Search Results (25)

Optical microscope (OM), energy dispersive spectrometer (EDS), electron backscatter diffractometer (EBSD), electrochemical test, and transmission electron microscope (TEM) were employed to conduct interface microstructure observation and cladding corrosion resistance analysis on 304 SS/CS clad plates that have four different reduction ratios. The increase in rolling reduction ratio leads to larger grain size, gradually refined microstructure, and a decreased thickness of the interfacial martensite area. As the concentration disparity of the C element between carbon steel (CS) and 304 stainless steel (SS) is small, no evident carburization layer or decarburization layer can be detected. The ferrite microstructure on the CS side has greater stress distribution and greater local orientation deviation, and deformed grains are dominant. Austenite undergoes strain-induced martensitic transformation with the transformation mechanism of γ→twinning→a’-martensite. The martensite microstructure within the interface region grows in the direction of the interior of austenite grains. The reduction ratio increases sharply, leading to an increase in dislocation density, which promotes the nucleation, growth, and precipitation of carbides and seriously reduces the corrosion resistance of the cladding. Subsequently, the reduction ratio keeps on increasing. However, the degree of change in the reduction ratio diminishes. High temperature promotes the dissolution of carbides and improves the corrosion resistance. From this, it can be understood that by applying the process conditions of raising the reduction ratio and keeping a high temperature at the carbide dissolution temperature, a clad plate that has excellent interface bonding and remarkable corrosion resistance can be acquired. Full article

This work deals with the effects of two individual isothermal aging experiments (450 °C/5000 h and 700 °C/2500 h) and the subsequent room-temperature electrolytic hydrogen charging of TP316H stainless steel on its Charpy V-notch (CVN) impact toughness and fracture behavior at room temperature. Microstructural analyses revealed that aging at 700 °C resulted in the abundant precipitation of intermediary phases, namely, the Cr₂₃C_6-based carbide phase and Fe₂Mo-based Laves phase, whereas aging at 450 °C resulted in much less pronounced precipitation of mostly intergranular Cr₂₃C₆-based carbides. The matrix phase of 700 °C-aged material was completely formed of austenitic solid solution with a face-centered cubic (FCC) crystal structure, whereas an additional formation of ferritic phase with a base-centered cubic (BCC) structure was detected in 450 °C-aged material. The performed microstructure observations correlated well with the obtained values of CVN impact toughness, i.e., a sharp drop in the impact toughness was observed in the material aged at 700 °C, whereas negligible property changes were observed in the material aged at 450 °C. The initial, solution-annealed (precipitation-free) TP316H material exhibited a notable hydrogen toughening effect after hydrogen charging, which has been attributed to the hydrogen-enhanced twinning-induced plasticity (TWIP) deformation mechanism of the austenitic solid solution. In contrast, both aging expositions resulted in significantly lowered hydrogen embrittlement resistance, which was likely caused by hydrogen trapping effects at the precipitate/matrix interfaces in thermally aged materials, leading to a reduced TWIP effect in the austenitic phase. Full article

Vanadis 8 steel is a tool steel manufactured by powder metallurgic processing. Its main alloy elements are V, Cr and Mo. By implementing an experimental design with five factors—all of them are related to the thermal processing of this steel and with ionic nitriding—the effects of said factors on adhesive wear resistance and corrosion resistance were studied. For this purpose, Pin-on-Disc wear tests and lineal polarization resistance tests were carried out using an aqueous solution with 3.5% NaCl by weight. The main aim was to increase this steel use in more aggressive environmental conditions, such as in coastal environments. By means of XRD, the percentage of retained austenite was determined, and by SEM-EDX, the microstructure was revealed. The conclusion is that adhesive wear resistance is improved if thermal processing parameters are at such levels that increase austenite destabilization and reduce retained austenite content. This means to destabilize austenite at 1180 °C during 1 h, with oil quenching, tempering at 520 °C during 2 h and ionic nitriding at 520 °C during 2 h. Corrosion resistance is highly improved with ionic nitriding. At the same time, to compensate for the negative effect on corrosion resistance of a high density of primary and secondary carbides, it is essential to carry out the ionic nitriding treatment. The harmful effect of electrochemical microcells that appear in the carbide/matrix interface is compensated by the passivating effect generated by the nitrided surface. Full article

