Martensitic Specimens Research Articles

Partial austenitisation and TBF steel composed of ferrite, bainitic ferrite, and austenite

A TRIP-aided bainitic-ferritic (TBF) steel with a chemical composition of Fe–0.19C–1.7Mn–1.09Si–0.51Al–0.05Nb (wt-%) was partially austenitised from a hot-rolled martensitic initial microstructure. After the hot rolling, the martensitic specimens were reheated to different intercritical temperatures and then austempered at 350°C. Thus, the effect of the initial microstructure of TBF steel on intercritical austenite formation during partial austenitisation was studied. The microstructures were investigated by scanning electron microscopy and electron backscatter diffraction (EBSD), and the tensile properties were tested. Microstructural observations revealed that a final microstructure of fine ferrite, bainitic ferrite, and retained austenite can be obtained. The steel partial austenitised at 770°C showed a good combination of ultimate tensile strength and total elongation.

Dependence of solute carbon concentration on electrical resistivity of retained austenite

The relationship of solid solution carbon and electrical resistivity in retained austenite was investigated by using as-quenched martensitic specimens of Fe-10mass%Ni alloys with added carbon up to 0.9 mass%. With increasing carbon content, the volume fraction of retained austenite increased, and simultaneously, the electrical resistivity of the bulk specimen also increased. Four-types of multi-phase models were applied to evaluate the electrical resistivity of each phase. As a result, the electrical resistivity of retained austenite was found to be increased with the increment of solute carbon concentration monotonously. The dependence of the solute carbon concentration (Csol) on the electrical resistivity of retained austenite (Δργ) could be formulated as Δργ [mΩ•mm] = 0.26 × Csol [mass%].

Role of Retained Austenite in Corrosion Resistance of Si-Mn Steel

Si-Mn steels are used for many industrial applications because of their high strength. One of the most important characteristics of the microstructure of Si-Mn steels is the retained austenite embedded in the matrix of martensite or bainite. The existence of the retained austenite promotes TRIP (Transformation-Induced Plasticity) effect, and therefore, provides Si-Mn steels with their excellent strength. While the retained austenite has the beneficial effect on the mechanical properties, corrosion mechanisms of the steels with the retained austenite have been unclear. In reality, the actual application of high-strength steels almost always involves outdoor corrosion environments which can be characterized as predominantly near-neutral pH environments with chloride ions. Even if the steels are successfully protected from corrosion by coating and/or painting, pitting corrosion readily occurs at the cut edges and in coating defect areas due to the presence of chloride ions. In this research, the objective is to clarify the effect of the retained austenite on the corrosion resistance of the Si-Mn steel in near-neutral pH environments.The specimens were Si-Mn steel (0.39%C, 1.51%S, 2.01%Mn, 0.010%P, 0.002%S, 0.04%Al, 0.004%N). The metallographic inspection was performed to ascertain the characteristics of the microstructure of the specimen by using EPMA (Electron Probe Micro Analyzer). It was confirmed that the microstructure consisted of the retained austenite and the matrix of martensite. To assess the corrosion behavior of the specimen, immersion tests were performed in the boric-borate buffer solutions with NaCl (pH 7.0). It was confirmed that the pits were initiated only in the matrix and not in the retained austenite.To clarify the reason for the high corrosion resistance of retained austenite, the corrosion behavior of two types of specimens were compared: Full martensitic specimen (0.44%C, 0.20%S, 0.85%Mn, 0.008%P, 0.002%S, 0.035%Al, 0.003%N) and full austenitic specimen (0.45%C, 0.32%S, 29.40%Mn, 0.014%P, 0.002%S, 0.025%Al, 0.005%N). In the case of the austenitic specimen, higher amount of Mn was added to form full austenitic structure even at room temperature. The austenitic and martensitic specimens simulated the retained austenite and matrix phases in the Si-Mn steel. The depassivation pH values of the austenitic and martensitic specimens were measured in boric-borate buffer solutions. The pH of the solution was decreased stepwise with 0.35 M H3BO3, and the open circuit potentials were measured. It was confirmed that the depassivation pH of the austenitic specimen was lower than that of the martensitic specimen. In other words, the depassivation occurred more readily on the martensite than the austenite. AES (Auger Electron Spectroscopy) depth analysis was performed to investigate the thickness of the surface oxide films on the austenitic and martensitic specimens. It was clarified that the oxide film on the austenite was about 1 nm thicker than that on the martensite, suggesting that the protective ability of the oxide film formed on the austenite is higher than that on the martensite. According to the above results, the high corrosion resistance of the retained austenite in Si-Mn steels appeared to be related with the nature of the oxide film formed on the austenite.

