ASTM E975-00
(Practice)Standard Practice for X-Ray Determination of Retained Austenite in Steel with Near Random Crystallographic Orientation
Standard Practice for X-Ray Determination of Retained Austenite in Steel with Near Random Crystallographic Orientation
SCOPE
1.1 This practice covers the determination of retained austenite phase in steel using integrated intensities (area under peak above background) of X-ray diffraction peaks using chromium K[alpha] or molybdenum K[alpha] X-radiation.
1.2 The method applies to carbon and alloy steels with near random crystallographic orientations of both ferrite and austenite phases.
1.3 This practice is valid for retained austenite contents from 1% by volume and above.
1.4 If possible, X-ray diffraction peak interference from other crystalline phases such as carbides should be eliminated from the ferrite and austenite peak intensities.
1.5 Substantial alloy contents in steel cause some change in peak intensities which have not been considered in this method. Application of this method to steels with total alloy contents exceeding 15 weight% should be done with care. If necessary, the users can calculate the theoretical correction factors to account for changes in volume of the unit cells for austenite and ferrite resulting from variations in chemical composition.
1.6 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.
General Information
Relations
Standards Content (Sample)
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
Designation: E 975 – 00
Standard Practice for
X-Ray Determination of Retained Austenite in Steel with
Near Random Crystallographic Orientation
This standard is issued under the fixed designation E 975; 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 (e) 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 (bodycen-
tered 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 practice 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
practice.
1. Scope austenite and ferrite resulting from variations in chemical
composition.
1.1 This practice covers the determination of retained aus-
1.6 This standard does not purport to address all of the
tenite phase in steel using integrated intensities (area under
safety concerns, if any, associated with its use. It is the
peak above background) of X-ray diffraction peaks using
responsibility of the user of this standard to establish appro-
chromium K or molybdenum K X-radiation.
a a
priate safety and health practices and determine the applica-
1.2 The method applies to carbon and alloy steels with near
bility of regulatory limitations prior to use.
random crystallographic orientations of both ferrite and auste-
nite phases.
2. Significance and Use
1.3 This practice is valid for retained austenite contents
2.1 Significance—Retained austenite with a near random
from 1 % by volume and above.
crystallographic orientation is found in the microstructure of
1.4 If possible, X-ray diffraction peak interference from
heat-treated low-alloy, high-strength steels that have medium
other crystalline phases such as carbides should be eliminated
(0.40 weight %) or higher carbon contents. Although the
from the ferrite and austenite peak intensities.
presence of retained austenite may not be evident in the
1.5 Substantial alloy contents in steel cause some change in
microstructure, and may not affect the bulk mechanical prop-
peak intensities which have not been considered in this
erties such as hardness of the steel, the transformation of
method. Application of this method to steels with total alloy
retained austenite to martensite during service can affect the
contents exceeding 15 weight % should be done with care. If
performance of the steel.
necessary, the users can calculate the theoretical correction
2.2 Use—The measurement of retained austenite can be
factors to account for changes in volume of the unit cells for
included in low-alloy steel development programs to determine
its effect on mechanical properties. Retained austenite can be
measured on a companion sample or test section that is
This practice is under the jurisdiction of ASTM Committee E04 on Metallog-
raphy and is the direct responsibility of Subcommittee E04.11 on X-Ray and
included in a heat-treated lot of steel as part of a quality control
Electron Metallography.
practice. The measurement of retained austenite in steels from
Current edition approved July 10, 2000. Published October 2000. Originally
service can be included in studies of material performance.
published as E 975 – 84. Last previous edition E 975 – 95.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
E 975
A
TABLE 1 Calculated Theoretical Intensities Using Chromium K Radiation
a
2 B 2
hkl Sinu/lu f Df8 Df9 /F/ LP P T N R
(a iron, body-centered cubic, unit-cell dimension a = 2.8664Å):
o
B C
110 0.24669 34.41 18.474 −1.6 0.9 1142.2 4.290 12 0.9577 0.001803 101.5
B C
200 0.34887 53.06 15.218 −1.6 0.9 745.0 2.805 6 0.9172 0.001803 20.73
B C
211 0.42728 78.20 13.133 −1.6 0.8 534.6 9.388 24 0.8784 0.001803 190.8
(g iron, face-centered cubic, unit-cell dimension a = 3.60Å):
o
B C
111 0.24056 33.44 18.687 −1.6 0.9 4684.4 4.554 8 0.9597 0.0004594 75.24
B C
200 0.27778 39.52 17.422 −1.6 0.9 4018.3 3.317 6 0.9467 0.0004594 34.78
B C
220 0.39284 64.15 14.004 −1.6 0.8 2472.0 3.920 12 0.8962 0.0004594 47.88
A
Data from “International Tables for X-Ray Crystallography,” Physical and Chemical Tables, Vol III, Kynoch Press, Birmingham, England, 1962, pp. 60, 61, 210, 213;
Weighted K and K value used (l = 2.29092Å).
a1 a2
B −2M 2 2
Temperature factor (T =e ) where M=B (sin u)/l and 2B = 0.71. Also N is the reciprocal of the unit-cell volume.
