Research on magnetic memory inspection signal characteristics of multi-parameter coupling pipeline welds

Magnetic memory inspection technology is an effective method for detecting early damage in ferromagnetic materials. However, the commonly used force-magnetic coupling models are insufficient for analyzing the distribution characteristics of magnetic signals in ferromagnetic material welds. This study first analyzes the influence of metal phase transformation and residual stress on the magnetic hysteresis characteristics of materials. Based on the principle of magnetic hysteresis characteristics, the magnetic charge density under the combined action of stress and phase transformation is determined, and a magnetic memory analysis model is established for multi-parameter coupling. The study quantitatively calculates the effects of phase transformation volume fraction, residual stress, the lift-off effect, and cracks on the magnetic signal features at welds. The research results show that the magnetic memory signals at welds exhibit extreme values in the tangential component and sinusoidal fluctuations with zero-crossings in the normal component. The peak-to-peak values of both the normal and tangential components of the magnetic signal increase with an increase in metal phase transformation and residual stress, while they decrease with the increase of the lift-off effect. When cracks appear in the weld, the normal component exhibits a double-peak and valley structure, with the inner side representing crack features and the outer side representing weld features. The tangential component shows a maximum value at the center of the crack. Based on the peak and valley values of the magnetic signal, a damage grade index F is determined. The damage index F can characterize the level of weld damage, decreasing approximately linearly with increasing stress.