Microstructure engineering for superior wear and impact toughness strength of hypereutectoid powder metallurgy steel

ABSTRACT In this study, powder metallurgical steel samples containing 1.5% C by weight were produced by sintering in a vacuum atmosphere at 1200°C. The impact test and dry sliding wear test specimens were prepared in accordance with ISO 5754 and ASTM G-99 standards, respectively. The wear and impact toughness properties of spherical cementite in the bainitic matrix (SCBM) microstructured specimens produced with different heat treatment routes were compared with pearlite plus primary cementite, fully tempered martensitic, fully bainitic and spherical cementite microstructured specimens in ferritic matrix (SCFM). Microstructural characterisations of all specimens were performed by SEM, EBSD and XRD analyses methods. Compared to fully tempered martensitic and fully bainitic microstructured specimens with similar hardness, both the dry sliding wear and the impact toughness properties of the specimens having SCBM microstructures were improved with the increased austempering time after spheroidisation treatment.

Characteristics and mechanisms of hydrogen-induced quasi-cleavage fracture of lath martensitic steel

This study presents an in-depth characterization of the microstructures, crystallographic orientations, and dislocation characteristics beneath hydrogen-induced quasi-cleavage fracture features of an as-quenched, lath martensitic (α') 22MnB5 steel. The fracture surfaces of gaseous hydrogen-embrittled martensitic specimens were analyzed by a combination of multiple analytical tools: scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and transmission Kikuchi diffraction (TKD). The dominant fracture mode in the hydrogen-affected zones was quasi-cleavage fracture, which involved significant plasticity, evidenced by plastic zones near tear ridges and a high density of dislocations beneath the quasi-cleavage facets. The martensite constituent sizes, variant orientations, and boundaries influenced the quasi-cleavage surface morphologies. Fractography revealed the occurrence of {100}α'-type cleavage across martensitic laths, developing relatively “flat” quasi-cleavage surfaces, in addition to {110}α'-type cracking likely along lath and block boundaries, and fracture along non-cleavage planes. The likelihood of the formation of the relatively “flat” quasi-cleavage surfaces increased with increasing martensitic constituent sizes. Substantial dislocation bands formed on intersecting {112}α' slip planes within a martensite block below the hydrogen-induced cleavage fractures on {100}α' planes. River markings on the quasi-cleavage surfaces were found to originate from the complex, hierarchical lath martensitic microstructure. Steps and ridges on the quasi-cleavage facets generally linked with the various martensitic boundaries, suggesting that they were produced as a result of crack deviations at those boundaries. The fracture paths and martensite quasi-cleavage mechanisms are discussed in the context of the role of hydrogen and Cottrell cleavage model.

