The effect of carbon segregation and carbide precipitation on the mechanical response of martensite

The influence of carbon distribution and carbide precipitation on the mechanical properties of the as-quenched and quenched and tempered 300M martensitic steel has been investigated. The microstructure, investigated by transmission electron microscopy (TEM) and three dimensional atom probe tomography (APT) was found to be relatively homogeneous in the as-water-quenched state, but signifi cantly evolved upon tempering and variation of quench rate. This evolution included carbon segregation to dislocations and grain boundaries and carbide precipitation. A simple mean- field precipitation model assuming heterogeneous nucleation onto the dislocations proved to satisfactorily capture the evolution of precipitation upon tempering at 120C and 150C. The material was found to behave, mechanically, as a composite and in accordance, the Bauschinger stress-strain behaviour was successfully modeled using a continuous Masing model. This model, when related to the microstructure, showed that the composite behaviour arose from the mechanical contrast between the laths, this being controlled by the local dislocation density and carbon segregation and/or precipitation onto them. Carbon segregation and carbide precipitation were observed to have a direct impact on alpha in the Taylor-like equation that was shown to control the local yield stress within the laths. When applied to martensites containing various amount of carbon, the model allowed for an empirical assessment of the e ffect of the nominal carbon content on alpha , which was found to be linearly dependent on the nominal carbon content.Applied Science, Faculty ofMaterials Engineering, Department ofGraduat

Relation microstructure-propriétés mécaniques d'un acier martensitique inoxydable

The relationship between microstructure and mechanical properties of MaX (1.4006) martensitic stainless steel has been studied. Optical microscopy was used to characterize the microstructure and the volume fraction of retained ferrite was measured by image analysis. Mechanical properties were measured in uni-axial tensile testing and a composite model has been developed to capture the effect of both the retained ferrite and the carbon content of the martensitic phase. First results show a reasonable correlation between the experimental stress-strain curves and the model. Results are discussed in view of a previous study on plain martensitic carbon steels

The Mechanisms of Transformation and Mechanical Behavior of Ferrous Martensite

International audienceMartensitic steels are among the most important structural materials used today, yet they are far from being completely understood. In this article we survey the academic literature pertaining to ferrous martensite, from seminal works performed starting in the mid 20th century to the insight gained from the availability of modern characterization technology within the last several years. The mechanisms governing the formation of the martensitic microstructure and its dependence on chemistry and processing route are critically evaluated. This is followed by a description of our current understanding of the relationship between microstructure and mechanical properties, particularly yield strength and work hardening rate. Throughout, we aim to highlight topics that remain open as unanswered areas for continued research

Building a Geological Reference Platform Using Sequence Stratigraphy Combined with Geostatistical Tools

International audienceThis paper presents a methodology that is currently tested at the French geological survey in order to validate drill holes interpretation. Validated drill holes are intended to be included in the future French geological reference platform which is under construction. To validate drill holes, a first subset of high-quality holes is selected. This data is interpreted in terms of geology and a geostatistical analysis is performed. A 3D geological model is built to assess the overall geological consistency. Then the rest of the drill holes is progressively and iteratively validated by geostatistical cross validation. As several thousands of drill holes are to be validated, specific software and workflows have been developed and are presented here

The Role of Chromium Carbides Volume Fraction on Plastic Instability Under Three-Point Bending Test in Martensitic Stainless Steel

Martensitic stainless steels (MSS) are valid candidates for automotive applications as they present a good combination of mechanical and functional properties which improves the safety and efficiency of new vehicles. However, these steels show limited fracture strain under three-point bending conditions. The fracture strain is controlled by the evolution of ductile damage as the material is plastically deformed, which in turn is influenced by microstructural features such phase ratio and carbides distribution. In this work, a modified AISI 410 MSS is heat treated in order to change the carbides distribution by dissolving part of the Cr-rich carbides while keeping the same phase ratio of ferrite and martensite. Each sample is tested under three-point bending and uniaxial tensile loading. The importance of the carbides distribution parameters (volume fraction, interparticle distance and size) on the crack propagation mechanism is assessed in order to link the material ductility to the critical microstructure constituents

Microstructural heterogeneity and its relationship to the strength of martensite

International audienceThe complex microstructure of ferrous martensite is reflected in its complex mechanical response. In an attempt to highlight how carbon redistribution during quenching and/or low temperature tempering can affect mechanical response, a set of controlled experiments were performed. By rapidly quenching samples it was possible to limit autotempering allowing the evolution of mechanical response and microstructure to be followed with low temperature tempering. This was compared to a situation where the material was more slowly quenched, leading to a highly autotempered state. The gradual transition from elastic to plastic deformation is interpreted based on possible sources of microstructural heterogeneity. Lath-to-lath variations of dislocation density are discussed as a contributor to the development of microstructural heterogeneity during tempering. These results shed light on the possible origins of the continuous-composite like mechanical response of lath martensite proposed in recent work

Effects of cooling path and resulting microstructure on the impact toughness of a hot stamping martensitic stainless steel

International audienceThe present study examined the effect of microstructural characteristics on the toughness properties of a hot stamping martensitic stainless steel. Moderately slow cooling during the martensitic transformation leads to the auto-tempering of the martensite laths and the stabilization of thin austenite films. The amounts of retained austenite and cementite precipitates were quantified for various cooling conditions. Charpy impact toughness tests were performed over a large range of temperatures to characterize the ductile-to-brittle transition. Decreasing the cooling rate from 300 °C/s down to 3 °C/s increased the retained austenite fraction from 0.6% up to 2.6% and decreased the ductile-to-brittle transition temperature by 140 °C. The critical cleavage fracture stress was determined to be around 2400 MPa whatever the cooling rate, by applying the local approach to fracture. However, it has been demonstrated that a higher retained austenite fraction modifies incipient plasticity and decreases the yield stress by 60 MPa. As a result, retained austenite delays cleavage fracture by increasing the strain necessary to reach the critical cleavage fracture stress required to trigger cleavage initiation in the ductile-to-brittle transition domain. In this way, retained austenite plays a determining role to decrease the ductile-to-brittle transition temperature. It is thus beneficial to design cooling rates in order to increase the retained austenite fraction and to improve impact toughness at low temperatures

Influence Of Microscopic Strain Heterogeneity On The Formability Of Martensitic Stainless Steel

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Microstructural heterogeneity and its relationship to the strength of martensite

The complex microstructure of ferrous martensite is reflected in its complex mechanical response. In an attempt to highlight how carbon redistribution during quenching and/or low temperature tempering can affect mechanical response, a set of controlled experiments were performed. By rapidly quenching samples it was possible to limit autotempering allowing the evolution of mechanical response and microstructure to be followed with low temperature tempering. This was compared to a situation where the material was more slowly quenched, leading to a highly autotempered state. The gradual transition from elastic to plastic deformation is interpreted based on possible sources of microstructural heterogeneity. Lath-to-lath variations of dislocation density are discussed as a contributor to the development of microstructural heterogeneity during tempering. These results shed light on the possible origins of the continuous-composite like mechanical response of lath martensite proposed in recent work.Nouveaux aciers à gradient de propriétés (Graded Composite Steels Design