This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles

for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

Designation: A799/A799M − 10 (Reapproved 2020)

Standard Practice for

Steel Castings, Stainless, Instrument Calibration, for
Estimating Ferrite Content1
This standard is issued under the fixed designation A799/A799M; the number immediately following the designation indicates the year
of original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval.
A superscript epsilon (´) indicates an editorial change since the last revision or reapproval.

1. Scope 2. Referenced Documents

1.1 This practice covers the procedure for calibration of 2.1 ASTM Standards:2
instruments to be used for estimating the ferrite content of the A941 Terminology Relating to Steel, Stainless Steel, Related
microstructure of cast stainless steels by magnetic response or Alloys, and Ferroalloys
measurement of permeability. This procedure covers both B499 Test Method for Measurement of Coating Thicknesses
primary and secondary instruments. by the Magnetic Method: Nonmagnetic Coatings on
1.1.1 A primary instrument is one that has been calibrated Magnetic Basis Metals
using National Institute of Standards and Technology-Standard E562 Test Method for Determining Volume Fraction by
Reference Material (NIST-SRM) thickness coating standards. Systematic Manual Point Count
It is a laboratory tool to be used with test specimens. Some 2.2 NIST Standard:
primary instruments may be used to directly measure the ferrite NIST-SRM Coating Thickness Standards
content of castings. NOTE 1—The specific coating thickness standards previously refer-
1.1.2 A secondary instrument is one that has been calibrated enced in this practice are no longer available. Similar ones are now
by the use of secondary standards that have been measured by available from NIST.
a calibrated primary instrument. Secondary instruments are to
be used to directly measure the ferrite content of castings. 3. Terminology
1.2 The values stated in either SI units or inch-pound units 3.1 Definitions—The definitions in Terminology A941 are
are to be regarded separately as standard. The values stated in applicable to this standard.
each system may not be exact equivalents; therefore, each 3.2 Definitions of Terms Specific to This Standard:
system shall be used independently of the other. Combining 3.2.1 ferrite, n—the body-centered cubic microconstituent
values from the two systems may result in nonconformance in stainless steel.
with the standard. 3.2.2 ferrite percentage, n—a value designating the ferrite
1.2.1 Within the text, the SI units are shown in brackets. content of stainless steels.
1.3 This standard does not purport to address all of the 3.2.2.1 Discussion—The Steel Founders’ Society of
safety concerns, if any, associated with its use. It is the America (SFSA) has assigned ferrite percentages to the series
responsibility of the user of this standard to establish appro- of NIST coating thickness standards.3 This assignment was
priate safety, health, and environmental practices and deter- based on the magnetic attraction for a standard magnet by the
mine the applicability of regulatory limitations prior to use. coating standards when compared with the magnetic attraction
1.4 This international standard was developed in accor- of the same magnet by a series of cast stainless steels whose
dance with internationally recognized principles on standard- ferrite content had been determined by an accurate metallo-
ization established in the Decision on Principles for the graphic point count. A similar assignment based on magnetic
Development of International Standards, Guides and Recom- permeability was also established. Algebraic equations have
mendations issued by the World Trade Organization Technical now been derived from a plot of the thickness of these
Barriers to Trade (TBT) Committee. standards and the assigned ferrite percentages. By the use of

2
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
1
This practice is under the jurisdiction of ASTM Committee A01 on Steel, contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Stainless Steel and Related Alloys and is the direct responsibility of Subcommittee Standards volume information, refer to the standard’s Document Summary page on
A01.18 on Castings. the ASTM website.
3
Current edition approved March 1, 2020. Published March 2020. Originally Aubrey, L. S., Weiser, P. F., Pollard, W. J., and Schoefer, E. A., “Ferrite
approved in 1982. Last previous edition approved in 2015 as A799/A799M – 10 Measurement and Control in Cast Duplex Stainless Steels,” Stainless Steel Castings,
(2015). DOI: 10.1520/A0799_A0799M-10R20. ASTM STP 756, ASTM International, 1982, p 126.

