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Effects of thermomechanical pretreatment on abnormal grain growth during simulated carburization, The
Gruich, James M.
Gruich, James M.
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2024
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Abstract
Carburization followed by quenching, and tempering is frequently utilized in the automotive industry to increase the surface hardness and compressive residual stress of a steel alloy while retaining toughness and ductility in the core. At carburizing temperatures, austenite grains are formed and abnormal grain growth (AGG) can occur. During AGG, the steel microstructure undergoes bimodal grain growth with some grains growing exponentially faster than others. The growth of large austenite grains through AGG compromises the fatigue performance of carburized steels. AGG has been shown to be further exacerbated by cold work introduced into the alloy prior to carburizing.
Warm and cold work are also sometimes utilized in part forming prior to carburizing. In this study, the effects of warm work and warm work in combination with cold work on austenite grain size were investigated. AISI 4121with added Al and N and a modified AISI 4121 with additions of Nb and Mo were homogenized, warm reduced by 0-50 pct at a temperature of 900 ℃, subjected to cold reduction ranging from 0-25 pct, and then heated in a furnace for 0 328 min at a temperature of 930 ℃ after ramping up 8 ℃ min-1 from room temperature to simulate a carburizing heat treatment without a carburizing atmosphere.
WR resulted in refinement of the resulting ferritic and secondary microconstituent microstructures. The average prior austenite grain size (PAGS) and AGG also tended to decrease as WR and CR increased. The refinement due to WR and CR were roughly equivalent, but the addition of CR tended to decrease the effectiveness of grain refinement due to WR. The alloy condition with the coarsest average PAGS and highest degree of AGG was 4121Nb after a 328 min hold time, equivalent to a full carburization cycle. 4121Al had a higher relative grain refinement across a majority of experimental conditions, as well as a lower degree of AGG on average. The precipitates present in 4121Al were exclusively AlN and those present in 4121Nb included both AlN and finer precipitates interpreted to be Nb(C,N).
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