M23C6 Carbides Research Articles

Room Temperature Corrosion Behavior of Selective Laser Melting (SLM)-Processed Ni-Fe Superalloy (Inconel 718) in 3.5% NaCl Solution at Different pH Conditions: Role of Microstructures

Inconel 718 (UNS N07718) is a nickel-base superalloy containing iron that is used at cryogenic temperatures (arctic pipe components) and at high temperatures (gas turbines). This alloy is also used in off-shore oil drilling due to its high overall strength and resistance to corrosion. Inconel 718 components are created by a selective laser melting (SLM) additive manufacturing route and result in isotropic fine-grained microstructures with metastable phases (such as Laves phases) that are not usually present in conventional manufacturing processes. In this work, SLM Inconel 718 alloy specimens were investigated in four different conditions: (1) As-manufactured (AS-AM), (2) Additively manufactured and hot isostatically pressed (AM-HIP), (3) As-manufactured and heat-treated (solution annealing followed by two-step aging), and 4) AM-HIP and heat-treated. Localized corrosion behavior was evaluated at room temperature in a 3.5% NaCl solution at three different pH conditions (pH 1.25, 6.25, and 12.25). Electrochemical tests, including linear polarization, cyclic polarization, potentiostatic conditioning, electrochemical impedance spectroscopy, and Mott–Schottky analyses, were used to compare the corrosion behaviors of the SLM specimens with that of the conventionally wrought IN718 samples. The results showed that the additively manufactured specimens showed better corrosion resistance than the wrought material in the acidic chloride solution, and the AM-HIP specimens exhibited superior corrosion resistance to the as-manufactured ones. Hot isostatic pressing resulted in the visible elimination of the dendritic structure, indicating compositional homogeneity as well as a significant decrease in porosity. In addition, the deleterious secondary phases, such as Laves and δ phases, were not observed in the microstructure of the HIPed samples. The AM-HIP material showed the highest corrosion resistance in all the pH conditions. The two-step aging treatment, in general, resulted in the deterioration of corrosion resistance, which could be attributed to the formation of γ′ and γ″ precipitates that increased the cathodic reaction catalytic activities. In the additively manufactured samples, the presence of the Laves phase was more detrimental to corrosion resistance than any other phases and MC carbide and grain boundary δ phase increased the susceptibility to corrosion in wrought materials.

Numerical and experimental study of underwater friction stir welding of 1Cr11Ni2W2MoV heat-resistant stainless steel

Due to the benefits of solid-state joining, friction stir welding (FSW) has seen an increase in applications in the aircraft and automotive industries. However, short tool life and heat-affected zone (HAZ) softening are the two main issues with the FSW of advanced high-strength steel. In this study, the underwater friction stir welding (UFSW) of 1Cr11Ni2W2MoV steel was investigated and compared to the Normal friction stir welding (NFSW). Besides, a 3-D thermo-mechanical finite element model was developed to understand the effect of the different cooling mediums and tool rotation rates on the thermal cycles, plastic deformation, tool wear, and microstructure evolution. The simulation results revealed that the stir zone (SZ) of the UFSW exhibits lower peak temperatures and smaller plastic strain compared to the NFSW. The experimental results showed that a high tool rotation rate can be used during UFSW without causing overheating of the workpiece and tool materials resulting in high joint quality and low tool wear. The M23C6 and Fe3C carbide fractions in the HAZ at UFSW are lower than that at NFSW. The UFSW reduces the soft HAZ's width and increases its hardness. The ultimate tensile strength was increased by 17 %, and the HAZ width was decreased by 35 % at UFSW compared to that at NFSW.

Failure analysis of the end surface of the burner nozzle made of UMCo50 material in long-term high temperature oxygen-enriched sulfur-chlorine environment

As the core device of coal water slurry gasification technology, the failure of the burner nozzle will bring serious technical and safety problems. The purpose of this paper is to determine the mechanism of burner failure based on the characteristics of corrosion products and cracks. The results showed that the Cr2O3 oxide film on the surface of UMCo50 was damaged under the effect of mechanical erosion, high temperature and stress, and the corrosive elements S and Cl in the combustion chamber made it further cracked and peeled off. Therefore, the matrix was corroded and cracks occurred. During the crack propagation process, more M23C6 carbides were precipitated at the grain boundary, it caused Cr-depleted at the grain boundary, which reduced the corrosion resistance of the matrix near the grain boundary. The exfoliation of carbides distributed along the grain boundary also accelerated crack propagation. Through XPS and XRD analysis, it was found that the main corrosion products on the crack surface of the failed parts were: Cr2O3, CoCr2O4, CoO, Fe2O3, FeS. According to the causes of failure analysis, the corresponding improvement measures are put forward.

