ASTM A976-18

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Designation: A976 − 13 A976 − 18
Standard Classification of
Insulating Coatings for Electrical Steels by Composition,
Relative Insulating Ability and Application
This standard is issued under the fixed designation A976; 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
1.1 This document classifies insulating coatings for electrical steels according to their composition, relative insulating ability,
and functionality. The purpose of this classification is to assist users of insulating coatings by providing general information about
the chemical nature and use of the coatings, as well as to provide important data concerning limits to their use, that is, relative
insulating ability, punchability, temperature stability, weldability, and fabricability. Specific surface insulation resistivity values for
each coating are not included in this classification. The user is referred to the flat-rolled electrical steel specifications noted in 1.2
should more detailed information concerning surface insulation resistivity values be required.
1.2 This classification is to be used in conjunction with the various specifications for flat-rolled electrical steels under the
jurisdiction of Committee A06, including Specifications A345, A677, A683, A726, A840, A876and , and A876A1086. However,
in those instances in which the coating descriptions and characteristics differ between this classification and any of the
specifications, this classification shall supersede the specification.
1.3 The values stated in customary (cgs-emu and inch-pound) SI units are to be regarded as standard. The values given in
parentheses are mathematical conversions to SI customary (cgs-emu and inch-pound) units which are provided for information
only and are not considered standard.
1.4 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 safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
1.5 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.
2. Referenced Documents
2.1 ASTM Standards:
A340 Terminology of Symbols and Definitions Relating to Magnetic Testing
A345 Specification for Flat-Rolled Electrical Steels for Magnetic Applications
A677 Specification for Nonoriented Electrical Steel Fully Processed Types
A683 Specification for Nonoriented Electrical Steel, Semiprocessed Types
A717/A717M Test Method for Surface Insulation Resistivity of Single-Strip Specimens
A726 Specification for Cold-Rolled Magnetic Lamination Quality Steel, Semiprocessed Types
A840 Specification for Fully Processed Magnetic Lamination Steel (Withdrawn 2011)
A876 Specification for Flat-Rolled, Grain-Oriented, Silicon-Iron, Electrical Steel, Fully Processed Types
A937/A937M Test Method for Determining Interlaminar Resistance of Insulating Coatings Using Two Adjacent Test Surfaces
A1086 Specification for Thin-Gauge Nonoriented Electrical Steel Fully Processed Types
3. Terminology
3.1 Definitions of Terms Specific to This Standard:
This classification is under the jurisdiction of ASTM Committee A06 on Magnetic Properties and is the direct responsibility of Subcommittee A06.02 on Material
Specifications.
Current edition approved Nov. 1, 2013Oct. 1, 2018. Published November 2013October 2018. Originally published in 1997. Last previous edition approved in 20082013
as A976 – 03 (2008).A976 – 13. DOI: 10.1520/A0976-13.10.1520/A0976-18.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
The last approved version of this historical standard is referenced on www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
A976 − 18
3.1.1 interlaminar resistance, n—the average resistance of two adjacent insulating surfaces in contact with each other, in
accordance with Test Method A937/A937M.
3.1.2 stress-relief anneal, n—heat treatment that improves the magnetic properties of electrical steel by relieving internal
stresses which are introduced during fabrication of magnetic cores.
3.1.3 surface insulation resistivity, n—the effective resistivity of a single insulating layer tested between applied bare metal
contacts and the base metal of the insulated test specimen, in accordance with Test Method A717/A717M.
3.2 Definitions for other terms and symbols used in this specification are defined in Terminology A340.
4. Significance and Use
4.1 This classification establishes categories of insulating coatings based on their chemical nature, relative insulating ability, and
typical applications. These categories describe general physical and chemical characteristics of the coatings that are useful in
making broad estimates of their insulating ability and suitability for various applications.
5. Basis of Classification
5.1 The insulating coatings are categorized according to general composition, relative insulating ability, and functionality
(Table 1). The purpose of this classification is to create a nomenclature for the various coating types. It is not the intent of this
classification to specify coating requirements. Specific properties required by coating users should be negotiated with the steel
producer.
