We were asked to perform material characterizations of AISI-SAE 4140 Miner Torque Shafts.
Chemical, microstructural, and mechanical characterizations were performed on three continuous miner drive torque shafts for comparison purposes. Two of the shafts were fabricated from AISI-SAE 4140 steel (Shafts S & J). Shaft X was fabricated from a non-standard medium carbon resulfurized free machining grade steel. All shafts were hardened and tempered. Variations were noted between the material chemistries, melt practice and heat treatments. Mechanical properties were used to calculate the torque values required for shear overload failure of each shaft.
The mechanical properties were determined using tensile testing of standard 0.505 inch diameter specimens per ASTM E8 along with Rockwell C hardness. Shaft S exhibited the lowest ultimate and yield strengths and hardness compared to Shafts J and X, with Shaft X exhibiting lower ductility as determined by elongation and reduction of area results.
The mechanical property results and cross section dimensions at the notch were used to calculate the torque required for shear overload of each shaft. The shear overload is defined as the torque required to initiate yielding in pure shear at the radius cross section.
Metallographic evaluations were performed to characterize the shaft material microstructure and to evaluate for case hardening of the spline and shaft surfaces. Knoop 500 gram microhardness depth profiles performed at both splined shaft ends found the same hardness at the spline teeth surface and the core, indicating no case hardening. Shafts S and X were calculated to fail at lower torque loads than Shaft J.
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