(Invited) Beneficial Role of Interstitial Carbon on Corrosion Resistance of Carbon Steels

Martensitic carbon steels with fine grains are used as important industrial materials. Interstitial carbon atoms are located in the octahedral sites in the body-centered tetragonal (bct) crystal structure, and bring high strength to martensite. Although it has been reported that the interstitial carbon has the beneficial effect on the corrosion resistance of steels,1,2 the role of interstitial carbon is still unclear. In this research, first-principles calculations were applied to explore the role of interstitial carbon on the corrosion resistance of martensite.The specimens were martensitic carbon steels with three carbon contents: 0.001 mass%, 0.44 mass%, and 0.88 mass% C. The specimens were heat-treated at 1223 K and then, quenched in water to form martensitic structure. Potentiodynamic polarization of the specimens was conducted using 1 M H2SO4. In addition to the electrochemical measurements, the first-principles calculations based on density functional theory (DFT) were carried out. For the calculation, the Vienna Ab initio Simulation Package (VASP) was used with the projector augmented wave (PAW) method 3 and GGA-PBE potential for both of the bulk and surface phases of martensite. The bct-based supercells are adopted and the interstitial carbon atoms were located at the octahedral sites in bct crystal structures.In the potentiodynamic polarization curves of the martensitic specimens in 1M H2SO4, the current densities of all specimens increased continuously with the electrode potential. This indicates that the specimen surfaces dissolved actively and were not passivated in this solution. The active dissolution current decreased with increasing in carbon content, suggesting that the active dissolution of martensite was suppressed by the interstitial carbon.By using the first-principles calculations, the effect of interstitial carbon atoms on the electronic structure of bct iron (martensite) was investigated. It was clarified that the electronic density of states (DOS) of iron atoms at and near the Fermi level decreased by increasing carbon concentration. Considering that the valence electrons near and at the Fermi level are responsible for the electrochemical reactivity (reduction/oxidation reactivity), this is evidence that interstitial carbon enhances the electrochemical stability of bct iron. Also, the effect of interstitial carbon on the work function of bct iron was investigated by the first-principles calculations. The work function on bct (110) and bct (100) surfaces increased with increasing interstitial carbon content, which might result in the suppression of the active dissolution of martensite.References; M. Kadowaki, I. Muto, Y. Sugawara, T. Doi, K. Kawano, and N. Hara, J. Electrochem. Soc., 164, C962 (2017).M. Kadowaki, I. Muto, Y. Sugawara, T. Doi, K. Kawano, and N. Hara, J. Electrochem. Soc., 165, C711 (2018).G. Kresse and J. Furthmüller, Phys. Rev. B, 54, 11169 (1996).

On the crack onset and growth in martensitic micro-structures; a phase-field approach

In this work, a phase-field approach (PFA) is presented to study crack nucleation and propagation in martensitic micro-structures resulted from multi-variant martensitic phase transformations (MPT) within the framework of a finite element method (FEM). To this end, first, a coupled system of the time-dependent Ginzburg-Landau (TDGL) equation and the equilibrium equation is established based on the micro-elasticity theory, which reveals the nucleation and growth of diffusionless martensitic multi-variants forming a twinned martensitic micro-structure. The Helmholtz free energy used in this work consists of a second-degree polynomial of the phase variable, which leads to a nonlinear dependence on the order parameter in the TGDL equation. Thereafter, the nucleation and propagation of a crack is scrutinized in the obtained martensitic specimen, with and without pre-existing crack, according to three types of martensitic embryos. To do so, a damage variable is introduced to the multi-variant MPT model to study the interactions between the martensitic transformation and fracture. The key contributions of this study are not only to shed light on the evolution of the martensitic variants in the micro-structure with three types of pre-existing martensitic embryos, but also to investigate onset and growth of a crack in the martensitic specimen.

