Austenitic Steel Matrix Research Articles

A study on fatigue damage and crack initiation in austenitic steel matrix during very high cycle fatigue

Fatigue damage and crack initiation behavior has been studied in an austenitic stainless steel using a novel progressive stepwise load increasing method with a cycle step greater than 108 cycles and investigated with a FIB-SEM. Subsurface crack origin with a fine granular area in the matrix was formed. Heterogeneous plastic deformation causes strain localization and grain fragmentation. Localized plasticity exhaustion induces crack initiation. A crack origin is completed once a short crack reaches the stress intensity factor threshold for stage II crack propagation. This study provides a fundamental discussion how damage and crack initiation in matrix occur during very high cycle fatigue.

(Fe,Mn)3AlCx κ-carbide formation and characterization in pack aluminization of Fe–29Mn–9Al–0.9C lightweight steel

The Fe–Mn–Al–C lightweight steel has been widely studied due to its higher strength-to-weight ratio and potential applications for automotive vehicles, energy, transportation and mining industries. In general, the strength and ductility of the Fe–Mn–Al–C lightweight steel can be improved by the formation of nano-sized (Fe,Mn)3AlCx κ-carbide precipitates by a proper heat treatment below 800 °C. In this work, a unique micrometer scale κ-carbide phase transformation phenomenon induced by the pack aluminization in the Fe–29Mn–9Al–0.9C lightweight steel was firstly reported.Extremely large columnar-like (Fe,Mn)3AlCx κ-carbide grains with average length of 18.1 μm embedded in the Fe3Al matrix were discovered in the interdiffusion zone of aluminization layer, which were totally different from these nanoscale κ-carbides precipitated in the austenitic steel matrix. The increased Al content in the pack aluminide coating/substrate interface reduced the chemical potential of carbon and became the driving force for carbon diffusion toward the coating/substrate interface. Therefore, due to the extremely low solubility of carbon in the Fe3Al phase and the inward diffusion of carbon atoms, very long columnar-like (Fe,Mn)3AlCx κ-carbide phases were thus produced in the interdiffusion zone. The average chemical concentration of the κ-carbide was 43.9Fe–27.3Mn–20.3Al–8.5 C (in at.%). The average hardness, elastic modulus and the lattice parameter of κ-carbide were 8.8 ± 0.3 GPa, 285 ± 10 GPa and 0.3764 nm, respectively. This study provided detailed chemical compositions, hardness and elastic modulus of (Fe,Mn)3AlCx κ-carbide phase for better understanding the carbide precipitation and strengthening mechanism of Fe–Mn–Al–C lightweight steel.

Direct energy deposition of high strength austenitic stainless steel matrix nanocomposite with superior ductility: Microstructure, tensile properties, and deformation behavior

Direct energy deposition (DED) was used to additively manufacture 22–13-5 austenitic stainless steel (SS), of which the mechanical properties and corrosion resistance are superior to those of SS316L and SS304L. Additionally, in-situ formed oxide-driven strengthening was utilized in this study. The DED-processed austenitic SS matrix nanocomposite (SSMNC) exhibited unique microstructural features of heterogeneous grains and dislocation networks. The nano-sized precipitates existed at the sub-structure boundaries and decorated the dislocation network. The results of the transmission electron microscopy (TEM)-energy loss spectroscopy (EELS) analyses revealed nano-sized precipitates with an average size of 21.1 nm that were identified as (Mn,Cr)-rich oxides. This means that, during the DED process, the oxygen in the powder feedstock transformed into nano-sized oxide particles by rapid solidification. The DED-processed SSMNC revealed a yield strength of 705.4±5.3 MPa, which is higher than those of reported AM-processed stainless steels. In addition, the elongation-to-failure was measured as 41.8±3.5%. This suggests that the DED-processed SSMNC has an excellent combination of strength and ductility at room temperature. The high ductility of the alloy developed in this work was found to be achieved by a twinning-induced plasticity (TWIP) mechanism that operated during the deformation. Based on the above findings, the relationships between the microstructure, mechanical properties, and deformation mechanism are also discussed.

