5.5_231127_Eargle_VT-5 ISO5817 Fundamentals of MT_Yoke method.pptx

Click to insert Picture Fundamentals of Magnetic Testing (MT) — Yoke method     Sidney Eargle CWI KION NA - Summerville, SC DEC 04, 2023 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Applicable documents • DIN EN ISO 17638 - Non-destructive testing of welds - Magnetic particl...

Click to insert Picture Fundamentals of Magnetic Testing (MT) — Yoke method     Sidney Eargle CWI KION NA - Summerville, SC DEC 04, 2023 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Applicable documents • DIN EN ISO 17638 - Non-destructive testing of welds - Magnetic particle testing • DIN EN ISO 9934 - Non-destructive testing of welds − Magnetic particle testing - Part 1: General principles • DIN EN ISO 9934 - Non-destructive testing of welds − Magnetic particle testing - Part 2: Detection media • DIN EN ISO 9934 - Non-destructive testing of welds − Magnetic particle testing - Part 3: Equipment 2 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Scope and purpose • Scope − To review the basic principles and applications of the MT - yoke method • Purpose − To qualify KION personnel for performance of MT - yoke method on in-house pructsod 3 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Topics • Principles of magnetics • Magnetic fields • Magnetization using electric currents • Equipment • Selection of technique • Application of MT • Discontinuity detection 4 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Terms and definitions related to magnetism • Magnetism − The ability of matter to attract other matter to itself • Magnet − A material having the power to attract iron and other magnetic materials to itself, and that exhibit poles • Magnetic field − The region around a magnet within which ferromagnetic 5 © 2021 Holdren Engineering Fundamentals of MT — Yoke method materials are attracted Terms and definitions related to magnetism • Magnetic flux (lines of force) − The lines of force that represent the flow of magnetism within a magnetic field − Magnetic field strength defined by number of flux lines crossing a unit area at right angles to these lines 6 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Characteristics of magnetic flux • They seek the path of least resistance between opposite magnetic poles. In a single bar magnet as shown to the right, they attempt to form closed loops from pole to pole. • They never cross one another. • They all have the same strength. • Their density decreases (they spread out) when they move from an area of higher permeability to an area of lower permeability. • Their density decreases with increasing distance from the poles. • They are considered to have direction as if flowing, though no actual movement occurs. • They flow from the south pole to the north pole within a material and north pole to south pole in air. 7 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Terms and definitions related to magnetism • Flux density • The number of flux lines per unit area • A measure of magnetic field strength • Unit of measurement is the "Gauss'', designated on hysteresis curve by letter "B" • One Gauss = one line of flux per square centimeter 8 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Terms and definitions related to magnetism • Magnetograph − A picture of the magnetic lines of force made using iron powder show the pattern of the magnetic field 9 © 2021 Holdren Engineering Fundamentals of MT — Yoke method which is arranged to Terms and definitions related to magnetism • Material classifications − Paramagnetic materials Stainless steel • Materials that are attracted to a magnetic field Carbon steel • Ferromagnetic materials − Materials that are strongly attracted to a magnetic field − These are the materials that may be examined using MT • Diamagnetic materials − Materials that are repelled from a magnetic field 10 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Terms and definitions related to magnetism • Demagnetization − The process of removing magnetism existing in a part • Induction − The magnetism induced in a ferromagnetic object by some outside magnetizing force − Unit of measurement is the "Gauss" 11 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Terms and definitions related to magnetism • Residual magnetism − The magnetic field remaining in some material after the magnetizing force has been eliminated − Point "b" on the hysteresis curve • Retentivity − The property of a given material to retain residual magnetism 12 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Terms and definitions related to magnetism Hysteresis • The reaction of a material to a magnetizing force in terms of the induced magnetic field • Described graphically by a "hysteresis curve" or "hysteresis loop" • Horizontal axis of hysteresis curve represents the intensity of the applied magnetizing force, "H“ • Vertical axis of hysteresis curve represents the intensity of the induced magnetic field in the material, "B" 13 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Terms and definitions related to magnetism • Permeability − A measure of the ease with which a material can be magnetized − Expressed as a ratio of flux density, B, to magnetizing force, H, so permeability = B/H • Reluctance − The opposition to the establishment of a magnetic field • Determines the magnitude of a magnetic field produced by a magnetizing force • Analogous to resistance in an electric circuit 14 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Terms and definitions related to magnetism • Saturation J − That degree of magnetization where further increase in magnetizing force, H, produces no further increase in the magnetic field strength, B − Point "a" on the hysteresis curve • Coercive force − The amount of negative or reverse magnetism necessary to bring the flux density back to zero after saturation, resulting in demagnetization of the part − Distance “co” on the hysteresis curve 15 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Hysteresis curve characteristics • Material with low retentivity − Low residual field • Material with high retentivity − High residual field • Material with Low reluctance − Easy to magnetize • Material with high reluctance − Difficult to magnetize 16 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Characteristics and types of magnetic fields • Bar magnet • Poles • Magnetic attraction • Flux leakage • Effect of flux direction • Longitudinal fields • Circular fields 17 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Magnetic field around a bar magnet • Field made up of curved lines linking north and south poles • Continuous lines, flowing through bar, out into the air, and circling to re-enter bar at opposite end • Lines of force do not cross one another 18 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Magnetic poles • Flux not actually flowing, but does have directional properties • Two ends of bar magnet where most flux lines leave and re-enter are called poles − North pole: lines leave bar − South pole: lines re-enter bar • Like poles repel, unlike poles attract 19 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Magnetic attraction • Each flux line considered continuous loop, traveling through some path • Will always seek path of least reluctance − Ferromagnetic objects, when placed in magnetic field, will be attracted, because this provides higher permeability, lower reluctance path • This same action causes magnetic particles to be attracted at locations where leakage fields exist at discontinuities − Discontinuity represents high reluctance − Magnetic particles provide lower reluctance path, and are therefore drawn to, and bridge, the air gap 20 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Flux leakage • Cracked bar magnet creates multiple bar magnets, with separate north and south poles • Fields set up at cracks or other physical or magnetic discontinuities called leakage fields • Field strength will determine how many magnetic particles will be attracted 21 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Effect of flux direction • Only discontinuities oriented transverse (across) lines of force will cause flux leakage • MT will only detect discontinuities oriented across flux lines • Strongest indication when discontinuity oriented 90° to flux lines 22 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Longitudinal magnetization • A method of magnetization where the lines of force are oriented along the length of the component • Will result in a magnetic field capable of detecting discontinuities oriented transverse to the component length 23 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Circular magnetism • A method of magnetization where the lines of force are oriented such that they encircle the component • Will detect discontinuities oriented along length of component 24 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Magnetic field directions - portable methods Longitudinal Magnetic Field with Yoke Method 25 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Distorted Circular Magnetic Field with Prod Method Magnetization using electric currents • Numerous ways to develop magnetic fields in ferromagnetic materials − Earth's magnetic field − Permanent magnets − Induced fields from electric currents − Loop − Solenoid (coil) − Yoke − Part as conductor & central conductor − Prods • Types of magnetizing currents 26 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Earth's magnetic field • Earth is huge permanent magnet with a north and a south magnetic pole • Relatively weak compared to other fields used for magnetic particle testing • Long ferromagnetic bars will become magnetized when placed such that their lengths are in line with the earth's magnetic field − Magnetization is hastened by striking the bars with a hammer to vibrate them 27 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Permanent magnets • May be used for magnetic particle testing as long as limitations are understood − Essentially only produce longitudinal fields − Poles created in part from contact of permanent magnet poles, causing confusing indications − Control of field direction possible over limited areas 28 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Permanent magnets 29 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Permanent magnets • Other limitations of permanent magnet MT − Difficult to vary the strength of the field − Difficult to magnetize large areas or masses with sufficient field strength to produce satisfactory crack indications − If magnet is very strong, removal from the part could be difficult 30 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Electromagnetism • Use of electric currents to generate magnetic fields best method for MT − Either longitudinal or circular magnetic fields can be produced − Strength of field can be easily varied − Using different types of electric current, useful variations in field strength and distribution can be accomplished 31 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Magnetic field around a conductor • When electric current passes through some conductor, a magnetic field is produced in the space surrounding it − Field strength will be uniform along its length − Field strength will decrease with increasing distance out from conductor − Field strength directly proportional to strength of electric current − Direction of magnetic field follows "right-hand rule" 32 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Loop • If conductor