Martensite Layer Research Articles

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.

Microstructural and hardness investigations on a dissimilar metal weld between low alloy steel and Alloy 82 weld metal

The investigation on microstructure and hardness at the fusion boundary (FB) region of a dissimilar metal weld (DMW) between low alloy steel (LAS) A508-III and Alloy 82 weld metal (WM) was carried out. The results indicated that there were two kinds of FBs, martensite FB and sharp FB, with obvious different microstructures, alternately distributed in the same FB. The martensite FB region had a gradual change of elemental concentration across FB, columnar WM grains with high length/width ratios, a thick martensite layer and a wide heat affected zone (HAZ) with large prior austenite grains. By comparison, the sharp FB region had a relatively sharp change of elemental concentration across the FB, WM grains with low length/width ratios and a narrow HAZ with smaller prior austenite grains. The martensite possessed a K-S orientation relationship with WM grains, while no orientation relationship was found between the HAZ grains and WM grains at the sharp FB. Compared with sharp FB there were much more Σ3 boundaries in the HAZ beside martensite FB. The hardness maximum of the martensite FB was much higher than that of the sharp FB, which was attributed to the martensite layer at the martensite FB.

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.

Effect of the amount of CH4 gas on the characteristics of surface layers of low temperature plasma nitrided martensitic precipitation-hardening stainless steel

Abstract Plasma nitriding was performed on AISI 630 martensitic precipitation hardening stainless steel samples at 400 V with a gas mixture of H 2 and N 2 for 19 h with changing N 2 percentage, temperature and adding various percentages of CH 4 . After treatment the behavior of the surface layer was investigated by optical microscopy, X-ray diffraction, GDOES analysis and micro-hardness testing. A potentiodynamic polarization test was also used to evaluate the corrosion resistance of the samples. With increasing N 2 percentage from 15% to 35% at 380 °C, the thickness of the expanded martensite (α′ N ) layer increases to about 20 μm and surface hardness also increases to about 1280 HV 0.05 but the corrosion behavior is worse than the untreated sample. When nitrided with 25% N 2 , increasing nitriding temperature from 380 °C to 460 °C also increases the α′ N layer thickness to around 38 μm and surface hardness to around 1270 HV 0.05 (at 460 °C), but decreases the corrosion resistance. Thus in order to improve the corrosion resistance, various amounts of CH 4 (0% to 6%) were introduced in the nitriding atmosphere at 400 °C with 25% N 2 percentage. Increasing CH 4 percentage slightly decreases the α′ N layer thickness but greatly improves corrosion behavior. Moreover, with 4% CH 4 percentage, the best corrosion behavior is obtained which also has around 11 μm α′ N layer thickness and about 1290 HV 0.05 surface hardness. Therefore, the conclusion can be drawn that, the highly suitable condition for plasma nitriding for a thicker expanded martensite layer, higher surface hardness and good corrosion resistance is plasma nitriding at 400 °C with 25% N 2 and 4% CH 4 for 19 h.

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.

High-Strength Steel Hot Forming Mechanics Performance and Characteristic Experimental Study

In high-strength steel hot forming, under the heating and quenching interaction, the material is oxidized and de-carbonized in the surface layer, forming a gradual change microstructure composed of ferrite, ferrite and martensite mixture and full martensite layers from surface to interior. The experiment enunciation: Form the table to ferrite, ferrite and martensite hybrid organization, completely martensite gradual change microstructure,and make the strength and rigidity of material one by one in order lower from inside to surface, ductility one by one in order increment in 22MnB5 for hot forming;Changes depends on the hot forming process temperature and the control of reheating furnace gas content protection, when oxygen levels of 5% protective gas, can better prevent oxidation and decarburization;Boron segregation in the grain boundary, solid solution strengthening, is a major cause of strength increase in ;The gradual change microstructure in outer big elongation properties, make the structure of the peak force is relatively flat, to reduce the peak impact force of structure, keep the structure of high energy absorption capacity;With lower temperature, the material yield strength rise rapidly,when the temperature is 650 °C, the yield strength at 950 °C was more than 3 times as much.