Precipitation, Growth, and Aggregation Behavior of Primary Carbides during Continuous Casting of 6Cr13 Slabs

The types, distribution, morphology, orientation, growth, and aggregation mechanism of primary carbides in the continuous casting slab of 6Cr13 high‐carbon martensitic stainless steel are investigated. In the results, it is shown that M7C3 carbides precipitate from the liquid steel at the end of solidification. The area fraction of primary carbides in the slab thickness direction shows an overall decreasing trend from the center to the edge. And, the closer to the center region, the greater the difference in carbide distribution due to the presence of spot segregation defects. At 1/8 thickness and edges in the slab, the fast cooling rate and fine dendrites make the carbide morphology to be a small‐sized brain like that aggregates a large number of spherical carbides. At 1/4 thickness in the slab, blocky, fibrous, and spherical carbides precipitate successively and eventually aggregate together. At 3/8 thickness and center in the slab, the cooling rate is slower and spot segregation defects are randomly distributed in this region. The carbide morphology is mostly small‐sized brain like in the non‐spot segregation region. However, the alloying elements are highly supersaturated and the carbides can grow sufficiently to form less oriented, large‐sized brain‐like carbide in the spot segregation region.

Influence of Cooling Rate and Alloying by “Cr, V, and Ni” on Microstructure and High-Temperature Wear Behavior of SiMo Ductile Iron

AbstractSiMo ductile irons, typical heat-resistant materials, are subjected to varied wear environments during operation in high-temperature applications. SiMo ductile iron castings of different thicknesses were cast in investment and greensand molds, achieving a wide range of cooling rates. The present work aims to investigate the effect of the cooling rate and alloying elements (Cr, V, and Ni) on the microstructure and the abrasive wear behavior of these grades of SiMo ductile iron at high-temperature 700 °C under different loads. Thermodynamic calculations were used to propose the phase diagrams, critical transformation temperatures, and phase volume fractions in all SiMo alloys by using the Thermo-Calc software then verified by and Differential Scanning Calorimetry (DSC). The microstructure of unalloyed SiMo ductile cast iron consists of graphite nodules and carbides embedded in the precipitates at the grain boundary regions in a ferrite matrix. The alloyed SiMo microstructure contains nodular graphite and the carbides promoted by the alloying elements (Cr and V). The alloyed SiMo alloys exhibit higher wear resistance than unalloyed ones. These wear results support that the microstructure plays a chief role in wear loss. The combination of M6C, VC, and M7C3 carbides embedded in a ferrite-pearlite matrix (alloyed SiMo) seems to be more resistant to wear than the ferritic matrix with lamellar pearlite and eutectic M6C carbides (unalloyed SiMo).

Effect of carbon content and cooling rate on the microstructure and hardness of TiC-Fe-Cr-Mo cermets

TiC-FeCrMo cermets have been obtained in fully dense form by Sinter HIP at 1400 °C. Significant microstructural changes have been observed in these materials for relatively small variations in their carbon content after sintering. In the cermets with higher carbon content Cr-rich likely M7C3 carbides are observed to precipitate at the (Ti1-x,Mox)yCz - metal interface. In addition, these cermets present a significant amount of retained austenite as part of the metal matrix. No retained austenite and many fewer M7C3 carbides are found in alloys with a reduction of 0.2 wt% in the total C content. Continuous cooling diagrams have been obtained from an austenitizing temperature of 950 °C. Hardness increases by 30% with respect to that of as sintered specimens after cooling at 1 °C/s confirming that these TiC-FeCrMo cermets are suitable for hardening by air-quenching. At this cooling rate, it is observed that the relatively small carbon changes mentioned before have a significant effect on the bainitic transformation, displacing its onset to higher temperatures as the C content is reduced. Slower cooling rates result in complex microstructures, in which, in addition to martensite, ferritic bainite, M7C3 and M23C6 carbides are also found. Microstructure and hardness of TiC-FeCrMo materials can be modified by the use of standard heat treatments to obtain a wide variety of mechanical properties suitable for certain hot rolling applications.

