M23C6 Carbides Research Articles

Multiple morphologies and twin structure generated by a definite growth behavior of grain boundary M23C6 in tempered Fe–15Mn–3Al-0.7C steel

M23C6 carbide can precipitate simultaneously on both sides of austenite grain boundary in Fe–15Mn–3Al-0.7C steel tempered at 600 °C. In this paper, the morphology and growth behavior of M23C6 are studied by means of OM, SEM and TEM. The results show that the morphology of M23C6 located on both sides of grain boundary is different. On one side of the grain boundary, the carbides appear to be dispersed and disconnected, each with an irregular particle-like morphology, while on the opposite side, they are dispersed along the grain boundary in a short-rod morphology, and the adjacent ones tend to join together to form a network structure. In fact, regardless of which side of the grain boundary the carbides are on, they exhibit a definite growth behavior, growing along the close-packed plane and grain boundary. Also, the morphology of M23C6 depends on the angle between the close-packed plane and the grain boundary. In addition, twin structures with the same orientation can be found in both austenite and M23C6. A notable distinction between them is that there is a flat twin plane in the austenite twin structure but not in the carbide twin structure. Therefore, this paper highlights that the formation of these two twin structures is different. The austenite twin structure is caused by quenching treatment, while the carbide twin structure is formed by a new method called “growing-contacting”.

Effect of local carbide formation behavior on repair weld liquation cracking susceptibility for long-term-serviced 247LC superalloy

This study assessed the liquation cracking temperature range (LCTR) for 247LC superalloy repair welds subjected to long-term and high-temperature exposure. To achieve this, the exposed specimens underwent simulations at 1273 K for 500, 1000, 3000, and 5000 h to replicate the long-term service conditions of gas turbines. Spot-Varestraint testing was employed to quantitatively measure LCTR variations. The metallurgical mechanism of the evaluated LCTR in the exposed 247LC superalloys was elucidated by analyzing the microstructural characteristics at the liquation cracking surface and performing thermodynamic calculations. The LCTR was evaluated to be 410 K for specimens exposed for 500 h, 440 K for 1000 h, 385 K for 3000 h, and 610 K for 5000 h. This indicates that the liquation cracking susceptibility of the repaired weld heat-affected zone significantly increased during the long-term service of the 247LC superalloy. After 5000 h of exposure, the formation of fine MC, M6C, and M23C6 carbides on the liquation cracking surface was characterized using transmission electron microscopy. The welding metallurgical mechanism underlying the severe liquation cracking susceptibility can be attributed to the fractions of MC (liquation initiation temperature: 1125–1356 K) and M6C (liquation initiation temperature: 1390–1420 K), which have lower liquation initiation temperatures than the solidus of the 247LC alloy (1530 K). Notably, the area fraction of MC and M6C increased during 5000 h of exposure, which led to an increase in LCTR from 410 K to 610 K.

Microstructure and abrasive wear performance of a novel CALPHAD-inspired wear-resistant steel containing multiphase and multiscale carbides

The microstructure and abrasive wear performance of a novel wear-resistant steel designed based on the CALPHAD approach were characterized and evaluated. The steel exhibited distinct improvements over conventional high chromium cast iron through the precipitation of micron-sized M7C3 carbides from austenite via desolation transformation, resulting in a more uniform distribution with smaller sizes and desirable shapes. Moreover, the steel incorporated micron-sized carbides (M7C3, VC, and Mo2C) and submicron carbides (M7C3 and VC) as hard reinforcing phases, while nano-sized carbides (Mo2C, M7C3, and M3C) contributed to dispersion hardening. The wear resistance and impact-abrasive wear resistance of the steel were 1.8 times and 1.7 times superior to that of KmTBCr26 under identical wear conditions, respectively. These results indicated that the modified features of the micron-sized M7C3 carbides effectively mitigated undesirable phenomena such as cracking and spalling during the wear process, leading to improved impact-abrasive wear resistance in this steel. The exceptional wear resistance was attributed to the synergistic effect of multiphase and multiscale carbides, demonstrating the effectiveness and potential of CALPHAD-inspired design strategies in developing advanced wear-resistant materials.