Numerical Simulation of Solid–Solid Phase Transformations During Induction Hardening Process

A multiscale multiphysical finite-element (FE) analysis is presented in this paper to predict the microstructural evolution during induction hardening processes. An ad hoc code has been developed in order to calculate the metallurgical phase changes that occur during heating and cooling steps. This routine has been coupled with an FE method code able to solve the coupled electromagnetic and thermal problem that typically describes the induction heating processes. In induction contour hardening of steels, an external layer of the piece is heated up to the austenitization temperature, and then it is cooled down to obtain a layer of martensite. In the thermo-metallurgical model, material properties depend on the temperature distribution but also on the microstructure, since the material is a mixture of different phases. From the solution of the coupled steady state, at a given frequency, electromagnetic and transient thermal problem, temperature distribution, as well as heating and cooling rates are used for the evaluation of the existing metallurgical phases at every time step. The effect of the latent heat of solid–solid phase transformations has also been considered. The numerical results are compared with the experimental ones.

Metallurgical characterization of coupled carbon diffusion and precipitation in dissimilar steel welds

The complex microstructures developed during post-welding heat-treatment in the vicinity of the fusion line between a ferritic and austenitic steel were examined in the case of submerged arc welded 18MND5/309L dissimilar joints. Quantitative measurements of the carbon distribution in the as-welded and post-weld heat-treated conditions were performed by both wavelength dispersive spectrometry and secondary ion mass spectrometry. The extent of carbon diffusion was confirmed by hardness profiles performed by nanoindentation. On the low-alloy ferritic side, decarburization resulted in cementite dissolution allowing the evolution of the bainitic structure toward a large-grained ferritic region. In the weld metal, the carbon content reached unusually high levels and an intense precipitation of chromium-rich carbides was observed in both the interfacial martensitic layer and the austenitic weld metal. The evolution of the precipitation as a function of the distance from the interface was analyzed in terms of crystallography, chemistry, volume fractions, and size distributions. Automated crystal orientation mapping in a transmission electron microscope allowed identification of the precipitates extracted on carbon replicas from both the martensitic and austenitic matrices. A 3D reconstruction of the carbides population in the martensitic layer was performed by serial cutting with a focused ion beam: M 7 C 3 and M 23 C 6 were found to coexist in the two carburized regions, but displayed different sizes, compositions, and morphologies, depending on their location with respect to the fusion line. This evolution in terms of precipitation was analyzed taking into account the local microstructure and composition.

Surface crack propagation and morphology in cutting tools

Purpose – The purpose of this paper is to investigate the driving mechanisms for crack propagation regarding the related microstructures. Cracks in white etching layers have been found at the surface of submerged steel blades subjected to frictional sliding conditions. Design/methodology/approach – In-situ monitoring revealed a fluctuation between mixed lubrication and hydrodynamic lubrication conditions. One lamella including a crack tip was prepared for transmission electron microscopy (TEM) using focused ion beam milling. Transmission electron microscope analysis was performed with the aim to understand the characteristics of the crack propagation, especially considering the influence of the microstructural configuration (grain refinement, carbides, martensite and ferrite grains). Findings – The investigations have shown a grain-refined plastically deformed layer (friction martensite with grain sizes of < 100 nm) which influences the propagation direction of cracks introduced at the frictionally stressed surface. Thereby, the crack propagation is dominantly parallel to the margin of the grain-refined martensitic layer at the surface and the base material. Cracks were split into side cracks what mostly appears at present carbides. In this case, the crack propagation might strike through the carbide or separate it from the matrix due to the mechanical misfit. Originality/value – For obtaining the results of this paper, a very special preparation of tribologically stressed samples was performed. Accordingly, specific findings of the crack propagation behavior under such conditions were achieved and are documented in the presented work. Moreover, the described crack propagation process is a combination of several mechanisms which occur in very limited region underneath the surface and are investigated by high-resolution TEM.

The effect of phase transformation in the plastic zone on the hydrogen-assisted fatigue crack growth of 301 stainless steel

The effects of rolling and stress ratio (R) on the fatigue crack growth rates (FCGRs) of AISI 301 stainless steel (SS) in gaseous hydrogen were investigated. Accelerated fatigue crack growth occurred for the solution-annealed (SA) and cold rolled (CR) specimens tested in gaseous hydrogen, and especially for the CR specimen at high R. Moreover, a sufficiently high ΔK and/or Kmax were required to activate hydrogen-assisted fatigue crack growth of the 301 SS. Fatigue crack growth within a narrow α′ martensite layer was responsible for the rapid FCGRs of the specimens in hydrogen. The plastic zone size, identified by hardness distributions, was much larger than the region that underwent phase transformation.

