The amount of ferrite present in an austenitic stainless steel
has been shown to influence the strength, toughness and corrosion
resistance of this type of cast alloy. The amount of ferrite present
tends to correlate well with the magnetic permeability of the steel. The
methods described in this standard cover calibration practice for
estimating ferrite by the magnetic permeability of the steel. The
practice is inexpensive to use over large areas of the cast part and is
non-destructive.
This practice has been used for research, alloy development, quality control, and manufacturing control.Many instruments are available having different designs, and different principles of operation. When the probe is placed on the material being investigated, a closed magnetic circuit is formed allowing measurement of the magnetic permeability. When calibrated with standards having known ferrite content, this permeability indicates the ferrite content of the material being analyzed. The estimated ferrite content is read from a calibrated dial or from a digital-readout dial. Follow the manufacturer's instructions for proper calibration of the instrument.
Since this practice measures magnetic attraction and not ferrite directly, it is subject to all of the variables that affect magnetic permeability, such as the shape, size, orientation, and composition of the ferrite phase. These in turn are affected by thermal history. Ferrite measurements by magnetic methods have also been found to be affected by the surface finish of the material being analyzed.
Magnetic methods should not be used for arbitration of conflicts on ferrite content except when agreed upon between manufacturer and purchaser.
1. Scope
1.1 This practice covers the procedure for
calibration of instruments to be used for estimating the ferrite content
of the microstructure of cast stainless steels by magnetic response or
measurement of permeability. This procedure covers both primary and
secondary instruments.
1.1.1 A primary instrument is
one that has been calibrated using National Institute of Standards and
Technology-Standard Reference Material (NIST-SRM) thickness coating
standards. It is a laboratory tool to be used with test specimens. Some
primary instruments may be used to directly measure the ferrite content
of castings.
1.1.2 A secondary instrument is one
that has been calibrated by the use of secondary standards that have
been measured by a calibrated primary instrument. Secondary instruments
are to be used to directly measure the ferrite content of castings.
1.2
The values stated in either SI units or inch-pound units are to be
regarded separately as standard. The values stated in each system may
not be exact equivalents; therefore, each system shall be used
independently of the other. Combining values from the two systems may
result in non-conformance with the standard.
1.2.1 Within the text, the SI units are shown in brackets.
1.3 This
standard does not purport to address all of the safety concerns, if
any, associated with its use. It is the responsibility of the user of
this standard to establish appropriate safety and health practices and
determine the applicability of regulatory limitations prior to use.
2. Referenced Documents (purchase separately)
ASTM Standards
A941 Terminology Relating to Steel, Stainless Steel, Related Alloys, and Ferroalloys
B499 Test Method for Measurement of Coating Thicknesses by the Magnetic Method: Nonmagnetic Coatings on Magnetic Basis Metals
E562 Test Method for Determining Volume Fraction by Systematic Manual Point Count
NIST Standard
NIST-SRMCoatingThick
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