Investigation on laser hardened and WC-Co-Cr based HVOF coated 13Cr 4Ni martensitic stainless steel materials under erosive environment

Abstract Coating materials are used on turbine blades. Coating plays an important role to protect the turbine blades from the silt erosion. In this study, comparison for erosion resistance between uncoated specimens of 13Cr 4Ni martensitic stainless, WC-Co-Cr based HVOF coated and laser hardened 13Cr 4Ni martensitic stainless steel specimens was carried out by performing air jet erosion tests. The micro structural analysis of eroded samples was done to ascertain the mode of erosion. From investigations it was observed that WC-Co-Cr based HVOF coated 13Cr 4Ni martensitic stainless steel specimens displayed maximum erosion resistance at all impingement angles i.e. 30°, 45° and 90° in comparison to uncoated and Laser hardened 13Cr 4Ni martensitic stainless steel specimens. Mass loss of WC-Co-Cr based HVOF coated 13Cr 4Ni martensitic stainless steel specimens were 0.0075, 0.007 and 0.005 g at 30°, 45° and 90°, respectively. Taguchi based design of experiment is implemented for optimizing the input process parameters. The calculated value of the whole mean of signal to noise (S/N) ratio for erosion wear is −89.2554 db for Bare material, −127.9290 db for HVOF coted material and −109.4730 db for laser hardened material observed. According to findings surface treatment of testing samples significantly affect erosion resistance.

The effects of a ferritic or martensitic matrix on the tensile behavior of a nano-precipitation strengthened ultra-low carbon Ti–Mo–Nb steel

This work investigates the correlation between microstructure and mechanical properties of a nano-precipitation strengthened ultra-low carbon (NPS- ULC) Ti–Mo–Nb steel. Two types of matrix microstructures (ferrite and martensite) with nano-precipitates were obtained through hot rolling and isothermal transformation in the case of ferrite, or by quenching and tempering, for martensite. The martensitic microstructure showed increases in both YS and UTS by ~110 and ~100 MPa, respectively, over the ferritic microstructure without sacrificing tensile ductility. All ferritic and martensitic specimens exhibited a two-stage work hardening behavior with different work hardening rates at high strain levels. Quantitative analysis of the strengthening contributions confirms that the increase in yield stress of the NPS-ULC specimens with a martensitic matrix was a result of the fine martensitic laths as well as the higher dislocation density and nano-precipitates.

Evolution of microstructure and mechanical properties during tempering of M50 steel with Bainite/Martensite duplex structure

The evolution of microstructure and mechanical properties during tempering of M50 steel with bainite/martensite (B/M) duplex structure have been investigated. The microstructural observation shows that the redistribution of carbon atoms and precipitation of transition carbides in the B/M duplex specimens are slightly decreased during the initial stage of tempering, as compared with the fully martensitic specimens. After tempering of 550 °C, not only finer ferrite packets are obtained, but also the precipitation of nano-scale carbides is enhanced for the B/M duplex specimens. The mechanical characterization indicates that the hardness is decreased while the impact toughness is significantly improved when tempering temperature increases to 300 °C. When the tempering temperature increases to 550 °C, due to the decomposition of retained austenite (RA) and precipitation of alloy carbides, the hardness exhibits an obvious improvement while the toughness first decreases and then increases. The impact toughness of the B/M duplex specimens is higher than that of the fully martensitic specimens during the whole tempering process, although they have the same tendency varying with tempering temperature. Furthermore, the final wear resistance is significantly improved in the austempered specimens due to the enhanced precipitation during tempering.

Parameterization of a Non-local Crystal Plasticity Model for Tempered Lath Martensite Using Nanoindentation and Inverse Method

Crystal plasticity (CP) models have proven to accurately describe elasto-plastic behavior on micro- and nanometer length scales in numerous applications. However, their parameterization requires series of experiments and inverse analysis of the results. In this regard, nanoindentation promises to be a well-suited tool for realizing a parameterization approach to determine all model parameters. The objective of this work is to develop a parameterization technique for a non-local CP model by means of an accessible and reproducible workflow. To determine its feasibility, tempered lath martensite with two different carbon contents is used as testing material. The workflow combines nanoindentation tests with finite element simulations. First, indentation into single packets of tempered lath martensitic specimen is yielding the load-displacement curves and the residual imprint topology on the surface with the help of atomic force microscopy. In a second step, a finite element simulation of the indentation using non-local crystal plasticity as constitutive model is performed with estimated model parameters. In the next step, non-local CP parameters are systematically adapted in an optimization scheme to reach optimal agreement with experiments. As a final validation step it is successfully demonstrated that the CP model parameterized by nanoindentation is able to predict the macroscopic stress-strain response of polycrystals. Two observations are made: on the one hand, the material properties locally scatter very strongly, which is caused by fluctuations in microstructure and chemistry. On the other hand, a novel method has been demonstrated were an inverse analysis is used to parameterize a non-local CP model for highly complex microstructures as those of tempered lath martensite.