Studying the Damage Evolution and the Micro-Mechanical Response of X8CrMnNi16-6-6 TRIP Steel Matrix and 10% Zirconia Particle Composite Using a Calibrated Physics and Crystal-Plasticity-Based Numerical Simulation Model

The mechanical behavior of newly developed composite materials is dependent on several underlying microstructural phenomena. In this research, a periodic 2D geometry of cast X8CrMnNi16-6-6 steel and 10% zirconia composite is virtually constructed by adopting microstructural attributes from literature. A physics-based crystal plasticity model with ductile damage criterion is used for defining the austenitic steel matrix. The zirconia particles are assigned elastic material model with brittle damage criterion. Monotonic quasi-static tensile load is applied up to 17% of total strain. The simulation results are analyzed to extract the global and local deformation, transformation, and damage behavior of the material. The comprehensively constructed simulation model yields the interdependence of the underlaying microstructural deformation phenomena. The local results are further analyzed based on the interlocked and free regions to establish the influence of zirconia particles on micro-mechanical deformation and damage in the metastable austenite matrix. The trends and patterns of local strain and damage predicted by the simulation model results match the previously carried out in-situ tensile tests on similar materials.

A modified model for the prediction of yield strength of nano-ZrO2 particle-reinforced austenitic steel matrix nanocomposites

A predictive analytical model has been modified to estimate the yield strength of austenitic-steel-based nanocomposites reinforced with nano-ZrO2 particle. The effective volume ratio of the reinforcement particles that actively contribute to strengthening mechanisms and the negative influence of porosity have been integrated into the predictive models. The results obtained using the modified quadratic summation model were in quite good agreement with the experimental results. Mechanical properties of austenitic steel could be significantly improved by nano-ZrO2 particles. Compared to deformation-induced strengthening mechanisms, particle-strengthening mechanisms due to nano-ZrO2 particles have had a much considerable impact on improving the strength of austenitic-steel/ZrO2 nanocomposites. All results indicated that the effective volume ratio and porosity should be considered in the predictive models to estimate the mechanical properties of austenitic-steel/ZrO2 nanocomposites more reliable.

Selective laser melting of low-content graphene nanoplatelets reinforced 316L austenitic stainless steel matrix: Strength enhancement without affecting ductility

Metal matrix composites reinforced by carbon nanostructured material are beneficial in achieving a balance between strength and ductility. However, achieving this balance in stainless steel through the selective laser melting (SLM) process remains a challenge. In this study, SLM is used to fabricate 316 L stainless steel matrix composites, which are reinforced with 0.1 wt.% single-layer graphene nanoplatelets (GNPs). The optimal process parameters are obtained by combining the analyses of the relative density and mechanical properties. The results show that the GNPs in the 316 L/GNPs composites are dispersed at the grain boundary, either parallel or at a certain angle to the grain boundary. Based on the tensile test, the ultimate tensile strength of the 316 L/GNPs composites is 738 MPa, which was 17.9 % higher than that of 316 L. Furthermore, both SLM-printed 316 L and 316 L/GNPs composites have an elongation of ∼38 %. The interaction between the GNPs and the molten pool, and the redistribution of the GNPs leads to the formation of a lamellar and relatively high-density distribution area of GNPs in the molten pool. The strengthening mechanism of 316 L/GNPs composites is also elucidated. These results provide insights into enhancing the combined strength-ductility property of other SLM-printed GNPs reinforced metal matrix systems.

