Austenite Layer Research Articles

Effect of initial texture and microstructure of Mg on mechanical properties of Mg – Stainless steel laminated metal composites

Laminated metallic composites consisting of an AZ31 magnesium alloy layer and two SS304 austenitic stainless steel layers were fabricated by a reactive transient liquid phase bonding method to achieve a metallic composite exhibiting an exceptional combination of specific strength and ductility. Three different initial microstructures were selected for the Mg layer on the basis of initial texture and grain size distribution (hot-rolled small grains, hot-rolled large grains and hot-extruded large grains). During deformation, the overall texture of all three samples approached the same preferential orientation. Since the original texture of the hot-extruded sample was very different from the final preferential orientation, its slip systems were able to accommodate more deformation in the Mg layer before saturation. It was also found that the initial texture plays an important role in the coalescence of micro cracks initiating failure. A strong initial texture facilitates the joining of micro-cracks, which leads to premature failure considerably before the predicted diffuse necking.

Effects of Ni on austenite stability and fracture toughness in high Co-Ni secondary hardening steel

Three kinds of high Co-Ni secondary hardening steels with different Ni contents were studied. The nanoscale austenite layers formed at the interface of matensite laths were observed. Both observation and diffusion kinetic simulation results showed that both Ni and Co did not obtain enough time to get the equilibrium content in this system. The Ni content in austenite layers decreased significantly, and Co content increased slightly with the decrease of Ni content in overall composition. The austenite stability was estimated by Olson-Cohen model, in which both chemical and mechanical driving force could be calculated by equilibrium thermodynamic and Mohr's circle methods, respectively. Simulation and mechanical test results showed that the decrease of Ni content in austenite layers would cause the change of austenite stability and decrease the fracture toughness of the steels. When the Ni content in the overall composition was lower than 7 wt. %, the Ni content in y phase would be lower than 20 wt. %. And the simulation value of Msσ (stress-induced critical martensite transformation temperature) would be up to 80 °C, which was about 60 °C higher than room temperature. Based on the analysis, the Ni content in the overall composition of high Co-Ni secondary hardening steels should be higher than 8 wt. % in order to obtain a good fracture toughness.

放射光X線回折法を用いた積層型高強度鋼板におけるマルテンサイト層の局所変形挙動評価

The strain distribution in a multilayered steel composite, which consists of martensite and austenite layers, was evaluated by energy dispersive X-ray diffraction to investigate the homogeneity of deformation in the martensite layer under uniaxial loading. A dog-bone shape specimen with a martensite layer with 0.2 mm in a thickness, sandwiching by austenite layers with 0.4 mm in thickness, was utilized in this study. A change in strains as a function of tensile loading was measured at the center of the martensite layer as well as near the interface between the martensite and austenite layers. Furthermore, the residual strain distributions were measured in the martensite layer of the unloaded specimens after different applied strains. As a result, slight inhomogeneous deformation was found at the beginning of the plastic deformation in the martensite layer although no local deformation was recognized even near the interface between austenite and martensite layers. After further deformation, mismatch of plastic deformation between austenite and martensite layers was reduced, and the martensite layer deformed homogeneously. Therefore, an unstable interface phenomenon might not be caused by the intrinsic factors with a mesoscopic scale discussed in this experiment.

Characterization of the expanded austenite developed on AISI 316 LM steel by plasma nitriding

AISI 316 LM samples were plasma nitrided at a temperature of 380?C for different times between 0.5 and 8 h in a 85%N2- 15%H2 gas mixture. Different experimental techniques such as: optical microscopy (OM), X- ray diffraction (XRD ) and glow discharge optical emission spectroscopy (GDOES) were used to characterize the expanded austenite layer formed at the surface of AISI 316 LM stainless steel. The microscopical observations revealed the presence of the expanded austenite with a mean layer thickness between 1.90 and 4.31 ?m. The growth kinetics of expanded austenite was also investigated. In addition, both the compressive stresses in the expanded austenite layer and the compositional strains were estimated by means of a simple mechanical model based on the XRD results.

