M23C6 Carbides Research Articles

Achieving excellent mechanical properties of ODS steel by Y2O3 addition

As a candidate material for fuel cladding in fission reactors, the mechanical properties of ODS-Ferritic/martensitic (F/M) steel should be further improved. Incorporating oxide nanodispersions with a high number density into the matrix is a promising strategy. In this study, the incorporation of the ultra-high number density of Y2O3 oxide nanoparticles in ODS steel was achieved by adding Y2O3 during Hot Isostatic Pressing (HIP), and the microstructure and properties were optimized by performing hot rolling deformation. The addition of Y2O3 has revealed two effects: the non-loss part of Y2O3 powder forms part of Y2O3 particles during HIP; the loss part generates a certain concentration of oxygen in the tank and reacts with yttrium element to generate another part of Y2O3 particles. Following HIP, a bimodal ferrite structure and reticular M23C6 carbides distributed along grain boundaries are revealed. The grain size and M23C6 carbide are remarkably refined following hot rolling while the Y2O3 particle size did not increase significantly. Due to dispersion and fine grain strengthening, and the effect of elimination of reticular M23C6, an excellent combination of strength (1474 MPa) and ductility (13%) has been achieved in ODS steel prepared by the HIP with Y2O3 addition and hot rolling.

Role of W addition in reducing heat checking and enhancing the mechanical properties of hot work tool steel

The influence of carbide precipitation on the mechanical properties of hot work tool steels was investigated in this study. Compared to the commercial H13 hot work tool steel, the KMoW and RW steels containing W exhibited a reduced high-temperature softening and an enhanced thermal fatigue resistance during long-term aging. These improvements can be attributed to the increased structural and chemical stability of V-rich MC carbides. The increased (W + Mo)/(V + Fe) atomic ratios at MC metallic sites increased their coherency as well as reduced the driving force required for the precipitation of Cr-rich carbides. These findings suggest that the addition of W effectively improves the heat-checking resistance and mechanical properties of hot work tool steels.

Study on Solidification and Phase Transformation Behaviors in Ni-based Superalloy IN625

The solidification and phase transformation behaviors of IN625 with low C and Fe were investigated through directional solidification and quenching experiments. The primary and secondary dendrite arm spacing decreased exponentially as the solidification rate increased, demonstrating good agreement with theoretical equations. The MC carbide formed at a constant temperature gradient regardless of the solidification rate. It is reasonable to assume that the Laves phase solidified in the final stage of solidification, as it was found at the bottom of the mushy zone. The morphologies and sizes of the MC carbide and the Laves phase were closely related to the solidification rate. Increasing the solidification rate at a constant thermal gradient resulted in the development of the MC carbide from blocky to Chinese script, and eventually spotty shapes, along with decreasing size. As a result of this research, it is believed that the weldability of the alloy could be improved at the expense of mechanical properties, especially with low C and Fe contents and a relatively high solidification rate, which are able to generate the finely distributed MC and Laves phase.

Effects of Cr Carbides Formation on the High Temperature Creep Property of Alloy 690 for Steam Generator Tube Material

The creep properties of Alloy 690, used as a steam generator tube material in nuclear power plants, were evaluated at 650°C, 750°C, and 850°C. The parameters of creep life prediction models were derived using the Larson-Miller (LM), Manson-Haferd (MH), and Orr-Sherby-Dorn (OSD) models, to use as mechanical properties under a virtual severe accident condition like station black out (SBO). The yield strength (YS) and creep property of Alloy 690 were compared with those of Alloy 600, and the effects of the precipitation behavior of Cr carbides on creep properties were analyzed. The YS of Alloy 600 decreased rapidly above the temperature of 750°C, but the YS of Alloy 690 decreased linearly up to the temperature of 850°C because of the formation of M23C6 carbides. The creep stress exponent (n) of Alloy 690 was between 5 and 6, and this indicated that dislocation creep was the major creep mechanism at the test temperatures. The results of creep tests were well matched with the LM, MH, and OSD models for Alloy 690, and there were no significant differences in accuracy between the models. The stress-rupture test results of Alloy 600 and Alloy 690 using the LM model showed that the decrease in creep strength with rupture time of Alloy 690 was steeper than that of Alloy 600 at high temperatures. This indicated that Alloy 690 was more susceptible to creep degradation under longterm creep conditions. The precipitation of Cr carbides in Alloy 690 increased YS, benefitting creep properties for short-term creep. However, the Cr carbides coarsened significantly under loading conditions at high temperature, and this deteriorated the creep properties for long-term creep.