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 A799/A799M − 10 (2020)
these equations, any primary instrument will have its calibra- 5.1.2 The spring is then wound until the force of the coiled
tion traceable to the SFSA’s instruments or any other calibrated spring overcomes the magnetic attraction of the magnet for the
instrument and thus afford comparable reproducible ferrite material being tested, causing the magnet to break contact and
percentages. It also allows traceability to NIST. the lever arm to rise.
3.2.3 secondary standards, n—a piece of cast stainless steel 5.1.3 The amount of force that the coiled spring has
whose ferrite percentage has been determined by a calibrated developed is determined from a marked dial securely attached
primary instrument. to the shaft that is used to coil or uncoil the spring.
3.2.3.1 Discussion—Secondary statements are used to cali- 5.1.3.1 A weighted No. 2 magnet (catalog number J5-
brate secondary instruments (see Calibration of Secondary 0664W) is used with this instrument.
Instruments). 5.2 When using a Feritscope,5 follow the manufacturer’s
instructions for calibration. When traceability is required,
4. Significance and Use confirm the calibration using the appropriate NIST standards.
5.2.1 Newer versions of this instrument have a single-point
4.1 The amount of ferrite present in an austenitic stainless
probe, while older versions have a two-point probe as the
steel has been shown to influence the strength, toughness, and
sensing device. When this probe is placed on the material being
corrosion resistance of this type of cast alloy. The amount of
investigated, a closed magnetic circuit is formed and energized
ferrite present tends to correlate well with the magnetic
by a low-frequency magnetic field. The voltage induced in the
permeability of the steel. The methods described in this
probe coil by this field is a measure of the permeability. When
standard cover calibration practice for estimating ferrite by the
calibrated with standards having known ferrite content, this
magnetic permeability of the steel. The practice is inexpensive
permeability indicates the ferrite content of the material being
to use over large areas of the cast part and is nondestructive.
analyzed. The estimated ferrite content is read from a cali-
4.2 This practice has been used for research, alloy brated dial or from a digital readout dial.
development, quality control, and manufacturing control.
5.3 One secondary instrument consists of a balance arm that
4.2.1 Many instruments are available having different de- has a rod-shaped magnet attached to one end.6 The opposite
signs and different principles of operation. When the probe is end is counterweighted to balance the magnet.
placed on the material being investigated, a closed magnetic 5.3.1 This arm with its magnet and counterweight is en-
circuit is formed allowing measurement of the magnetic closed in a transparent box. The top face of this container has
permeability. When calibrated with standards having known a threaded hole directly over the magnet. Marked inserts that
ferrite content, this permeability indicates the ferrite content of have metal plates on their bottom faces are screwed into this
the material being analyzed. The estimated ferrite content is hole. These plates have different strengths of attraction for the
read from a calibrated dial or from a digital readout dial. magnet.
Follow the manufacturer’s instructions for proper calibration of 5.3.2 In use, the bottom end of the magnet is touched to the
the instrument. material being investigated. The other end of the magnet is in
4.3 Since this practice measures magnetic attraction and not contact with the metal plate on the bottom of the insert. The
ferrite directly, it is subject to all of the variables that affect container is then raised. If the material being measured has a
magnetic permeability, such as the shape, size, orientation, and greater attraction for the magnet than does the plate on the
composition of the ferrite phase. These in turn are affected by bottom of the insert, the magnet will be pulled away from the
thermal history. Ferrite measurements by magnetic methods insert. If not, the magnet will pull away from the material being
have also been found to be affected by the surface finish of the measured. The inserts are changed and the test repeated until
material being analyzed. the inserts that are just weaker and just stronger than the
material being investigated are found.
4.4 Magnetic methods should not be used for arbitration of
5.3.3 The results of a measurement with this instrument are
conflicts on ferrite content except when agreed upon between
reported as less than A and greater than B.
manufacturer and purchaser.
5.4 NIST-SRM Coating Thickness Standards—These are
5. Apparatus mild steel plates that are covered by an electroplated copper
layer which in turn is covered by a flash coat of chromium. The
5.1 One primary instrument that uses magnetic attraction thickness of the copper coat varies from standard to standard
consists of a spring-loaded balance arm from which a rod- and is certified by NIST. The strength of the magnetic
shaped magnet is suspended.4 The opposite end of the balance attraction of each standard varies with the thickness of the
arm from the magnet has counterweights that balance most but coating. These are primary standards for calibration.
not all of the weight of the magnet.
5.1.1 When this instrument is used, the spring load is 5.5 Other instruments such as the Elcometer7 may be used.
relaxed sufficiently to allow the magnet to make contact with
the material being tested. 5
Feritscope, produced by Fischer Technology, Inc., 750 Marshall Phelps Road,
Windsor, CT 06095; http://www.fischer-technology.com.
6
Severn Gage, Severn Engineering Co., Old Stage Business Park, 555 Old Stage
Road, Suite 1-A, Auburn, AL 36830; http://www.severnengineering.com.
4 7
Magne Gage, produced by Magne Gage Sales and Service Co., Inc., 629 Packer Elcometer, Elcometer Instruments Ltd., Edge Lane, Manchester, UK M43 6BU;
Street, Avoca, PA 18641; http://www.magne-gage.com. http://www.elcometer.com.

