The analysis on causes of rupture for a sucker rod made of 20CrMo alloy

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. This analysis is on the causes of rupture for a sucker rod made of 20CrMo alloy, and the analysis is trusted by the Quality Supervision Test center of an oilfield in China. A sucker rod is used in the oil industry to join together the surface and down-hole components of a reciprocating piston pump installed in an oil well; when counting top-down, it is the very first among a series of rods. The sucker rod connects the nodding donkey with the down-hole components to transfer the reciprocating drive from surface, and during its repeating movement, the rod is subject to the pulling and pressing stress and the corrosion of crude oil. Due to the sinking of pumping unit basis, improper installation or other reasons, the rod may work in a non-concentric mode; suffer biased wearing in the well head; and develop cracks. In normal cases sucker rods made of AOC alloy have a lifespan of over 200 days, however in this case the rod broke after 30 days in use. The production of sucker rod follows the standard of certain enterprise, and the materials should follow YT 5029-2006 -sucker rod. Table 1 summarized the requirement on material and mechanics properties HL for steel sucker rods, the specification is AOC/GGD-29-H-11m (nominal diameter of 29.6mm,length of 10973mm).