Effects of cerium addition on the microstructure and stress rupture properties of a new nickel-based cast superalloy

The microstructure and stress rupture properties of a new nickel-based cast superalloy were investigated with the cerium (Ce) additions of 0, 19, 50, 96, 150, and 300 ppm, respectively. The results indicated that Ce was mainly found in MC and M23C6 carbides, and it was also found to exist in the form of cerium-rich phases or inclusions. According to the microstructure evidence, Ce promoted the formation of MC carbides and aggravated the inhomogeneity of M23C6 carbides along grain boundaries. It was also identified that the average sizes of primary and secondary γ' phases all decreased with the rising Ce content. The acceleration of Ti, Nb, and C segregations during solidification was attributed to the influence of cerium on the variation of carbides along grain boundaries. The stress rupture life experienced a significant drop as the Ce content increased from 19 to 300 ppm. Explorations showed that the degradation was mainly attributed to the severe degradation of MC carbides and the easily forming micro-voids around them caused by the Ce addition. In addition to that, the increments in the inhomogeneous distribution of M23C6 carbides at grain boundaries and the accelerated coarsening rate of γ' phases both induced the fracture under complex stress conditions.

Electron microscopy of carbides in annealed 28 wt% Cr - 1 wt% (Mo/W) cast irons

Phase identification and the microstructure of carbide phases present in annealed 28 wt% Cr-2.6 wt% C high chromium cast irons with the addition of Mo or W have been characterized using optical microscopy, scanning electron microscopy, X-ray diffraction and transmission electron microscopy. Electron energy-loss spectroscopy was also used to identify M7C3 and M23C6 carbides in these irons. Annealing was performed by holding at the temperature of 800 °C for 4 h, followed by slow cooling at 20 °C/s to 500 °C, and then furnace cooling. Multiple eutectic carbides including, M23C6 and M6C were observed in the as-cast iron containing 1.42 wt% Mo. In contrast, the addition of 0.99 wt% W led to the change from hypoeutectic to hypereutectic iron and only M7C3 carbide was found in the as-cast condition. The post-annealing microstructures of all irons contained eutectic carbides, coarse secondary carbides next to the eutectic carbides, fine secondary carbide precipitated within the ferrite matrix, and some lamellar pearlite at the central areas of prior austenite dendrites. The results of TEM observation via selected-area electron diffraction and energy-dispersive X-ray spectrometry confirmed that the coarse and fine secondary carbides and lamellar pearlitic carbides, formed during the annealing treatment, were M23C6 carbide. In addition, the fine structures in C K-edge spectra were found to be useful for distinguishing between M7C3 and M23C6 carbides.

Insights to the fracture toughness, damage tolerance, electronic structure, and magnetic properties of carbides M2C (M = Fe, Cr)

The fracture toughness, damage tolerance, electronic structure, and magnetic properties of M2C (M = Fe, Cr) carbides were analyzed using first-principles calculations. Calculations of formation energy and modulus of elasticity indicate that a Cr/Fe ratio of 1/3 is a critical threshold which triggers a significant increase in the corresponding stability and related mechanical properties. Cr atomic content enhances the crack resistance, while Cr has a significantly detrimental effect on damage resistance. The electronic properties demonstrated that the Cr atom content enhances the metallic, ionic and covalent bonding. Furthermore, the reduction in the coordination number of Fe atoms is the main reason for the reduction in the local magnetic moment of the low-spin Cr atoms, which is strongly supported by the electronic structure. These studies provide detailed insights into Cr-containing carbides, providing valuable theoretical and technological information for the knowledge-based design and prediction of the mechanical properties of chromium-containing iron-based materials.

