Material fold under head of cold headed hex bolt caused by severe cold heading process
Magnetic particle inspection (MPI) 
MPI process is a non destructive method for the detection of surface and near surface defects in ferrous materials. It makes use of an externally applied magnetic field or half-wave DC (rectified AC) current through the material, and the principle that the magnetic susceptibility of a defect is markedly poorer than that of the surrounding material. In a dark room using black light the defect will appear much brighter than the surrounding material and is detected.
MPI is utilized after heat treating, welding, or at any time during or after the manufacturing process. Besides having qualified operators and being accredited to perform MPI, our metallurgical staff, if needed, can analyze the defect and categorize it further and determine its root cause.
