Study On Ultrasonic Flaw Detection Of 304L Stainless Steel Welding Defects

Austenitic stainless steel is the most widely used stainless steel,especially the high Cr-Ni stainless steel.It is widely used in petroleum and petrochemical equipment,paper and bleach equipment,power plant flue gas desulfurization equipment,marine systems or seawater treatment,desalination industry and heat exchangers.The austenitic stainless steel welding structure are most widely used in the petroleum,petrochemical equipment,offshore systems or seawater treatment and other industrial applications,in which the argon arc welding is the main method of welding.However,due to the improper welding parameters,materials or other factors,the lack of fusion,crack,porosity and other defects often occurs,resulting in the failure of welding structure,and restrict the application of the welding of stainless steel structure.In order to avoid the structural failure and reduce the accident rate,the detection of welding defects has become an important part of the application of these welding structures.In this study,ultrasonic testing method,which is a typical testing techniques in welding structures was studied.The welding process,defects and microstructure of 304 L stainless steel T-welded joints were mainly studied.The ultrasonic testing equipment selection,parameter setting,testing and positioning for the ultrasonic testing were investigated,and the reliability of detection methods were verified.The main conclusions are as follows:1.The T-joint 304 L stainless steel is welded by argon arc welding machine with the design of the groove shape.The surface of the welded joint is relatively smooth,and there is no external defect of lack of fusion.2.The metallurgical quality and microstructure of TIG welded 304 L stainless steel welded joint were observed and characterized by optical microscope.The results show that the heat affected zone of welded joints is relatively small,and the weld defects between the welded joints and the base zone are the main defects of the welded joints.There is no porosity and crack defects.3.The microstructure of the weld zone in 304 L stainless steel TIG welded joint is epitaxial growth of dendrites,which grows perpendicular to the weld line.The microhardness of the weld fluctuates is in the range of 172 HV to 187 HV,which indicates that the microhardness is basically uniform in the weld joints.4.A 2.5P9 × 9K2 type CTS-8008 ultrasonic flaw detector with an oblique probe was used to detect and characterized the weld defects.The test results show that the size of the defect in the welded joint is about 2.3mm,which is in agreement with the results of metallographic characterization,indicating that non-destructive ultrasonic flaw detection is an effective means of detection on the mm-level welding defects.