Magnetic Particle testing is also known as MPI or MPT is a well accepted NDT method to reveal surface and below surface flaws. Applied for ferro-magnetic metals such as Iron, Nickel and Cobalt alloys. Due to its effectiveness in detecting flaws, industries such as Automobile, Oil and Gas, Processing and Aerospace accepted as a means of verifying quality of components and structures.
How to Start Magnetic particle testing?
Before start of any NDT method a detailed procedure shall be prepared. MPI testing also needs a procedure that outlines, essential and non essential parameters. The MPI procedure shall address all key elements of testing. For instance, MPI techniques, powders, light requirements, medium, current calculations, sequence of testing, equipment to be used, personnel qualification etc.,
Therefore, an MPI testing procedure shall be prepared considering the parameters on site. Procedure shall be prepared by a certified NDT Level II and shall be approved by a NDT Level III in Magnetic Particle testing.
Here are key elements and procedure for MPI testing as per ASTM E709/ASTM E1444
What is the difference between ASTM E709 and ASTM E1444
Both the international standards are issued by The American Society for Testing Materials (ASTM). Both the standards are widely used in every industry involving MPI. ASTM E 709 is a mother standards for many of the worlds country specific standards being used today.
ASTM E709 – A Standard Guide for Magnetic Particle testing, covers in detail about every requirement, recommendation pertaining magnetic particle testing.
ASTM E1444 – A standard practice for Magnetic Particle Testing is specifically applicable for Aerospace NDT applications. This is a replacement for US military standard – MIL-STD-1949. Covers minimum requirements for performing MPI testing. Also recommended to use in conjunction with ASTM E709.
Though both the standards are widely accepted in industry, where stringent requirements are to be followed ASTM E1444 is a better choice. Because it has close acceptance limits, this standard is especially used in Aerospace sector for MPI testing. More specifically this standard covers requirement for aerospace industry.
It covers the techniques for both dry and wet magnetic particle inspection. Applicable for raw materials and semi-processed materials such as blooms, billets, castings, rolled products, forgings and weld joints. It is also can be used in in-service maintenance inspection of plants and structures.
ASTM E709 is a guide that helps you in preparing MPI procedures, establishing techniques. This can also be used for evaluating and reviewing customer specifications. This standard can be applied for parts of any size, material(ferro) and any shape for any application. Therefore, users of this standard are required to exercise to evaluate specific requirements pertaining to their job and conditions.
Does This Standard Specify Acceptance Criteria?
No. ASTM E709 does not specify any acceptance or rejection criteria. This only covers the procedure for magnetic particle inspection. As this standard is used for variety of applications such as automotive, structural, oil and gas and even for aerospace, it is left to the user to specify the criteria for acceptance or rejection.
Therefore, the contracting parties shall specify acceptance or rejection criteria in the MPI procedure. It may also be cross referenced in place of specifying in procedure. An ASNT Level III or an expert in design shall be consulted for deciding on the criteria. This shall be based on criticality of application, risk associated with failure of the part.
What MPI techniques are used?
- Dry Powder Technique
- Wet powder technique
and other techniques which are not much use in industries.
What is the personnel qualification requirement to do MPT testing?
MPI testing inspector should be performed by qualified and certified as per ASNT recommended practice SNT TC 1A or ANSI, CP189 or NAS410(aerospace). The document also gives freedom to specify certification scheme based on agreement between contacting parties such as ISO9712 certifications.
A number of specifications and standards are listed for the benefit of users. It is good if you can buy there standards from ASTM website for additional knowledge. Also, a standard ASTM E1316 gives definitions related to terminology applicable for Nondestructive testing.
Summary of Magnetic particle Testing
In MPT testing, initially magnetic flux is introduced by a suitable means. This could be using directly passing current techniques such as Head Shot, Prod or Indirect techniques that passes only magnetic field, such as Yoke, Central conductor etc.,
By applying Fleming’s right hand rule we can find the direction of magnetic field if we know the direction of electric current. Once magnetic field is introduced into any ferro magnetic metals, flux lines will be travelling through the materials. If there are any flaws, flux will be distorted and leakage field is created. As we cannot sense leakage flux, a finely powdered ferro-magnetic powder is uniformly sprinkled on the surfaces.
