Grain size-acoustic emission relationships in hydrogen induced delayed cracking
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I.
INTRODUCTION
STRESS corrosion cracking is particularly pronounced in ultrahigh strength low alloy steels, which become extremely susceptible to hydrogen attack even at a few parts per million concentration. In many applications, exposure to aggressive environment is unavoidable. Acoustic emission analysis may detect stress corrosion cracking during service and assist in understanding the behavior of the material in such environments. ~-5 Acoustic emissions are generated during transient changes in the local stress and strain fields within a material. When a stress corrosion crack propagates, that part of the total energy which is released as an elastic stress wave, if large enough, can be detected by a transducer coupled to the specimen surface. In this study, the characteristics of the acoustic emissions generated during stress corrosion cracking of 300M steel were investigated.
II.
EXPERIMENTAL PROCEDURE
Commercially available 300M steel was used. Its chemical composition and heat treatment schematics are given in Table I and Figure 1, respectively, in the earlier paper. 6 Bolt loaded double cantilever beam (DCB) specimens were used for evaluating threshold stress intensity (K]~cc) and compact tension (CT) specimens conforming to ASTM E399-78a specifications, for crack growth studies. The environment was a 3.5 pct NaC1 solution in water. A crack opening displacement (COD) gauge, with a sensitivity of 0.25 mm/V, was used to measure continuously the CT specimen displacements as a function of time. Data were stored on a magnetic tape for subsequent computer analysis. Using a previously generated compliance curve (Figure 2), 6 the crack length and the corresponding applied stress intensities were computed for specific time intervals. Specimens were etched in a 5 pct picral and nital solution containing zephryon trichloride and examined in an optical microscope for grain size measurement. The failure mode under stress corrosion cracking was studied using a scanning electron microscope under a secondary electron accelerating voltage of 25 KV. R. PADMANABHAN, Research Scientist and Instructor, N. SURIYAYOTHIN, Graduate Student Research Associate, and W.E. WOOD, Department Chairman and Professor, are all with Oregon Graduate Center, 19600 N.W. Walker Road, Beaverton, OR 97006. Manuscript submitted August 2, 1982. METALLURGICALTRANSACTIONS A
A block diagram of the acoustic emission (A.E.) monitoring system is given in Figure 1. An AET Model AC175L resonant frequency piezoelectric transducer was attached, using a viscous couplant, to the CT specimen undergoing the stress corrosion test. The transducer location was kept identical for all samples. The transducer output was passed through an AET Model 160 filter with a band-pass range of 125 to 250 KHz to eliminate extraneous noise below 125 KHz, and the signal was further amplified with a 60 dB preamplifier. This signal was then processed using an AET model 201 signal analyzer (which can provide an additional gain of up to 40 dB) to record the cumulative threshold c
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