ASTM E975-22

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: E975 − 22
Standard Test Method for
X-Ray Determination of Retained Austenite in Steel with
Near Random Crystallographic Orientation
This standard is issued under the fixed designation E975; 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.
INTRODUCTION
The volume percent of retained austenite (face-centered cubic phase) in steel is determined by
comparing the integrated chromium or molybdenum X-ray diffraction intensity of ferrite (body-
centered cubic phase) and austenite phases with theoretical intensities. This method should be applied
to steels with near random crystallographic orientations of ferrite and austenite phases because
preferred crystallographic orientations can drastically change these measured intensities from
theoretical values. Chromium radiation was chosen to obtain the best resolution of X-ray diffraction
peaks for other crystalline phases in steel such as carbides. No distinction has been made between
ferrite and martensite phases because the theoretical X-ray diffraction intensities are nearly the same.
Hereafter, the term ferrite can also apply to martensite. This test method has been designed for
unmodified commercial X-ray diffractometers or diffraction lines on film read with a densitometer.
Other types of X-radiations such as cobalt or copper can be used, but most laboratories examining
ferrous materials use chromium radiation for improved X-ray diffraction peak resolution or
molybdenum radiation to produce numerous X-ray diffraction peaks. Because of special problems
associated with the use of cobalt or copper radiation, these radiations are not considered in this test
method.
1. Scope necessary, the users can calculate the theoretical correction
factors to account for changes in volume of the unit cells for
1.1 This test method covers the determination of retained
austenite and ferrite resulting from variations in chemical
austenite phase in steel using integrated intensities (area under
composition.
peak above background) of X-ray diffraction peaks using
chromium K or molybdenum K X-radiation. 1.6 Units—The values stated in inch-pound units are to be
α α
regarded as standard. The values given in parentheses are
1.2 The method applies to carbon and alloy steels with near
mathematical conversions to SI units that are provided for
random crystallographic orientations of both ferrite and aus-
information only and are not considered standard.
tenite phases.
1.7 This standard does not purport to address all of the
1.3 This test method is valid for retained austenite contents
safety concerns, if any, associated with its use. It is the
from 1 % by volume and above.
responsibility of the user of this standard to establish appro-
1.4 If possible, X-ray diffraction peak interference from
priate safety, health, and environmental practices and deter-
other crystalline phases such as carbides should be eliminated
mine the applicability of regulatory limitations prior to use.
from the ferrite and austenite peak intensities.
1.8 This international standard was developed in accor-
dance with internationally recognized principles on standard-
1.5 Substantial alloy contents in steel cause some change in
ization established in the Decision on Principles for the
peak intensities which have not been considered in this
Development of International Standards, Guides and Recom-
method. Application of this method to steels with total alloy
mendations issued by the World Trade Organization Technical
contents exceeding 15 weight % should be done with care. If
Barriers to Trade (TBT) Committee.
This test method is under the jurisdiction of ASTM Committee E04 on
2. Significance and Use
Metallography and is the direct responsibility of Subcommittee E04.11 on X-Ray
and Electron Metallography.
2.1 Significance—Retained austenite with a near random
Current edition approved Nov. 1, 2022. Published February 2023. Originally
crystallographic orientation is found in the microstructure of
approved in 1984. Last previous edition approved in 2013 as E975 –13, which was
withdrawn in July 2022 and reinstated in November 2022. DOI: 10.1520/E0975-22. heat-treated low-alloy, high-strength steels that have medium
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E975 − 22
(0.40 weight %) or higher carbon contents. Although the
p = multiplicity factor of the (hkl) reflection,
presence of retained austenite may not be evident in the
θ = Bragg angle,
microstructure, and may not affect the bulk mechanical prop-
LP = Lorentz Polarization factor for a Bragg-Brentano
2 2
erties such as hardness of the steel, the transformation of powder diffractometer is equal to (1 + cos 2θ)/sin
retained austenite to martensite during service can affect the
θ cos θ for normal diffractometric analysis but
2 2 2
performance of the steel. becomes (1 + cos 2α cos 2θ)/(sin θ cos θ)
(1 + cos 2α) when a monochromator is used in
2.2 Use—The measurement of retained austenite can be
which diffraction by monochromator and specimen
included in low-alloy steel development programs to determine
take place in the same plane; 2α is the diffraction
its effect on mechanical properties. Retained austenite can be
angle of the monochromator crystal. If diffraction
measured on a companion specimen or test section that is
by the monochromator occurs in a plane perpen-
included in a heat-treated lot of steel as part of a quality control
dicular to the plane of specimen diffraction, then
practice. The measurement of retained austenite in steels from
2 2 2 2
LP = (cos 2α + cos 2θ)/(sin θ cosθ) (1 + cos 2α),
service can be included in studies of material performance.
