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Influence of microstructural variation on hydrogen embrittlement susceptibility of 9260 bar steel, The
Hoyt, Ethan M.
Hoyt, Ethan M.
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2022
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The influence of microstructure on hydrogen embrittlement (HE) of high strength steels for fastener applications was explored in this study. Space limiting applications in areas such as the automotive or agricultural industries provide a need for higher strength fasteners, though HE susceptibility typically increases with strength. Using a 9260 steel alloy containing 1.92 wt pct silicon, the influence of microstructure on HE was investigated by examining conditions generated by quenching and tempering (Q&T), austempering, quenching and partitioning (Q&P), and prior austenite grain size (PAGS) refinement. The target hardness for all testing conditions was 52-54 HRC. All testing conditions were initially austenitized at 880 ºC for 20 min. The Q&T conditions were tempered at 450 ºC. Additional austenitization and quench cycles were conducted on one Q&T condition to refine the prior austenite grain size. The austempered condition was held at an isothermal bainitic hold temperature of 275 ºC upon cooling after austenitization. Q&P microstructures were created using quench temperatures that varied from 140 ºC up to 220 ºC, and two partitioning temperatures of 290 ºC and 400 ºC to attain the desired constant hardness level. HE susceptibility was assessed for these microstructures using slow strain rate tensile (SSRT), and circumferentially notched tensile (CNT) testing in a cathodic hydrogen pre charging environment with a hydrogen concentration of 1.0-1.5 ppm.
At comparable hardness, tensile mechanical properties varied by microstructure, with significant variations in yield point, ultimate tensile strength, work hardening behavior, and reduction in area at fracture. The retained austenite (RA) volume fraction and carbon content, as measured by X-ray diffraction, varied with the initial quench temperature for Q&P specimens. The baseline Q&T condition exhibited the highest HE resistance of all conditions for both smooth and notched testing geometries. The refined Q&T condition exhibited comparable HE resistance to the baseline Q&T condition for notched geometries. The austempered condition exhibited the lowest HE resistance of all testing conditions for both smooth and notched specimen testing geometries. HE resistance in Q&P conditions was influenced by quench temperature for smooth testing geometries; lower quench conditions, which had higher austenite carbon content, exhibited greater HE resistance. HE resistance was independent of quench temperature in Q&P conditions for notched testing geometries. The most HE resistant microstructure, the baseline Q&T condition, exhibited a ductile, microvoid coalescence fracture morphology. Microstructures with lower HE resistance exhibited multi mode fracture with microvoid coalescence, quasi cleavage, brittle transgranular, and intergranular fracture morphologies. Fresh martensite formed either during heat treatment or from transformation induced plasticity (TRIP) is responsible for the lower HE resistance in the Q&P and austempered microstructures. The TRIP phenomenon is believed to have occurred predominantly from plastic deformation during SSRT testing of smooth geometries, and from either a triaxial stress state near the notch surface or machining induced plasticity at the notch surface for quasi static CNT testing of notched geometries.
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