C
Calculated intensity includes the variables listed that change with X-ray diffraction peak position.
3. Principles for Retained Austenite Measurement by
LP = Lorentz Polarization factor which is equal to
X-Ray Diffraction
(1 + cos 2u)/sin u cos u for normal diffractomet-
2 2
ric analysis but becomes (1 + cosu 2a cos
3.1 A detailed description of a retained austenite measure-
2 2
2u)/(sin u cos u) (1 + cos 2a) when a mono-
ment using X-ray diffraction is presented by the Society
chromator is used in which diffraction by mono-
ofAutomotive Engineers. Since steel contains crystalline
chromator and sample take place in the same
phases such as ferrite or martensite and austenite, a unique
plane; 2a is the diffraction angle of the mono-
X-ray diffraction pattern for each crystalline phase is produced
chromator crystal. If diffraction by the mono-
when the steel sample is irradiated with X-irradiation. Carbide
chromator occurs in a plane perpendicular to the
phases in the steel will also produce X-ray diffraction patterns.
plane of sample diffraction, then LP = (cos
3.2 For a randomly oriented sample, quantitative measure-
2 2 2
2a + cos 2u)/sin ucos (1 + cos 2a),
ments of the relative volume fraction of ferrite and austenite
−2 M
e = Debye-Waller or temperature factor which is a
can be made from X-ray diffraction patterns because the total
2 2 2
function of u where M=B(sin u)/l , B =8p
integrated intensity of all diffraction peaks for each phase is
2 2
(μ ) , where μ is the mean square displacement
proportional to the volume fraction of that phase. If the s s
of the atoms from their mean position, in a
crystalline phase or grains of each phase are randomly ori-
direction perpendicular to the diffracting plane,
ented, the integrated intensity from any single diffraction peak
and
( hkl) crystalline plane is also proportional to the volume
V = volume fraction of thea -plane.
a
fraction of that phase:
K is a constant which is dependent upon the selection of
hkl hkl
I 5 KR V /2μ
a a a
instrumentation geometry and radiation but independent of the
where:
nature of the sample. The parameter, R, is proportional to the
theoretical integrated intensity. The parameter, R, depends
e 2 4 3
K 5 ~I /m c ! 3 ~l /32pr!
o A
upon interplanar spacing (hkl), the Bragg angle, u, crystal
and
structure, and composition of the phase being measured. R can
2 22M
be calculated from basic principles.
1~/F/ pLPe !
hkl
R 5
a 2
3.3 For steel containing only ferrite (a) and austenite (g)
v
and no carbides, the integrated intensity from the ( hkl) planes
where:
of the ferrite phase is expressed as:
hkl
I = integrated intensity per angular diffraction peak
a hkl hkl
I 5 KR V /2μ
a a a
(hkl)inthe a-phase,
3.3.1 A similar equation applies to austenite. We can then
I = intensity of the incident beam,
o
μ = linear absorption coefficient for the steel, write for any pair of austenite and ferrite hkl peaks:
e,m = charge and mass of the electron,
hkl hkl hkl hkl
I /I 5 @~R /R !~V /V !#
a g a g a g
r = radius of the diffractometer,
c = velocity of light, 3.3.2 The above ratio holds if ferrite or martensite and
l = wavelength of incident radiation,
austenite are the only two phases present in a steel and both
A = cross sectional area of the incident beam,
phases are randomly oriented. Then:
v = volume of the unit cell,
V 1 V 5 1
a g
/F/ = structure factor times its complex conjugate,
p = multiplicity factor of the (hkl) reflection, 3.3.3 The volume fraction of austenite ( V ) for the ratio of
g
u = Bragg angle, measured integrated intensities of ferrite and austenite peak to
R-value is:
V 5 I /R / I /R ! 1 I /R ! (1)
@ ~ ~ #
g g g a a g g
3.3.4 For numerous ferrite and austenite peaks each ratio of
Retained Austenite and Its Measurement by X-ray Diffraction, SAE Special
Publication 453, SAE, Warrendale, PA 15096. measured integrated intensity to R-value can be summed:
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
E 975
q P q
beam at all angles of 2u required for the X-ray diffraction
1 Igj 1 1
V 5 / Iai/Rai 1 Igj/Rgj
FS D S D S DG
g ( ( (
q Rgj P q
analysis to prevent errors in the analysis. In most cases, a 1- in.
j 5 1 i 5 1 j 5 1
(2)
square area is sufficient, but sample size depends upon the
dimensions of the incident X-ray diffraction. When using
3.3.5 If carbides are present:
molybdenum radiation, select peaks in the range from 28 to 40°
V 1 V 1 V 5 1
a g c
2u for best results.