New insight into the relationship between the C-Cr ratio and carbides, mechanical property of cold working die steel

An new insight into the relationship between the C-Cr Ratio and carbides, mechanical property Cr Alloyed of cold working die Steel were investigated by optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), rockwell hardness and impact toughness tests. The ‘C-Cr ratio’ and ‘Cr equivalent E Cr ‘ parameters were introduced to characterize the carbide characteristics and mechanical properties. The results show that the precipitation temperature of M7C3 eutectic carbides grows linearly with E Cr when the E Cr value is less than about 22, and the precipitation temperature increases significantly when the E Cr value exceeds 22, and the growth curve takes a plateau turn. The precipitation temperature of MC carbides decreases approximately linearly with E Cr value, and the precipitation of MC eutectic carbides will be completely suppressed when the E Cr value exceeds 19. The content of eutectic carbides in the as-cast organization is more, the shape is more complex and the size is larger. The ratio of Cr and Fe content in the composition of M7C3 eutectic carbides is linearly related to the Cr-C ratio; the content of carbides in the steel after forging and the E Cr value are basically linear. The average particle size and the average length of longitudinal carbides after forging are basically proportional to the E Cr value. The average particle size of longitudinal carbides after forging is greater than 4 μm and the E Cr value are basically linear; when the Cr content is greater than 4%, the quenching peak hardness of steel and the C/Cr have a good linear relationship; as the E Cr value gradually increases, the impact toughness of steel gradually decreases. These results are important not only for understanding the strengthening mechanisms of die steel, but also for the composition design and carbide control of cold working die steel.

Reaction with hot air of high entropy alloys strengthened by monocarbides formed from heavy metals: assessment of the oxidation kinetics from the analysis of the chemically changed subsurfaces

As in classical superalloys based on nickel or cobalt, MC carbides can be easily obtained in cast high entropy alloys (HEA) for enhancing their mechanical properties at high temperatures. These carbides may have also some influence on the hot oxidation properties. They can be also not neutral for the oxidation behavior. To explore their possible influence in this field, an equimolar CoNiFeMnCr reference alloy and two versions containing TaC or HfC carbides were prepared by casting and exposed to air at 1000°C. Their oxidation behaviors were studied by characterizing the obtained corrosion products and the modifications induced in the subsurfaces of the samples. Results show that TaC and HfC are moderately involved in the oxidation phenomena but they obviously influence the Cr and Mn diffusion toward the oxidation front. This seemingly aggravates more the already poor oxidation high temperature resistance of the base alloy. However the harmful effect of the TaC seems to be much lower than the one of the HfC, and the TaC–strengthened CoNiFeMnCr alloy is only a little less resistant against oxidation than the carbide–free reference equimolar alloy.

Microstructural stability of secondary phases in an ODS ferritic steel after thermal aging at 873 K

An oxide dispersion strengthened (ODS) steel with nominal composition Fe–14Cr–2W–0.4Ti–0.3Y2O3 (wt%) has been manufactured by mechanical alloying of pre-alloyed powders with nanosized Y2O3, compacted by hot isostatic pressing and hot cross rolled. In order to evaluate the long-term thermal resistance of the alloy, it has been subjected to 2000 h of thermal aging at 873 K, which is a relevant temperature for nuclear reactor applications. A thermodynamic equilibrium simulation indicates the precipitation of Laves phase under the aging parameters used. This prediction is confirmed from the detailed multi-technique characterization performed. Before aging, Ti-rich oxides, Cr-W-rich precipitates (M23C6 type) and a homogeneous Y-rich nanoprecipitate dispersion are observed. After aging, some W-rich precipitates are identified as Laves phase, while M23C6 carbides, Ti-rich oxides and Y-rich nanoprecipitates remain stable. Mechanical characterization performed in a previous research showed higher hardness, a loss of total elongation and a slight shift of the ductile-to-brittle transition temperature (DBTT) towards a higher value after aging, with similar strength values before and after the long-term thermal treatment. These changes can be due to the redistribution of precipitates together with the Laves phase formation. The mechanical properties not being dramatically affected seem to be due to the observed stability of Y-rich nanoprecipitates distribution.

