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Microstructure development and fatigue crack growth studies on high-carbon plate-martensite austenite microstructures
Agnani, Milan
Agnani, Milan
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2022
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Abstract
Carburized components such as gears and drive shafts experience bending fatigue loading during service. At stresses above the endurance limits, fatigue cracks nucleate at an early stage and most of the fatigue life of the component is spent in the fatigue crack propagation regime, particularly in the case region of a carburized component. The goal of this PhD work was to study the influence of microstructural features on the fatigue crack growth (FCG) resistance of high-carbon plate martensite austenite microstructures for potential application in carburized and bearing steels. Using commercial high-carbon 52100 steel (1 wt pct carbon), high-carbon plate-martensite austenite microstructures (microstructures comparable to the case regions of carburized steel) were developed, fatigue tested and characterized. Thermal processing strategies, namely thermal cycling and step-quenching were implemented and the subsequent refinement in prior austenite grains size (PAGS), plate-martensite and RA was quantitatively characterized. Lower austenitizing temperatures and shorter hold times resulted in the greatest PAGS refinement as well as refinement in plate-martensite and RA constituent size. Multiple thermal cycling did not further refine the PAGS but resulted in a narrower PAGS distribution. Step-quenching (interrupted quenching below Ms temperature) mechanically and/or thermally stabilized the RA, resulting in a refinement in plate-martensite size and an increase in the stability of RA. The mode I fatigue crack growth (FCG) behavior of high-carbon plate-martensite austenite microstructures with varying amounts and stability of RA was investigated. The interaction between the fatigue cracks and the surrounding microstructure was characterized using microscopy, electron backscatter diffraction and quantitative fractography. Higher initial amounts of RA, mechanical stability of RA, and greater degree of martensite tempering improved FCG resistance of high-carbon plate martensite austenite microstructures. Stress-assisted RA to martensite transformation was observed in the vicinity of the fatigue cracks in the microstructures with substantial amounts of RA (more than 15 vol pct). The fatigue fracture surfaces of the different microstructures exhibited varying combinations of IG fracture, mixed-ductile brittle (MDB) and TG (cleavage, quasi-cleavage, and ductile striations) fracture as a function of stress intensity range at the crack tip. Microstructure conditions showing greater fractions of MDB, and TG fracture had better fatigue crack growth resistance. The presence of a higher initial RA content suppressed brittle IG fracture and promoted TG fracture, potentially due to the transformation-toughening associated with RA to martensite transformation or transformation induced micro-crack coalescence in the vicinity of the fatigue crack. Finally, transformation toughening models were developed to predict the role of deformation induced martensite transformation (DIMT) on the FCG behavior of high-carbon plate-martensite austenite microstructures. The model predictions of FCG rates were highly sensitive to small variations in the amount and distribution of DIMT, suggesting that transformation toughening may be used as a microstructure design strategy for improving FCG resistance of high-carbon plate-martensite austenite microstructures.
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