TEM Observation of Martensite Layer at the Weld Interface of an A508III to Inconel 82 Dissimilar Metal Weld Joint

A lenticular martensite layer at the weld interface in an A508III/Inconel 82 dissimilar metal weld (DMW) joint was studied by TEM. The martensite/weld metal boundary was observed as the fusion boundary. There was a K–S orientation relationship between martensite and weld metal. The formation of the martensite was mainly determined by the distribution of alloy elements. The martensite was responsible for the hardness peak in the DMW.

Metallurgical investigation of wire breakage of tyre bead grade

Tyre bead grade wire is used for tyre making application. The wire is used as reinforcement inside the polymer of tyre. The wire is available in different size/section such as 1.6–0.80mm thin Cu coated wire. During tyre making operation at tyre manufacturer company, wire failed frequently. In this present study, different broken/defective wire samples were collected from wire mill for detailed investigation of the defect. The natures of the defects were localized and similar in nature. The fracture surface was of finger nail type. Crow feet like defects including button like surface abnormalities were also observed on the broken wire samples. The defect was studied at different directions under microscope. Different advanced metallographic techniques have been used for detail investigation. The analysis revealed that, white layer of surface martensite was formed and it caused the final breakage of wire. In this present study we have also discussed about the possible reason for the formation of such kind of surface martensite (hard-phase).

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.

Surface Integrity in Micromilling of Hardened AISI D2 Tool Steel

This paper presents workpiece surface integrity of micromilling hardened tool steel AISI D2. These include 3D mapping of surface texture, analysis of microstructure and microhardness variation. The results indicate no evidence of major surface damage such as microcracks, un-tempered/over-tempered martensite, phase transformations or white layer formation was detected in any of the specimens analysed. No discernible deformation of the material grain structure beneath the machined surface was observed. This was mainly attributed to the comparatively low levels of mechanical force and temperatures generated during the micro milling operation.

Laser-Plasma Treatment of Structural Steel

To conduct high production hardening modification of iron-carbon and titanium alloy surface layers a laser-plasma method (LPM) is developed. The method is based on the use of optical pulse discharge plasma. A discharge is ignited with laser pulses repeated with a high frequency by a CO2-laser oscillator - amplifier system. A laser pulse is focused on the treated surface. To form plasma in alignment with the beam in the laser head, a high velocity gas flow (air, nitrogen, argon, and carbon dioxide) is created. The pressure of the plasma-forming gas can reach 0.5 MPa, and the output speed of the laser head can be 300 m/s.The results of the experiment on the impact of laser-plasma action on the structure and microhardness of the structural steel surface are presented. Laser-plasma treatment leads to the formation of a layer with the martensitic structure on the surface of structural low-alloyed steel 40Kh. This layer is formed due to quenching in a liquid state (QLS) and quenching in a solid state (QSS). The microhardness of the martensitic layer is 11-13 GPa, the hardened zone depth reaches 0.3 mm. It is proposed to use laser-plasma treatment of structural steel as a method for the local surface hardening of machine parts and tools.

Effect of thermal cycling on microstructure evolution and elements diffusion behavior near the interface of Ni/NiAl diffusion couple

The interdiffusion behavior of pure Ni vs. NiAl alloy was investigated via diffusion couple after diffusion annealing treatment under two different thermal cycling (20 and 60 cycles) conditions and isothermal condition. A combination of scanning electron microscope, transmission electron microscope and electroprobe microanalysis was used to characterize the microstructures and measure the concentration profiles across diffusion zones in the Ni/NiAl couples. A thick Ni-rich NiAl layer and a thin γ′-Ni3Al layer were formed near the interface in each couple. The martensite layer with Ni concentration of 63–67at.% was formed in the Ni-rich β-NiAl layer near the Ni/NiAl interface upon cooling. Meanwhile, the Kirkendall plane was formed close to the initial Ni/NiAl interface due to the different diffusion rate of Ni and Al near the interface. Additionally, results indicated that thermal cycling influenced the microstructure evolution behavior of Ni/NiAl diffusion couple. The γ′-Ni3Al/γ-Ni and γ′-Ni3Al/β′-martensite interfaces were rumpled after the diffusion annealing treatment under thermal cycling conditions and the rumpling became more severe under thermal cycling condition with 60 cycles. Furthermore, the slope of interdiffusion coefficients vs. Al concentration curve under thermal cycling condition with 60 cycles was higher than that under cycling condition with 20 cycles and isothermal condition, suggesting that thermal cycling might have influence on the interdiffusion behavior near the Ni/NiAl interface.

