Evaluation of Heat Treatment Effect on the Tensile Strength of Mild Steel Welded Joints Using Ultrasonic Testing

The tensile strength of welded joints subjected to thermal stresses degrades with age. Tensile strength is generally assessed using tensile test, which is a destructive technique. This degradation measurement using the nondestructive testing (NDT) method has merit as specimens are intact even after the test. In situ NDT offers another significant advantage that testing of material can be performed without removing the sample from its service. Ultrasonic testing (UT) is a widely used NDT technique for material characterization and flaws detection. Motivated by this, we aim to investigate the tensile strength of mild steel post-weld heat-treated samples through advanced signal processing of the acquired ultrasonic signals through UT of mild steel heat-treated welded specimens. To this end, mild steel welded specimens were prepared for this study using electric arc welding with E6013 electrodes. After welding, specimens were heat-treated at different temperatures and then normalized. Ultrasonic signals were acquired using the pulse-echo technique on different samples, heat-treated at different temperatures, and later ultrasonic signal’s attenuation was measured. The acquired UT signals were then processed using advance signal processing techniques i.e., fast Fourier transform (FFT) and power spectral density (PSD). Measured attenuation of UT signals and discriminatory features computed through the application of signal processing techniques on acquired UT signals then correlated with the specimens’ tensile strength. Analysis of results showed that the tensile strength, FFT power of UT signals, and UT signals’ PSD energy decreases as UT signal attenuation, increases with an increase in heat treatment temperature. Results also revealed that there exists a relationship between FFT signals strength, PSD energy, and UT signals attenuation with respect to heat treatment temperatures. This study will help weld quality inspectors to use in situ UT for predicting the tensile strength of mild steel welded specimens using the relationship established through the proposed scheme.