| Austenitic stainless steel S30408 has been widely used in nuclear power industry,petrochemical, medical equipment manufacturing and other fields, due to the excellent corrosion resistance and mechanical properties of materials. Thermal cracks are easy to produce during welding, it is necessary to develop nondestructive testing to prevent potential dangers and improve its safety. Ultrasonic testing is used widely because of its reliable and easy to operate. Phase transformation does not occur in welding process of austenitic stainless steel, thus coarse column-like grains will develop in weld due to thermal cycling, which result in heterogeneous and anisotropic of weld. The sound beam will have a serious scattering phenomenon when implement ultrasonic testing, which impact the detection sensitivity and accuracy greatly. The phased array ultrasonic testing technology can flexibly change the beam direction, focus size and position by exciting each element of an ultrasonic array transducer with independent phase delay, which has improved the resolution, sensitivity and signal to noise ratio.S30408 austenitic stainless steel test blocks of 10 mm, 20 mm, 30 mm, 40 mm in thickness were fabricated in this paper. The welding method is manual welding, and automatic submerged arc welding backing cover. U- and V- type slot of 5.0mm, 10 mm and 15 mm in length, 1.5mm in height at the bottom of the heat affected zone of the test blocks were processes. This research were carried out on the blocks combining theoretical and experimental investigation as follows:(1) The microstructure of the matrix, weld and heat affected zone of the test blocks were analyzed. The grain size was calculated by area method, and the length and width of column-like grains were studied. The acoustic characteristic of blocks were measured to analyze the time domain waveform, frequency spectrum frequency shift, sound velocity and attenuation coefficient variation law. It is found, that the damping factor of 4 blocks is less than welding area. The sound velocity is uniform and fluctuation is less than 1.13%. CIVA simulation software was used to determine the depth of focus by the ultrasonic field simulation of phased array ultrasound, and the phased array usltrasonic probe aperture was optimized through the experiment.(2) The frequency of phased array ultrasonic testing is 5MHz, and 32-element phased array probe was chosen. Scanning were developed along the weld cooperated with encoder,and with the scanning mode of A-B-S and grid scanning. The notch height of the test blocks were determined by the position of the upper and lower tip of the notch in the sector scan, and the length of the notches were determined by 6d B method. L-wave detection results show that the difference of the height quantitative is between 0~0.3mm. And difference of the length of5 mm, 10 mm and 15 mm slot are 0.3~0.7mm, 0.2~1.1mm and 0.5~2.0mm respectively. The S-wave was experimented on a 10 mm and 20 mm thick block, the difference of the height quantitative is between 0.2~0.4mm. And difference of the length of 5mm, 10 mm and 15 mm slot are 0.4~1.1mm, 0.3~1.0mm and 0.7~1.2mm respectively. |