First-Principles Investigation of the Effect of Interstitial Carbon on Corrosion Resistance of Martensitic Medium-Carbon Steel

Martensitic carbon steels with fine grains are used for many industrial applications. Interstitial carbon provides high strength to martensitic structure. Although it has been reported that the interstitial carbon has the beneficial effect on the pitting corrosion resistance of steels,1,2 the role of interstitial carbon is still unclear. In this research, first-principles calculations were applied to the clarification of the effect of interstitial carbon on the anodic dissolution of martensite. The base specimen was AISI 1045 carbon steel, and the chemical composition of this steel (mass%) was 0.44%C, 0.20%Si, 0.85%Mn, 0.008%P, 0.002%S, <0.01%Ni, <0.01%Cr, <0.01%Mo, <0.01%Cu, <0.001%Ti, <0.002%Nb, 0.035%Al, 0.003%N, and 0.002%O. The specimens were heat-treated at 1123 K for 10 h in air and then, quenched in water. Since the surface of the specimens was decarburized during this heat-treatment, the low carbon martensitic structure was formed in the surface region of the specimens, and high carbon martensitic structure remained in the core area. Potentiodynamic polarization of the low and high carbon martensitic specimens was conducted using boric-borate buffer solution with 1 mM NaCl at pH 8.0. In addition to the electrochemical measurement, the first-principles calculations based on density functional theory (DFT) were carried out. For the calculation, the carbon concentration was varied from 0.40 at.% to 5.88 at.%. In the potentiodynamic polarization curves of the low and high carbon martensitic specimens in 1mM NaCl containing boric-borate buffer at pH 8.0, the region of active dissolution appeared on both specimens at about -0.5 V. The dissolution current density for high carbon martensitic specimen was lower than that of the low carbon martensitic one, suggesting that the interstitial carbon suppressed the active dissolution. Next, the effect of interstitial carbon atoms on the electronic structure of the martensitic structure was analyzed. It was clarified that the electronic density of states (DOS) of iron atoms at and near the Fermi level decreased by increasing carbon concentration. This result indicates that the interstitial carbon atom makes weak atomic bonding with the nearest iron atom, and stabilizes the electronic structure of martensite. This change in the energy state of iron atom was, therefore, attributed to the high corrosion resistance of martensitic carbon steels. References; M. Kadowaki, I. Muto, Y. Sugawara, T. Doi, K. Kawano, and N. Hara, J. Electrochem. Soc., 164, C962 (2017).M. Kadowaki, I. Muto, Y. Sugawara, T. Doi, K. Kawano, and N. Hara, J. Electrochem. Soc., 165, C711 (2018).

Neutron diffraction study of temperature-dependent elasticity of B19′ NiTi---Elinvar effect and elastic softening

