Materials Engineering Department
BS in Materials Engineering
This project seeks to increase the lifespan of steel mandrels used by Schlosser Forge in the saddling process, which is part of the production of forged rings for the aerospace and power generation industries. All saddle mandrel samples were taken from used 4340 steel mandrels. Hardness profiles were measured across rectangular bars cut from the transverse direction of a completely fractured piece of a 7″ diameter mandrel. The hardness near one outer surface of the tested bar was 36 HRC and gradually decreased to 30.5 HRC at the opposite surface. Macroscopic examination of the primary fracture surface along with light microscope imaging of fracture surface feature details suggests that the failure mechanism is likely related to reversed bending fatigue rather than deterioration of the mechanical properties of the steel due to microstructural changes from in-process overtempering during saddling. Cyclic softening of the quenched and tempered steel caused by the saddling process adversely affects the ability of the steel to withstand fatigue. Different heat treatments were conducted to compare the resulting toughness, measured by Charpy impact test, to that of the current process of quenching and tempering the mandrel to a hardness of 38 – 42 HRC. Each heat treatment involved austenitizing at 1500°F for one hour. By varying the tempering temperature, with a constant two hour tempering time, samples were obtained at 28.5 HRC, 39.8 HRC, and 42.5 HRC. Charpy impact energy decreased significantly as hardness increased. An additional heat treatment was conducted where a 4340 steel sample was austenitized, quenched in a 660°F NaNO3-based molten salt bath and held for two hours. The result of this heat treatment is a bainitic structure, which may show improved toughness.