The stepped growth mechanisms of micro-sized hexagonal M7C3 carbides: Self-assembly shell and epitaxial-layered core

The stepped growth mechanisms of micro-sized hexagonal M7C3 carbides: Self-assembly shell and epitaxial-layered core

Microstructural characteristics and mechanical response of diffusion bonding Inconel 617 superalloy

This study focuses on the microstructural characteristics and mechanical response of the diffusion-bonded joints of Inconel 617 superalloy. The joints exhibited two types of interfacial microstructure involving the straight interface (i.e., bond line) and the interface with partial migration of grain boundaries (GB). The straight interface was composed of the GB of γ-Ni, M23C6 carbide and α-Al2O3 oxide. The M23C6 carbide was precipitated with a semi-coherence of 33̅3M23C6//111̅γ−Ni, and an amorphous interface was detected between the α-Al2O3 and γ-Ni phase. The migration of GB across the interface was mainly caused by dynamic recrystallization and strain-induced GB migration. In addition, the α-Al2O3 oxide was remained inside the migrated grain. The formation of the M23C6 and α-Al2O3 was governed by diffusion mechanism, and the γ′-Ni3Al phase was formed surrounding the α-Al2O3 due to the abundant diffusion of Al. The straight interface, particularly embedded with the hard particles (i.e., the M23C6 carbide and α-Al2O3 oxide), restricted GND slipping, resulting in concentration of stresses and initiation of cracks. The migrated interface, in contrast, effectively prevented crack propagation and induced crack deflection, supporting more continuous and uniform plastic deformation. The elongation of 51.2 % was obtained for the joint with partial migrated interface, reaching 71 % of the base metal. Nevertheless, the optimum impact toughness of the joints (20.38 J/cm2) was merely 24 % of the base metal, mainly affected by the remained straight interface.

Influence of gadolinium on solidification behavior of Ni–Mo-Cr-Gd alloy

The influence of Gd content on the solidification behaviors and as-cast microstructures of Ni–Mo-Cr-Gd alloy have been investigated systematically. It was found that the γ/Ni5Gd eutectic phases were formed in the interdendritic region, which exhibits the divorced character. With the Gd adding, the γ/M6C eutectic phases were replaced by the γ/M2C eutectic ones. Most M2C carbides locate at the interfaces of γ/Ni5Gd eutectic phases or are enclosed by Ni5Gd phases. The segregation of alloying elements was mitigated by Gd adding. Based on the thermal analysis, composition analysis and thermodynamic calculation, the solidification behaviors, and the affecting mechanism of Gd adding were briefly discussed. During the solidification process, the γ/Ni5Gd eutectic reaction occurs first, which interrupts the solute redistribution process and thus mitigates the segregation of some alloying elements. The Ni5Gd phases accommodate a mass of Si and keep the Si content in the residual liquid phase low, which decreases the nucleation rate of M6C carbides. Then, the γ/M2C eutectic phases become the dominant ones at the final stage of the solidification process. Our results are useful in future optimization and processing research of Ni-Mo-Cr-Gd alloy for applications in molten salt reactors.

Effect of W/Mo ratio on the microstructure of Ni-Mo-W-Cr-Nb based superalloys after thermal exposure

The structural materials used in molten salt reactors (MSRs) require enough high-temperature strength, good chemical compatibility with fuels and irradiation damage resistance. The Nb addition and W alloying are two effective optimization paths, which have been proven to respectively improve the embrittlement problems and high-temperature strength of alloys. It is worth exploring whether one full-featured alloy can be developed for the high-temperature MSRs by combining the benefits of Nb addition and W alloying. In this work, four Ni-Mo-W-Cr-Nb based superalloys with different W/Mo ratios were designed by substituting W for Mo in the standard Nb-modified alloy. The microstructural evolution and mechanical properties of these alloys were evaluated systematically after thermal exposure at 750 °C. As the W/Mo ratio increased, more undissolved primary MC carbides remained, while fewer secondary carbides formed in the matrix and at the grain boundaries after thermal exposure. Micro-hardness and tensile tests showed that the substitution of Mo by W can provide an obvious strengthening effect for alloys in the solid-solution treated state. After thermal exposure, all alloys exhibited precipitation-induced strengthening, but the strengthening degree decreased with the W/Mo ratio due to the fewer secondary MC carbides. Based on experimental observations and thermodynamic calculations, it was found that W atoms cannot easily occupy the lattice sites of MC carbides as Mo atoms do, and the absence of secondary carbides in the high-W alloys is due to the undissolved primary MC carbides and the low Mo concentration.