Wear behaviour of Cr3C2–Ni cermet reinforced hardfacings

Submerged arc welding (SAW), plasma transferred arc welding (PTAW), and tungsten inert gas welding (TIG) technologies were used to produce austenitic stainless steel (SS) and nickel alloy (Ni) matrix based hardfacings without reinforcement and reinforced by Cr3C2–Ni cermet. As a substrate material normalized structural steel S355 has been chosen. Mechanical properties of produced hardfacings were assessed in terms of hardness (HV/30) and wear resistance tests. The latter property was analysed under different wear conditions: two-body (ASTM G132) and three-body (ASTM G65) abrasive wear tests. Microstructure and phase evolution were analysed by scanning electron microscopy (SEM) and X-ray diffraction analysis (XRD), respectively. Cermet particles in all the conducted tests proved its’ selection – hardness and wear resistance of the reinforced hardfacings substantially increased. The addition of Cr3C2–Ni cermet increased the hardness twice on average in all utilized technologies leading to the higher wear resistance in further tests. Tendency of wear rate reduction is less expressed in SAW hardfacings, but in PTAW and TIG cases the strengthening effect is obvious: 8.6 and 8.1 times higher wear resistance of SS and Ni based PTAW hardfacings, respectively, and 7.5 and 4.8 times higher for TIG hardfacings. XRD analysis supported wear tests results proving the formation of the complex compounds.

Development of thermo-mechanical processing to form high density of uniformly distributed nanosized carbides in austenitic stainless steels

Model alloys and thermomechanical processing (TMP) were developed to form a high density of uniformly distributed nanosized (<10 nm) carbides in an austenitic stainless steel matrix. The composition of the model alloys was determined to control the formation of precipitates during the TMP. The TMP consisted of homogenization at 1200 °C for 1 h, multiple hot-rolling passes above and below the non-recrystallization temperature (TNR), and precipitation heat treatment at 800 °C for 2 h. By proper division of the rolling passes above and below TNR, a microstructure with a homogeneous grain size and a well-developed dislocation structure was obtained. Then, subsequent precipitation heat treatment produced a microstructure containing a high density (~1.1 × 1022/m3) of uniformly distributed and coherent NbC precipitates in the austenitic matrix. The evolution of the nanosized NbC during the precipitation heat treatment was coupled with the dislocation characteristics, such as the density and distribution, which were controlled by the hot-rolling conditions.

Microstructure and Microhardness of ZrO2 Reinforced PM 316L Austenitic Stainless Steel Composites Sintered in Ambient and Argon Atmosphere

The present study examines the microstructure and microhardness of zirconia (ZrO2) reinforced PM 316L austenitic stainless steel matrix composites. ZrO2 was added in a proportion of 1 wt% to 3 wt%. Powders were compacted into a disc of 11mm diameter and 2mm thick at 70kN. Compacts were sintered in an ambient and argon atmosphere at 1250oC for 30 minutes. Sintered compacts were then analyzed for microhardness using Vickers hardness testing machine, and microstructure was examined using a scanning electron microscope. The study revealed that the reinforcement of ZrO2 significantly enhanced the microhardness of PM 316L SS matrix composites with a microstructure consisting of irregular porosity and zirconia encapsulating the 316L SS particles.

Additive manufacturing of ultrafine-grained austenitic stainless steel matrix composite via vanadium carbide reinforcement addition and selective laser melting: Formation mechanism and strengthening effect

Selective Laser Melting (SLM) was applied to Additive Manufacturing (AM) of carbide ceramic particulate reinforced 316 L stainless steel matrix composites. The dense and ultrafine-grained Austenite matrix with uniformly distributed and nano-sized VCx particulate reinforcements were obtained according to the tailored SLM process parameters and 3 wt% submicron-sized V8C7 ceramic particle addition into 97 wt% 316 L powder via ball-milling treatment. The influences of SLM process parameters on the SLM printed composites in terms of density, surface roughness, phase constitution, microstructure characteristics and mechanical tensile performance were investigated in detail, comparing with the SLM printed 316 L in absence of vanadium carbides. Based on the electron back-scattered diffraction analyses, the number of grains with grain size less than 2 µm accounted for more than 90% in the printed composites. The formation mechanisms of the printed composites with their unique microstructure characteristics were elaborated. The room-temperature ultimate tensile strength of the printed 316 L matrix composites achieved higher than 1400 MPa, which was more than twice as high as that of printed 316 L stainless steel. It contributed substantially to the strengthening effect on the printed composites of the ultrafast laser-induced and highly nonequilibrium melting-solidification nature of SLM process, the dispersed carbide ceramic particulate reinforcements, the nanoscaled nucleation sites of VCx for the equiaxed Austenite grains, and the inhibition against Austenite matrix growth by particulate dispersedly distributed along Austenite grain boundaries.