Effect of CH<sub>4</sub> Content on the Characteristics of Surface Layers of Low Temperature Plasma Nitrided 2205 Duplex Stainless Steel

Plasma nitriding was performed on the 2205 duplex stainless steel samples at 400 V with a gas mixture of H2 and N2 for 15 hrs with changing N2 percent, temperature and adding various amounts of CH4. After treatment the behavior of the surface layer was investigated by optical microscopy, X-ray diffraction, GDOES analysis and micro-hardness testing. Potentiodynamic polarization test was also used to evaluate the corrosion resistance of the samples. With increasing both N2 percentage from 10% to 25% and nitriding temperature from 370°C to 430°C, the thickness of nitrogen expanded austenite (S-phase) layer and surface hardness increase up to 16 μm and 1200 HV0.1 at the treatment temperature of 430°C with 25% N2, but decreases the corrosion resistance due to the formation of Cr2N and γ`(Fe, Cr)4N. Thus in order to further increase the thickness of S-phase layer and the corrosion resistance, the influence of adding various amount of CH4 (1% to 5%) in the nitriding atmosphere was investigated. Adding CH4 in the nitriding atmosphere increases the layer thickness compared with that of nitrided sample. The highest thickness can be obtained at 1 % CH4, but addition of CH4 beyond 1 % slightly decreases the layer thickness. Moreover, when nitrided at 400°C with 10% N2 and 5% CH4 content, best corrosion behavior is obtained which also have around 10 μm layer thickness and about 870 HV0.1 surface hardness.

Strong bimetallic structural material alloyed with nitrogen

The production of strong two-layer steel sheet—a bimetallic structure with a basic layer of low-carbon manganese microalloyed bainitic steel and an applied layer of two-phase austenite–ferrite stainless steel alloyed with nitrogen—is considered. In trials, the production of bimetallic material on the basis of the electrical-arc surfacing is investigated: specifically, the application of high-alloy steel to a plane microalloyed steel blank, by means of welding wire (under a flux layer), with subsequent hot deformation. The trials include the simulation of forced sheet cooling on the exit conveyer of the broad-strip mill and slow cooling of the final coiled strip. The microstructure, mechanical strength, and corrosion resistance of the bimetallic structural material are investigated. The proposed material matches the corrosion resistance of existing bimetals. Its yield point and the adhesive strength of the two layers exceed those of traditional two-layer steel by at least 30–50% and are three times the standard requirements.

Effect of Annealing Temperature and Time on Microstructure and Mechanical Properties of Multilayered Steel Composite Sheets

Multilayered composite steels consisting of alternating layers of martensitic phase and austenitic phase exhibit an excellent combination of strength and elongation compared with conventional advanced high-strength steels. The deformation processes underlying these properties are of considerable interest. In this article, microstructure, grain size, and phase are characterized by scanning electron microscopy (SEM) and electron backscattering diffraction. The hardness of each layer is analyzed by a microindentation hardness testing system. Finally, the deformation and failure processes in multilayered steel are investigated by in-situ SEM. The hardness results indicate that various hardening modes occur in the soft austenitic layer and the hard martensitic layer. In-situ SEM results combined with microstructure analysis and hardness results reveal that annealing temperature and annealing time have a significant impact on final microstructure, fracture behavior, strength, hardness, and ductility.

Formation and Growth Kinetics of Reverted Austenite During Tempering of a High Co-Ni Steel

It is well known that high Co-Ni steels exhibit excellent toughness. Since the good toughness in these steels is supposed to be related to thin layers of austenite between martensite crystals, this work presents an experimental study corroborated with diffusional calculations to characterize the evolution of reverted austenite. Atom probe measurements were conducted for analyzing the element distribution in austenite and martensite during tempering. These results were correlated with crystallographic information, which was obtained by using transmission electron microscopy investigations. Additionally, the experimental findings were compared with kinetic calculations with DICTRA™. The investigations reveal that reverted austenite formation during tempering is connected with a redistribution of Ni, Co, Cr, and Mo atoms. The austenite undergoes a Ni and Cr enrichment and a Co depletion, while in the neighboring martensite, a zone of Ni and Cr depletion and Co enrichment is formed. The changes in the chemical composition of austenite during tempering affect the stability of the austenite against phase transformation to martensite during plastic deformation and have thus decisive influence on the toughness of the material.