The effect of small additions of Nb and Ti on the sliding wear behavior of a Co–30Cr–5Mo alloy

This work studies the addition of 0.5%Nb and 0.5%Ti in a Co–30Cr–5Mo alloy as well as the increase of carbon from 0.1 to 0.3%. The main goal is to increase the wear resistance of the alloy with such additions due to the formation of primary carbides (TiC and NbC). The additions of carbon, niobium and titanium were as ferroalloys, which implies the addition of iron into the chemistry of the alloy. Microstructural prediction studies by JMatPro indicate that the addition of all of these elements promotes an increase in the amount of the sigma phase, an undesirable phase because it is a hard brittle intermetallic compound. However, this undesirable phase could be eliminated from the structure by means of a solubilization heat treatment undertaken at 1200 °C for 2 h; this heat treatment lowered the values of hardness of the alloys. An additional aging heat treatment promoted the precipitation of chromium carbides of the type M23C6 as dispersed elongated bars within the cobalt matrix, which notably increased the hardness of the alloys. Each alloy was wear tested under two loads 52 and 78 N and under dry sliding and also under lubricated sliding using Hank solution as lubricant; the tests were undertaken at a sliding speed of 0.5 m/s. Results indicate that the addition of Ti and Nb produced small amounts of TiC and NbC in addition to a light increase in the amount of the sigma phase which increased hardness in the as-cast conditions. These alloys, with Ti and Nb additions, shown the higher hardness values in the as-aged conditions. The wear resistance of the alloys are in agreement with the hardness results particularly for the dry sliding tests. Wear resistance was higher for the harder alloys (those with Nb and Ti additions in the as-aged conditions) and it is lower for the higher applied load. For the case of the lubricated wear tests, such a trend is also observed but not so clearly as for the unlubricated tests. However, friction and wear for the lubricated experiments shown values extremely low compared to these observed for the unlubricated sliding wear tests.

Microstructure Stabilities of Newly Developed Hot‐Working Die Steel and H13 Steel at Elevated Temperature

Herein, the microstructure stability of a newly developed hot‐working die (HWD700) steel at elevated temperatures up to 700 °C is investigated using transmission electron microscopy (TEM), selected‐area electron diffraction, energy‐dispersive spectrometer, and high‐resolution transmission electron microscope. The results show that the HWD700 steel exhibits a higher thermal stability than the commercial H13 steel when the temperature is up to 700 °C. After quenching and tempering, high densities of intra‐lath rodlike and inter‐lath spherical MC nano‐carbides and inter‐lath strip‐like M7C3 nano‐carbides are observed in the HWD700 steel. These carbides remain nanoscale at temperatures up to 700 °C. For comparison, rod‐shaped MC nano‐carbides in the tempered H13 steel tend to coarsen and transform into larger M23C6 carbides at temperatures up to 650 °C. The TEM observation further reveals that the interface misfit between the MC nano‐carbide and matrix in the HWD700 steel is as low as 0.88%, lower than that of 14.8% in the H13 steel. This can be the main reason for the improved stability of the HWD700 steel by pinning dislocations and grain boundaries and thereby hindering the recrystallization process at an elevated temperature.

The effects of chemistry variations on hot cracking susceptibility of Haynes® 282® for aerospace applications

Hot cracking susceptibility of Haynes® 282® with varying amount of C (0.05–0.09 wt%), Mn (0.03–0.12 wt%), Si (0.03–0.16), B (0.005–0.006 wt%) and Zr (0–0.01) are investigated. Synergistic role of C and B is found on solidification and heat affected zone (HAZ) liquation cracking susceptibility. High amount of C and B promote formation of eutectic constituents during final stages of solidification and promote crack healing by backfilling effect. When C and B are added in low amount the crack healing does not occur due to the absence of eutectic consituents therefore cracking extent increases. Thermodynamics simulations indicate C and B tie up to MC carbides and M3B2 borides during solidification. Scanning Electron Microscopy and Nanoscale secondary ion mass spectrometry analysis reveal C and B to be present both in solid solution and in form of precipitates to Ti-Mo rich carbides and Mo rich borides, respectively. In HAZ, these phases promote liquation cracking where cracking extent correlates to the amount carbides and borides. Lower C and B is found to reduce the liquation cracking in the HAZ. Furthermore, a high temperature homogenization heat treatment at 1190 °C excarbates the cracking by dissolving the borides and releasing B to the grain boundaries.

