Use of Electron Fractography to Diagnose Cracking in a Chromium-Plated AISI 4340 Pivot Bolt

Overhaul mechanics discovered a crack in an AISI 4340 Cr-Mo-Ni alloy steel pivot bolt when grinding off the chromium plating. The bolt had served for an estimated 10,000 h and was replated when last overhauled. On checking the bolt, several fine cracks were found on the surface. A 6500x micrograph revealed the intergranular nature of a crack. By trying different grinding procedures, investigators were able to reproduce this type of failure in the laboratory. It was concluded that grinding cracks initiated the failure. It should be noted that governing specifications prohibit grinding on high-strength steel; chromium should be stripped by electrochemical methods.