On the process, structure, property relationship for the optimization of strength and bendability of martensitic stainless steels

Progresses in the structural engineering of materials for automotive is essential to ensure safe and sustainable means of transportation. The development cycle of new materials can be shortened through the application of design tools that enable rapid identification of pathways for materials optimization. This thesis investigates the property - structure - processing relationship of a novel Niobium-added martensitic stainless steel, with a special focus on the enhancement of crash performance. The first part of the thesis unravels the relationship between properties and structures of the alloy. A multi-scale approach combining mechanical characterizations at the macroscale with finite element simulations at the microscale allows identifying critical microstructural features that control strength, ductility and bendability. Above all, the presence of undissolved Cr-rich carbides and residual islands of soft ferrite, is found detrimental for the bendability of the steel. On the other hand, the presence of thin films of retained austenite have a beneficial effect on the overall mechanical properties. In the second part of this work, CALPHAD-based methods are used to understand the thermodynamics and kinetics of the physical processes leading to the formation of those critical microstructural features. On the one hand, the presence of clusters of Cr-rich carbides contributes to the stabilization of ferrite even after prolonged heat treatments at high temperatures. On the other hand, the quenching rate significantly influences the presence of austenite in the room temperature microstructures. Finally, different original modelling tools are proposed, which can serve for the design of novel heat treatments and alloy compositions, contributing to the development of high strength stainless steel grades with improved mechanical properties.(FSA - Sciences de l'ingénieur) -- UCL, 201

Overcoming the Diffusion Bottleneck: Effect of Alloying Elements on Phase Transformations and Carbides Dissolution in Martensitic Stainless Steels

Martensitic Stainless Steels (MSS) become an attractive choice for automotive industry in view of their good combination of properties such strength, formability and corrosion resistance. MSS are Fe-Cr-C alloys, which are heat treated in the austenitisation range and then quenched to room temperature to form martensite. However, depending on the chemical composition and the processing route, residual ferrite and chromium carbides can also be found in the final structure. These sub-products are detrimental for both mechanical and functional properties and therefore should be minimised. An in-depth understanding of the kinetics of sub-products formation is thus needed to optimise industrial practices. To fulfil this task, thermodynamic and kinetic models can be used to calculate carbon diffusion, elements distribution and phase transformation during heat treatments. A one-dimension moving phase boundary simulation was performed with DICTRA 26, in which the dissolution of chromium-rich carbides, diffusion of carbon and subsequent ferrite to austenite transformation reactions are treated simultaneously in a one-cell simulation. This extra complexity is needed to capture the relationship between the carbide dissolution and the ferrite to austenite transformation. Results show that the diffusion kinetics of interstitial carbon atoms is controlled by the substitutional diffusion of carbide formers elements (Cr and Mo), which decrease carbide dissolution rate and consequently inhibit the phase transformation kinetics. Numerical predictions are compared to experimental results for a newly developed AISI 410 martensitic stainless steel, confirming the trend highlighted by DICTRA calculations. Optimised steel composition and processing practices are finally suggested

Austenization stasis in Fe-12Cr-0.1C martensitic stainless steel

Residual ferrite, a common sub-product of the austenization process in martensitic stainless steels (MSS), has serious detrimental effects on the mechanical properties of these alloys [1]. Due to its technological relevance, austenization is one of the most well-known phase transformation in material science. For high-Cr steels, a transformation in multiple stages is often reported. However, the mechanisms dictating the onset of the different transformation rates are not entirely clear [2]. Here, using both experimental and simulation techniques, we show that the austenization reaction in MSS occurs in three stages: (1) fast growth of austenite driven by Cr diffusion in ferrite and partial dissolution of M23C6, (2) soft-impingement and reaction stasis, (3) slow austenite growth driven by Cr homogenization in austenite. The moving boundary model in DICTRA is used to study the transformation. Based on experimental observations, austenite is set to nucleate from ferrite grain boundaries and to grow towards the M23C6 particle, which is initially embedded in the ferrite matrix. DICTRA calculations are in good agreement with dilatometric experimental data, which show the presence of residual ferrite even after prolonged holding time in the austenite temperature range. An analysis of the Cr profiles in the simulation domain shows that the transformation stasis is caused by soft-impingement between M23C6 /α and α/γ interfaces in the residual ferrite matrix. Next, the effect of heating rate and initial M23C6 particle size are investigated to optimize the process parameters. For heating rates greater than 1°C/s, simulations predict ferrite growth, which immediately follow the point of maximum austenite volume fraction. Based on thermodynamic considerations, this phenomenon is qualitatively explained with Thermo-Calc. Moreover, as the initial carbide size is reduced, the volume fraction of austenite transformed before soft-impingement increased. Finally, a set of process parameters are optimized with the objective function of minimizing time while maximizing the final volume fraction of austenite

