Steels manufactured via Laser powder bed fusion (LPBF) usually exhibit a good synergy of strength and ductility due to their ultrafine microstructure. Yet, their toughness, in particular cryogenic ...toughness is intrinsically inferior as the formation of micro-voids and oxide inclusions can hardly be fully prevented during LPBF. In this study, a toughening strategy based on chemically heterogenous metastable austenite was proposed to improve the impact toughness of LPBF manufactured high strength steels. As demonstrated in a maraging stainless steel, cryogenic (-196 °C) impact toughness can be enhanced by three times without a sacrifice of strength via tailoring chemically heterogenous austenite in the strong martensitic matrix. Both experiments and molecular dynamic simulations demonstrate that upon impact deformation chemically heterogenous austenite could transform into martensite in a stepwise manner, which could not only absorb massive energy via deformation induced martensite transformation but also make a contribution to local stress mitigation, crack passivation and deflection. The chemically heterogenous austenite strategy has the potential to be utilized for improving the toughness of other high-strength steels.
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A two-steps tempering-partitioning process is designed for redistributing carbon and austenite stabilizer such as manganese in a cold rolled medium-manganese steel. After tempering at 450℃ for 0.5h ...and 1h, a large number of metastable Mn-rich M12C carbides and high density of NiAl-type nanoparticles were found, and the segregation of carbon at the dislocations/interfaces around NiAl precipitates was revealed by 3D atom probe tomography (APT). The competition between the M12C carbides and the dislocations/interfaces for carbon inhibits transformation of the metastable M12C carbides to more stable carbides. During the following partitioning at 630℃ for 0.5h, the metastable Mn-rich M12C dissolved and the fine reversed austenite nucleating on the metastable M12C carbides exhibits a sharp gradient Mn distribution. The tailored metastable austenite inherited from metastable M12C carbide shows strong enrichment of Mn and C (Mn: 18.1 at.%, C: 1.56 at.%), dispersed distribution (9.8 μm−2) and fine size (50-200 nm). With the nature of both hard particle and transitionable phase, the austenite couples Orowan strengthening and transformation induced plasticity (TRIP) assisted work hardening. Outstanding dynamic mechanical properties (yield strength: 1350 MPa, total elongation: 30%) were achieved due to this microstructure. Here we report a strategy for the design of ultrahigh-strength steels by making metastable austenite serve as a strengthening second phase in addition to the TRIP effect.
It is desirable to break the relationship between the wear resistance and bulk hardness when researching a new wear-resistant material because of the requirement for high formability. The sliding ...wear behaviors of 5Mn steel treated by different quenching and partitioning (Q&P) processes were investigated at three wear test temperatures (300 ℃, 20 ℃ and −50 ℃). The results showed that when the wear test was conducted at 300 ℃ and 20 ℃, the wear rates of the steels decreased with increasing initial hardness. The wear performance of QP220 steel, which contained ~22% austenite with a lower initial hardness, was better than that of martensitic steel at − 50 ℃ due to the improved strain hardening ability, which was controlled by the enhanced transformation-induced-plasticity (TRIP) effect.
•Sliding wear behaviors of medium Mn Q&P steels were studied at different temperatures.•Wear resistance of softer Q&P steels was superior to harder martensitic steel at − 50 ℃.•Decreasing wear temperature stimulated TRIP effect and thus enhancing strain hardening.•TRIP effect of metastable austenite remarkably increased ductility and wear resistance.•This work provides good guidance for designing next-gen formable wear-resistant steels.
Introducing reverted austenite at lath martensite boundaries has been shown to be an effective method to increase the ductility of martensitic steels. However, significant strength reductions can ...also be introduced during the reversion process in martensitic stainless steels. To overcome the strength-ductility trade-off, we explore the effects of alloying elements in delaying the overaging of the precipitates and accelerating the austenite formation. To this end, we carry multi-objective optimization considering (1) lattice misfit and diffusivity for kinetics of precipitation, and (2) austenite fraction and thermodynamic driving force of austenite reversion for austenite formation. With the insights from calculations, model alloy compositions are then proposed, and mechanical testing and microstructural analysis are applied. The model alloy confirms that a relatively higher volume fraction of austenite can form prior to overaging, which doubles the total elongation without significant strength loss.
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Warm caliber rolling at 550, 600, and 650 °C were carried out to refine the austenite grains of medium Mn steel by dynamic recrystallization (DRX). The effect of rolling temperature on the ...microstructure characteristics and mechanical properties of metastable austenitic steel was investigated in detail. The results show that the hierarchical structure of coarse fiber grains (CFGs) and ultrafine grains (UFGs) is obtained by warm caliber rolling method. With the increasing rolling temperature, more obvious DRX behavior occurs, resulting in a high percentage of UFGs. The preactive TRIP effect shortens the yield stress plateau in tensile stress-strain curves caused by CFGs with low stability. Furthermore, the unique hierarchical structure induces crack deflection and delamination, and significantly improves the impact toughness of medium Mn steel at room temperature. However, at the low temperature of −80 °C, CFGs are easy to induce martensite transformation, which seriously deteriorates the impact toughness.
