BS in Materials Engineering
Materials Engineering Department
Oliver Racing Parts (ORP) is looking to optimize the manufacturing process for high-strength connecting rod bolts. A high yield strength is desired for the bolts because deformation would result in a loss of engine performance. A high tensile strength is also necessary to prevent catastrophic failure if deformation occurs. A high ductility as a proxy for toughness is attractive for resisting crack nucleation and growth during cyclic loading. Bolts were made of MP35N, a nickel-cobalt multiphase alloy; MLX-17, a precipitation hardenable stainless steel; 8740, a low alloy steel; and H11, a chromium hot-work tool steel. The temperature of the most important heat treatment step was varied for each material. Tensile testing was performed to determine the tensile and yield strength of the bolts. Ductility was measured through a percent reduction in area. Fracture surfaces were imaged via scanning electron microscopy to observe the mode of failure. In order of highest to lowest tensile strength: H11 (270 ksi), 8740 (220 -250 ksi), MLX 17 (220 – 230 ksi), and MP35N (130 ksi). In order of highest to lowest yield strength: H11 (265 ksi), 8740 (205 -230 ksi), MLX-17 (215 – 220 ksi), and MP35N (85 – 120 ksi). In order of lowest to highest %RA: 8740 (21% - 52%), H11(48%), MLX-17 (55% - 64%), and MP35N (80%). The low strength and high ductility of the MP35N bolts can be attributed to insufficient cold work, necessary for strengthening, prior to aging. It is suggested that room temperature aging or aging during the bolt forming process resulted in MLX-17 bolts that were weaker than predicted. 8740 bolts achieved maximum strengths but at a possible cost to ductility. H11 bolts were the strongest bolts tested.
Available for download on Tuesday, June 14, 2022