Characterization of the Microstructure and Residual Stress in Quenched and Tempered AISI 4135 Steel Using Magnetic Nondestructive Testing Techniques

Thin-walled cylinders are commonly used in the oil and gas pipeline industry. To ensure processing quality, the microstructure and residual stress of AISI 4135 steel samples before and after heat treatment were investigated nondestructively by magnetic hysteresis loop and Barkhausen noise techniques. Surface hardness measurements and microscopic investigations using optical, atomic force and scanning electron microscopies were also conducted to analyze microstructural behavior. The evolution process that occurs in the microstructure and magnetic domains during heat treatment has been characterized in terms of Barkhausen noise activity and coercivity mapping. With an appropriate precalibration procedure, the residual stress variations in the samples under the as-received, quenched and tempered states were monitored quantitatively. The results show that there is a linear correlation between the coercivity and hardness values, and the internal dependence was also validated by the observed microstructural and morphological features of the samples. Finally, the hysteresis behavior dependence of the steel samples on the quenching and tempering process was interpreted from the micromagnetic domain structure. It is shown that the magnetic hysteresis loop and Barkhausen noise techniques could be reliable tools for the quality control of steel components in line production.