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The microstructure evolution and precipitation behavior of hot-rolled Ti–Mo–V micro-alloyed high-strength steel processed at different final rolling temperatures (FRT) and coiling temperatures (CT) were investigated. The results indicated that the decrease of FRT (880 °C–820 °C) promoted the ferrite refinement and strain-induced precipitation in austenite, but it also led to the decrease of pearlite content, which finally led to a slight difference in properties, and CT obviously changed the phase component, grain size, and precipitation behavior. Appropriate reduction of the CT (650 °C–600 °C) could promote the precipitation of (Ti, Mo, V)C and refine ferrite grains, thus improving strength. As the CT further lowered to 550 °C, the precipitation hardening effect would be seriously lessened owing to the significant reduction of (Ti, Mo, V)C volume fraction, finally resulting in the loss of strength. The optimum mechanical properties were attained for the FRT850-CT600 sample, and the ultimate tensile strength (UTS), yield strength (YS), and total elongation (TE) were 908 MPa, 848 MPa, and 21.5 %, respectively, with precipitation hardening up to 383.51 MPa.

The microstructure, mechanical properties and three-body impact abrasive wear behavior were investigated. The results indicated that martensite transformation occurred even under air-cooling, and fully martensitic structure consisted of lath martensite and a small amount of twinned martensite was obtained. And the martensite auto-tempering occurred during the subsequent air cooling process, resulting in precipitation of fine ellipsoidal V(C, N) particles and acicular η-Fe2C cementite in the matrix. Meanwhile, the tensile strength, yield strength, Brinell hardness and impact energy (−40 °C) of experimental steel were 1526 MPa, 973 MPa, 454.3 HB and 28 J, respectively. The strengthening mechanism analysis showed that solution strengthening was still the dominant strengthening method even though a large number of carbides were precipitated in the matrix. The dislocation strengthening was limited due to lower dislocation density, and the hot rolled recrystallization led to considerable grain refinement strengthening. Moreover, the results of three-body impact abrasive wear tests indicated that the wear resistance of experimental steel reached a same level as commercial Hardox450 steel. The wear mechanisms of experimental steels were plastic deformation fatigue, abrasive embedded and cutting. The mass loss rates of experimental steel and Hardox450 steel were 0.0033 g/min and 0.0032 g/min under the impact energy of 2.5 J, respectively.

In this work, the precipitates in Ti−Mo−V steel were systematically characterized by high-resolution transmission electron microscopy (HRTEM). The thermodynamics and kinetics of precipitates in Ti−Mo and Ti−Mo−V steels were theoretically analyzed, and the effect of vanadium on the precipitation behavior was clarified. The results showed that the precipitation volume fraction of the Ti−Mo−V steel was significantly higher than that of Ti−Mo steel. The randomly dispersed precipitation and interphase precipitation (Ti, Mo, V)C particles coexisted in the Ti−Mo−V steel. When the temperature was higher than 872 °C, the addition of vanadium could increase the driving force for (Ti, Mo, V)C precipitation in austenite, resulting in an increased nucleation rate and shortened incubation period, promoting the (Ti, Mo, V)C precipitation. When the temperature was lower than 872 °C, the driving force for (Ti, Mo, V)C precipitation in austenite was lower than that for (Ti, Mo)C precipitation, and the incubation period of (Ti, Mo, V)C precipitation was increased. Moreover, it was also found that the precipitated-time-temperature curve of (Ti, Mo, V)C precipitated in the ferrite region was “C” shaped, but that of (Ti, Mo)C was “ε” shaped, and the incubation period of (Ti, Mo, V)C was significantly shorter than that of (Ti, Mo)C.

