Measurement of Variation in Residual Stress using Ultrasonic Technique

It is known that residual stresses may adversely affect the engineering properties of materials by affecting their fatigue life, distortion, dimensional stability, corrosion resistance, etc. Therefore, residual stress is an important parameter for steel components in respect of their design, construction andservice life. Since it is quite difficult to measure residual stress pretty accurately and moreover the measurement is destructive in nature conventionally, the recent developments and research have put non-destructive methods in limelight. Among the existing NDT methods ultrasonic technique (UT) has shown great potential as an effective stress measurement approach. Using this technique one can measure residual stress in the same point many times, study the variations under service loading conditions or heat treatment or machining operations. An attempt has been made in this work to arrive at residual stress based on the ultrasonic thickness measurements on the weld, HAZ, away from the weld seam of a SA210C boiler tube made of steel on either side followedby the analysis of the data.The UT is based on the acousto-elastic law, which relates stress and acoustic wave velocity in materials. In this study an approach involving variations in the measurements of welding residual stress at different points at different distances from the weld in a 12mm thick SA210C boiler tube using thickness variation caused by thermally induced residual stress and measuring it using ultrasonic thickness gauge. The Time of Flight (T.O.F) can be calculated for varying thickness inthe parent metal for the welded tube and can be related to stress for evaluating residual stress pattern.