Effect of WC on the joint of Ti(C,N)-based cermet/steel joint by autogenous pressure-assisted interlayer-free diffusion bonding

Adherent joint of Ti(C,N)-based cermet and steel was achieved by a novel diffusion bonding method, which was free of external pressure and interlayer. The effect of WC content of the cermet on the microstructure and properties of the joint was investigated. The results showed that WC addition resulted in grain size decrease and formation of white core/black rim phase apart from conventional black core/gray rim microstructure. The joints appeared to consist of four bonding zones, i.e., binder-enriched surface layer (A), Fe–Ni bonded cermet layer (B), austenite layer (C) and martensite layer (D). The thickness of the bonding zones firstly increased after WC addition due to severe denitrification reaction and tighter contact of the base materials. The thickness slightly decreased after 15wt.% WC addition since the dissolution and diffusion of Ti are inhibited by W and Mo. The gradient bonding zone structure mitigated the CTE mismatch of cermets and steel, and the maximum residual stress decreased with the increase of bonding zone thickness. The location of the maximum residual stress region transfers from the cermet side to the ductile austenite layer in the vicinity of steel, which relieves the residual stress by plastic deformation. The shear strength was also enhanced due to the decrease of residual stress and transition of the maximum residual stress region.

A novel roll-bonding methodology for the cross-scale analysis of phase properties and interactions in multiphase structural materials

Abstract We introduce a new thermo-mechanical approach for producing layered bulk samples built-up from the constituent phases of structural materials for the analysis of multiphase co-deformation phenomena. Following a thermo-mechanically controlled roll-bonding procedure, the intrinsic properties of the microstructural components as well as their mutual mechanical interaction and interfacial phenomena can be systematically investigated in highly controlled model microstructures of reduced complexity. The effectiveness of the approach is demonstrated on two examples where austenite or martensite layers, respectively, are introduced in a bulk ferritic matrix, representing in either case components of high strength steels. Special emphasis is laid on how the plasticity of martensite within ferrite, as a key parameter required for understanding and optimising dual phase steels, can be investigated following the proposed approach.

Microstructure and Stress Corrosion Cracking of Dissimilar Metal Weld 16MND5/309L/308L/Z2CND18-12N Used for Connecting Reactor Pressure Vessel to Piping in Nuclear Power Plants

The microstructure and stress corrosion cracking (SCC) in simulated primary water at 290°C of a dissimilar metal weld 16MND5/309L/308L/Z2CND18-12N made in China were studied for reliability and lifetime assessment and management of relevant pressurized water reactor (PWR) nuclear power plants. Results of microstructure characterization showed that the base metal of the low alloy steel (LAS) 16MND5 was upper-bainitic and stainless steel (SS) Z2CND18-12N was equiaxial austenitic grains, the bulk weld metal of SS 308L was austenite with dendritic structures plus δ-ferrite islands. The interfaces areas around 16MND5/309L and 308L/Z2CND18-12N exhibited apparent fusion lines with complicated microstructure/chemical composition changes, especially the former. The area between the 16MND5 base metal and the bulk weld metal was made up of three parts, that is, the heat-affect zone of 16MND5, fusion line and transition zone. There was a thin martensitic layer along the fusion line, which possessed the highest micro-hardness among the weld. The transition zone was an austenitic layer within which Cr% and Ni% changed significantly. SCC test results showed that this area around 16MND5/309L was the most susceptible to SCC among the weld. Both intergranular and transgranular SCC occurred in this area when tested in at high electrode potential which is mainly related to bad water chemistry with high oxygen contamination. The crack mechanism and the significance in engineering are discussed.

Microstructure and Mechanical Properties of an Underwater Wet Welded Dissimilar Ferritic/Austenitic Steel Joint

EH40 ship steel was underwater wet-welded by shielded metal-arc welding at a depth of 7 m in the open sea, using austenitic electrodes. The microstructure and mechanical properties of the joint were estimated with microhardness measurements, bending and tensile testing, optical microscopy, scanning electron microscopy, and energy dispersive X-ray spectroscopy. The highest microhardness was recorded on the coarse-grained heat-affected zone of the base metal, which displayed a mixed bainite-martensite microstructure, while the welds possessed a fully columnar dendritic austenitic structure. The specimens exhibited higher ultimate strength and elongation in longitudinal tensile tests than in transverse ones. Moreover, the specimens satisfied face bending tests but failed in root bending ones. Hydrogen-induced microcracks, which were observed in a lath martensite layer at the fusion boundary, became the fracture initiation region both in bending and transverse tensile tests.