Can ultrasonic testing be used for austenitic stainless steel welded joints? Why?

Features such as twin boundaries present in austenitic stainless steels significantly affect the attenuation and propagation of ultrasonic waves; therefore, ultrasonic testing has not been widely applied in such stainless steels.

I. Characteristics of the material structure: During the solidification of austenitic stainless steel welds, no phase transformation occurs; at room temperature, these welds retain their cast-like columnar austenite grains. These columnar grains are large in size, the structure is uneven, and there is significant anisotropy, which poses many difficulties for ultrasonic testing. In butt welds of austenitic stainless steel, the grain orientation is generally perpendicular to that of the columnar grains in the groove. A characteristic of such grains is that they exhibit different sizes when measured from different directions. This results in varying levels of attenuation and signal-to-noise ratio when detecting these grains from different angles; when the wave beam is perpendicular to the columnar grains, the attenuation is higher and the signal-to-noise ratio is lower. In austenitic stainless steel welds formed by multiple manual weld passes, differences in welding processes and specifications result in varying microstructures in different parts of the weld. This leads to changes in sound speed and acoustic impedance, causing the direction of sound wave propagation to deviate and thereby making it difficult to locate defects. II. Selection of inspection conditions 1. Waveform: In ultrasonic flaw detection, the signal-to-noise ratio and attenuation are related to the wavelength; when the grain size of the material is large and the wavelength is short, the signal-to-noise ratio is low and the attenuation is high. Therefore, in austenitic stainless steel welds, longitudinal wave testing is generally used, as shear waves do not propagate in austenitic welds. 2. Probe angle (K value): The direction of hazardous defects in austenitic welds is usually at an angle to the inspection surface. To effectively detect such hazardous defects in welds, oblique P-wave probes are generally used. Since the welds of austenitic stainless steel consist of columnar grains, the signal-to-noise ratio and attenuation vary depending on the detection direction; therefore, the refraction angle of the longitudinal wave probe must be selected appropriately. Practice has shown that for butt welds, using a longitudinal wave refraction angle of bL=45° results in a high signal-to-noise ratio and low attenuation when detected by a K1 longitudinal wave oblique probe. When the weld is thin, a probe with bL=60° can also be used for inspection, but the sensitivity decreases significantly. 3. Frequency ； When inspecting austenitic stainless steel welds, frequency has a significant impact on attenuation; the higher the frequency, the greater the attenuation and the lower the penetration capability. Since the grains in austenitic stainless steel welds are coarse, it is advisable to use a lower inspection frequency, typically ranging from 0.5 to 2.5 MHz. Practice has shown that 2 MHz is an appropriate choice. 4. Selection of calibration blocks and comparison blocks: As can be seen from the structural characteristics of material type “1”, austenitic stainless steel, as well as its welds, differ significantly from ordinary steel. Many standards require the use of CSK-IA blocks for distance calibration and CSK-IIA blocks for the distance-amplitude curve. Tests show that significant errors occur; due to the large variations in the longitudinal wave speed of different types of austenitic materials, it is best to use the same type of material to create the blocks shown in Figure 2 for the type of material being tested, and to form the welds using the same welding method.

Characteristics of coarse grains in austenitic stainless steel welds and three challenges in ultrasonic testing: ① Columnar grains cause the sound beam to bend, leading to inaccurate positioning； ②Coarse grains cause severe ultrasonic attenuation, preventing the ultrasonic beam from penetrating the weld ； ③The coarse-grained forest-like echoes significantly reduce the signal-to-noise ratio of defects.

These problems were resolved long ago; as long as it’s not too thick (under 50mm), it’s completely fine

Appendix N of JB/T4730.3 states that ultrasonic testing can be used for butt welded joints of austenitic stainless steel with a thickness of 10–50 mm