is bent into single loop, lines of force surrounding conductor will pass through loop in one direction − North pole on one side, south pole on opposite side 33 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Solenoid (Coil) • Multiple loops of conductor − Strength of field proportional to the product of current (in amperes) and the number of turns of wire in the solenoid (coil), that is ampere-turns − Example: 1000 amperes x 4 turns = 4000 ampere-turn 34 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Yokes • U-shaped cores of soft iron with a coil wrapped around the base of the U • When electric current passes through coil, longitudinal magnetic field produced between the two ends of the core • DC yokes may be used to produce fields similar to fields generated by permanent magnets 35 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Yokes • Advantages • Field strength ay be varied easily with DC yoke, compared to permanent magnet yoke, by varying the amount of electric current • They may be applied to or removed from the part in the unmagnetized condition when no current is flowing • AC yokes may be used for both testing and demagnetization of the part after testing • Since electricity does not pass through part, capable of being used on painted surfaces • Very portable 36 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Part as conductor - circular magnetization • Electric current passed through part to be examined • Results in circular magnetic field that will reveal discontinuities lying along length of part 37 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Part as conductor - circular magnetization • Advantages − Seldom any external leakage fields to cause background interference patterns, as can be the case for longitudinal magnetization • Since the part is the conductor, the magnetic field created is within the part is more intense • Maximum field strength at the outer surface of the part (solid part) • If a hollow part is magnetized by a conductor placed inside, the maximum field strength will be on the inside surface of the part 38 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Part as conductor - circular magnetization Field distribution in and around a solid conductor of magnetic material carrying direct current Field distribution in and around a hollow magnetic cylinder with direct current flowing through a central conductor Electric conductor 39 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Prods • Parts magnetized by passing electric current through the part between local contacts (prods) • Produces a distorted circular magnetic field 40 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Prods • Produces a distorted circular magnetic field Magnetograph of field around and between prod contacts 41 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Prods • Advantages − Very convenient and practical method for inspection of large weldments − Portable − Increased sensitivity to defects lying wholly below the surface compared to other methods of magnetization, especially when half wave current is used 42 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Prods • Disadvantages − Necessary to scan large parts in small sections, since prod spacing are seldom more than 300mm • External field existing between prods may interfere with observation of pertinent indications, limiting how much current may be used • Poor contact between prods and the part can result in arcing or burning of the part − Caused by − Dirty contacts − Insufficient contact pressure − Excessive current 43 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Types of magnetizing currents • Direct current (DC) − Sources − Motor generators − Storage batteries − Rectified AC (most effective source of DC) − Full-wave and half-wave − Single and three-phase • Alternating current (AC) − Source − Commercial power lines 44 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Types of magnetizing currents • Strength, direction and distribution of fields greatly affected by type of current used • Understanding magnetizing characteristics of these types of current essential for proper application of MT 45 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Types of magnetizing currents • Direct current − Current flow in one direction − Magnetic fields produced using direct current generally penetrate th entire part cross section − Increased sensitivity to subsurface defects − Reduced particle mobility 46 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Types of magnetizing currents • Alternating current − Current flow alternately in two opposite directions − Usually 50 or 60 cycles/second, depending on commercial power source − Minimal penetration of the field below the surface, making it most sensitive for detection of surface defects − Increased particle mobility 47 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Types of magnetizing currents • Half-wave rectified single-phase AC • AC passed through rectifier, allowing current flow in only one direction − Reverse half cycle is completely blocked out − Result is a one-directional current which pulsates • Provides reasonably penetration and excellent particle mobility • Used extensively for weldment inspection with dry magnetic particles 48 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Types of magnetizing currents Field distribution in and around solid conductor of magnetic material carrying direct current 49 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Field distribution in and around solid conductor of magnetic material carrying alternating current Magnetizing methods - summary • Magnetic fields must be oriented goo with defects • Fields generated by electric currents lie at 90O to the direction of current