5.2 To reduce confusion regarding the various categories of coatings, this classification follows the “C” type of designation
initially used by the American Iron and Steel Institute for describing insulating coatings. The “C” is included in the coating
designation because insulating coatings for electrical steels have been historically referred to as “coreplate” coatings. This
classification includes new coatings and test methods not included in the most recent edition of the AISI classification. Note that
the electrical steel committee of the AISI is no longer active, and the 1983 edition of the coating classification document was the
last edition of the coating classification to be published.
6. Test Methods
6.1 The surface insulation resistivity of an insulating layer may be measured using Test Methods A717/A717M. In this test
method, ten metallic contacts of fixed area are applied to one of the surfaces of the test specimen, and electrical contact is made
with the base metal by two drills (Fig. 1 and Fig. 2). The effectiveness of the coating (surface insulation) then is indicated by a
measurement of the average electrical current flowing between the contacts and the base metal under specified applied voltage.
This current value often is referred to as the “Franklin Current” and may be used directly as an indicator of the quality of the
insulation. Specifically, a Franklin Current of zero corresponds to a perfect insulator. A Franklin Current of 1 ampere corresponds
to a perfect conductor. If desired, an apparent surface insulation resistivity value for the insulating layer may be calculated as
follows:
R 5 6.45 1 / I 2 1 in Ω·cm /lamination (1)
$~ ! %
i
R 5 6.45 $~1 / I! 2 1% in Ω·cm /lamination (1)
i
or
R 5 645 $~1 / I! 2 1% in Ω·mm /lamination (2)
i
R 5 645 $~1 / I! 2 1% in Ω·mm /lamination (2)
i
where:
2 2
R = surface insulation resistivity of test sample (two surfaces in series) in Ω·cm /lamination or Ω·mm /lamination and
i
2 2
R = surface insulation resistivity of test sample (two surfaces in series) in Ω·cm /lamination or Ω·mm /lamination and
i
I = ammeter reading in A (also known as Franklin Current).
Note that this test method often is referred to as the Franklin test. The Franklin test is a widely used method for evaluating the
effectiveness of surface insulation on electrical steels.
6.2 The average resistance of two adjacent insulating surfaces in contact with each other, interlaminar resistance, may be
measured using Test Method A937/A937M. Because the interlaminar resistance is influenced by coating-to-coating contact, this
test method is particularly useful for providing an estimate of the interlaminar resistance in a stacked or wound core in which
coated surfaces are in contact with each other. Furthermore, this test method is particularly useful for electrical steels coated with
2 2
insulating coatings having surface insulation resistivities in excess of 300 Ω·cmΩ·cm (30 kΩ·mm, or 30 kΩ·mm ), (that is, less
Steel Products Manual on Flat–Rolled Electrical Steel, American Iron and Steel Institute, 1101 17th St., N.W., Washington, DC 20036–4700, January 1983.
Loudermilk, D. S. and Murphy, R. A., “Overview of Technology of Insulating Coatings for Grain–Oriented and Nonoriented Electrical Steels,” Fifteenth Annual
Conference on Properties and Applications of Magnetic Materials, Illinois Institute of Technology, Chicago, IL, May 1996.
A976 − 18
TABLE 1 Classification of Insulating Coatings for Electrical Steels
Coating
Coating Description/Characteristics
A
Name
C-0 Oxide that is formed naturally on the steel surface during mill processing. This oxide layer is thin, tightly adherent, and provides sufficient insulating quality
for most small cores. The oxide layer will withstand normal stress-relief annealing temperatures. The insulation quality is affected by the oxidizing
potential of the user’s anneal, that is, the oxidized surface condition may be enhanced by controlling the atmosphere to be more or less oxidizing to the
surface. It is not appropriate to assert a maximum acceptable Franklin test current for this coating.
C-1 User-formed oxide that is created on the steel surface by contact with an oxidizing furnace atmosphere at the end of the heat-treating cycle. This coating
usually is bluish to gray in color and used for various electrical steel applications. It is not appropriate to assert a maximum acceptable Franklin
test current for this coating.
C-2 Inorganic insulating coating predominantly comprised of magnesium silicate and used on grain-oriented electrical steel. The coating is formed from the
reaction of the annealing separator with the steel surface during high-temperature annealing. The resulting coating often is referred to as “mill glass”
or “glass film”even though the coating is not technically a glass. The coating is very abrasive, and hence, is not typically used for stamped laminations.
The primary application of this coating is air-cooled or oil-immersed wound distribution transfo
...