DIFFRACTION NOTES 5521 Fair Lane, Cincinnati, OH 45227 (513)561-0883 No.

33 Fall 2006

Determination of Volume Percent Retained

INTRODUCTION ANNOUNCEMENTS
Hardening of steels requires heating to an
austenitic phase and quenching to room
temperature to produce a hard martensitic phase. Fractographic and Failure Analyses
Austenite is an FCC phase that is stable above a Capabilities
temperature of 735 C. Due to incomplete
transformation some austenite is retained at room Optical (both micro and macro) and SEM
temperature. Retained austenite can dramatically fractography services are available at Lambda, to
decrease the mechanical properties of the steel. support the analysis of fatigue and SCC failures
Properties such as fatigue strength, toughness, and the development of surface enhancement
hardness, yield strength and machinability can be solutions. Lambda’s facilities enable quantitative
influenced by retained austenite. failure analysis including the measurement of
fatigue striation spacing, critical crack size,
Austenite can transform in service as a result of
cleavage fracture, ductile fracture, intergranular
thermal cycles, plastic deformation, or shock.
Shot peening, for example, will transform the fracture, etc. Lambda’s combined capabilities of
austenite on the surface of gear teeth. Exposure fractography and fracture mechanics
to extreme cold renders the austenite increasingly computations provide powerful tools for
unstable as the temperature diminishes. The characterizing failure mechanisms, and the
transformation of austenite to ferrite involves a associated chronology of events that include
nominal 4% volume increase. A linear understanding of the stress events leading to
dimensional increase on the order of the cube failure.
root of that would lead to seizure and excessive
interference in precision gearing and bearings. Upcoming Events
Accurate measurement of the retained austenite
levels is important in the development and control Stop by our booth at the Logistics Officer
of a heat treatment process.
Association (LOA) Exhibit. The Conference will be
held at the Henry B. Gonzales Convention Center
X-ray diffraction is considered to be the most
in San Antonio, TX, October 9th through the 12th.
accurate method of determining the amount of
retained austenite in steels. Lambda Research is This year’s topic is “Sustaining Today's Weapon
the only accredited independent laboratory that Systems with Tomorrow's Airmen.”
uses Bragg-Brentano diffractometers for
measurement of retained austenite, as required
(1) (2)
by ASTM and SAE . Most other labs now use For more information visit either of our web sites at
www.lambda-research.com, www.lambdatechs.com.

For more information about our testing capabilities, accreditations, or other publications,
visit our website at www.lambdatechs.com
 Lambda Technologies Diffraction Notes No. 33, Fall 2006 - Page 2

position sensitive detector (PSD) based The austenite fraction is determined from the
systems for speed and portability that cannot ratio of the austenite and ferrite diffraction
be calibrated by direct calculation of the peak intensities and the values of R for each
relative phase intensities and must use phase. This method is more commonly
reference samples that usually are not of the known as the “Direct Comparison” method.
correct structure. As a result, the retained
austenite measurements can have a Lambda uses the method of Averbach and
(3) (1)
proportional error as much as 20% of the Cohen in accordance with ASTM . The
value reported. It was for this reason that the integrated intensities of the austenite (200)
NIST austenite standards were withdrawn, and (220), and the ferrite (200) and (211)
based upon work performed at Lambda diffraction peaks are measured on the
Research, which showed that the various automated diffractometers, providing four
austenite standards were not self consistent, austenite/ferrite peak intensity ratios. The use
leading to inaccuracies in quality assurance of multiple diffraction peaks minimizes the
testing for retained austenite. effects of preferred orientation and allows
interference from carbides to be detected.
RETAINED AUSTENITE MEASUREMENT
TECHNIQUE Carbides can produce diffraction peaks that
interfere with the austenite and/or martensite
For a randomly oriented sample, the peaks potentially causing significant errors in
integrated intensity from any diffraction peak measurement. Lambda identifies any carbide
is proportional to the volume fraction of that phases that may be present by obtaining a
phase. Intensities of the austenite (γ) and diffraction pattern of the specimen to be
ferrite (α) diffraction peaks in a steel are: tested. The carbides are identified and the
austenite and martensite diffraction peaks are
KRγ cγ chosen to eliminate interference from the
Iγ = carbide phases.
2µ
A device known as a “Miller fixture” is used at
KRα cα
Iα = Lambda to rotate the specimen around the
2µ surface normal and oscillate it through an
angular range of + 45 deg. The fixture is
designed to minimize the effects of preferred
Where I is the integrated intensity, c is the
orientation during collection of the diffraction
volume fraction of each respective phase,
peak integrated intensities.
and µ is the linear absorption coefficient for
the steel. K is a constant that is dependent
upon the selection of instrumentation EXAMPLES
geometry and radiation but independent of
the sample. R depends upon the interplanar Figures 1 and 2 show examples of retained
spacing, the Bragg angle, crystal structure austenite distributions as functions of depth. A
and composition of specimen. R is a term that depth resolution of less than 0.0005 in. is
can be calculated from first principals. achievable by electropolishing to remove
Lambda calculates R for each steel alloy layers without inducing plastic deformation
(2)
tested per SAE . and transformation of unstable austenite.
Austenite measurements should be made on
Dividing the above equations, and given the electropolished surfaces to remove interfering
sum of the volumes of the austenite and oxides and eliminate any plastically deformed
martensite phases equals one, yields the surface layer. The austenite distributions
following equation. shown indicate low austenite content in the
decarburized near-surface layer. Beneath the
−1 decarburized layer the austenite content
 Rγ I α  diminishes through the hardened case.
cγ =  + 1
 Rα I γ 

Lambda Research is an accredited independent institute providing unique x-ray diffraction and
materials research services to industrial, government and academic clients since 1977.
 Lambda Technologies Diffraction Notes No. 33, Fall 2006 - Page 3

50 REFERENCES
VOLUME % RETAINED AUSTENITE
45
1. ASTM, “Standard Practice for X-Ray
40 Near Linear Decrease
Determination of Retained Austenite in Steel with
35 Near Random Crystallographic Orientation,”
30 Standard E975-03, American Society for Testing
25 and Materials, Nov. 1, 2003.
20 2. “Retained Austenite and Its Measurement by X-
15 Ray Diffraction,” SAE Special Publication 453,
Decarburized Surface SAE, Warrendale, PA 15096.
10
(Low Austenite) 3. B.L. Averbach and M. Cohen, Trans. AIME, Vol.
5 176, 1948, p. 401.
0
0 5 10 15 20 25 30 35
-3
DEPTH (x 10 in.)

Figure 1 – Percent retained austenite in 8620 steel

20
VOLUME % RETAINED AUSTENITE

15

10

5
Decarburized Surface
(Low Austenite)
0
0 20 40 60 80 100
-3
DEPTH (x10 in.)

Figure 2 – Percent retained austenite in 8720 steel

For more information about our testing capabilities, accreditations, or other publications,
visit our website at www.lambdatechs.com