The effect of quenching/double quenching and tempering heat treatment cycles on the microstructure, impact energy and hardness of AISI H13 tool steel

ABSTRACT In this study, microstructure modifications of AISI H13 steel were studied with different heat treatments such as conventional quenching, quenching + tempering/double tempering and double quenching + tempering/double tempering. Carbide volume ratio analyses were performed. The XRD analysis was used to determine the type of carbide, the average crystallite size, and the dislocation density in the samples. Charpy impact tests were performed. The hardness values of the samples were measured with the Rockwell C scale. The presence of MC, M23C6, and M7C3 type carbides was determined in the microstructures. Heat treatment cycles caused a high dislocation density in martensite laths. The dislocation density of martensite laths decreased with double quenching. The tempering process caused the formation of fine carbide particles in the microstructure. In addition, tempering processes gave rise to an increase in the volume ratio of M23C6-type strip carbides. The length/width ratio of the strip-type carbides increased with the effect of double tempering. As a result of the applied heat treatments, the impact energies of the samples decreased. The tempering process caused a secondary hardening of the samples. While the double quenching process increased the impact energy value by approximately 2%, it caused a decrease in hardness by 24%.

Influences of partial substitution of C by N on the microstructure and mechanical properties of 9Cr18Mo martensitic stainless steel

Large eutectic carbides presenting in the matrix deteriorate the mechanical properties of high carbon martensitic stainless steel. In the present work, nitrogen-alloyed martensitic stainless steel with different C/N ratios were designed by partial substitution of C by N of 9Cr18Mo, and their microstructure and mechanical properties were investigated. The results show that, with increasing the content of N up to 0.15 wt% in the experimental steel, the quantity and the size of eutectic carbides decrease； and the net-work eutectic carbides are gradually eliminated. The microstructure of all the steels is composed of lath martensite, twin martensite and M23C6 carbides. With partial substitution of C by N, the morphology of martensite blocks evolves from a lath shape to a blocky one, the quantity of twin martensite increases, and the nano-scaled Cr2N nitrides precipitate in the martensite matrix. The steel containing 0.15 wt% N exhibits the hardness of 60.5 HRC, a fracture strength of 1900 MPa and an elongation rate of 1.6 % due to hot rolling and heat treatment, which is attributed to the refinement of carbides and grains, solid solution of C and N, as well as the dense dislocation.

Formation and dissolution of carbides and precipitates in self-ion irradiated HT9 alloy

Microstructural changes of HT9 irradiated with 5 MeV Fe self-ions were studied at damage levels of 300 and 600 displacements-per-atom (dpa) and at temperatures ranging from 450 °C to 700 °C. Transmission electron microscopy and chemical mapping showed the formation of needle-like Cr-rich M2X carbides and spherical Ni/Si/Mn-rich G-phase precipitates at 450 and 500 °C. No M2X carbides or G-phase precipitates were observed at temperatures of 600 and 700 °C. On the other hand, pre-existing intergranular M23C6 carbides were found to be stable even at 700 °C. The M2X carbides have a semi-coherent interface with an orientation relationship of {100}BCC//{21¯1¯0}HCP, 〈011〉BCC//〈0001〉HCP, and the M23C6 carbides have a semi-coherent interface with mismatching lattice planes of {110}BCC//{211}FCC. The M23C6 carbides exhibited a concentration gradient of Fe/Cr over the interface region of approximately 100 nm. The mean sizes of both M2X carbide and G-phase precipitates increased with increasing damage, but the density of M2X carbides saturated while the density of G-phase decreased with increasing damage. G-phase precipitates were found to decorate M2X carbides, and such a spatial distribution correlation was believed to increase precipitate stabilities. Ion irradiation introduces defect imbalances due to the injected interstitial effect. This effect reduces the Cr diffusion and suppresses M2X carbide formation near the projected range. A spatial distribution difference between M2X carbides and G-phase precipitates was observed.

