Finite element modeling of creep damage effects on a magnetic detector signal for a seam weld/HAZ-region in a steel pipe

Creep damage in steel causes a reduction of magnetic properties. A mathematical model, previously formulated, accounts for this. Recently, this model was used in finite element modeling (FEM) of a magnetic C-core signal due to creep damage at a seam weld in Cr-Mo steam pipe. The FEM assumed unrealistically that in the absence of creep damage, the weld material and heat-affected zone (HAZ) and base metal all had the same magnetic properties, in this paper, new finite element simulations are presented for worst case relative permeabilities of 1271, 784 and 571 for base metal, HAZ, and weld material. Reduced permeability at the weld results in a considerably reduced emf at low probe magnetic fields. However, creep damage does produce an additional emf reduction that is large enough to be detected, even when the creep damage does not extend to the pipe wall surface. A method is suggested for calibrating the magnetic signal for weld, HAZ, and base metal effects