scope:

Purpose

This recommended practice establishes the requirements and procedures for Barkhausen Noise (BN) inspection of ferromagnetic steel components. Applications of the method are listed in 1.2 through 1.5.

Application to Grinding Burns

This test method may be used for the nondestructive detection of grinding-induced thermal damage of base ferromagnetic material in finish ground parts. This test method is primarily utilized on high-strength low-alloy steel parts which have been ground in accordance with MIL-STD-866 or some commercial standard, before and/or after coating or plating. This test method may be used to detect grinding burn, which can be confirmed in accordance with MIL-STD-867 in bare components, or upon removal of coatings, platings, etc., from the base material.

Application to Stresses

This test method may be used for the nondestructive detection of mechanically induced stresses in bare or coated/plated parts. Examples include overload yielding residual stress and localized induced stress resulting from manufacturing, maintenance, or in-service events.

Application to Microstructure/Heat Treatment Defects

This test method may be used for the nondestructive detection of defects induced during heat treatment including, but not limited to, lack of heat treatment, soft spots, decarburization, improper case depth, etc., as well as defects induced during other surface treatments such as shot peening.

Application to Miscellaneous Thermally Induced Defects

This test method may be used for the nondestructive detection of defects induced by any process which results in absorption of excess heat. This includes friction burns, arc burns, etc. Typical applications include thermal damage which results in microstructure transformation and/or stress relief.

Measurement Preconditions

The BN inspection method is an electromagnetic test and is sensitive to anything which affects the magnetic properties of the surface to be tested. Peripheral processes which affect the measurement must be considered and controlled. Examples include:

a. Coating/plating thickness will affect the measurement in through-coating/plating inspections. Coating/plating thickness should be determined to be constant or a minor factor.

b. Ferromagnetic coatings and patches (for example, Ni patches) will affect the measurement and can provide false rejects.

c. Grit/media blasting or shot peening processes can remove a surface layer of material which may contain otherwise detectable damage. This can decrease the ability of BN (or Nital etch, X-ray diffraction) to detect damage on the surface. Blasting also induces compressive residual stresses which affect the BN measurement and can result in reduced sensitivity. It is not recommended to grit/media blast, shot peen, or use any other stress-inducing cleanup method before testing with BN, if possible.

d. Measurements can be made in-situ with loaded components. Baseline values will be different for components when they are under load and, as a result, rejection limits may differ. Thermally induced defects detected in loaded components should be verified as detectable in the un-stressed/un-loaded state.

e. Parts to be tested should be free of defects due to earlier manufacturing processes. For example, defects present before grinding may be visible after grinding and can thus affect the measurement. Such defects include soft spots and edges due to defective heat treatment, decarburization, stress variations due to cooling rate changes, etc.

f. Sensor measurements can be made through a non-conductive protective layer/film. Protective films create an attenuating layer (i.e., lift-off) which will decrease the BN signal intensity as the layer thickness increases. Layer thickness is typically limited to 0.50 mm or less. Some applications allow for thicker layers (up to +1 mm), and some are limited to a thinner layer (0.05 mm). This depends on the alloy measured, the sensor geometry and electrical design, and the magnetizing parameters. Care must be taken to ensure that protective film thickness is maintained during measurements, as a varying thickness will result in an artificial influence on the measurement result.

g. Measurements made without sensor contact are possible. They are, however, subject to the same limitations as protective films as specified in 1.6.f.

h. BN measurements have depth sensitivity, which varies according to fundamental laws of electromagnetic damping. Material properties such as magnetic permeability and conductivity, as well as magnetizing frequency, voltage, and analysis frequency band, can affect depth sensitivity. Typical BN measurements made with magnetizing frequencies between 10 Hz and 500 Hz, along with analysis bands between 10 kHz and 500 kHz, have a depth penetration between 0.010 to 0.100 mm.