Microbially mediated reductive immobilization of chromium is a possible remediation technique for sites contaminated with Cr(VI). This study is part of a broader effort investigating the biogeochemical mechanisms for Cr(VI) reduction in Hanford 100H aquifer sediments using flow-through laboratory columns. It had previously been shown that reduced chromium in the solid phase was in the form of freshly precipitated mixed-phase Cr(III)-Fe(III) (hydr)oxides, irrespective of the biogeochemical conditions in the columns. In this study, the reduced Cr phases in the columns were investigated further using spectroscopy to understand the structure and mechanisms involved in the formation of the end products. Several samples representing potential processes that could be occurring in the columns were synthesized in the laboratory and characterized using X-ray absorption near edge structure (XANES) and X-ray scattering. The XANES of Cr(III) particles in the columns most closely resembled those from synthetic samples produced by the abiotic reaction of Cr(VI) with microbially reduced Fe(II). Microbially mediated Cr-Fe reduction products were distinct from abiotic Cr-Fe (hydr)oxides [CrxFe1-x(OH)3] and organically complexed Cr(III) sorbed onto the surface of a mixed ferrihydrite-goethite mineral phase. Furthermore, analyses of the abiotically synthesized samples revealed that even the end products of purely abiotic, iron-mediated reduction of Cr(VI) are affected by factors such as the presence of excess aqueous Fe(II) and cellular matter. These results suggest that CrxFe1-x(OH)3 phases made under realistic subsurface conditions or in biotic cultures are structurally different from pure Cr(OH)3 or laboratory-synthesized CrxFe1-x(OH)3. The observed structural differences imply that the reactivity and stability of biogenic CrxFe1-x(OH)3 could potentially be different from that of abiotic CrxFe1-x(OH)3 [ABSTRACT FROM AUTHOR]

Cr(VI) is a widespread groundwater contaminant that is a potent toxin, mutagen, and carcinogen. In situ reductive immobilization is a favored approach for Cr(VI) bioremediation, and Cr(VI) reduction has been reported in a variety of aerobic, facultative, and anaerobic bacteria, including a number of pseudomonads. However, studies comparing Cr(VI) reduction under aerobic and denitrifying conditions in the same organism are not available. We have conducted studies with stain RCH2, a bacterium similar to Pseudomonas stutzeri that we isolated from a Cr-contaminated aquifer. Cell suspension studies with lactate demonstrated that Cr(VI) reduction could occur under either denitrifying or aerobic conditions (at comparable specific rates) and that reduction was at least 20-fold more rapid when the terminal electron acceptor (i.e., nitrate or O2) was present. Our results suggest that Cr(VI) reduction by strain RCH2 under either aerobic or denitrifying conditions is primarily cornetabolic in the sense that the physiological electron acceptor (oxygen or nitrate) appears to be required. Under both aerobic and denitrifying conditions, the gene(s) associated with chromate reduction are not inducible by Cr. Continuous culture (chemostat) studies showed strong correlations (r² values >0.93) between nitrate reduction rate and the transcript copy number of either nirS (cytochrome cd1 nitrite reductase) or narG (nitrate reductase α subunit). As our studies indicate that anaerobic Cr(VI) reduction by this pseudomonad requires active denitrification and that denitrification and chromate reduction rates are highly correlated (r² > 0.99), monitoring expression of such denitrification genes in biostimulated aquifers could provide valuable proxy information for in situ chromate reduction by similar bacteria even if the specific genes involved in chromate reduction have not been identified. We also report incomplete removal of reduced Cr tram solution and on artifacts in the widely used diphenylcarbazide assay for Cr(VI), most notably, its complete inactivation in the presence of millimolar nitrite. [ABSTRACT FROM AUTHOR]

We studied Cr isotopic fractionation during Cr(VI) reduction by Pseudomonas stutzeri strain RCH2. Despite the fact that strain RCH2 reduces Cr(VI) cometabolically under both aerobic and denitrifying conditions and at similar specific rates, fractionation was markedly different under these two conditions (ϵ was -2%o aerobically and -~0.4%o under denitrifying conditions). [ABSTRACT FROM AUTHOR]