The temperature-dependent elasticity of the B19′ NiTi is unknown today. To gain insights into the lattice-level temperature-dependent elasticity of the B19′ crystal, we present results of in-situ neutron diffraction experiments performed on polycrystalline martensitic specimens in the temperature range of 300 down to 50 K. The experimental results are compared with the density functional theory molecular dynamics (DFT-MD) and Quasi Harmonic Approximation (QHA) calculations. The results confirm that the temperature-dependent Young's modulus (TDYM) of the B19′ crystal is strongly anisotropic. For different crystallographic orientations, the change in Young's modulus over the temperature range of 300–50 K (ΔE(hkl)=E(hkl)50K−E(hkl)300K), ranges from ΔE(102¯) = 2.8 ± 3.5 GPa (extremely weak dependence) to ΔE(103) = 59.6 ± 9.1 GPa (strong dependence). Moreover, it is found that the orientation-specific TDYM and thermal expansion (TE) of the B19′ crystal are correlated. The crystallographic orientations with weak and negative TE responses exhibit a weaker TDYM than the orientations with positive TE. The DFT-MD and QHA results capture qualitatively the above experimental observations and further show that there are orientations in a B19′ crystal exhibiting elastic softening (ΔE(hkl)<0) and ideally no change in Young's modulus (ΔE(hkl)= 0) with cooling. This is found to originate from the strong negative temperature dependence of c35 stiffness constant. The experimental results along with the first-principles calculations confirm that the Elinvar and Invar are two confluent properties in NiTi SMAs and can be tailored by texturing B19′ crystallographic orientations.

Impulse Excitation Internal Friction Study of Retained Austenite in Ferrous Martensite

Internal friction (IF) measurements were conducted using ferrous lath martensite specimens with different retained austenite fractions. While a distinctive Snoek-Ke-Koster (SKK) peak was detected in the IF spectra, the peak shape did not conform to the peak shape observed for fully martensitic steel. Peak analysis suggests that the peak shape is related to the overlap of the SKK peak with a Finkelstein–Rosin peak, additional Debye-type peak related to the retained austenite.

The Effect of Interstitial Carbon on Pitting Corrosion Resistance of Martensitic Carbon Steels

Ultra-high strength steels with excellent ductility are required to reduce the weight of automobiles and other transportation vehicles. While martensite including high amount of interstitial carbon is favored for strengthening steels, the ductility is relatively low. To optimize the balance between ductility and strength, tempering is conducted. In the case of carbon steels, the precipitation of carbides occurs, and the amount of interstitial carbon decreases during tempering. Although tempering provides the balance between the strength and the ductility, it was reported that the pitting corrosion resistance tends to decrease by tempering.1 To achieve a balance between pitting corrosion resistance and mechanical properties, it is important to determine the quantitative effect of tempering on the pitting corrosion resistance. The specimens were AISI 1045 carbon steel (0.47%C, 0.19%Si, 0.85%Mn, 0.015%P, 0.015%S, 0.02%Ni, 0.18%Cr, 0.01%Mo, 0.01%Cu, <0.001%Ti, <0.002%Nb, 0.024%Al, 0.005%N, 0.002%O). The specimens were heat-treated at 1123 K for 3.6 ks and then quenched in water to form a full martensitic structure. To obtain a tempered martensitic structure, the as-quenched specimens were heated to 873 K and kept at this temperature for 0.1, 1, 10, 15, and 20 h in an Ar atmosphere. The metallographic inspection was performed to ascertain the characteristics of the microstructures of the as-quenched and tempered specimens by using FE-SEM. In tempered martensitic specimens, many small white dots were observed. This is evidence of the carbides formed by tempering. The sizes of the carbides increased with tempering time, meaning that the interstitial carbon decreased. The micro-scale anodic polarization measurements of the as-quenched and tempered specimens were conducted in a boric-borate buffer solution with 500 mM NaCl (pH 8.0) under naturally aerated conditions at 298 K. A small area without non-metallic inclusions was selected for the working electrode. The polarization measurements started at -0.1 V to eliminate the influence of active dissolution on pitting corrosion initiation. For 20 h tempered specimen, a stable pit was initiated immediately after immersion. The pitting potential of 20 h tempered specimen was thought to be less than -0.1 V. In the case of 15 h tempered specimen, the sharp increase in current density was observed at 0.05 V due to the pit initiation. In contrast, no pit was initiated on the as-quenched specimen and the specimens tempered for less than or equal to 10 h. It was found that the pitting corrosion resistance of the martensite tempered for less than or equal to 10 h was higher than that of the martensite tempered for more than or equal to 15 h. Pitting corrosion resistance of martensite decreased with tempering time. To clarify the role of the interstitial carbon on the pitting corrosion resistance, the macro-scale anodic polarization curves of as-quenched, 10 h tempered, and 20 h tempered specimens in a boric-borate buffer solution with 1 mM NaCl (pH 8.0) were measured. The polarization started at -1.0 V after a cathodic treatment (-1.2 V, 600s). For the macro-scale polarization, the region of active dissolution appeared on all specimens in the potential region from -0.7 to -0.6 V. The dissolution current density for 20 h tempered specimen was certainly higher than that of other two specimens. It is suggested that the higher pitting corrosion resistance of the as-quenched and short-time tempered specimens was due to the low dissolution rate of those specimens. It is proposed that the inhibition of the active dissolution rate of these specimens is because of the existence of interstitial carbon, and interstitial carbon improves the pitting corrosion resistance. Short-time tempering is one of the answers for striking an optimal balance between pitting corrosion resistance. References; 1. M. Kadowaki, I. Muto, Y. Sugawara, T. Doi, K. Kawano, and N. Hara, J. Electrochem. Soc., 164, C962 (2017).