Molecular dynamics modeling of helium bubbles in austenitic steels

Abstract The austenitic steel devices from pressurized water reactors are continuously subjected to neutron irradiation that produces crystalline point defects and helium atoms in the steel matrix. These species evolve into large defects such as dislocation loops and helium filled bubbles. This paper analyzes, through molecular dynamics simulations with recently developed interatomic potentials, the impact of the helium/steel interface on the helium behavior in nanosize bubbles trapped in an austenitic steel matrix. It is shown that the repulsive helium-steel interactions induce higher pressures in the bubble compared to bulk helium at the same temperature and average density. A new equation of state for helium is proposed in order to take into account these interface effects.

Stainless steel matrix composites reinforced with ceramic particles through ingot casting process

The aim of the study was to assess the influence of adding Al2O3 nano-particles of 0.5 wt. % with the mean particle size of 500 nm on the mechanical properties and wear behaviour of the austenitic stainless steel matrix reinforced with nano-particles produced by conventional ingot casting. The focus was on the methods and possibilities of homogeneous and uniform distribution of the particles within the steel matrix using conventional casting routes. The main drawback of the casting method used is the agglomeration of the particles and poor interface between the particles and the metal matrix. The results show that through a proper insertion method, nano-particles can be successfully introduced into the metal matrix. The Al2O3 nano-particles were successfully incorporated into the steel matrix with no signs of clustering and intermetallic reactions taking place between the nano-particles and the steel matrix. This led to improved mechanical properties as well as the wear behaviour of the stainless steel, achieved by using conventional casting routes.

Effect of Laser Shock Peening on the Microstructures and Properties of Oxide‐Dispersion‐Strengthened Austenitic Steels

Oxide‐dispersion‐strengthened (ODS) austenitic steels are promising materials for next‐generation fossil and nuclear energy systems. In this study, laser shock peening (LSP) has been applied to ODS 304 austenitic steels, during which a high density of dislocations, stacking faults, and deformation twins are generated in the near surface of the material due to the interaction of laser‐driven shock waves and the austenitic steel matrix. The dispersion particles impede the propagation of dislocations. The compressive residual stress generated by LSP increases with successive LSP scans and decreases along the depth, with a maximum value of −369 MPa. The hardness on the surface can be improved by 12% using LSP. In situ transmission electron microscopy (TEM) irradiation studies reveal that dislocations and incoherent twin boundaries induced by LSP serve as effective sinks to annihilate irradiation defects. These findings suggest that LSP can improve the mechanical properties and irradiation resistance of ODS austenitic steels in nuclear reactor environments.

Influencia de la presión de compactación en compuestos de acero AISI 316 reforzados con carburo de titanio frente al desgaste

In the manufactured materials with the powder metallurgy technique, it is important to analyse the different manufacturing conditions that lead to get a better behaviour of the metallic matrix composite (MMC) against the wear, since it is one of the main industry problems largely affecting part of the production sectors. In this study the stainless steel AISI 316 was selected as a matrix reinforced at 3, 6 and 9% Vol of titanium carbide (TiC). The effect of the compaction pressure (700 – 800 MPa) and the reinforcement percentage were evaluated. The sintering temperature (1200±5°C) is reached by means of the generation of an abnormal glow discharge of direct current that transfers electric energy to a gaseous medium, generating an electric discharge between the electrodes (anode and cathode, where the sample is located) in an atmosphere of H2 – N2 and with a residence time of 30 minutes. The investigation results evidence that the ceramic particles incorporation (TiC) in an austenitic steel matrix (AISI 316) demonstrates a significant improvement in the wear resistance, getting the best behaviour when the composite is compacted at 800 MPa and with a reinforcement content of 6%. In these manufacturing conditions there is a good interaction in the matrix and the reinforcement, low porosity, superior densification with the highest hardness and lowest friction coefficient.

