Magnetic Particle Testing

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Contents

General

A. Magnetic particle inspection (MPI) procedures can only be done on Ferromagnetic materials (cobalt, iron, nickel and some steel alloys).

B. Magnetic particle inspection procedures can involve portable and mobile equipment, which permit procedures to be done without removing components from their locations, or stationary, bench-type equipment such as can be found in workshops.

C. MPI can detect surface-breaking and subsurface discontinuities. The sensitivity does, however, decrease quickly with the increasing depth of discontinuities below the surface.

Basic Principles of Magnetic Particle Inspections

A. When a ferromagnetic component is magnetized, lines of flux are established in that component. If a discontinuity exists in the component and that discontinuity cuts across the path of the lines of flux, the flux is diverted and new, local poles can be set up on the surface of the component. This is known as flux leakage.

B. If fine particles of magnetic material are applied to the surface of the magnetized component, these particles will be attracted to any flux leakages and will gather at the site of the new poles.

C. Maximum sensitivity is achieved when discontinuities lie at right angles to the direction of magnetic flux but sensitivity is not seriously reduced with discontinuities orientated at angles up to 45˚ from the optimum direction. Beyond 45˚ sensitivity diminishes quickly and discontinuities which lie parallel to the direction of flux will not, cause flux leakages of sufficient strength to be detected.

NOTE : Because flux leakages can be caused by discontinuities and also by changes of geometry, indications can be relevant (cracks, laps, nonmetallic inclusions, pipe porosity, seams etc.) or non-relevant (edges, holes, recesses, thread roots etc.) to the condition of the component. Relevant indications must be recorded before the component is demagnetized and cleaned upon completion of the inspection.

Definitions

A. Longitudinal Magnetization (Magnetic Flow) (1) The component is placed into a magnetic field so that the field couples with the component and lines of flux flow from one pole to the other, through the component.

(2) If the magnetic flow is mainly parallel to the long axis of the component, it is said to be longitudinally magnetized.

B. Circular Magnetization (Current Flow)

(1) An electrical current, passing through a conductor, creates a magnetic field in and around the conductor at an angle 90˚ to the direction of current flow.

(2) When a component is magnetized, directly or indirectly in this way, it is said to be circularly magnetized.

NOTE : Because the magnetic circuit is completed within the component there are no apparent poles with this method (except where discontinuities or extreme changes of geometry occur).

C. Magnetic Flux Density (B) (1) If the magnetic flux is cut at right angles to its direction of flow, the number of lines of flux, for a given unit area, is referred to as the magnetic flux density.

D. Magnetizing Force (H)

(1) The force, from an existing magnetic field, or from a field created by an electrical current, which is required to establish a certain flux density within a magnetic circuit. E. Permeability (μ) (1) The ratio of flux density (B) to magnetizing field strength (H), designated by the Greek symbol ”mu” (μ) and indicates the ease with which a material may be magnetized :

B/H=μ

F. Hysteresis Loop

(1) If, for a given ferromagnetic material, a graph is plotted of the changes in flux density (B) against variations in magnetic field strength (H), a curve (loop) will be formed. This loop will be characteristic of the material and will be of a different shape for other ferromagnetic materials.

(2) After the initial curve the loop shows the lag (hysteresis) between the flux density and the changing magnetizing force.

G. Tangential Magnetic Field (1) It corresponds to the existing magnetic field just on component surface. It is generally measured in direction tangentially to the circulating path.

Standards

Standard Description Links
ISO standards
ISO 3059 Non-destructive testing - Penetrant testing and magnetic particle testing - Viewing conditions
ISO 9934
Non-destructive testing - Magnetic particle testing
ISO 9934-1 Part 1: General principles
ISO 9934-2 Part 2: Detection media
ISO 9934-3 Part 3: Equipment
ISO 17638 Non-destructive testing of welds - Magnetic particle testing
ISO 23279 Non-destructive testing of welds - Magnetic particle testing of welds - Acceptance levels (2009)
US standards
ASTM E-1444 Standard Practice for Magnetic Particle Testing
ASTM A-275 Test Method for Magnetic Particle Examination of Steel Forgings
ASTM A-456 Specification for Magnetic Particle Inspection of Large Crankshaft Forgings
ASTM E-709 Guide for Magnetic Particle Testing Examination
ASTM E-1316 Terminology for Nondestructive Examinations
ASTM E-2297 Standard Guide for Use of UV-A and Visible Light Sources and Meters used in the Liquid Penetrant and Magnetic Particle Methods
ASME BPVC Section V, Article 7 Magnetic Particle Examination
CEN standards
EN 1290 "Non-destructive examination of welds. Magnetic particle examination of welds" (1998). Withdrawn, replaced by EN ISO 17638
EN 1291 "Non-destructive testing of welds. Magnetic particle testing of welds. Acceptance levels" (1998). Withdrawn, replaced by EN ISO 23278
EN 1330-7 "Non-destructive testing. Terminology. Terms used in magnetic particle testing" (2005)
EN 1369 "Founding - Magnetic particle inspection"
EN 10228-1 "Non-destructive testing of steel forgings - Part 1: Magnetic particle inspection"
EN 10246-12 "Non-destructive testing of steel tubes - Part 12: Magnetic particle inspection of seamless and welded ferromagnetic steel tubes for the detection of surface imperfections"
EN 10246-18 "Non-destructive testing of steel tubes - Part 18: Magnetic particle inspection of the tube ends of seamless and welded ferromagnetic steel tubes for the detection of laminar imperfections"


References

used literature: NDT manual A320

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