Characterization of Austenite Microstructure from Quenched Martensite Using Conventional Metallographic Techniques and a Crystallographic Reconstruction Procedure

Recently, procedures for recovering austenite fcc crystallographic information from EBSD (Electron Back-Scatter Diffraction) data recorded from martensite or other bcc transformation products have been proposed. Due to the difficulties in revealing prior austenite grain boundaries using available etching techniques, these reconstruction methods appear as promising tools for understanding austenite hot deformation behavior in low carbon steels. In a previous work, the accuracy of an in-house developed reconstruction code was validated using an Fe-30Ni alloy. Validation of this method in low carbon steels is more difficult due to the loss of austenitic orientation information. In addition, it is not known how the results of conventional metallography correlate with those of the reconstruction or which scan parameters or post-processing treatments are necessary for obtaining comparable results. To study this, two martensitic specimens obtained from water quenched recrystallized and deformed austenite were characterized in this study, using both conventional metallographic techniques and the above mentioned reconstruction procedure applied to EBSD scans acquired using different parameter values. The comparability of austenite grain size and morphology was analyzed as a function of the post-processing treatment. The results show that in order to obtain comparable results, it is critical to analyze the coherence of the twins present in the microstructure.

Effects of microstructures on the fatigue crack growth behavior of laser additive manufactured ultrahigh-strength AerMet100 steel

In order to evaluate the effects of microstructure characteristics on fatigue crack growth (FCG) resistance of laser additive manufactured (LAM) AerMet100 steel, microstructures and FCG behaviors (in Paris region) of as-deposited specimen and three types of tempered martensite specimens were examined. Results indicate as-deposited specimens of LAM AerMet100 steel have apparent texture characteristics of epitaxy unidirectional growth prior-austenite columnar grains and grain-interior inter-dendritic blocky retained austenite with [001] crystallographic orientation. And poor boundary cracking resistance of these texture characteristics along deposition direction mainly contributes to the FCG rate anisotropy of as-deposited specimens. After post-LAM heat treatments, the FCG resistance of all heat-treated specimens apparently improves with the fracture mode of transgranular cracking. With the increase of yield strength, the value of Paris coefficient C of the steel increases, but the value of Paris exponent m decreases. Compared to the poor dislocation slip resistance of bainite plates in as-deposited specimens, the improved dislocation slip resistance of martensite plates is mainly related to the strong dislocation pinning effect of fine dispersive rod-like coherent M2C carbides, resulting in the stronger FCG resistance of the heat-treated specimens. In the Paris region of low ΔK (< ~ 20 MPa m1/2), fatigue cracks mainly propagate along the bainite (or martensite) plate interfaces, and the FCG rate of the steel can be effectively decreased by containing higher contents of thick film-like retained/reverted austenite; with the increase of ΔK, besides propagating along the soft inter-plate film-like austenite, fatigue cracks can also directly pass through the harder bainite (or martensite) plates with the striations and secondary cracks observed on fracture surfaces; in the Paris region of high ΔK (> ~ 70 MPa m1/2), higher contents of retained/reverted austenite inversely accelerate the FCG rate of heat-treated LAM AerMet100 steel. In contrast, grain refinement has the little influence on the FCG rate (in most of Paris region) of the heat-treated specimens.