Development of a sintering methodology through abnormal glow discharge for manufacturing metal matrix composites

In this study, a sintering methodology is presented by using abnormal glow discharge to metal matrix composites (MMC), consisting of 316 steel, reinforced with titanium carbide (TiC). The wear behaviour of these compounds was evaluated according to the standard ASTM G 99 in a tribometer pin-on-disk. The effect of the percentage of reinforcement (3, 6, and 9%), with 40 minutes of mixing in the planetary mill is analysed, using compaction pressure of 700MPa and sintering temperature of 1,100°C±5°C, gaseous atmosphere of H2 - N2, and sintering time of 30 minutes. As a result of the research, it shows that the best behaviour against wear is obtained when the MMC contains 6% TiC. Under this parameter the lowest percentage of pores and the lowest coefficient of friction are achieved, ensuring that the incorporation of ceramic particles (TiC) in 316 austenitic steel matrix significantly improves the wear resistance. Also, it is shown that it is possible to sinter such materials using the abnormal glow discharge, being a novel and effective method in which the working temperature is reached in a short time.

Corrosion-fatigue properties of plasma-based low-energy nitrogen ion implanted AISI 304 L austenitic stainless steel in borate buffer solution

Abstract AISI 304 L austenitic stainless steel was modified by using the plasma-based low-energy nitrogen ion implantation (PBLEII) at a process temperature of 400 °C for a processing time of 4 h in order to improve the corrosion-fatigue resistance of the austenitic stainless steel. A single high-nitrogen face-centered-cubic phase (γN) layer with a maximal nitrogen concentration of about 25 at.% was formed on the nitrogen-modified austenitic stainless steel. Compared with the original austenitic stainless steel, the γN phase layer on the austenitic stainless steel possessed a significant improvement in corrosion resistance in the borate buffer solution with a pH value of 8.4. The corrosion-fatigue properties of the γN phase layer on the austenitic stainless steel were examined by the push–pull fatigue experiments with a ratio R of tensile and compression of − 1 in the borate buffer solution. The γN phase layer has an increased corrosion-fatigue strength up to 230 MPa from 180 MPa of the original austenitic stainless steel with an apparent increase of about 28%. The corrosion-fatigue crack initiation in the γN phase layer was found as a controllable stage in the fracture process at the interface between the γN phase layer and the austenitic stainless steel matrix with the arc corrosion-fatigue source. Some tiny corrosion-fatigue striations were obtained on the corrosion-fracture surfaces of the γN phase layer. The high density of slip bands and dislocations in the γN phase layer was able to prevent the crack initiation and propagation, leading to improvement of the corrosion-fatigue properties in the borate buffer solution.

Corrosion Behavior of TiC and (Ti,W)C-Reinforced Fe-17Mn and Fe-17Mn-3Al Austenitic Steel Matrix In Situ Composites

The corrosion behavior of Fe-17Mn and Fe- 17Mn-3Al austenitic steel matrix composites reinforced with TiC and (Ti,W)C particles, during exposure to a 3.5% NaCl aqueous solution has been reported. The corrosion behavior of these composites has been evaluated by potentiodynamic polarization curves and linear polariza- tion resistance measurements at a scan rate of 1 mV/s at room temperature (25 ± 2 � C). Tafel extrapolation tech- nique has been used in calculating the corrosion current density (Icorr) and corrosion potential (Ecorr). It has been observed that the corrosion resistance of the composites is less than that of their corresponding unreinforced matrix materials. The corrosion resistance increases with the addition of aluminum. The corrosion resistance of (Ti,W)C-reinforced composite is more than that of TiC- reinforced composite in the case of Fe-17Mn matrix while the same is not very different in the case of TiC and (Ti,W)C-reinforced Fe-17Mn-3Al austenitic steel matrix composites.

