FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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Following a brief description of the sample, its mechanical properties, and
the experimental techniques, attenuation measurements are presented as a
function of temperature and frequency. They are interpreted in terms of
scattering mechanisms and discussed in terms of possible correlations with
mechanical properties.
EXPERIMENTAL
Sample
The material chosen is an AISI type 403 stainless steel which has well
characterized properties because of its use as end fittings to hold zirconium
pressure tubes in Canada-Deuterium Uranium (CANDU) nuclear reactors. The
sample was part of a compact tension (CT) specimen used in a previous study on
the effects of impurity concentration and temperature on the fracture
toughness of type 403 stainless steel (14). The particular sample used
here, identified as LR in reference 14, has chromium as main alloying element
(11.8 wt %) and a total impurity concentration of 1620 ppm. (See reference 14
for exact composition and heat treatment). The microstructure consists of
tempered martenslte. Carbides outline the original austenite grains (6= 25um)
and also the martensite lath boundaries. Fig. 1 is a scanning electron
micrograph of the sample.
The Kic values were measured using compact tension (CT) specimens according
to ASTM Test for Plane-Strain Fracture Toughness of Metallic Materials
E 399-74. The yield stress, ov, and the plane strain fracture toughness,
Kic, taken from reference 14 are given in Fig. 2 for the temperature range
-60°C to +40°C. Note that the yield stress decreases by = 7% and Klc
increases by approximately a factor three over this temperature range.
Ultrasonic attenuation
Ultrasonic attenuation was measured using the pulse-echo technique with two
undamped 12 mm diameter X cut quartz piezoelectric transducers having center
frequencies f0 of 5 to 10 MHz used successively. Using overtones,
measurements were made at frequencies of 10 to 50 MHz in 5 MHz intervals. The
ultrasonic pulses were generated and detected with a Matec heterodyne
pulser-receiver and the attenuation was measured by electronically fitting an
exponential curve over the decaying echos. The velocity of longitudinal waves
was also measured (6075 m/s at 20°C varying by less than 1% in the temperature
range studied) and used to convert the decay time into neper/m attenuation
units. Temperature was varied by mounting the 5 mm thick sample on a Peltier
element which allowed both cooling and heating of the sample depending on the
polarity of the current supplied to the element. The temperature was measured
with a thermocouple mounted directly on the sample. The experimental
apparatus, along with a typical oscillogram showing a series of echos fitted
with an exponential decay curve, is illustrated in Fig. 3.