•Low caliber rolling temperature contributes to the formation of fiber grains.•Fiber grain structure improves the impact property at ambient temperature.•V-Carbide reduces the tensile and impact properties of the Medium Mn steel.•The martensitic transformation decreases impact properties at low temperature.
An AISI 304 austenitic stainless steel (ASS) with average grain size of approximately 48µm was selected to explore the effect of the low-temperature on mechanical behavior for commercial metastable ...ASS, which basing on tensile tests and Charpy V-notch impact tests under temperatures of 20–298K, Feritscope testing and physical metallurgy. The results showed that both yield strength and tensile strength were enhanced by lowering temperatures; however, the uniform strain decreased with reducing temperatures. The yield strength at 20K and 77K were much higher than that at other temperatures, accompanying with an abrupt increase of thermally-induced martensite before tensile testing. The Charpy V-notch impact energy decreased faster in the range of 77–298K and kept almost unchanged in the range of 20–77K, and the ASS at 20K still exhibited a dimple shaped fracture. Generally, the work-hardening rate (Θ) of the ASS at testing temperatures of 20–298K dropped rapidly at the initial plastic strain range (Stage I) and then grew with the increase of tensile strain (Stage II), then following by a continuous decline to necking (Stage III), i.e., the generation of a peak of work-hardening rate. Specifically, the Stage I, II and III were terminated in advance and the peak value at the Stage II were increased obviously by reducing temperature from 298K down to the range of 20–253K. Furthermore, the work-hardening behavior of the ASS was discussed in view of the evolution of microstructure basing on Olson-Cohen model.
Steels with a certain amount of retained metastable austenite derived from e.g. heat treatment can undergo martensitic transformation induced by mechanical loads. Pre-machining is of high relevance ...for the following processes. Milling and grinding were chosen here for the pre-machining with subsequent deep rolling of 18CrNiMo 7-6 (AISI 4820) in carburized and carbonitrided states. Deep rolling is used to improve the surface and subsurface strength by inducing compressive residual stresses, strain hardening, and in this case martensitic transformation. The results of this paper show clearly that the pre-machining is sufficient to generate mechanically induced martensite and enhance the surface regarding the resulting hardness. These changes of the surface integrity are affecting the following deep rolling process, which is why a constant pre-machining is necessary for future investigations in order to compare deep rolling processes across each other.
The present work investigates the role of martensite induced by either stress or strain on the tensile properties of medium Mn steel treated by quenching and partitioning/tempering. Different from ...the strain-induced martensite with isolated fine martensite blocks, the stress-assisted martensite is featured with large and continuous martensite blocks, which facilitates the propagation of cracks and results in the premature fracture and thus limited tensile ductility (12.8%). A pre-deformation and tempering strategy is employed to deplete the stress-assisted martensite while promote the strain-induced martensite, turning an original soft and brittle medium Mn steel into a strong and ductile one, leading to an increment of ultimate tensile strength and total elongation of 533 MPa and 28.4%, respectively.
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Abstract Cryogenic turning of metastable austenitic stainless steels can improve wear resistance of the resulting surface due to the phase transformation of γ‐austenite into α’‐ and/or ϵ‐martensite ...in the near surface layer. By using a cryogenic two‐step turning process, the amount of deformation‐induced α’‐martensite in the subsurface regime can be further increased. To determine the influence of the implemented and optimized two‐step turning strategy on the tribological properties of countersurfaces for radial shaft seals, an evaluation of wear behavior of the shaft seal countersurface as well as microstructural analyses in subsurface regime is presented and compared to the cryogenic single step turning process. The results show that not only the integral phase transformation in the overall subsurface region, but also the local phase distribution plays an important role when it comes to the surface performance in tribological applications.
Translation abstract Das kryogene Drehen von metastabilen austenitischen rostfreien Stählen kann die Verschleißfestigkeit der resultierenden Oberfläche aufgrund der Phasenumwandlung von γ‐Austenit nach α’‐ und/oder ϵ‐Martensit im oberflächennahen Bereich verbessern. Durch den Einsatz eines zweistufigen kryogenen Drehprozesses kann der verformungsinduzierte α’‐Martensitgehalt weiter erhöht werden. Um den Einfluss des implementierten und optimierten zweistufigen kryogenen Drehprozesses auf die tribologischen Eigenschaften von Gegenlaufflächen für Radialwellendichtringe zu ermitteln, werden die Verschleißuntersuchungen der Wellendichtring‐Gegenlauffläche sowie die Mikrostrukturanalysen in den oberflächennahen Bereichen durchgeführt und mit der einstufigen Drehstrategie verglichen. Die Ergebnisse zeigen, dass nicht nur die integrale Phasenumwandlung in der gesamten Einflusszone, sondern auch die lokale Phasenverteilung eine wichtige Rolle für die Oberflächenqualität in tribologischen Anwendungen spielen.
