E.W. Metters, Oilwell Division of United States Steel Corporation
During the early 1960's much work was done with compressive pre-stressing of material used in oilfield equipment to overcome fatigue problems which had plagued the industry for years. The API adopted the undercut-thread sucker rod pin design to overcome pin failures which had become a serious problem as sucker rod loads increased. The undercut pin design made it possible to produce the threads by rolling, with further increases in pin fatigue strength. Micro-hardness profiles were run on these new pins, and it was found that on a typical API Class C normalized sucker rod, the hardness at the thread root had increased significantly. At a depth of about 0.001 of an inch from the surface of the thread root, the hardness value was approximately 34 on the Rockwell C scale; at 0.002 it was about 30.3 Rc: at 0.004 it was about 25.5 Rc and so on to a depth of about 0.012 of an inch where it leveled out at a Brine11 hardness of about 185. Photomicrographs proved some of the success of the new API pin could be attributed to the compressive prestress of the thread root.