Multiple types of analysis and examinations are applied to analyze the causes of rupture for a sucker rod made of 20CrMo alloy. Based on the results from macro- and micro-fracture analysis, chemical composition analysis, mechanics test and metallographic examination, the generation of cracks can be attributed to the irregularity and impurity in the coating of sucker rod; the stress fatigue from pulling and pressing and the stress corrosion. There is severe banded segregation, non-metal impurity and un-dissolved ferrite in the interior of the rod, meanwhile the poor heat treatment causes the property differences between the surface and core structures of the rod, thus reduces the overall properties of fatigue- and corrosion-resistance. Stress corrosion makes pits from cracks; the expansion of such corrosion pits reduces the size of effective loading area; and the failure to endure service load due to the reduction of loading area finally causes sudden rupture of the rod.