Decoupling the contributions of constituent layers to the strength and ductility of a multi-layered steel

Multi-layered steel (MLS) consisting of alternating soft/ductile austenitic and hard/brittle martensitic stainless steel layers is a new class of hybrid material for structural application as it offers excellent combinations of strength and ductility. In this study, the contributions of each of the constituent layers to the overall strength and ductility of an MLS (having tensile strength > 1.4 GPa and ductility > 20%) were examined by recourse to nanoindentation experiments on each of them. By adapting two different indenter tip radii for the spherical nanoindentation experiments, constituent layers' stress-strain responses within the plastic regime were obtained and then compared with the macroscopic flow curve of the MLS that was obtained through tensile tests, to show that the strength contributions of the constituent steels to the global strength of MLS is as per the rule of mixtures. In order to examine the sources of tensile ductility of the MLS, sharp tip nanoindentation experiments were conducted on specimens extracted from tensile coupons that were subjected to predetermined plastic strains a priori. Results of these experiments show that the tensile failure occurs at a strain at which hardness of the austenitic layer, which is found to be dependent on the prior-plastic strain, is almost equal to the strain-independent hardness of the martensitic layer. The results are discussed in terms of martensitic transformation within austenitic layer and the role of the mechanical environment change imposed by the neighboring martensite layers on it.

Upset Resistance Welding of Carbon Steel to Austenitic Stainless Steel Narrow Rods

Effects of welding current (at the range of 2-4 kA) on the microstructure and mechanical properties of upset resistance welds of AISI-1035 carbon steel to AISI-304L austenitic stainless steel rods were investigated. The results showed that the joint strength first increased by raising the welding current up to 3 kA and then decreased beyond it. Increasing trend was related to more plastic deformation, accelerated diffusion, reduction of defects and formation of mechanical locks at the joint interface. For currents more than 3 kA, decrease in the joint strength was mainly caused by formation of hot spots. Using the optimum welding current of 3 kA, tensile strength of the joint reached to ~76% of the carbon steel base metal strength. Microstructural observations and microhardness results confirmed that there was no hard phase, i.e., martensite or bainite, at the weld zone. Moreover, a fully austenitic transition layer related to carbon diffusion from carbon steel was observed at the weld interface.

Modification of Nanostructured Maraging Steels Surface with Atmospheric Nitrogen at Steel Hardening in Confined Space

The specific features of atmospheric nitrogen surface nitriding of cold-rolled foils (the value of the coherent-scattering region is 10 nm) from stainless maraging steels after hardening in a confined space containing air are considered. Thermodynamic evaluation of the probability of influence caused by the confined space air on the phase transformations and solubility of the atmospheric nitrogen shows that the surface modification takes place in two stages: at the first stage, oxidation dominates; and at the second stage, predominantly the nitrogen absorption by the surface takes place. While changing, the nitrogen solubility reaches the maximum in the range of 10 – 22 % of Cr and has the largest values at the air pressure in the range of 0.001 – 0.04 MPa. Based on the nitrogen solubility diagrams, optimal conditions for the nitrogen austenite formation without the formation of nitrides are established. The developed method for nitriding allows the formation of nitrogen austenite layer of about 20 μm in thickness in the surface of nanostructured maraging steels without deteriorating the surface electrochemical properties.

Mixed-mode thermal fracture of AISI 304 stainless steel with temperature-dependent material properties

Mixed-mode thermal fracture of cracked AISI 304 austenitic stainless steel layers under severe thermal gradients of cryogenic and elevated temperatures is studied. Taking into account the variation of thermo-mechanical properties with temperature, the J k -integral method, incorporating temperature-dependent material properties, is used to determine mixed-mode stress intensity factors from the results of finite element (FE) analysis. Effects of the convection heat transfer coefficient and the temperature of the contacting fluid on the mixed-mode fracture of the steel layers are investigated and it is shown that the mixed-mode stress intensity factors increase nonlinearly with these parameters. Results indicate that for accurate determination of crack tip fracture parameters when severe thermal gradients are present in the material, it is necessary to consider the variation of thermo-mechanical properties with temperature.

