse Field Magnetic Fields – Circular – Longitudinal Defect Orientation Field Distribution 47 48 47 48 ©NDT Consulting Group Inc. 12 MT Module 2, Fundamentals of MT Section Agenda Current types Magnetizing Current AC Skin effect HWDC Demagnetization FWDC Curie Point Why demag Three Phase FWDC Circular Field versus Longitudinal 49 50 49 50 Alternating Current (AC) Skin Effect Changes directions in Inductive effect limits accordance with the AC to the surface of incoming current cycle. For surface exams only. the part Easier to remove because it Excellent for surface is surface only. defects like fatigue Direction changes gives mobility to particles. cracks. 51 52 51 52 ©NDT Consulting Group Inc. 13 MT Module 2, Fundamentals of MT Half Wave Direct Current Half Wave DC (DC) Rectifier used to block Continuous current flow in one the flow of AC current. direction establishes strong and Cyclic current reaching penetrating magnetic fields a maximum then – Flux density zero at part center dropping to zero until increasing to maximum at part surface the next cycle. – Pulsating effect give particles mobility 53 54 53 54 Depth of Penetration Full Wave DC (FWDC) Subsurface difficult to detect Negative Portion of AC flow is inverted unless larger than 0.060” and if Same penetrating power as HWDC over 0.060” deep. 55 56 55 56 ©NDT Consulting Group Inc. 14 MT Module 2, Fundamentals of MT Rectification Three Phase FWDC Three phase AC is rectified to create Three phase FWDC. Design allows a “quick break” circuit that improves the formation of indications. 57 58 57 58 Demagnetization Demagnetization The process of reducing or removing the Reverse the field magnetism of a ferromagnetic material. Reduce field strength A suitably intense magnetic field applied Part orientation should in a direction opposite to that of the be such that the long existing magnetization will serve to axis of the part is in an reduce or eliminate that magnetization. east/west direction 59 60 59 60 ©NDT Consulting Group Inc. 15 MT Module 2, Fundamentals of MT Demagnetization Equipment DC Step Down Demag A more even and complete penetration of even large To remove cross sections. magnetic field DC current flows in one direction for a short time, it then is slightly reduced in magnitude, and completely reversed in Part is passed direction. through coil while The process of automatically reversing and reducing the current is continued until the current reaches zero and the coil is energized part is effectively demagnetized. DC step down best Very effective but requires large, heavy equipment. 62 1/28/25 61 61 62 DC demag Yoke Demag A step-down reversing demagnetization is usually Put yoke in AC completed in about 30 seconds - one second per step. Place yoke on part and energize The one second at each step allows time for the Pull yoke slowly away from part field in the part to reach a steady state, at which time induced currents become zero, permitting while energized. maximum penetration of the field into the part. 63 64 63 64 ©NDT Consulting Group Inc. 16 MT Module 2, Fundamentals of MT Curie Point Why Demag Could affect other processes. The temperature point where a Could interfere with electronic equipment such as a material becomes nonmagnetic. compass. Low carbon steel is 770 degrees C Could affect welding process. Arc blow may cause the weld arc to wander or filler metal to be repelled or 1390 degrees F from the weld. Could cause abrasive particle to cling to bearing or faying surfaces and increase wear. 65 66 65 66 Check for Demag Circular Vs Longitudinal Check part with Longitudinal field can be detected field indicator Circular field not detectable Must be three Therefore, longitudinal is done last gauss or less IAW E1444. so we can tell if demaged. 67 68 67 68 ©NDT Consulting Group Inc. 17 MT Module 2, Fundamentals of MT Section Review Module Review Magnetizing Current In this lesson we have discussed: – Magnetic Field Theory Skin effect – Characteristics of Magnetic Fields – Types of Magnetic Fields Demagnetization – Defect Orientation Curie Point – Types of Magnetizing Currents – Demagnetization Why demag Later you will be using this information when you perform MT on parts. Any Questions? Circular Field versus Longitudinal 69 70 69 70 ©NDT Consulting Group Inc. 18