Simulation of thermal desorption spectrum of hydrogen from austenite embedded in the martensite matrix

Thermal desorption spectra of hydrogen of martensite (or ferrite), austenite and a composite of them in which austenite is a minor phase are simulated using McNabb–Foster theory. Whereas the peak temperature of desorption rate is markedly dependent upon the specimen thickness in all specimens, it did not decrease below a certain limit in thin martensite specimens where hydrogen desorption is controlled by detrapping of hydrogen from the trap site. The peak temperature in the two-phase specimen depends strongly upon the thickness of austenite if the initial amount of hydrogen in the ferrite matrix can be ignored, becoming close to or lower than the room temperature for the austenite thickness less than ∼10 μm. For larger austenite particles the release rate of hydrogen from the austenite governs the peak temperature. For small particles such as interlath retained austenite the release time of hydrogen from the particle is so short that re-capture of hydrogen at matrix dislocations and martensite/austenite interfaces can enhance the peak of martensite and/or cause an independent peak at a higher temperature.

The Cause of Premature Tensile Fracture of Gas-Nitrocarburized Steel

The objective of the present study was to investigate the mechanism of premature tensile fracture of the gas-nitrocarburized steel. For this purpose, tempered martensitic steel specimens were nitrocarburized at 853 K (580 °C) for maximum 3.5 hours, and tensile-strained at room temperature. The prolongation of nitrocarburizing time caused the high strength, the short elongation, and the heavy serrations at the yield point elongation range of the tensile flow curve. The serrations were generated by vertical cracking at the specimen surface, which started at the bottom of the gauge portion, and were propagated to the top of the gauge portion with increasing tensile strain up to the yield point elongation. The vertical cracks were triggered at the porous compound layer when the local deformation occurred by the propagation of the Luders band from the bottom to the top of the gauge portion. The cracks readily passed through the diffusion layer with an assistance of e-carbonitride particles, which formed at the boundaries of tempered martensite during nitrocarburizing, resulting in brittle fracture at the diffusion layer and finally in premature failure of the gas-nitrocarburized specimens.

Effects of microstructure and surface roughness on the fatigue strength of high-strength steels

Rotating bending fatigue (RBF) tests were conducted on a high-carbon steel 100CrMnMoSi8 (martensite and bainite), and a tough-tempered medium-carbon steel 50CrMo4. The specimens were surface-finished to different conditions: polished surface and ground surface with a range of roughness levels. The experimental results indicate that the hardened high-carbon steel specimens with rough surface failed predominantly by surface crack initiation, whereas the fatigue fracture of the specimens with smoother surface tend to fail by subsurface crack initiation from non-metallic inclusions. Moreover, the fatigue strength of the martensitic specimen is lower than that of the bainitic specimen in the low stress-cycle range in which failure is dominated by surface initiated fatigue fracture, whereas this difference diminishes in the high-cycle fatigue regime where subsurface initiated fatigue prevails. The non-hardened medium-carbon steel samples, however, fail only by surface crack initiation, and the fatigue strength is much less sensitive to surface roughness. A unified model is developed to predict the fatigue strength of the tested samples with different microstructures and surface finish.