Effect of Silica Size on the Relative Wear of Microstructures Containing Solidification Carbides and Tempered Martensite Using a Dry Sand Rubber Wheel Test

The objective of this study was to examine the impact of the size of silica sand particles on the wear resistance of two microstructures containing solidification M7C3 carbides in tempered martensite matrix (namely, AISI D2 steel and high-chromium white cast iron). Wear tests were conducted using abrasive silica sand particles of three different sizes (0.20, 0.50, and 1.0 mm) in a dry sand rubber wheel tribometer (DSRW) following the ASTM G65 standard. Subsequently, these samples were analyzed using microscopy and three-dimensional optical profilometry. The results showed that the wear loss increased with increased abrasive particle size, particularly when the abrasive size increased from 0.5 to 1.0 mm. The microscopic observation revealed silica sand incrustation and plastic deformation of the tempered martensite matrix, cracking of the solidification M7C3 carbides, and detachment of the carbides and matrix. Furthermore, the largest abrasive particle size triggered a more damaging wear mechanism, leading to severe fracture damage of the solidification carbides.

Effects of yttrium content on solidification, microstructure, and mechanical properties of laser powder bed fused IN718 superalloy

The microstructural evolution and mechanical properties of IN718 superalloy with different yttrium (Y) contents were investigated. In this study, three alloys containing 0 wt%, 0.07 wt%, and 0.58 wt% of Y were fabricated by laser powder bed fusion (L-PBF). The experimental results showed that with a minor addition of Y content (0.07 wt%), the tensile ductility at 650 ºC increased from 10.3% to 20.3%, and the creep life increased from 203 h to 1545 h. The increased tensile ductility and creep life are attributable to the formation of fine MC carbides, which induce serrated grain boundaries, and to the strong affinity of Y for oxygen, resulting in the formation of Y2O3 to strengthen the grain boundaries. However, when the Y addition increased (0.58 wt%), the formation of a large Y-rich phase at the grain boundary reduced the tensile ductility and creep life.

Design of a 2.7 GPa ultra-high-strength high Co–Ni secondary hardening steel by two-step nano-size precipitation tailoring

High Co–Ni secondary hardening steels are valuable in the field of aviation materials. With the increasing demand of lightweight, further enhancing its strength has received widespread attention. In this study, instead of using only M2C carbides as the reinforcements in the traditional high Co–Ni secondary hardening steels, nano-size NiAl phases were introduced to provide additional strength. To obtain the collaborative precipitation of these two reinforcements, systematically thermodynamic calculation, including the driving force, coarsening rate, etc., was made to quantitatively tailor the composition and the two-step aging parameters. The calculation results showed that adding 1 % Al into the standard composition of M54 is a plan with high potential and a second aging process at 450 °C should be added to promote NiAl precipitation. Based on the experimental validation, the target microstructure formed as the design. With the dual-phase strengthening of NiAl particles and M2C carbides, the yield and tensile strength are greater than 2.3 and 2.7 GPa, respectively, which increases the strength of M54 steel by >30 %, retaining sufficient elongation and relatively low cost. This new designed alloy increased the strength–ductility balance of high Co–Ni secondary hardening steels and aviation maraging steels from ∼12.5 to 13.5 GPa%.

Microstructural characteristics, hardness and tribological behavior of additive manufactured CM247LC nickel super alloy

Nickel superalloy CM247LC was fabricated by laser powder bed fusion (LPBF) and its microstructure, hardness and tribological properties were investigated. X-ray diffraction analysis showed the presence of γ, γ′ and metastable MC carbides. Microstructure revealed cellular structure with fine MC carbides at cell boundaries. Higher hardness of 408 HV was attributed to grain refinement from rapid solidification and strengthening from MC carbides. Wear testing showed lowest wear rate of 1.46x10-9 mm3/m at 10 N load and predominant wear mechanisms were abrasive and adhesive wear. This study recommends LPBF as a promising method for manufacturing CM247LC superalloy for load bearing and tribological applications.

Microstructures Evolution of a Second Generation Single Crystal Superalloy after Long Term Aging at 980°C

The microstructure evolution of the second-generation single crystal superalloy DD412 aged at 980°C for 1000 h were investigated. The results show that needle-shaped topologically close packed (TCP) phases rich in Re and W were firstly found in the dendritic core region after aging at 980°C for 400h, while the size and amount of TCP phases increased gradually with aging time, which is due to the fact that high melt point elements Re and W segregated to the dendritic core region. With the increment of aging time, the size of γ′ phase increased and volume fraction of γ′ phase decreased. The γ′ phase in dendrite core and inter-dendrite region remained cuboidal basically, and the γ′ phase in second dendritic arm showed directional coarsening after aging for 800 h. The MC carbides in inter-dendritic region decomposed gradually and transformed to the M6C carbides partially during long term aging, while few fine blocky M6C carbides precipitated in dendritic core after aging for 400h. The different microstructure evolution in dendritic region is due to the dendrite segregation of alloying elements and elements diffusion during aging.