Buckling and crush resistance of high-density TRIP-steel and TRIP-matrix composite honeycombs to out-of-plane compressive load

The mechanical and structural responses of high-density TRIP steel and TRIP-steel/zirconia composite honeycomb structures were studied under uniaxial compression in the out-of-plane loading direction over a wide range of strain rates. Their mechanical response, buckling, and failure mechanisms differ considerably from those of conventional thin-walled, low-density cellular structures. Following the linear-elastic regime and the yield limit of the bulk material, the high-density square honeycombs exhibited a uniform increase in compression stress over an extended range of (stable) plastic deformation. This plastic pre-buckling stage with axial crushing of cell walls correlates with the uniaxial compressive response of the bulk specimens tested. The dominating material effects were the pronounced strain hardening of the austenitic steel matrix accompanied by a strain-induced α’-martensite nucleation (TRIP effect) and the strengthening effect due to the zirconia particle reinforcement. The onset of critical plastic bifurcation was initiated at high compressive loads governed by local or global cell wall deflections. After exceeding the compressive peak stress (maximum loading limit), the honeycombs underwent either a continuous post-buckling mode with a folding collapse (lower relative density) or a symmetric extensional collapse mode of the entire frame (high relative density). The densification strain and the post-buckling or plateau stress were determined by the energy efficiency method. Apart from relative density, the crush resistance and deformability of the honeycombs were highly influenced by the microstructure and damage evolution in the cell walls as well as the bulk material’s strain-rate sensitivity. A significant increase in strain rate against quasi-static loading resulted in a measured enhancement of deformation temperature associated with material softening. As a consequence, the compressive peak stress and the plastic failure strain at the beginning of post-buckling showed an anomaly with respect to strain rate indicated by minimum values under medium loading-rate conditions. The development of the temperature gradient in the stable pre-buckling stage could be predicted well by a known constitutive model for quasi-adiabatic heating.

Numerical Model of Machining Considering the Effect of MnS Inclusions in an Austenitic Stainless Steel

Austenitic stainless steels are generally difficult to machine when compared to carbon or low alloy steels. The presence of Cr, Ni, and Mo raises the temperature in the flow zone during machining. These steels are strongly work hardening, and usually bond to the tool forming a build-up-edge. The increase of the tool life and better removal rates can be achieved through the use of free cutting austenitic stainless steels. In these steels, sulfide and manganese can be added to form sulfide inclusions (MnS) that can induce stress concentration in the deforming material and facilitate the chip fracture. Finite element method simulation of the machining was carried out to investigate the influence of the inclusions in the machinability of the austenitic stainless steel. The simulation considers that the microstructure of the steel has two main constituents; the matrix of austenitic stainless steel and inclusions. Two simulation groups were carried out, the first one analyzing the machining cutting process of the workpiece and tool, and the second one was performed in a small area corresponding to a region inside the primary shear zone, where the stress concentrator effect of the inclusions in the austenitic stainless steel matrix was analyzed. This procedure was repeated for microstructures with different inclusions size, volume fraction and morphologies. The results obtained for the stress concentrations and strain near the inclusions showed that inclusions with elongated shapes usually give higher stresses than circular ones. Additionally,it was found that increasing the size of inclusions also resulted in higher stresses.

Corrosion Behaviour of TiC-Reinforced Hadfield Manganese Austenitic Steel Matrix In-Situ Composites

The corrosion behaviour of Hadfield manganese austenitic steel matrix composite reinforced with the varying amount of TiC and unreinforced Hadfield manganese austenitic steel matrix alloy has been evaluated in 3.5% NaCl aqueous solution with the pH value of 6 by the potentiodynamic polarization curves and linear polarization resistance measurements at a scan rate of 1 mV/s at room temperature (25°C ± 2°C). The corrosion rate of the composites is higher than that of their unreinforced matrix alloy and it increases with the increasing volume fraction of TiC. The poor corrosion resistance of the composites can be attributed to the galvanic effects between the matrix and reinforcement.