In this work emphasis was given to determine the evolution of the retained austenite phase fraction via X-ray diffractometry technique in the as-hardened AISI 440C martensitic stainless steel surface subjected to cavitation for increasing test times. Scanning electron microscopy results confirmed the preferential carbide phase removal along the prior/parent austenite grain boundaries for the first cavitation test times on the polished sample surface during the incubation period. Results suggest that the strain-induced martensitic transformation of the retained austenite would be assisted by the elastic deformation and intermittent relaxation action of the harder martensitic matrix on the austenite crystals through the interfaces between both phases. In addition, an estimation of the stacking fault energy value on the order of 15 mJ m⁻² for the retained austenite phase made it possible to infer that mechanical twinning and strain-induced martensite formation mechanisms could be effectively presented in the studied case. Finally, incubation period, maximum erosion rate, and erosion resistance on the order of 7.0 h, 0.30 mg h⁻¹, and 4.8 h μm⁻¹, respectively, were determined for the as-hardened AISI 440C MSS samples investigated here. Full article

High-carbon nano bainitic steel is currently a hot research topic. The effect of the matrix’s carbon content and carbon atom distribution on the toughness of high-silicon, high-carbon bainitic steel is studied. The microstructure under an incomplete austenitization process consists of undissolved carbides, bainitic ferrite, and retained austenite. Using this process, the carbon content in bainitic ferrite is relatively low. Under the complete austenitization process, the carbon content in the bainite ferrite in the sample is high, and there is more retained austenite in the blocky type. The sample exhibits high impact toughness under an incomplete austenitization process, which is mainly affected by the low carbon content of bainite ferrite, high coordination ability of retained austenite, and high interface density of microstructure. The EBSD results show that the crack easily propagates between parallel bainite laths with low interface density compared with the high interface density perpendicular to the laths. Full article

Secondary phase precipitation in Fe-22Mn-9Al-0.6C low-density steel was investigated during a continuous cooling process with different cooling rates through a DIL805A thermal expansion dilatometer, and the changes in microstructures and hardness by different cooling rates were discussed. The results showed that the matrix of the Fe-22Mn-9Al-0.6C was composed of austenite and δ-ferrite; moreover, the secondary phases included κ-carbide, β-Mn and DO₃ at room temperature. The precipitation temperatures of 858 °C, 709 °C and 495 °C corresponded to the secondary phases B2, κ-carbide and β-Mn, respectively, which were obtained from the thermal expansion curve by the tangent method. When the cooling rate was slow, it had enough time to accommodate C-poor and Al-rich regions in the austenite due to amplitude modulation decomposition. Furthermore, the Al enrichment promoted δ-ferrite formation. Meanwhile, the subsequent formation of κ-carbide and β-Mn occurred through the continuous diffusion of C and Mn into austenite. In addition, the hardness of austenite was high at 0.03 °C/s due to the κ-carbide and β-Mn production and C enrichment, and it was inversely proportional to the cooling rate. It can be concluded that the presence of κ-carbide, DO₃ and β-Mn produced at the austenitic/ferrite interface when the cooling rate was below 0.1 °C/s resulted in κ-carbide and β-Mn precipitating hardly at cooling rates exceeding 0.1 °C/s, which provides a guideline for the industrial production of Fe-Mn-Al-C low-density steel in the design of the hot working process. Full article

Q690 steel is widely used as building steel due to its excellent performance. In this paper, the microstructure evolution of the heat-affected zone of Q690 steel under simulated high heat input welding conditions was investigated. The results show that under the heat input of 150–300 kJ/cm, the microstructures of the heat-affected zone are lath bainite and granular bainite. The content of lath bainite gradually decreased with the increase in heat input, while the content of granular bainite steadily increased. The proportion of large-angle grain boundaries decreased from 51.1% to 40.3%. Overall, the average size of original austenite increased, and the precipitates changed from Ti (C, N) to Cr carbides. During the cooling process, the nucleation position of bainitic ferrite was from high to low according to the nucleation temperature, and in order of inclusions at grain boundaries, triple junctions, intragranular inclusions, bainitic ferrite/austenite phase boundaries, twin boundaries, grain boundaries, and intragranular inclusions at the bainitic ferrite/austenite phase interface. The growth rate of bainitic ferrite nucleated at the phase interface, grain boundary, and other plane defects was faster, while it was slow at the inclusions. Moreover, it was noted that the Mg-Al-Ti-O composite inclusions promote the nucleation of lath bainitic ferrite, while the Al-Ca-O inclusions do not facilitate the nucleation of bainitic ferrite. Full article