Achieving polycrystalline transformation and microstructural segregation reduction of nickel-based single crystal super-alloys by ultrasonic pulse arc welding

The sub-grains coarsening and low melting point eutectic are the internal causes of cracking of single crystal super-alloys in welding. Aiming to overcome two internal causes, in this paper, the effect of ultrasonic pulse arc welding on the grains, sub-grains and the precipitations of single crystal super-alloys (IN738) has been comprehensively studied, and the microstructure evolution of IN738 in ultrasonic pulse arc welding has been systematically revealed. The results show that (1) the combined effect of ultrasonic vibration and electromagnetic oscillation not only realizes the synchronous refinement of grains and sub-grains in WMZ (weld metal zone), and the polycrystalline transformation of single crystal super-alloys. A novel 3D schematic model of sub-grains is established to evaluate the size and orientation of sub-grains in 3D direction. (2) The ultrasonic vibration and electromagnetic field induced by ultrasonic pulse current can effectively reduce the microstructural segregation of welds. With the increase of ultrasonic pulse frequency, the size and volume fraction of harmful precipitations (η + δ eutectic, mixed eutectic and MC carbides) decrease greatly, especially the morphology and distribution characteristics of γ′ in HAZ (heat affected zone). Thanks to the above outstanding advantages, ultrasonic pulse arc welding has great application value in single crystal super-alloys.

Effect of carbide orientation during single scratch test in directionally solidified and heat-treated high chromium cast irons

In this work, a billet of eutectic high chromium cast iron, material commonly used in highly abrasive environments, such as in grinding and milling, was unidirectionally solidified. The resulting microstructure was composed of oriented M7C3 carbides and eutectic matrix. Specimens were cut parallel and transversally to the billet axis and split into two groups. Both were heat treated with different austenitization temperatures. The temperature for the first group was 1100 °C, and the value for the second was 950 °C, providing matrix instrumented indentation hardness values of 8.0 and 6.3 GPa, respectively. This procedure resulted in specimens with three carbide directions dispersed in two different matrices. Samples were evaluated by means of single scratch test with a cono-spherical diamond stylus under progressive and constant normal load conditions. To support the analysis, a non-contact 3D surface profiler and scanning electron microscopy (SEM) were used to characterize the scratched surfaces. Progressive load tests indicated a transition in wear micro-mechanisms which was mainly observed in the samples with the long axes of the carbides parallel to the wear surface and oriented orthogonal to the scratch direction and with matrix hardness of 6.3 GPa. On the other hand, constant load tests showed that carbides with the long axes parallel to the wear surface and oriented parallel to the scratch direction tend to produce deeper scratchers with higher ridges than in the other directions. Results also indicate that the hardness, despite being an important variable, does not entirely explain the behavior of the material under single scratch, since the orientation of the carbides with respect to the scratch direction must also be considered.

Processing Fundamentals and Performance Investigation of Selective Laser Melting of High‐Speed Steel in Reactive N2 Atmosphere

The distinct deposition method of selective laser melting (SLM) enables the near‐net‐shaping of high‐speed steel tools with complex geometries and integrated functions, such as adapted cooling channels, which offers economic advantages and has, thus, attracted considerable attention. However, SLM‐processed high‐speed steels are prone to cracking due to their high carbon and alloying element contents and high internal stresses. Varying the steel composition to stabilize austenite can help to reduce the residual stresses. Herein, the effect of nitrogen atmosphere on the densification behavior, microstructural evolution, and mechanical properties of high‐speed steel in the SLM process is investigated. The results demonstrate that nitrogen can dissolve in the steel. In combination with suitable SLM parameters, this leads to a fully austenitic microstructure with no cracks. After heat treatment, the microstructure of steel transforms into tempered martensite, along with the precipitation of tiny V‐rich M(C,N) carbonitride and V‐rich MC carbide. The hardness and bending strength of the tempered sample reach the highest of 61.3 HRC and 3659 MPa, respectively. Its abrasive resistance is also improved. An alloy design idea is provided that is based on the reactive atmosphere of high‐speed steel and other materials for processing by SLM.