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 A799/A799M − 10 (2020)
6. Calibration 6.1.4.5 Tabulate the black dial readings and the thickness of
6.1 Calibrate primary instruments that use magnetic attrac- the coatings on the standards.
tion as criterion as follows: 6.1.5 Preparation of Ferrite Percentage Curve—Calculate
6.1.1 When calibrating magnetic instruments, make sure the assigned ferrite percentage value for each NIST-SRM
there is no magnetic material within the area that could affect coating thickness used in 6.1.4.3 by using Eq 1, where F is the
the calibration. This includes beneath the surface on which the assigned ferrite content and T is the coating thickness as
instrument rests. reported by NIST, and the coefficients B0 through B4 from
6.1.2 Magnet—Use weighted standard No. 2 magnet for Table 1. Round the calculated value to the nearest 0.1 % ferrite.
measurement of ferrite content of cast stainless steel. B1 B2 B3 B4
F 5 B 01 1 1 1 (1)
6.1.3 Zeroing—Before calibration, zero each primary instru- T T2 T3 T4
ment.
6.1.5.1 Plot on an arithmetic scale the ferrite percentage for
6.1.3.1 When zeroing the instrument, use the T-shaped
each standard and the black dial reading obtained for that
handle to lower the spring-loaded balance arm until the plastic
standard. This is the calibration curve for the instrument. It is
protection cylinder around the magnet is in contact with a
used to designate the ferrite percentage of any sample mea-
nonmagnetic object. The base plate of the unit is satisfactory.
sured with the instrument.
6.1.3.2 Turn the large knurled knob on the central shaft
counter-clockwise to a dead stop. 6.2 Calibrate primary instruments that use magnetic perme-
6.1.3.3 Rotate the large knurled knob clockwise until the ability as criterion as follows:
magnet lifts off the nonmagnetic object and the pivot arm from NOTE 2—This calibration procedure applies only to the older (pre-
which the magnet is suspended is parallel to the base plate. 1980), analog instruments with the two-point probe. Analog instruments
Loosen the set screw holding the black dial in position. may have either an analog meter or a digital meter. Newer (post-1980)
6.1.3.4 Set the “0” position on the black dial at the index instruments with digital readouts or single-point probes must be calibrated
using the procedure given under Calibration of Secondary Instruments.
position and tighten the set screw.
6.1.4 Determining Black Dial Values for the NIST-SRM 6.2.1 If the instrument has more than one measuring range,
Thickness Standards—Use the NIST-SRM coating thickness set the instrument to the desired range.
standards. 6.2.2 Connect the measuring probe to the instrument.
6.1.4.1 Insert, one at a time in random order, the various 6.2.3 Zeroing—Bring the needle opposite “0” on the dial by
NIST-SRM coating thickness standards under the plastic pro- means of the “zero” knob. Be sure the probe is at least 1 ft
tection cylinder. Lower the instrument each time until the [305 mm] away from any magnetic material when this adjust-
cylinder contacts the standard. ment is made.
6.1.4.2 If the magnet is attracted to the plate when the 6.2.4 Calibration of “End of Range”:
plastic protection cylinder is placed in contact with the 6.2.4.1 Apply the sensing probe to an NIST-SRM with
standard, rotate the large knurled knob clockwise slowly until 2.00 mil [51.0 µm] coating thickness. Using the “end point”
the magnet breaks contact with the standard. Record the black control knob, bring the meter needle opposite “29” on the
dial reading. meter. If a digital readout meter is being used, bring the
6.1.4.3 If the magnet is not attracted to the plate when the maximum digital reading to “29.”
plastic cylinder comes in contact with the standard, push the 6.2.4.2 If an NIST-SRM with coating thickness other than
magnet into contact by using the push-rod located over the 2.00 mil [51.0 µm] is used, determine the meter setting to be
magnet. If the magnet does not adhere to the standard, turn the used from Table 2. It is recommended that coating thickness of
large knurled knob counterclockwise a few divisions at a time 3.25 mil [82.5 µm] or less be used.
until the magnet does adhere when it comes in contact with the 6.2.4.3 When measuring ranges are changed, the “zero”
standard. When the magnet remains in contact with the setting must be adjusted to “0.”
standard, rotate the large knurled knob clockwise slowly until 6.2.5 Determining the Meter Readings for the NIST-SRM
the magnet breaks contact with the standard. Record the black Thickness Standards:
dial reading. 6.2.5.1 Apply the probe to the various NIST-SRM standards
6.1.4.4 Repeat 6.1.4.3 several times, more than three, with several times, more than three. Rotate the probe 90° between
each standard moving the standard under the plastic protection readings. Record the readings and average them.
cylinder after each reading. Take the reading in the central area 6.2.5.2 Tabulate the average meter readings and the thick-
of the standard. Average the readings. ness of the measured standards.