The hydrogen embrittlement (HE) behavior of a commercial QP980 steel is studied in this work. The HE susceptibility results indicate that QP980 suffers from a severe HE, and the fracture mode transforms from ductile dimpling to brittle quasi-cleavage under the attack of hydrogen. The EBSD results show that strain-induced martensite transformation can rarely occur at a strain close to the HE fracture strain, which is mainly attributed to the high mechanical stability of austenite. The TKD-KAM analysis results indicate that hydrogen-induced strain localization in martensite can be mitigated by the hydrogen-trapping effect of surrounding austenite, while it is most pronounced in martensite adjacent to ferrite. Correspondingly, HE cracking is considered to initiate in martensite adjacent to ferrite under the synergistic action of HELP and HEDE mechanisms, and then cracks can propagate through ferrite or along phase interfaces. Our findings suggest that to further improve the HE resistance of QP steel with stable austenite, it is necessary to consider introducing effective hydrogen-trapping sites (such as carbides, film austenite) into martensite, which is deemed to be beneficial for increasing the resistance against hydrogen-induced cracking initiation in martensite. Full article

Austenite formation was numerically investigated using Thermo-Calc/DICTRA in a deformed ferrite/pearlite microstructure to produce dual-phase steels. This work aims to better understand how the interface conditions (local equilibrium with negligible partitioning—LENP—or local equilibrium with partitioning—LEP) control the austenite growth kinetics during the intercritical annealing. Inspired by our experimental observations, two nucleation sites were considered. The austenite formed from pearlite islands showed a regime transition from LENP to LEP when the holding stage started. For the growth of austenite from isolated carbides, three stages were identified during the heating stage: first, slow growth under LEP; then, fast growth under LENP; and finally, after dissolution of the carbide, slow growth again. LENP and LEP interface conditions may coexist thanks to these regime transitions. In the case of competition, LEP conditions hinder austenite growth while it is promoted by LENP interface conditions. Such differences in growth kinetics explain, in part, the morphogenesis of dual-phase microstructures. Full article

In this paper, we investigate the effects of vanadium on the strength and ductility of medium-manganese steels by analyzing the microstructural evolution and strain hardening rates and performing quantitative calculations. Two significantly different contents of vanadium, 0.05 and 0.5 wt.%, were independently added to model steel (0.12C-10Mn) and annealed at different intercritical temperatures. The results show that higher vanadium addition increases the yield strength but decreases the ductility. The maximum yield strength can increase from 849 MPa to 1063 MPa at low temperatures. The model calculations reveal that this is due to a precipitation strengthening increment of up to 148 MPa and a dislocation strengthening increment of 50 MPa caused by a higher quantity of V₄C₃ precipitates. However, the high density of vanadium carbides leads them to easily segregate at grain boundaries or phase interfaces, which prevents strain from uniformly distributing throughout the phases. This results in stress concentrations which cause a high strain hardening rate in the early stages of loading and a delayed transformation-induced plasticity (TRIP) effect. Additionally, the precipitates decrease the austenite proportion and its carbon concentrations, rendering the TRIP effect unsustainable. Accordingly, the ductility of high vanadium steels is relatively low. Full article

This paper focuses on the microstructure and tribological properties of novel hardfacing alloy based on Fe-C-Mn-B doped with Ni, Cr, and Si. The 4 mm-thick coating was deposited on the AISI 1045 carbon steel by the MIG-welding method using flux-cored wires in three passes. The transition zone thickness between the weld layers was ~80 μm, and the width of the substrate-coating interface was 5–10 μm. The following coating constituents were detected: coarser elongated M₂B borides, finer particles of Cr₇C₃ carbides, and an Fe-based matrix consisting of ferrite and austenite. The nanohardness of the matrix was ~5–6 GPa, carbides ~16–19 GPa, and borides 22–23 GPa. A high cooling rate during coating fabrication leads to the formation of a fine mesh of M₇C₃ carbides; borides grow in the direction of heat removal, from the substrate to the friction surface, while in the transition zone, carbides become coarser. The dry sliding friction tests using a tribometer in PoD configuration were carried out at contact pressure 4, 7, 10, and 15 MPa against the AISI 1045 carbon steel (water-quenched and low-tempered, 50–52 HRC). The leading wear phenomenon at 4 and 7 MPa is fatigue, and at 10 and 15 MPa it is oxidation and delamination. Full article

The early-stage M₂₃C₆ morphology at the phase boundary between austenite and δ ferrite grain in Type 304L austenitic stainless steel was investigated with transmission electron microscopy (TEM). The M₂₃C₆ has coherency with austenite grains at phase boundary. The phase boundary between the M₂₃C₆ and austenite grains has curved appearance. The curved phase boundary might be a faceted interface composed of (111) plane and the other low-index planes on the atomic scale. The M₂₃C₆ morphology at the phase boundary was identified to be a complex pyramid-like shape that has {111}, {110} and {100} interfaces. The slight deviation in the angle measured between edges of carbide in the TEM images from that calculated between edges expected from the M₂₃C₆ morphology might be attributed to multilayer growth in the interior region of the M₂₃C₆ face. Full article