Generating duplex microstructures by nitriding; nitriding of iron based Fe–Mn alloy

Nitriding of Fe–2 at-%Mn alloy at 650°C employing a nitriding potential of 0·05 atm− 1/2 resulted in a highly complex microstructural development as a function of depth below the specimen surface: a surface adjacent layer exhibiting an austenite–martensite duplex microstructure, followed by an intermediate region showing a ferrite–austenite duplex microstructure, and at even larger depths a region where an austenite layer covers the grain boundaries of the ferrite matrix grains. Development of this complex microstructure is attributed to the strong austenite stabilising effects of Mn and N. This work demonstrates the power of a relatively simple nitriding treatment to realise a highly complex microstructure known to be associated with strongly enhanced mechanical properties.

Liquid phase sintering-based diffusion bonding of Ti(C,N)-based cermet and steel

Sound joint of Ti(C,N)-based cermets and steel is achieved by liquid phase sintering of cermet green compact and steel, in which external pressure and interlayer metals are not required. The bonding process, joint microstructure, and properties are investigated. Liquid phase sintering occurs and typical core/rim structure forms in the cermet during diffusion bonding at 1593 K. Full densification of the cermet can be achieved after bonding for 2 h, the relative density, porosity, and grain size do not vary remarkably with the dwelling time during bonding. Bonding zones are composed of an austenite phase-based layer in the cermet side and a martensite phase-based layer in the steel side. There exists a Ni-enriched layer in the vicinity of cermet/bonding zone interface, which forms due to the outward diffusion of N and inward diffusion of Ti on the surface of cermet. The bonding zone thickness follows a parabolic relationship with the dwelling time. The microhardness of the bonding zones increases from the Ni-enriched layer, austenite layer to the martensite zone. The maximum value of residual stress locates in the bonding zone adjacent to the cermet, and the maximum residual stress value decreases with the dwelling time from the simulation results. Due to the existence of the Ni-enriched ductile bonding layer, a reliable joint can be achieved.

Cladding of low-carbon steel to austenitic stainless steel by hot-roll bonding: Microstructure and mechanical properties before and after welding

The aim of this contribution was to study the performance of cladded low-carbon steel (ASTM A283 Grade C) to austenitic stainless steel (ASTM A240 type 316/AISI 316) using hot-roll bonding. The mechanical and metallurgical properties of the cladded steel have been investigated before and after welding. Three weld samples with different configurations are prepared from cladded materials and, then, these joined metals were studied.Results on the microstructure, microhardness and mechanical strength were reported. Experimental studies showed that austenitic stainless steel AISI 316 (clad layer) could be cladded to low-carbon steel A283 (parent metal) with a good quality of bonding by hot-roll bonding. The bond interface of the cladded metals indicated that there is an intermediate zone between the clad layer and parent metal with heterogeneous microstructures due to the carbon diffusion. It was also found that microhardness measured from the clad layer to the parent metal showed an abrupt change adjacent to the interface due to a coarse grained soft ferritic band near the interface with larger microhardness than the adjacent metals. Tensile results revealed that the bonding was acceptable in all weld samples. It was also observed in bending tests that interfaces of bonded samples were safely carried out for some welding configurations. Toughness strength of the cladded metals at a given test temperature was found significantly higher than that of parent plate alone because of the high impact toughness of austenitic stainless steel layer. Consequently, mechanical properties of the low-carbon steel can be improved by hot-roll bonding with austenitic stainless steel.

