Identification of magnetic anomaly characteristics in stress concentration and macroscopic defects in martensitic stainless steel based on lift off effect

Martensitic stainless steel plays a role in modern industry, but damage to martensitic components poses a safety hazard. This article adopts magnetic testing under natural magnetisation, using 07Cr15Ni5MoWVNb martensitic precipitation hardening stainless steel as an example. By utilising magnetic anomalies at different lift‐off heights, the goal is to qualitatively identify stress concentration and macroscopic defects in martensitic stainless steel. Based on magnetic coupling model under natural magnetization, a finite element model of martensitic stainless steel with stress concentration and macroscopic defects was established using simulation software. The simulation results showed that the characteristic values of the magnetic induction intensity curve at the macroscopic defect approached zero when the lift‐off height exceeded 6 mm, the characteristic values of the magnetic induction curve at the stress concentration remained significant, showing fluctuations in the curve. The gradient magnetic sensing device was utilized to acquire the magnetic induction intensity curves of stainless steel specimens near the surface and at a lift‐off height of 7 mm. The experimental results indicated when both the upper and lower sensors exhibited magnetic anomalies, it created a stress concentration area. When the lower sensor displayed anomalies, but the upper sensor did not show any magnetic anomalies, these were considered macroscopic defects.