On the effect of tempering temperature on the long-term creep behavior of a 10% Cr steel with low nitrogen and high boron contents

The effect of a decrease in the tempering temperature from 770 to 750 °C on the long-term creep behavior of a 10% Cr martensitic steel with 0.008 wt % B and 0.003 wt % N was studied. The evolution of microstructure and precipitation distribution during creep was examined using the interrupted tests under an applied stress of 120 MPa at 650 °C. It was found that a higher dislocation density, a finer lath structure and finer carbides after tempering at 750 °C did not provide higher long-term creep strength than after tempering at 770 °C. The creep rupture time was about 30% shorter. Effect of changes in the dislocation density, lath width, sizes of M23C6 carbides and Laves phase particles, and precipitation of V-rich MX phase on the strengthening contribution at the onset of creep and at the transient, apparent steady-state and tertiary stages was analyzed. The higher dislocation density formed by the tempering at a lower temperature provided a higher climb velocity at the transient creep stage. The higher driving force for recovery was not fully compensated by the higher strength exerted by smaller M23C6 carbides. This led to faster annihilation of the lath boundaries and a decrease in internal stress and, consequently, to an order of magnitude higher minimum creep rate of the steel after tempering at 750 °C as compared to tempering at 770 °C.

Aging response of a Fe–C–Co–Ni secondary hardening steel: The significance of matrix ordering

The evolution of precipitates and dislocations during aging dictates the mechanical properties of secondary hardening steels. Such microstructural response of a Fe–C–Co–Ni secondary hardening steel has been investigated via high-resolution transmission electron microscopy in this study. The primary source of strengthening is linked to coherent Molybdenum carbides, which form in a sequence of cementite, cluster and M2C. It is shown that the ordering of the martensitic matrix occurs rapidly upon aging and results in the formation of dispersed B2–FeCo precipitates. The co-location of ordered domains with the cluster and M2C carbides indicates that the ordering greatly affects the nucleation and growth of M2C and thus leads to a more homogeneous spatial distribution of M2C. This yields excellent mechanical properties including an ultimate tensile strength of 2150 MPa, yield strength of 1829 MPa, and total elongation of 12 %. It should be noted that the ordering not only contributes to the mechanical properties by promoting the dispersion of M2C, it also causes strengthening directly as demonstrated by an additive model of yield strength. However, the model fails to predict the yield strength of the as-quenched sample since it contains extra mobile dislocations which considerably modify the mechanical property.

Solidification Segregation Behavior and Homogenization Process of a Difficult-to-Deform Superalloy Used at 850 °C

Solidification segregation behavior and homogenization heat treatment processes of a difficult-to-deform superalloy for use at 850 °C were studied. Additionally, the effect of carbon content on homogenization, the thermal treatment process, and pore growth behavior within the alloy were discussed. Our results revealed that Al, Ti, and Nb elements are distributed in the interdendritic space, while W and Mo elements are distributed in the dendrite. There is a significant quantity of γ-γ′ eutectic and MC carbide precipitates in the interdendritic space. Notably, for the alloy containing 0.1 wt% C, a homogenization heat treatment at 1200 °C for 48 h can effectively eliminate the segregation and undesirable phases. As carbon content increased, the γ-γ′ eutectic phases diminished, and the homogenization time decreased accordingly. In this context, pores are smaller and more dispersed, which may enhance alloy forging properties.

Effect of Si on δ‐Ferrite/γ‐Austenite Transformation and M23C6 Precipitation During Solidification of X10CrAlSi18 Ferritic Heat‐Resistant Stainless Steel

Herein, the δ‐ferrite/γ‐austenite transformation and the precipitation behavior of M23C6 carbides in X10CrAlSi18 ferritic heat‐resistant stainless steel (FHSS) with various Si contents at a cooling rate of 100 °C min−1 using confocal scanning laser microscopy (CSLM) are investigated. The findings reveal that γ‐austenite preferentially forms along the δ‐ferrite phase boundaries, and it progressively precipitates into the δ‐ferrite phase as the temperature decreases. The increase in the Si content reduces the δ‐ferrite/γ‐austenite transformation temperature. It also inhibits the martensite transformation in the subsequent cooling process, decreasing the volume fraction of γ‐austenite/martensite. M23C6 carbides are mostly found at the δ‐ferrite and γ‐austenite/martensite phase boundaries. Meanwhile, the nucleation of M23C6 carbides becomes more difficult as the volume fraction of γ‐austenite/martensite decreases. Furthermore, the complex solidification mechanism of the nucleus is addressed.

