基于试验统计经不同温度(800~1 350℃)固溶处理后薄壁304L奥氏体不锈钢焊管母材与焊缝的晶粒尺寸,利用基于连续小波变换的超声衰减法对母材和焊缝超声信号进行三层小波分解后提取特征能量,研究超声衰减法评估焊管晶粒尺寸的可行性。结果表明:随着固溶温度的升高,母材晶粒尺寸与对应的特征能量均呈减小趋势,实际检测中可设置超声信号的相对特征能量阈值在600~1 000之间,此时晶粒度在7~8.5之间,晶粒尺寸合格;随着固溶温度的升高,焊缝的晶粒尺寸增大,特征能量先升高后降低再升高,并在1 050℃附近达到最大值,实际检测中可设置超声信号的相对特征能量阈值在140~180之间,此时晶粒度为8.5~9,晶粒尺寸合格。超声衰减法可实现薄壁304L奥氏体不锈钢焊管晶粒尺寸的快速无损评估。

Grain size of base metal and weld of thin-walled 304L austenitic stainless steel welded tube after solid solution treatment at different temperatures (800-1 350℃) were counted by tests. The ultrasonic signals of base metal and weld were triple wavelet decomposed by ultrasonic attenuation method based on continuous wavelet transform to extract the characteristic energy. The feasibility of ultrasonic attenuation method to evaluate grain size of welded tube was studied. The results show that with increasing solid solution temperature, the base metal grain size and the corresponding ultrasonic characteristic energy decreased; in the actual detection, the relative characteristic energy threshold of ultrasonic signals could be set between 600 and 1 000; in this case, the grain size was between 7 and 8.5, and the grain size was qualified. With increasing solid solution temperature, the grain size of weld increased, and characteristic energy first increased and then decreased and then increased, and reached the maximum at about 1 050℃; in the actual detection, the relative characteristic energy threshold of ultrasonic signals could be set between 140 and 180; in this case, the grain size was 8.5-9, and the grain size was qualified. Ultrasonic attenuation method could be used to quickly and nondestructively evaluate the grain size of thin-walled 304L austenitic stainless steel welded tube.