−2 M
e = Debye-Waller or temperature factor which is a
2 2 2 2
function of θ where M = B( sin θ)/λ , B = 8π (μ ) ,
s
3. Principles for Retained Austenite Measurement by
where μ is the mean square displacement of the
s
X-Ray Diffraction
atoms from their mean position, in a direction
3.1 A detailed description of a retained austenite measure-
perpendicular to the diffracting plane, and
ment using X-ray diffraction is presented by the Society of
V = volume fraction of the α-phase.
α
Automotive Engineers. Since steel contains crystalline phases
K is a constant which is dependent upon the selection of
such as ferrite or martensite and austenite, a unique X-ray
instrumentation geometry and radiation but independent of the
diffraction pattern for each crystalline phase is produced when
nature of the specimen. The parameter, R, is proportional to the
the steel sample is irradiated with X-irradiation. Carbide
theoretical integrated intensity. The parameter, R, depends
phases in the steel will also produce X-ray diffraction patterns.
upon interplanar spacing (hkl), the Bragg angle, θ, crystal
3.2 For a randomly oriented specimen, quantitative mea-
structure, and composition of the phase being measured. R can
surements of the relative volume fraction of ferrite and
be calculated from basic principles.
austenite can be made from X-ray diffraction patterns because
3.3 For steel containing only ferrite (α) and austenite (γ) and
the total integrated intensity of all diffraction peaks for each
no carbides, the integrated intensity from the (hkl) planes of the
phase is proportional to the volume fraction of that phase. If the
ferrite phase is expressed as:
crystalline phase or grains of each phase are randomly
hkl hkl
I 5 KR V /2μ
oriented, the integrated intensity from any single diffraction
α α α
peak (hkl) crystalline plane is also proportional to the volume
3.3.1 A similar equation applies to austenite. We can then
fraction of that phase:
write for any pair of austenite and ferrite hkl peaks:
hkl hkl
I 5 KR V /2μ
hkl hkl hkl hkl
α α α
I /I 5 @~R /R !~V /V !#
α γ α γ α γ
where:
3.3.2 The above ratio holds if ferrite or martensite and
4 3
I e λ A austenite are the only two phases present in a steel and both
o
K 5 ×
S D S D
2 4
phases are randomly oriented. Then:
m c 32πr
V 1V 5 1
and α γ
2 22M
3.3.3 The volume fraction of austenite (V ) for the ratio of
1~/F/ pLPe !
γ
hkl
R 5
α
v measured integrated intensities of ferrite and austenite peak to
R-value is:
where:
V 5 I / R !/ I /R 1 I /R (1)
hkl @~ ! #
γ ~ γ γ α α ~ γ γ!
I = integrated intensity per angular diffraction peak
α
(hkl) in the α-phase,
3.3.4 For numerous ferrite and austenite peaks each ratio of
I = intensity of the incident beam,
o measured integrated intensity to R-value can be summed:
μ = linear absorption coefficient for the steel,
q P q
1 1 1
e,m = charge and mass of the electron,
V 5 I ⁄R / I /R 1 I /R (2)
S D FS D S DG
γ ( γj γj ( αi αi ( γj γj
q P q
j51 i51 j51
r = radius of the diffractometer,
c = velocity of light,
where:
λ = wavelength of incident radiation,
q = total number of austenite peaks, and
A = cross sectional area of the incident beam,
P = total number of ferrite peaks.
v = volume of the unit cell,
/F/ = structure factor times its complex conjugate,
3.3.5 If carbides are present:
V 1V 1V 5 1
α γ c
3.3.6 Then the volume fraction of austenite (V ) for the ratio
2 α
Retained Austenite and Its Measurement by X-ray Diffraction , SAE Special
of measured ferrite and austenite integrated intensity to R-value
Publication 453, Society of Automotive Engineers (SAE), 400 Commonwealth Dr.,
Warrendale, PA 15096-0001, http://www.sae.org. is:
E975 − 22
V 5 1 2 V I /R / I /R 1 I /R (3) 4.1.5.1 When using molybdenum radiation, select peaks in
~ !~ ! @~ ! ~ !#
γ c γ γ α α γ γ
the range from 28° to 40° 2θ for best results.
