CHAPTER 3 MAGNETIC PARTICLE INSPECTION METHOD …

1 TO 33B-1-1 NAVAIR 01-1A-16-1TM 1-1500-335-23 CHAPTER 3 MAGNETIC PARTICLE INSPECTION METHODSECTION I MAGNETIC PARTICLE (MT) INSPECTION CAPABILITIES OF MAGNETIC PARTICLE terms MPI, MPT, and MT are used interchangeably in this Introduction to MAGNETIC PARTICLE INSPECTION (MPI). MAGNETIC PARTICLE INSPECTION is an NDT METHOD used toreveal surface and near surface discontinuities in MAGNETIC materials. This INSPECTION METHOD can only be used on materialsthat can be magnetized (known as ferrous). The MPI process, when properly performed, establishes a field leakage site on thesurface of the part below which the flaw lies.

2 This CHAPTER presents theory and practical guidance for the performance ofmagnetic PARTICLE INSPECTION . Process control and basic INSPECTION procedures are located in TO Benefit of MAGNETIC PARTICLE is the METHOD of choice on ferrous materials instead of liquidpenetrant because it is faster, requires less surface preparation, and in some instances is able to locate subsurface Basic Concept of MAGNETIC PARTICLE relies on the principle of magnetism (paragraph ). Verysmall ferrous particles, which are suspended in a bath of oil or water, are attracted to MAGNETIC field leakage sites, just as ironfilings are attracted to the poles of a magnet.

3 Cracks and similar types of discontinuities cause disruptions in the magneticfield of magnetized parts, in turn attracting these ferrous particles to the leakage site. This allows the inspector to visualizewhere the discontinuities are located in the part. The keys to a successful MAGNETIC PARTICLE INSPECTION are the correct amountof magnetization of the part, in an optimum direction with respect to flaws, and adequate contrast between the part s surfaceand the particles used to identify the flaw. The particles used are precipitated soft iron, and are stained or dyed in variouscolors, usually with a fluorescent dye or a red dye.

4 Fluorescent dyes on particles in a liquid suspension are used to find verytight surface flaws. Visible dyes on dry particles are less sensitive to tiny surface defects, but are better for finding sub-surface flaws. The type of flaw and/or the INSPECTION environment determines selection of the color or type of following paragraphs describe in detail the standard terminology used, the theory of magnetism, MPImagnetization and demagnetization techniques, process controls, and safety 33B-1-1 NAVAIR 01-1A-16-1TM 1-1500-335-23 section II MAGNETIC PARTICLE PRINCIPLES AND AND THEORY OF MAGNETIC PARTICLE Principles of parts made of ferrous materials, such as iron, are placed in a strong magneticfield or have electric current flowing through them.

5 They will become ''magnetized.'' The degree of magnetization is affectedby the strength of the magnetizing field or the amount of current flow. How strongly the ferrous part will be magnetized afterthe magnetizing force is removed is called ''retentivity.'' Permanent magnets have high retentivity and conductors normallyhave low retentivity. When a surface or near-surface discontinuity interrupts the MAGNETIC field in a magnetized part, some ofthe field is forced into the air above the discontinuity resulting in a leakage field. The size and strength of the leakage fielddepends on the size and proximity of the discontinuity to the MAGNETIC field.

6 The discontinuity is detected by the use of finelydivided iron particles applied to a part s surface and attracted to the leakage field. This collection of particles indicates thepresence and location of the Basic following terms and definitions are basic to an understanding of the MPI in parentheses refer to the hysteresis curve (Figure 3-17). Force. The negative or reverse applied magnetizing force (H) necessary to reduce the residual magnetizingforce (B) to zero in a ferromagnetic material, after MAGNETIC saturation has been achieved.

7 The line (O/G) represents themagnitude and direction of this Contact Magnetization. Use of current passed through the part via contact heads or prods to produce amagnetic A term that describes a material which exhibits both MAGNETIC hysteresis and saturation, also whosemagnetic permeability is dependent on the magnetizing force present. In MAGNETIC PARTICLE testing, we are concerned onlywith ferromagnetic MAGNETIC Field. A circular MAGNETIC field is a MAGNETIC field surrounding the flow of the electric MAGNETIC PARTICLE testing, this refers to current flow in a central conductor or the part MAGNETIC Field.

8 A longitudinal MAGNETIC field is a MAGNETIC field wherein the flux lines transverse thecomponent in a direction essentially parallel with its longitudinal Field. The term used to describe the volume within and surrounding either a magnetized part or a current-carrying conductor wherein a MAGNETIC force is Leakage Field. The MAGNETIC field outside of a part resulting from the presence of a discontinuity, achange in MAGNETIC permeability, or a change in the part s Flux Density (B). The strength of a MAGNETIC field is expressed in flux lines per unit cross-sectional Lines or Lines of Force.

9 A conceptual representation of MAGNETIC flux illustrated by the line pattern producedwhen iron filings are sprinkled on paper laid over a permanent Hysteresis. The phenomenon exhibited by a MAGNETIC system wherein its state is influenced by itsprevious Current Magnetization. Use of an externally applied MAGNETIC field to induce current in a part to produce amagnetic field having the flux direction needed for the INSPECTION . Useful for parts where flowing current directly through thepart would risk damaging the Current (I). The electric current passed through or adjacent to an object that produces a MAGNETIC fieldin the 33B-1-1 NAVAIR 01-1A-16-1TM Force (H).

10 The magnetizing field applied to a ferromagnetic material to induce Permeability (u). MAGNETIC permeability is the ease with which a ferromagnetic part can be is equal to the ratio of the flux density (B) produced to the magnetizing force (H) inducing the MAGNETIC field. It changes invalue with changes in the strength of the magnetizing force. A metal easy to magnetize, such as soft iron or low carbon steel,has a high permeability or is said to be highly Magnetism. This is the MAGNETIC field that remains in the part when the external magnetizing force hasbeen reduced to The property of a metal that remains magnetized after the magnetizing force has been removed.