Малоцикловая усталость 10 % Cr стали с высоким содержанием бора при комнатной температуре

High-chromium martensitic steels are a promising material for the production of elements of boilers and steam pipelines, as well as blades and rotors of steam turbines for new coal-burning thermal generating units. The use of such materials will give an opportunity for the transition to ultra-supercritical steam parameters (temperature of 600–620 °C and pressure of 25–30 MPa), which will allow increasing the efficiency of generating units to 45 %. Modifications of the chemical composition of high-chromium steels have led to significant improvements of high-temperature properties such as 100,000 h creep strength and 1 % creep limit, while resistance to softening due to low-cycle fatigue remains understudied in this field. This work covers the study of low-cycle fatigue at room temperature with different amplitudes of deformation of martensitic high-chromium 10%Cr–3%Co–2%W–0.5%Mo–0.2%Cu–0.2%Re–0.003%N–0.01%B steel. The steel was pre-subjected to normalizing at 1050 °С followed by tempering at 770 °С. After heat treatment, the steel structure was a tempered martensitic lath structure stabilised by the particles of secondary phases of M23C6 carbides, NbX carbonitrides, and M6C carbides. The average width of martensite laths was 380 nm, and the dislocation density was 1.4×1014 m−2. At low-cycle fatigue, with an increase in the strain amplitude from 0.2 to 1 %, the number of cycles before failure significantly decreases, and the value of plastic deformation in the middle of the number of loading cycles significantly increases. Maximum softening (18 %) is observed at a strain amplitude of 1 % in the middle of the number of loading cycles. In general, the steel structure after low-cycle fatigue tests does not undergo significant changes: the width of the laths increases by 18 % at a strain amplitude of more than 0.3 %, while the dislocation density remains at a rather high level (about 1014 m−2) at all strain amplitudes.

Influence of magnesium treatment on inclusions and primary carbides in As-cast H13 die steel

In this study, the inclusions and primary carbides in H13 hot-work tool steel were investigated, both with and without magnesium treatment, utilizing various analytical techniques such as optical microscope (OM), scanning electron microscope (SEM), energy-dispersive spectroscopy (EDS), and quantitative analysis methods. Furthermore, the experimental results were compared with the calculations obtained from Thermo-Calc software. The results revealed that as we move from the edge to the center of the steel, the equivalent diame-ter of inclusions increased. However, upon adding magnesium to the steel, the size of inclusions decreased, while the number of inclusions per unit area increased. The primary carbides found in H13 steel consisted of vanadium-rich MC carbides and molybdenum-chromium-rich M2C carbides. The addition of magnesium had a significant impact on the size of these primary carbides. The presence of MgAl2O4 particles acted as heterogeneous nuclei, providing nucleation sites that induced smaller sizes and more uniform distribution of primary carbides. Furthermore, it was observed that the banded segregation in the annealed microstructure became more uniform after magnesium treatment, leading to an improvement in the banded segregation.

Effect of Temperature and Ageing on Fracture Toughness of Alloy 617M

In recent years, Advanced Ultra Super Critical (AUSC) plants have used Ni-Cr-Co-Mo based alloy 617M for high-temperature components like steam piping and rotors, where material resistance against fracture after prolonged exposure in a hostile environment is a matter of concern to the designers. To this end, fracture toughness characterization of the material in as-received and aged conditions using an elastic-plastic fracture mechanics (J-R curves) approach can provide a significant input towards the component integrity assessments at service conditions. In this study, J-R curves of the alloy 617M have been determined for the as-received and differently aged (up to 20000 h at 710 °C) conditions using compact tension (CT) specimens following ASTM E 1820-13. Repeated tests have been carried out at temperatures 27 °C, 650 °C, 710 °C and 750 °C. The fracture toughness (JIC) thus determined, has shown a declining trend with test temperatures. This attributes to the ease of the crack initiation process owing to the γ′ induced matrix hardening and subsequent ageing-inducing precipitations of M23C6 carbides at the grain boundaries.