In this study, the process of reinforcing austenitic stainless steel with tungsten carbide (WC) particles prepared by an ex situ technique was investigated. More specifically, the effect of microstructural features on the properties of the resulting WC-metal matrix composite (WC-MMC) was studied. For that purpose, porous Fe-WC preforms, prepared by the ex situ technique, were fixed in the mold cavity where they reacted with the molten steel. As confirmed by scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS), the resulting composite showed a compositional and microstructural gradient in depth. The microstructure next to the surface is essentially martensite with large WC particles. From this region to the base metal, the dissolution of the original WC particles increased, being closely related to the formation of new carbides: (Fe,W,Cr)₆C, (Fe,Cr,W)₇C₃, and (Fe,Cr,W)₂₃C₆. At the interface bonding, a sound microstructure free of discontinuities was achieved. Furthermore, the mechanical tests indicated that the WC-MMC is four times harder and more wear-resistant than the base metal. Full article

In this work, austenitic stainless steel specimens were locally reinforced with WC particles. The reinforcements were fabricated via an ex situ technique based on powder technology. Mixtures of WC, Fe, and M0101 binder were cold-pressed to obtain powder compacts. After debinding and sintering, the porous WC–Fe inserts were fixed in a mold cavity, where they reacted with liquid metal. Microstructural analysis was conducted for characterization of the phases constituting the produced reinforcement zone and the bonding interface. The results revealed that the reinforcement is a graded material with compositional and microstructural gradients throughout its thickness. The zone nearest to the surface has a ferrous matrix with homogeneously distributed WC particles and (Fe,W,Cr)₆C and (Fe,W,Cr)₃C carbides, formed from the liquid metal reaction with the insert. This precipitation leads to austenite destabilization, which transforms into martensite during cooling. A vast dissolution of the WC particles occurred in the inner zones, resulting in more intense carbides formation. Cr-rich carbides ((Fe,Cr,W)₇C₃, and (Fe,Cr,W)₂₃C₆) formed in the interdendritic regions of austenite; this zone is characterized by coarse dendrites of austenite and a multi-phase interdendritic network composed of carbides. An interface free of discontinuities and porosities indicates good bonding of the reinforcement zone to stainless steel. Full article

This study is focused on isothermal and anisothermal precipitation of M₂₃C₆ carbides from the fully ferritic structure of the (γ + δ) austenitic-ferritic duplex stainless steel X2CrNiMo2253, (2205). During isothermal heat treatments, small particles of K-M₂₃C₆ carbide precipitates at the δ/δ grain-boundaries. Their formation precedes γ and σ-phases, by acting as highly potential nucleation sites, confirming the undertaken TEM investigations. Furthermore, anisothermal heat treatment leads to the formation of very fine islands dispersed throughout the fully δ-ferritic matrix. TEM characterization of these islands reveals a particular eutectoid, reminiscent of the well-known (γ-σ)—eutectoid, usually encountered in this kind of steel. TEM and electron microdiffraction techniques were used to determine the crystal structure of the eutectoid constituents: γ-Austenite and K-M₂₃C₆ carbides. Based on this characterization, orientation relationships between the two latter phases and the ferritic matrix were derived: cube-on-cube, on one hand, between K-M₂₃C₆ and γ-Austenite and Kurdjumov-Sachs, on the other hand, between γ-Austenite and the δ-ferritic matrix. Based on these rational orientation relationships and using group theory (symmetry analysis), the morphology and the only one variant number of K-M₂₃C₆ in γ-Austenite have been elucidated and explained. Thermodynamic calculations, based on the commercial software ThermoCalq^® (Thermo-Calc Software, Stockholm, Sweden), were carried out to explain the K-M₂₃C₆ precipitation and its effect on the other decomposition products of the ferritic matrix, namely γ-Austenite and σ-Sigma phase. For this purpose, the mole fraction evolution of K-M₂₃C₆ and σ-phase and the mass percent of all components entering in their composition, have been drawn. A geometrical model, based on the corrugated compact layers instead of lattice planes with the conservation of the site density at the interface plane, has been proposed to explain the transition δ-ferrite ⇒ {γ-Austenite ⇔ K-M₂₃C₆}. Full article