TABLE 1 Coefficients for Calculating Assigned Ferrite Values

Instrument Units B0 B1 B2 B3 B4
Magne-Gage mils –0.6727 164.8 –334.3 516.2 –352.6
mm –0.6727 6 486 –518 100 31 500 000 –847 100 000

Feritscope (Note 2) mils –2.042 296.5 –1656 5 440 –6 945

mm –2.042 11 670 –2 566 000 332 000 000 –16 680 000 000

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 A799/A799M − 10 (2020)
TABLE 2 “End Point” Settings to be Used with Coating 8.1.1.1 Measure each secondary standard with a primary
Thickness Other Than 2.00 mils [51.0 µm] instrument and note the black dial or meter reading. Tabulate
Coating Thickness, mils [µm] “End Point” the results.
1.66 [42.2] 30
2.04 [51.8] 28.5 8.1.1.2 Determine the reading from the secondary instru-
2.1 [53.3] 28 ment for each secondary standard. Make several readings on
2.2 [55.9] 27.7
2.3 [58.4] 27.5
each standard.
2.4 [61.0] 27 8.1.1.3 Plot the black dial or meter readings and the read-
2.5 [63.5] 26.5
2.6 [66.0] 26.3
ings from the secondary instrument. This curve can be used to
2.7 [68.6] 26 obtain the ferrite percentage of the material measured with the
2.8 [71.1] 25.7 secondary instrument from the primary calibration curve.
2.9 [73.7] 25.5
3.0 [76.2] 25.2 8.2 Secondary Instruments That Have Set Point Readings:
3.1 [78.8] 25
3.22 [81.8] 24.5 8.2.1 Measure each secondary standard with various inserts
in the secondary instrument set.
8.2.1.1 Note the lowest ferrite percentage of the secondary
6.2.6 Preparation of Ferrite Percentage Curve: standard set that will pull the measuring magnet of the
6.2.6.1 Calculate the assigned ferrite percentage for each instrument away from the insert.
NIST-SRM measured in 6.2.5.1 by using Eq 1, where F is the
assigned ferritic content and T is the coating thickness as 8.2.1.2 Note the highest ferrite percentage of the secondary
reported by NIST, and the coefficients B0 through B4 from standard set that will not pull the measuring magnet of the
Table 1. Round the calculated value to the nearest 0.1 % ferrite. instrument from the insert.
6.2.6.2 Plot on an arithmetic scale the ferrite percentage for 8.2.1.3 For each insert, tabulate the values obtained in
each standard and the meter or digital readings obtained for 8.2.1.1 and 8.2.1.2. Each insert will be designated as “greater
that standard. This is the calibration curve for the instrument. than ________ ferrite percentage less than __________ ferrite
percentage.”
7. Checking Calibration
7.1 Whenever any instrument is to be used after a period of 9. Checking of Calibration of Secondary Instruments
nonuse, the zero point and the black dial or meter readings of
one or more coating thickness standards must be determined to 9.1 Before using a secondary instrument to measure the
see if the instrument is in calibration. ferrite content of a casting, it should be checked with one or
more of the secondary standards.
8. Calibration of Secondary Instruments 9.1.1 Periodically, the set of secondary standards shall be
8.1 Correlating Ferrite Percentage of Secondary Standards checked with the primary instrument by determining the proper
with Primary Instruments: readings. Compare these values with those obtained in 8.1.1.
8.1.1 Determine the proper readings (black dial or meter)
with a calibrated instrument for each secondary standard.8 10. Keywords
10.1 calibration; ferrite; stainless steel castings
8
The secondary standards may be produced by a foundry that produces cast
stainless steel or purchased from an organization such as NIST. (Standard Reference
Materials 8480 and 8481 are available from the NIST Standard Reference Materials
Program at 100 Bureau Drive, Stop 2300, Gaithersburg, MD 20899-2300; http://
ts.nist.gov/ts/htdocs/230/232/232.htm.)

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