Influence of Boron on Stress-Relief Cracking Susceptibility of T23 Steel

The effect of boron on stress-relief cracking (SRC) sensitivity in the coarse grain heat-affected zone (CGHAZ) of ASME SA213-T23 was studied by thermo-mechanical simulation. Then, the fracture mode, microstructure, carbide evolution, and boron segregation were observed by an optical microscope, a scanning electron microscope, a transmission electron microscope, and an electron probe microanalyzer. Finally, the mechanism of increasing boron content to improve SRC resistance was described. The results showed that when the boron content is lower than 0.0038 wt.-%, T23 steel is sensitive to SRC at 550–750°C (1022–1382°F), and the sensitive temperature range narrows as the boron content increases. When the boron content increases to 0.010 wt.-%, SRC can be eliminated. Moreover, boron addition did not improve grain boundary (GB) strength nor did it fundamentally change the fracture mode at high temperatures, but it significantly improved intergranular ductility. This is because the boron segregation at the GB inhibits the precipitation of M23C6 carbides, which reduces the void nucleation and alloy element depletion near the GB, thus significantly improving intergranular plasticity. The improvement of intergranular plasticity gives the grain sufficient time to deform and greatly improves the overall plasticity of the CGHAZ. As a result, the SRC resistance of the welded joint is significantly improved because the stress can be released through enough plastic deformation during postweld heat treatment or service.

Effect of hot isostatic pressing on microstructure and properties of high chromium K648 superalloy manufacturing by extreme high-speed laser metal deposition

High chromium superalloys have good overall mechanical properties and are widely used in critical hot-end components. The current manufacturing process still faces the problem of low forming efficiency. Extreme high-speed laser metal deposition (EHLMD) is a new and efficient additive manufacturing technology that can quickly and directly shape the direct forming of difficult-to-process materials. However, EHLMD still possesses some inter-layer defects in EHLMD alloy due to its excessively fast scanning speed, implying that there is potential for further improvement in its properties. Hot isostatic pressing (HIP) as a post-heat treatment is primarily used for the removal of defects in additive manufacturing components. In this study, the defects, microstructure, and properties of the EHLMD K648 superalloy using Micro-CT, XRD, SEM and TEM under HIP treatment were investigated. The results show that the porosity of the HIP EHLMD K648 superalloy decreased, and slender needle-shaped α-Cr phase, γ′ phase and coarse M6C carbides were precipitated. The tensile strength of the HIP EHLMD K648 superalloy increased by about 240 MPa. However, its plasticity was reduced. To further improve the plasticity of the HIP EHLMD K648 superalloy, subsequent heat treatment was carried out. The outcomes of this study are expected to promote the manufacturing of high-quality chromium superalloys and the development of EHLMD technology.

Effect of Hf and Ta on Ni-based superalloy powders

The effect of Hf and Ta modifying on the microstructure and composition of Ni-based superalloy powders was investigated. The element concentrations on the powder surface and subsurface were quantitatively characterized by the Auger electron spectroscopy, and cross sections of powders were observed by the electron probe microanalysis. The results show that Hf and Ta hardly change the morphology of powders, but significantly change the powder surface and subsurface composition. Hf and Ta enhance the volume fraction of MC carbides in powders, thereby reducing the concentrations of C, Ti, and Nb on the powder surface and subsurface, which effectively inhibits the formation of the prior particle boundary during HIP. It reduces crack sources and thus improves the mechanical properties of the alloy.

Influence of MC Carbides on Pitting Corrosion Resistance of Weld Metal in Austenitic Stainless Steels

Influence of MC Carbides on Pitting Corrosion Resistance of Weld Metal in Austenitic Stainless Steels

A modified M2 high-speed steel enhanced by in-situ synthesized core-shell MC carbides

A modified M2 high-speed steel enhanced by in-situ synthesized core-shell MC carbides

Optimization of large cast Haynes 282 based on thermal induced cracks: formation and elimination

Haynes 282, a wrought material, attracts the attention from casting application due to its excellent mechanical properties. Fracture of large cast Haynes 282 were found before yielding during tensile test in previous researches, but the reasons have not been fully understood. In this study, various heat treatments were designed and applied for large cast Haynes 282 to reveal the fracture before yielding. Thermodynamic calculation, residual stress calculation combined with microstructural examination were utilized to analyze the reasons of fracture. It has been found that there are grain boundary cracking and grain boundary precipitate cracking in the tensile sample. The grain boundary cracking is mostly attributed to the melting of the low melting point phases such as some segregated matrix and μ phases. The grain boundary precipitate cracking is associated with the blocky Ti, Mo rich MC carbides, and it is more likely to be caused by the quenching after the second homogenization treatment. Optimized heat treatment has been proposed and proven to be workable for the large cast Haynes 282.