CHANGE OF ULTRASONIC ATTENUATION AND MICROSTRUCTURE EVOLUTION DURING CREEP OF A STAINLESS STEEL

We studied ultrasonic attenuation change during creep tests on austenitic stainless steel (JIS-SUS 316L). The material was exposed to the temperature of 973K at various stresses. We measured ultrasonic attenuation using electromagnetic acoustic resonance (EMAR), which is a contactless resonant method with the use of an EMAT and is free from extra energy losses, resulting in the measurement of intrinsic attenuation in solids. We launched polarized shear wave in the thickness direction of the plate sample to obtain the ultrasonic velocity from the resonant frequency and the attenuation coefficient from the ringdown curve at resonance. The attenuation exhibits much larger sensitivity to the damage accumulation than the velocity. Approaching the rapture, it becomes ten times as large as the initial value, which is interpreted as resulting from microstructural changes, especially, dislocation mobility. This is supported by TEM observations for dislocation structure. This technique has a potential to assess the damage advance and to predict the creep life of metals. Introduction: Many of fossil power plants, which were constructed during 1960's and 70's and exceed more than 100,000 working hours, have presently been operating while they have underwent progressive damage, mainly from creep, as the time proceeds[1, 2]. Furthermore, by shifting the base load of power from fossil power plants to nuclear power plants, they are faced to even more severe operating condition such as daily or weekly startup and shutdown in order to correspond to rapid change of the demand. As the consequence of this trend, the material's degradation is being accelerated. Therefore, a nondestructive technique is now highly required for safely operating plants and predicting the remaining life. It is also important for the technique to be simple and quick operating to cope with the large number of measuring points. We studied the microstructure evolution of on austenitic stainless steel (JIS-SUS 316L) during creep by monitoring ultrasonic attenuation. We applied tensile stress, ranging between 100 and 120MPa, to the material at 973K and measured ultrasonic attenuation using the electromagnetic acoustic resonance (EMAR) technique [3, 4]. EMAR is a combined technique of the resonant acoustic technique and a non-contacting electromagnetic acoustic transducer (EMAT) [4]. Incorporation of EMAT in a resonant measurement contributes to improve the weak coupling efficiency to a large extent. The attenuation measurement is inherently free from any energy loss, resulting in the pure attenuation in a metal sample. Conventional contact transducers usually cause large loss, which hinders measuring attenuation in metals. We used a bulk-wave EMAT for transmitting and receiving the polarized shear wave travelling in the thickness direction of the sample to obtain the ultrasonic velocity from the resonant frequency and then the attenuation coefficient from the ringdown curve at resonance. Experiments: We obtained the creep specimens from hot-rolled commercial plates of on austenitic stainless steel (JIS-SUS 316L). The gauge section is 5 mm thick, 18 mm wide and 35 mm long (Fig.1). The rolling direction was parallel to the longitudinal direction of the specimen [4]. The mechanical properties at room temperature are as follows 0.2% proof stress is 261MPa, ultimate tensile strength 580MPa and elongation 59%. We used a shear-wave EMAT of 10 x 10 mm active area, which consists of an elongated-spiral coil and a pair of permanent magnets in opposite directions being normal to the specimen surface (Fig.2). The measurement principle of EMAR appears in [3].