Leakage flux attracts the ferro-magnetic powder thereby bridging the space between the crack/flaw faces. The powder is added with a pigment for suitable viewing. Fluorescent powders are to be used only in darkened room. These powders emits yellowish green light when impinged by Ultraviolet (UV) light in the wavelength range of 320-365nm.
Fluorescent powder absorbs UV light and emit visible light at around 555nm. As Yellowish green light is highly sensitive to human eye, the MPI inspector will be able to locate the indication easily. Non-fluorescent powders are colored with black, red, grey to give contrast with respect to surface.
How to select fluorescent or Non-fluorescent techniques in Magnetic particle testing?
Fluorescent powder technique is suitable for high speed sensitive applications. Non-fluorescent techniques are good for field/site testing conditions where components cannot be moved to a darked area to maintain darkness. Later technique is economical and a cost effective solution at the cost of less sensitivity.
- Permanent Magnet
- Electromagnetic Yoke
- Head Shot
- Central Conductor
- Cable wrap
- Coil Shot
- Prod Technique
- What kind flaws can MPI testing detect?
Magnetic particle testing detect flaws located perpendicular to magnetic flux. Flaws located up to 45 degree may also be detected.
How many directions do we need to magnetize?
For effective testing, magnetic field shall be introduced in two mutually perpendicular directions. Inspector shall ensure this while establishing the technique for all surfaces, wherever practicable. The procedure must address techniques to generate the field in various directions.
What is multi-directional magnetization?
Equipment with multi directional magnetization are available. These equipment generate vector field so that the magnetic field rotates almost 360 degree in each shot. Therefore, whatever may be the flaw orientation at one point of time the vector will be perpendicular to the flaw. However, equipment that generate multi directional fields are expensive thus can be used only in critical applications such as aerospace.
Magnetic Field Strength
Magnetic field strength should be sufficient enough to generate leakage flux to detect flaws. Under magnetized components cannot generate the flux enough to detect the defects. Over magnetization causes the field to cause excess in whole of the materials leading to excessive field leakage and heavy accumulation of particles. This leads to shadowing of relevant indications. Because, the contrast is lost flaw identification becomes a challenge.
Therefore, while establishing magnetic field strength it is required to generate just sufficient to detect minimum size of flaw and should not over magnetize to mask indications.
Types of Magnetic Particles
Magnetic particles both Dry or Wet based can be used. Fluorescent and non-fluorescent dry or wet particles to be selected based on end user need. Powder concentrates also are specified. Wet particles can be dispersed in water(water based) or carrier oil based (petroleum distillate) that confirms to ASTM E709.
What is meant by indication?
Clustering of powder at a specific area under MPI testing is called Indication. Each indication shall be evaluated for relevance, acceptance or rejection.
How does surface indications looks like?
MPI Indications from surface flaws will be sharp, distinct pattern and tightly held to the surface.
How does subsurface indications looks like?
In MPI testing, subsurface(near surface) indications produce less distinct, fuzzy patterns and powder is loosely held. Indications will be broader than sharp. Just with a small puff of air from mouth can fully or partially eliminate from the surface.
Magnetic Particle testing Equipment Selection
A big challenge for inspectors in MPI testing is selecting right equipment. Numerous equipment and types are available today to choose for testing components and structures.
With the exception of permanent magnetic yoke, all other equipment needs electricity to generate magnetic field.
When do you choose permanent magnetic yoke?
If you want to perform the testing at fire hazard area or where spark can ignite the surrounding or getting electricity at a high altitude is a challenge, such as chimney, permanent magnet is preferred. This equipment does not need electricity. In almost all petroleum refineries, for testing weld joints and parts, permanent magnet is the best option. When all other equipment are prohibited to use, this equipment is the last option.
Before using permanent magnetic yoke, check for calibration and evaluate the strength. It is covered in this post else where.