Abstract
Three different commercially available American gear steels were heat treated using different processes and their microstructures and hardness profiles examined. These steels and corresponding heat treatments were compared to 18CrNiMo7-6 case carburizing gear steel for technical performance. A fractured gear tooth from a failed helical gear made of 18CrNiMo7-6 from a twin screw extruder gearbox was analyzed for case and core hardness, case depth and microstructure. The surface contact and bending stresses of the gear were then determined from the original gear geometry and torque rating using American Gear Manufactured Association (AGMA) 2001-D04. The surface contact and bending stresses were found to be approximately 196,500 psi and 57,600 psi, respectively. A test coupon of American Iron and Steel Institution (AISI) 9310 was nitrided to a case depth of 0.023 inches (based on AGMA recommendations). Test coupons of AISI 4140 and 4340 were induction hardened to a case depth of 0.093 inches (based on AGMA recommendations). The three samples were compared to 18CrNiMo7-6 for surface hardness, core hardness, case depth, and microstructure. AISI 9310 developed surface and core hardnesses of 56 HRC (Rockwell C) and 25 HRC, respectively and case depth of approximately 0.008 inches. AISI 4140 developed surface and core hardnesses of 65 HRC and 38 HRC, respectively and case depth of approximately 0.3 inches. AISI 4340 developed case and core hardnesses of 65 HRC and 54 HRC, respectively and did not drop below the threshold for an effective case depth.