Interfacial excess of solutes across phase boundaries using atom probe microscopy

Three-dimensional elemental mapping in atom probe microscopy provides invaluable insights into the structure and composition of interfaces in materials. Quasi-atomic resolution facilitates access to the solute decoration of grain boundaries, advancing the knowledge on local segregation and depletion phenomena. More recent developments unlocked three-dimensional mapping of the interfacial excess across grain boundaries. Such detailed understanding of the local structure and composition of these interfaces enabled advancements in processing methods and material development. However, many engineering alloys, such as Ni-based superalloys, have much more complex microstructures with various solutes and precipitates in close proximity to grain boundaries. The complex interaction of grain boundary segregation and grain boundary precipitates requires precise compositional control. However, abrupt changes in solute solubility across phase boundaries obscure the interfacial excess in proximity to grain boundaries.Therefore, this study provides a methodological framework of the quantitative characterization of phase boundaries in proximity to grain boundaries using atom probe microscopy. The detailed mass spectrum ranging of MC, M23C6, and M6C carbides is explored in order to achieve satisfactory compositional information. Proximity histograms and correlating concentration difference profiles determine the interface location, where a Gibbs dividing surface is not accessible. This enables reliable direct calculation of the interfacial excess across phase boundaries. Intuitively interpretable and quantitative ‘interface plots’ are introduced, and showcased for phase boundaries between γ-matrix, γ' precipitates, GB-γ', MC, M23C6, and M6C carbides. The presented framework advances access to the local composition in proximity to grain boundaries and may be applicable to other engineering alloys or materials with functional properties.

Effect of aging temperature on corrosion resistance of 316L stainless steel in simulated proton exchange membrane fuel cell cathode environment

The effects of aging temperature on the corrosion behavior of 316 L stainless steel in a simulated cathodic environment of a proton exchange membrane fuel cell (PEMFC) were investigated by potentiodynamic polarization curves, electrochemical impedance spectroscopy (EIS), double-loop electrochemical potential response (DL-EPR). The results show that the corrosion resistance of 316 L is significantly degraded after aging at 750–900 °C for 1 h in the simulated cathodic environment of PEMFC. A strong sensitivity to intergranular corrosion takes place after aging at 850 °C, and chromium rich M23C6 carbides precipitate on grain boundaries. The precipitation of M23C6 leads to discontinuity of the passivation film near the grain boundaries, resulting in Cr-depleted zones at the grain boundaries, increasing intergranular corrosion, and decreasing corrosion resistance. When the aging temperature exceeds 950 °C, grain boundary precipitation of M23C6 does not occur and the corrosion resistance is improved. At an aging temperature of 1050 °C, a dense Cr2O3 film is formed on the surface of the alloy during the polarization test, giving it the best corrosion resistance in the simulated cathodic environment of PEMFC.

Softening Behavior and Hardness Prediction of H13 Hot‐Work Die Steel during Isothermal Tempering

In this study, the microstructural evolution of H13 hot‐work die steel during isothermal tempering at 500 ≈ 650 °C is evaluated, and kinetic equations and a neural network are constructed to describe the softening behavior. The results show that the softening behavior appears as a loss of hardness with increasing isothermal tempering temperature and time. The softening mechanism is ascribed to the decomposition of martensitic laths, annihilation and rearrangement of dislocations, formation of sub‐grains, and precipitation and coarsening of secondary carbides. The evolution of secondary carbides includes the coarsening of V‐rich MC nanoprecipitates, and the precipitation and coarsening of secondary phases (Cr‐rich M23C6 and Mo‐rich M6C carbides) during long‐term isothermal tempering. Regarding the applicability of the model in precipitation evolution and hardness prediction, the Johnson–Mehl–Avrami model is considered to be more suitable than the Hollomon–Jaffe, Lifshitz–Slyozov–Wagner, and back‐propagation neural network models.