3.3.6 Then the volume fraction of austenite ( V ) for the
a
4.2 X-Ray Equipment:
ratio of measured ferrite and austenite integrated intensity to
4.2.1 A standard X-ray diffractometer with a pulse height
R-value is:
selector circuit is preferred for the measurement, but an X-ray
V 5 @~1 2 V !~I /R !/~I /R ! 1 ~I /R !# (3)
g c g g a a g g
camera plus densitometer readings of the film may be used.
3.3.7 For numerous ferrite and austenite peaks the ratio of
X-ray film and adequate photographic development techniques
measured integrated intensity to R-values can be summed:
are required to assure a linear response of the film to the X-ray
q p
intensity.
V 5 @~1 2 V !~1/q ~Igj/Rgj!!/1/P ~I i/R i!
( (
g c a a
j 5 1 i 5 1 4.2.2 A chromium X-ray source with a vanadium metal or
q
compound filter to reduce the K radiation is recommended.
b
1 1/q ~I j/R j!# (4)
( g g
j 5 1 Chromium radiation produces a minimum of Xray fluorescence
of iron. Chromium radiation provides for the needed X-ray
3.4 The volume fraction of carbide, V , should be deter-
c
diffraction peak resolution and allows for the separation of
mined by chemical extraction or metallographic methods.
carbide peaks from austenite and ferrite peaks.
Adequate X-ray diffraction peak resolution for the identifica-
4.2.3 Other radiation such as copper, cobalt, or molybde-
tion of carbide peaks is required to avoid including carbide
num can be used, but none of these provide the resolution of
peaks in the retained austenite measurement.
chromium radiation. Copper radiation is practical only when a
4. Procedure
diffracted-beam monochromator is employed, because iron
4.1 Sample Preparation: X-ray fluorescence will obscure the diffracted peaks.
4.1.1 Samples for the X-ray diffractometer must be cut with
4.2.4 A molybdenum source with a zirconium filter is used
a minimum amount of heat effect. Since most steels containing
to produce a large number of X-ray diffraction peaks.
retained austenite are relatively hard, abrasive cutoff wheels
4.3 X-Ray Method—X-ray diffraction peaks from other
are frequently used. If adequate cooling is not used, heat effects
crystalline phases such as carbides must be separated from
from abrasive cutoff wheels can be substantial and, in some
austenite and ferrite peaks. The linearity of the chart recorder
cases, can transform retained austenite. Saw cutting rather than
or photographic film shall be verified prior to utilizing this
abrasive wheel cutting is recommended for sample removal
method.
whenever it is practical.
4.3.1 Entire diffraction peaks minus background under the
4.1.2 Rough grinding using a milling tool or high-pressure
peaks shall be recorded to obtain integrated peak intensities.
coarse grinding can deform the surface and transform some of
Peaks without carbide or second phase interference can be
the retained austenite to a depth that is greater than the surface
scanned, and the total peak plus background recorded. Back-
depth analyzed. Final milling or rough grinding cuts limited to
ground counts are obtained by counting on each side of the
a depth of 0.010-in. or less should reduce the depth of
peak for one-half of the total peak counting time. Total
deformation.
background is subtracted from peak plus background to obtain
4.1.3 Standard metallographic wet-grinding and polishing
the integrated intensity. Alternatively, software supplied with
methods shall be used to prepare samples for X-ray analysis.
the diffractometer can be used. In general, a diffractometer
Grit reductions of 80, 120, 240, 320, 400, and 600 silicon
scanning rate of 0.5°2u/min or less is recommended to define
carbide or alumina abrasives may be used but other valid grit
the peaks for austenite contents of less than 5 %.
combinations may also be used. A final surface polish of 6-μm
4.3.2 Where carbide or other phase X-ray diffraction peak
diamond or an equivalent abrasive polish is required. Sample
interference exists, planimeter measurements of area under the
etching, observation for heat effects, and repolishing is a
austenite and ferrite peaks on X-ray diffraction charts can be
recommended safeguard.
used to obtain integrated intensity. Alternatively, software
4.1.4 Since deformation caused by dull papers or over-
supplied with the diffractometer can be used. Carbide interfer-
polishing can transform some of the retained austen
...
Questions, Comments and Discussion
Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.