Unfolding the effect of residual ferrite on damage and fracture resistance in a martensitic stainless steel for automotive application

In order to comply with new regulations on safety and pollution, the automotive industry is constantly seeking for new alloys to reduce the weight of chassis, while increasing strength and ductility. Martensitic stainless steels (MSS) exhibit good strength, acceptable ductility and corrosion resistance, which make these alloys a valid solution for automotive applications. MSS for hot-stamping are heat treated in the austenitic range for few minutes and then quenched to room temperature. Other than martensite, residual ferrite is usually present after this process. The ductile damage mechanism of a martensitic stainless steel with 15 vol.% of residual ferrite and Cr-carbide particles is investigated using a combined multiscale experimental and modelling approach. A preliminary study reveals that Cr-rich carbides are preferential damage nucleation sites. Hence, three different heat treatments are applied to partially dissolve these 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 Cr-carbide dissolution. According to finite element simulations of strain partitioning inside the dual phase microstructure, the stress triaxiality in ferrite increases with the mechanical strength contrast. This promotes nucleation and growth of primary voids, which reduces the fracture strain. In addition, a statistical study of FE simulations reveals that there is a critical phase configuration that maximizes damage. Voids are more likely to initiate in channels of percolated ferrite aligned perpendicular to the main tensile direction and constrained by the surrounding martensite. The present understanding of the role of residual ferrite in MSS should enable the design of microstructure with enhanced mechanical properties

Influence of 5 at.%Al-Additions on the FCC to BCC Phase Transformation in CrFeNi Concentrated Alloys

In the context of the recent developments in the field of high entropy alloys (HEAs), CrFeNi based alloys with the addition of Al have gained significant attention due to their interesting combination of mechanical properties and corrosion resistance, enabling an even wider range of applications for HEAs. A key feature of this system is the co-existence of multiple phases such as body centered cubic (BCC) and face centered cubic (FCC) phase, which significantly enhances the mechanical performance of the alloy. However, despite the ongoing research efforts, an in-depth study of the effect of Al on the phase transformation kinetics in this system is not yet available, which undermines the design of effective heat treatments, curbing the optimization of microstructures and properties. In this work, the influence of 5 at.%Al addition on the interdiffusion behavior and phase transformation kinetics from FCC to BCC phase at 700 C is studied by experiments and state-of-the-art phase field simulations. The kinetics of BCC precipitation are observed experimentally through the sequential characterization of samples heat treated up to 11 days. 2D phase field (PF) simulations of the growth and coarsening of BCC-precipitates in a dual-phase BCC/FCC system are performed on the same alloy system. Experimental observations reveal that the growth of BCC is greatly enhanced by the addition of Al to the ternary CrFeNi system. This result is consistent with findings from the phase field simulation. PF results show that the gradients of the diffusion potentials and interdiffusion mobilities are increased in the case of Al addition, which explains the significant increase of the phase transformation rate from FCC to BCC. Finally, the cross terms in multi-component diffusion equations are found to significantly affect the evolution of the phases. These findings demonstrate the need of multicomponent models to fully understand the complex interdiffusion behavior in high and medium entropy alloys to foster the design of these materials

Effect of Multi-Step Austempering Treatment on the Microstructure and Mechanical Properties of a High Silicon Carbide-Free Bainitic Steel with Bimodal Bainite Distribution

The effect of multi-step austempering treatments on the microstructure and mechanical properties of a novel medium carbon high silicon carbide-free bainitic steel was studied. Five different isothermal treatment processes were selected, including single-step isothermal treatments above martensite start temperature (at 350°C and 370°C, respectively), and three kinds of two-step routes (370°C + 300°C, 370°C + 250°C, and 350°C + 250°C). In comparison with single-step austempering treatment adopting a two-step process, a microstructure with a bimodal-size distribution of bainitic ferrite and without martensite was obtained. Bainitic transformation was studied using dilatometry both for single-step and two-step routes and the specimens were completely characterised by electron microscopy (SEM and TEM), X-ray diffraction (XRD) and standard tensile tests. The mechanical response of the samples subjected to two-step routes was superior to those treated at a single temperature