3.3.7 For numerous ferrite and austenite peaks the ratio of
4.2 X-Ray Equipment:
measured integrated intensity to R-values can be summed:
4.2.1 Any diffraction system may be used that consists of an
V 5 ~1 2 V ! (4)
γ c
x-ray source, an angular measurement capability, and an x-ray
q p q detection system. The system shall be capable of obtaining the
1 1 1
~Iγj/Rγj! / ~I i/R i! 1 ~I i/R i!
FS DG F G entire diffraction peak along with adjacent background levels,
( ( a a ( r r
q P q
j51 i51 j51
capable of detecting at least two separate austenite reflections
3.4 The volume fraction of carbide, V , should be deter-
c
and a ferrite reflection, and capable of normalizing any
mined by chemical extraction or metallographic methods.
equipment-specific intensity biases not accounted for by
Adequate X-ray diffraction peak resolution for the identifica-
R-factors. Two separate ferrite reflections should be measured;
tion of carbide peaks is required to avoid including carbide
however, in alloys with known carbide interference, only the
peaks in the retained austenite measurement.
unaffected ferrite reflection may be measured.
4.2.2 A chromium X-ray source with a vanadium metal or
4. Procedure
compound filter to reduce the K radiation is should be used.
β
4.1 Specimen Preparation:
NOTE 4—Chromium radiation produces a minimum of X-ray fluores-
4.1.1 Specimens for the X-ray diffractometer shall be cut cence of iron. Chromium radiation provides for the needed X-ray
diffraction peak resolution and allows for the separation of carbide peaks
with a minimum amount of heat effect. Saw cutting rather than
from austenite and ferrite peaks.
abrasive wheel cutting should be for specimen removal when-
4.2.3 Other radiation such as copper, cobalt, or molybde-
ever it is practical.
num may be used, but none of these provide the resolution of
NOTE 1—Since most steels containing retained austenite are relatively
chromium radiation.
hard, abrasive cutoff wheels are frequently used. If adequate cooling is not
used, heat effects from abrasive cutoff wheels can be substantial and, in
NOTE 5—Copper radiation is practical only when a diffracted-beam
some cases, can transform retained austenite.
monochromator is employed, because iron X-ray fluorescence will ob-
NOTE 2—Rough machining using a milling tool or coarse grinding can
scure the diffracted peaks.
deform the surface and transform some of the retained austenite to a depth
4.2.4 A molybdenum source with a zirconium filter may be
that is greater than the surface depth analyzed. Final milling or rough
used to produce a large number of X-ray diffraction peaks.
grinding cuts limited to a depth of 0.010-in (0.254 mm) or less will reduce
the depth of deformation.
4.3 X-Ray Method—X-ray diffraction peaks from other
4.1.2 Standard metallographic wet-grinding and polishing
crystalline phases such as carbides shall be separated from
methods shall be used to prepare specimens for X-ray analysis.
austenite and ferrite peaks. The linearity of the chart recorder
Grit reductions of 80, 120, 240, 320, 400, and 600 silicon
or photographic film shall be verified prior to utilizing this
carbide or alumina abrasives may be used, but other valid grit
method for older systems using these recording media.
combinations may also be used.
4.3.1 Entire diffraction peaks minus background under the
-4
4.1.2.1 The final surface polish shall be 2.36 × 10 in.
peaks shall be recorded to obtain integrated peak intensities.
(6-μm) diamond or an equivalent abrasive polish. Peaks without carbide or second phase interference may be
4.1.2.2 Specimen etching, observation for heat effects, and scanned, and the total peak plus background recorded. Obtain
background counts by counting on each side of the peak for
repolishing should be conducted as a safeguard.
one-half of the total peak counting time. Subtract the total
4.1.3 Since deformation caused by dull papers or over-
background from peak plus background to obtain the integrated
polishing can transform some of the retained austenite, elec-
intensity. Alternatively, software supplied with the diffracto-
trolytic polishing or chemical polishing of initial specimens of
meter may be used. In general, the diffractometer scanning rate
each grade and condition should be used to verify proper
should be 0.5°2θ/min or less to define the peaks for austenite
metallographic specimen preparation. Standard chromic-acetic
contents of less than 5 %.
acid for electropolishing 0.005-in. (0.127 mm) from specimens
4.3.2 Where carbide or other phase X-ray diffraction peak
ground to 600 grit or specific chemical polishing solutions for
-4
interference exists, planimeter measurements of area under the
a particular grade of steel polished to a 2.36 × 10 in. (
...