Significance and Use
2.1 Significance—Retained austenite with a near random crystallographic orientation is found in the microstructure of heat-treated low-alloy, high-strength steels that have medium (0.40 weight %) or higher carbon contents. Although the presence of retained austenite may not be evident in the microstructure, and may not affect the bulk mechanical properties such as hardness of the steel, the transformation of retained austenite to martensite during service can affect the performance of the steel.
2.2 Use—The measurement of retained austenite can be 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 included in a heat-treated lot of steel as part of a quality control practice. The measurement of retained austenite in steels from service can be included in studies of material performance.
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 Kα or molybdenum Kα 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 Units—The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical conversions to SI units that are provided for information only and are not considered standard.
1.7 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.
TABLE 1 Calculated Theoretical Intensities Using Chromium Kα RadiationA
hkl
Sinθ/λ
θ
f
Δf′
Δf”
/F/2
LP
P
TB
N2
R
(α iron, body-centered cubic, unit-cell dimension ao = 2.8664Å):
110
0.24669
34.41
18.474
−1.6
0.9
1142.2
4.290
12
0.9577
0.001803B
101.5C
200
0.34887
53.06
15.218
−1.6
0.9
745.0
2.805
6
0.9172
0.001803B
20.73C
211
0.42728
78.20
13.133
−1.6
0.8
534.6
9.388
24
0.8784
0.001803B
190.8C
(γ iron, face-centered cubic, unit-cell dimension a o = 3.60Å):
111
0.24056
33.44
18.687
−1.6
0.9
4684.4
4.554
8
0.9597
0.0004594B
75.24C
200
0.27778
39.52
17.422
−1.6
0.9
4018.3
3.317
6
0.9467
0.0004594B
34.78C
220
0.39284
64.15
14.004
−1.6
0.8
2472.0
3.920
12
0.8962
0.0004594B
47.88C
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 Kα1 and Kα2 value used (λ = 2.29092Å).
B Temperature factor (T = e−2M) where M = B(sin 2 θ)/λ2 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.