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 effect of oxygen on the precipitation behaviour of Nb in Fe-15Cr-1Nb-xO alloys

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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

Dissolution versus morphological evolution of residual δ-ferrite in model austenitic stainless steel

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

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Residual ferrite in martensitic stainless steels: the effect of mechanical strength contrast on ductility

Ductile damage process of a martensitic stainless steel with 15vol% of residual ferrite and two populations of carbide particles is investigated using a combined multiscale experimental and modelling approach. Whereas Nb-rich carbides contribute to grain refinement, coarser Cr-rich carbides are preferential damage nucleation sites. Three different heat treatments are applied to partially dissolve Cr-carbide particles while keeping the same ratio of ferrite versus martensite volume fraction. Surprisingly, ductility decreases with decreasing volume fraction of Cr-carbides. Nanoindentation mapping indicates that the strength contrast between ferrite and martensite increases with carbide dissolution. According to finite element simulations of strain partitioning inside the two phase microstructure, the stress triaxiality in ferrite increases with the mechanical strength contrast. This promotes void nucleation and growth, reducing the fracture strain

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