Segregation of alloying elements on the hot tear formation in friction melt bonding of Al/steel joints

Friction melt bonding (FMB) is a promising recent technique to join dissimilar metals with different melting temperatures, in which Al to steel joints are significantly attractive to the automotive industry for weight saving [1, 2]. In an Al/steel lap joint, steel is placed on top and Al beneath. As the tool passes, Al melts and then solidifies as the tool steps away. Occasionally, this process exhibits hot tearing defects in the solidified aluminum zone. A recent study from our research group by Jimenez-Mena et al. (2018), showed a possibility to suppress the hot tear by optimizing the thermal environment of the process [3]. In this study, DP600 steel welded to different aluminum alloys (AA1050, AA6061 and AA7039) were investigated to understand the severity of the hot tear formation due to segregation of alloying elements during solidification. Welds produced with various parameters are characterized to identify the critical elements which favor hot tearing. The base materials of DP600 steel had thicknesses of 0.8 mm while AA1050, AA6061 and AA7039 had the thicknesses of 3.0 mm, 3.0 mm and 17.5 mm respectively. Different material backing plates were used when welding 3mm aluminum plate to DP600 while no backing plate was used when welding AA7039 to DP600 steel. Microstructural characterization Energy dispersive X-ray spectroscopy (EDS) maps were obtained from the transverse section of the weld. A thermomechanical solidification model was also performed using DICTRA® to estimate the amount of segregated elements for an AA6061aluminum alloy. In the case of AA1050/DP600 welds, the Al reached 650°C (at least the melting temperature of Al), and at that temperature some iron dissolve in the aluminum melt pool and segregate at the grain boundary of the resolidified aluminum region. In the welding case of AA6061/DP600 without hot tear, welded with copper backing plate, it showed large amount of Mg2Si segregation near the interface. Thermomechanical simulation carried out with DICTRA® for the case of AA6061 solidification under FMB solidification condition predicts that Mg2Si is formed near the interface of the FMB welds. The hot tear defects are formed for the case of AA6061/DP600 welds performed with brass backing plate, the surrounding regions of the hot tear was severely segregated with Si and Mg. According to DICTRA solidification model, in AA6061, the segregation corresponding to the hot tear zone presents about 5.8% and 6.5% of Mg and Si, respectively. In the third welding case (AA7039/DP600) the aluminum plate thickness was 17.5 mm. Therefore, no backing plate was used. EDS maps obtained at 15 kV accelerating voltage show severe segregation of Zn and Mg near the interface, while some Si was observed near the interface. Quantitative EDS were also performed for longer duration in selected zones, which determine that the segregated zone composition contains large amount of MgZn2 and Mg3Zn5 while having small amount of Mg2Si. Literature shows that Zn combines with large amount of Cu or large Mn, which cause severe segregation [4]. Since the used AA7039 has very low Cu and low Mn fractions, the hot tear susceptibility remains low in our study. Based on these observations one can clearly see that AA1050 does not have any element to segregate, also never had a hot tear defect in the weld pool. In the case of AA6061, large segregation of Mg and Si occurs near the interface. Depending on the cooling rate during solidification, the segregated zone becomes further vulnerable, and favors the formation of hot tear zone. That is, the cooling rate affects the position of final segregates, Mg and Si, thus with a slow cooling rate (in the case of steel or brass backing plate), this zone forms hot tear defects. References [1] C. van der Rest, P.J. Jacques, A. Simar, On the joining of steel and aluminium by means of a new friction melt bonding process, Scripta Materialia 77 (2014) 25-28. [2] C. van der Rest, A. Simar, P.J. Jacques, Method for welding at least two layers, Patent No. WO 2013164294 A1, 2013. [3] N. Jimenez-Mena, P. Jacques, J.-M. Drezet, A. Simar, On the prediction of hot tearing in Al-to-steel welding by friction melt bonding, Metall and Mat Trans A 49(7) (2018) 2692-2704. [4] D.G. Eskin, Suyitno, L. Katgerman, Mechanical properties in the semi-solid state and hot tearing of aluminium alloys, Progress in Materials Science 49(5) (2004) 629-711

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

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

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

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