Liquation crack-free welding strategy for 247LC DS superalloy by control of pipeline diffusion via ultra-high-speed laser beam scanning

Under completely suppressed constitutional liquation of MC carbide, unexplored 247LC superalloy single-mode fibre laser welds experience unexpected liquation cracking. These cracks continuously propagate from a solidification crack, particularly at highly misoriented and epitaxially grown solidification grain boundaries, and dominated by pipeline diffusion of impurities. Two novel metallurgical preconditions were identified which, if satisfied, ensure complete suppression of liquation cracking of 247LC superalloy welds. These preconditions are (i) the constitutional liquation of MC carbide at the heat-affected zone and (ii) pipeline diffusion of impurities at the fusion zone. It is also highly recommended that the welding is carried out in a single directionally solidified grain under scanning conditions, avoiding the formation of highly misoriented solidification grain boundaries.

Effect of austenitizing temperature on microstructure and mechanical properties evaluation of microalloyed low-carbon RAFM steel

The microstructure and mechanical properties of microalloyed low-carbon reduced activation ferritic/martensitic (RAFM) steel after austenitization at 950 °C, 1000 °C and 1050 °C for 0.5 h and tempering at 750 °C for 1.5 h were investigated using scanning electron microscopy, transmission electron microscopy, electron back scattering diffraction, X-ray diffraction, tensile tests, and impact tests. The grain size and lath width obviously increase because more M23C6 carbides dissolve after austenitization at 1050 °C. Besides, more fine MX particles would precipitate and result in higher dislocation density. Furthermore, the strengthening mechanisms at room temperature were also systematically discussed, the predominant strengthening mechanism changes from grain boundary strengthening and dislocation strengthening to dislocation strengthening when the austenitizing temperature increases from 950 °C to 1000 °C and 1050 °C. After taking the microstructure, strength, ductility, and toughness into thorough consideration, the optimum austenitizing temperature for microalloyed low-carbon RAFM steel is about 1000 °C.

Creep anisotropy reduction and improvement via post-heat treatment in yttrium-added Hastelloy-X fabricated by laser powder bed fusion

The development of laser powder bed fusion (LPBF) has made it possible to produce complex three-dimensional components for high-temperature applications. The LPBF process needs to be refined to address several key factors, such as high-temperature elongation, microstructure heterogeneity, and mechanical anisotropy. Hastelloy-X Ni-based superalloy was used to illustrate these issues in this study. First, 0.046 wt.% of yttrium (Y) was added to Hastelloy-X (HX-y) to prevent grain boundary embrittlement. The second step involves two kinds of post-heat treatments; i) at 850 °C for 2 h (DA) for carbides and ii) solution heat treatment at 1240 °C for 8 h upon aging at 850 °C for 2 h (HTA). The creep properties of the samples were compared at 900 °C/ 80 MPa to understand the effect of Y addition and post heat treatments. The HX-y specimen was strengthened by solid solution and dispersion of Y-rich oxides, together with stabilization of oxygen-based contamination at the grain boundaries. The DA and HTA HX-y specimens had better creep properties than the HX specimens. The HX-y specimen showed superior creep properties to the HX specimen due to the presence of carbides M6C and Cr23C6 inside grains and at grain boundaries. However, carbides remained stable even at high temperatures within grains and at grain boundaries. Nevertheless, the HTA HX-y specimen exhibited superior isotropic creep properties. As a result of grain boundary pinning, serrated grain boundaries prevented grain boundary sliding. In contrast, HX specimens exhibited poor creep properties. This study confirmed that the optimal addition of Y together with the optimization of post-heat treatment drastically enhances the creep properties of materials fabricated by the LPBF process.