flow • When using electric currents, direction of electric current will be parallel to direction of detectable defects • Circular magnetism is generally preferred to longitudinal magnetism because few, if any, local poles are created that will cause confusing indications 50 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Magnetizing methods - summary • For circular magnetism, use 1000 amperes per inch of part diameter − May create too strong of a field, but it will certainly be strong enough • For coil magnetization, required field intensity is determined by the following formula, where NI= ampere-turns, L = part length and D = part diameter 51 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Magnetizing methods - summary • For prod magnetization, magnetizing current should be 100-125 amperes per inch of prod spacing − Prod spacing should be limited to approximately 200mm • For yoke magnetization, intensity of magnetic field shall be verified by testing the lifting capacity of the yoke with the legs and the spacing to be used for the inspection − For AC yokes: must be capable of lifting 4.5kg {10Ib) − For DC yokes: must be capable of lifting 18.1 (40Ib) 52 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Equipment • Means of producing magnetic field • Magnetic particles • Suitable light source 53 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Means of producing magnetic field • A number of ways of creating magnetic fields in parts, but most practical for portable use are: − Coils − Yokes − Prods 54 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Coils and Yokes Magnetizing yoke and coil 55 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Prods 56 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Magnetic particles • General characteristics • Effect of size • Effect of density • Effect of shape • Magnetic properties • Visibility and contrast 57 © 2021 Holdren Engineering Fundamentals of MT — Yoke method General characteristics • Finely divided ferromagnetic materials • Usually iron oxides • Numerous types available • Proper type must be selected for each application for optimum results 58 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Effect of particle size • Ability of a weak magnetic field to attract and hold particles dependent on size − Field may not be strong enough to hold large particles − Very small particles may be held at irrelevant indications, such as normal part surface irregularities • In general, the smaller the particles, the more sensitivity and mobility possible 59 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Particle size - dry powders • Most dry powders for MT are mixtures of particles of all sizes − Smaller ones add sensitivity and mobility − Larger ones aid in locating larger defects − Larger ones also, by their mechanical action, counteract the tendency for the finer particles to leave a "dusty" background 60 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Particle size - wet method materials • Ferromagnetic particles in a liquid suspension • Normally particle size much smaller than for dry method • Upper size limit generally 0.04-0.06mm − Larger particles difficult to hold in suspension − Larger particles can also become stranded and immobilized as the solution flows over a part, creating "drainage lines" that could be confused with indications of real discontinuities • Generally no minimum size • Groups of extremely small particles will tend to agglomerate to form clumps of particles • Fluorescent particles generally do not include extremely fine particles, so less tendency for agglomeration 61 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Effect of density • Most ferromagnetic materials relatively dense • Those in common use are 5 to 8 times the density of water • Larger particles not desirable because they tend to settle too rapidly, either in liquid or air • Smaller particles, having less mass per particle, as well as more surface area in proportion to mass, settle out much more slowly • Density may be altered by compounding or coating them with lower density pigments 62 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Effect of shape • Shape of particles has significant effect on their ability to locate defects • Elongated particles will tend to develop stronger polarity than do rounded particles − Because of this tendency, elongated particles will more easily align with applied magnetic fields, especially when the flux leakage occurs at wide, shallow discontinuities or those that lie wholly below the surface • Dry powders normally contain both shapes, to provide the best combination of sensitivity and mobility 63 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Magnetic properties • Generally, higher permeability materials are preferred − Increased permeability alone does not imply improved sensitivity, since there are numerous other variables that affect sensitivity • Particles should also have low coercive force and low retentivity − Otherwise, particle mobility and the tendency for indications formation of objectionable background • Overall magnetic property effects can be shown by their hysteresis curves 64 © 2021 Holdren Engineering Fundamentals of MT — Yoke method Visibility and contrast • Visibility and contrast enhances by choosing colors of particles that are easy to see against the color of the surface of the test object • Visibility and contrast can also be enhanced by coating the test object with a contrasting color, such as penetrant developer, which can be used in conjunction with dark particles • Use of fluorescent dyed particles will improve sensitivity because human eye can more readily detect fluorescent indication 65 © 2021 Holdren Engineering Fundamentals of MT — Yoke method