Prevention of Corrosion-Resistant Austenitic Steel Cladding Layer Surface Defects

Reasons are studied for surface defect formation (rolling crystallization cracks) for a bimetallic rolled product cladding layer of austenitic class corrosion-resistant steel prepared by electroslag welding. An unfavorable effect of liquation during welded layer steel crystallization is established for surface quality. Possibilities and examples are established for preventing the occurrence of surface defects by optimizing welded steel chemical composition, and also bimetallic rolled product start-to-finish manufacturing technology parameters.

Microstructure characteristics of high borated stainless steel fabricated by hot-pressing sintering

The present study investigated the microstructure of powder metallurgy (P/M) high borated stainless steel through hot-pressing sintering in a temperature range of 1000–1150 °C within 30 min under 30 MPa. Microstructure and phase examinations were carried out by applying scanning electron microscope, electron backscatter diffraction and X-ray diffraction analysis. The results of as-atomized powders demonstrated that many powders kept egg-type structure with an austenite outer layer and the eutectic borides were much finer than those in traditional cast products. Microstructure studies revealed that borides suffered Ostwald ripening and were significantly influenced by the sintering temperature. Orientation maps indicated that the inter-particle contact areas consisted of equiaxed grains and the regions consisting of large elongated grains partly inherited the microstructure characteristics of as-atomized powder particles. Furthermore, the mechanisms governing the morphological changes in microstructure were discussed.

Reduced crystal plasticity for materials with constrained slip activity

In a number of materials, plasticity occurs along preferential directions or slip systems, while other directions barely contribute to deformation. This can occur due to the specific crystalline nature of the materials (e.g. in polymers) or due to the morphology of the crystalline material itself, like in metal laminates. In the latter case, when the layers are very thin, the surrounding material acts as a constraint and only preferential slip directions are activated. This observation suggests the reduction of the underlying full crystal plasticity model within those regions to a more computationally efficient model which still retains the main deformation mechanism, i.e. plasticity occuring along a few slip systems only. In this paper we propose such a reduced crystal plasticity model in a finite deformation setting. In the limit of either no active slip system or five linearly independent slip systems, the model reduces to isotropic plasticity and standard crystal plasticity, respectively. The model is validated on a specific case, i.e. lath martensite microstructures consisting of alternating crystalline layers of martensite and austenite. The characteristic material behaviour (i.e. stress-strain response and slip activity on the most active slip systems) is correctly reproduced by the reduced model at a significantly lower computational cost compared to a fully resolved crystal plasticity model.

Multi-scale simulation of hydrogen influenced critical stress intensity in high Co–Ni secondary hardening steel

Abstract Hydrogen embrittlement was an important and long-standing problem in the fields of steels, especially ultra-high strength steels. In order to simulate the ability of hydrogen embrittlement resistance for high Co–Ni secondary hardening steels, a multi-scale simulation method with four steps was used to calculate the critical stress intensity (K IC ) and hydrogen influenced critical stress intensity (K ISCC ). For the four steps: the atomic scale and nm scale simulation were mainly used to simulate the effect of stress-assisted hydrogen diffusion at the crack tip; the μm scale simulation was used to handle the effect of microstructure; the cm simulation was used to analyze the size effect. As the effect of hydrogen concentration at the crack tip, the simulation results of critical cohesive strength of the Fe(110) at the crack tip decreased by 82.3%. The μm scale simulation showed the improvement of fracture toughness with the help of austenite layer between martensite laths. Compared with the mechanical properties of 300 M and AerMet100 steels, the accuracy of this simulation method was proved.

Analysis of fracture toughness in high Co–Ni secondary hardening steel using FEM

The microstructure and mechanical properties of ultrahigh strength steels (300M, Aermet100 and M54) were analyzed. The experiment results showed that the steels with austenite layers at the boundary of martensite laths had higher KIC than those without austenite layers. Finite element method (FEM) was used to establish models based on the actual microstructure of the steels. Extended finite element method (XFEM) was used to study the crack path and crack growth rate. Contour integral method (CIM) was used to study the best thickness of austenite layer, the effect of crack propagation direction and the value of KIC. According to the simulation results, 10–15nm was the best thickness of austenite layer for the fracture toughness of the steels with martensite as matrix. The simulation results of normalized value of KIC were close to the experimental results.