Influence of heat treatment on the microstructure and creep response of additively manufactured cobalt superalloy Mar M509

ABSTRACT This work explores the influence of various post-build heat treatment procedures on the microstructural characteristics and creep response of laser powder bed fusion (L-PBF) Mar M509 superalloy. Besides MC-type carbides formed primarily during the L-PBF process, the obtained results indicate the influence of heat treatment on the nucleation of M6C and M7C3 carbides due to elemental segregation. Short-term creep-rupture tests have been conducted at 900°C/110MPa on eight specimens, each subjected to a specifically individual heat treatment procedure. Rupture times are found to be sensitive to the heat treatment applied and vary from ~178 to ~641 hours. Detailed microscopic characterisation of ruptured specimens highlights the synergistic influence of oxidation and M7C3/M6C precipitate-free zone formation on macroscopic intergranular failure facilitated by grain boundary sliding along favourably oriented small grains. Besides notable grain growth, the presence of a serrated grain boundary structure with relatively less MC-type carbide coarsening is found to offer the best creep resistance among the eight L-PBF specimens. Further improvement of the elevated temperature performance of the alloy is seen to require a focus on the L-PBF process conditions to optimise the morphology of MC carbides, as they form.

Influence of interstitial carbon content on the microstructure, mechanical and electrochemical corrosion properties of CoFeNiMn multi-principal component alloys

In this study, the effects of carbon content on the microstructural evolution as well as compression and electrochemical corrosion properties of CoFeNiMn multi-principal component alloys (MPCAs) were investigated. A series of (Co25Fe25Ni25Mn25)100−xCx (x = 4, 5, 6, 7, 8) alloys were prepared by vacuum arc melting. Increasing carbon content, the structure transits from a single face-centered-cubic (FCC) phase into a mixed structure containing FCC phase and M23C6 carbides. The yield strength and hardness of the (Co25Fe25Ni25Mn25)100−xCx alloys were enhanced by increasing carbon content, which in turn decreases the ductility. The alloys containing eutectic carbides had a good combination of high yield strength (880 MPa), high fracture strength (2773 MPa), and large fracture strain (47%). Electrochemical measurements indicated that carbon content evidently affects the corrosion resistance of (Co25Fe25Ni25Mn25)100−xCx alloys in 3.5 wt% NaCl solution. Their corrosion resistance initially increases and then decreases with increasing carbon content. The increase in M23C6 carbides can accelerate pitting corrosion and degenerate their corrosion resistance. These findings not only provide comprehensive understanding of the carbon-alloyed behavior in FCC-type MPCAs but also show their potential engineering application as high-performance structural materials.

Toughening mechanism and delayed fracture analysis of 4145H steel under intercritical quenching heat treatment process

Tuning thermal treatment schedule has been confirmed as an effective technical route for achieving optimum properties of steels. In the present work, the intercritical quenching tempering(IQT) process was applied to steel 4145H for oil drilling tools, and the performance enhancement and delayed fracture mechanism of the IQT process were comprehensively elucidated using multiscale characterization methods and various mechanical tests. Compared with the quenching and tempering(QT), the IQT process results in a separate island ferrite plus finely tempered sostenite organization, which reduces the dislocation density in the tempered organization and increases the proportion of recrystallized grains. Due to the grain refinement effect of multiple quenching, the pristine austenite grains were refined by 58 %. At the same time, the proportion of large-angle grain boundaries increased from 68.6 % to 80.3 %. The results of the mechanical property tests show that the intercritical 790 °C quenching treatment and tempering at 600 °C exhibits a tensile strength of 1087 MPa, a high elongation of 21.72 %, and a high impact work of 104 J compared to QT. The improved impact toughness is mainly attributed to the grain refinement effect of multiple quenching and the large-angle grain boundaries provided by the composite ferrite organization and M23C6 carbides. The impact of different ratios of large-angle grain boundaries on crack extension was also evolved by means of the crystal phase field, and the delayed fracture effect of large-angle grain boundaries was verified.

Surface precipitate analysis of gas atomized Ni-Base superalloy powders

Powders surface precipitate has strong relations with the microstructure and properties of powder metallurgy components. Morphology, element distribution and crystal structure of the precipitate at gas atomized Ni-base superalloy powders surface were investigated. Results show that the precipitate is lamelliform and prefers to form at grain boundaries and interdendrite regions than dendrite arms. With powders size decreasing, the precipitate shape observed from surface varies from strip to rectangular and to nearly round, and the precipitate density increases, measured 17.75 μm−2, 21.42 μm−2, 26.50 μm−2 at nearly 45μm, 30μm and 10μm powders surface respectively, but the precipitate average size changes conversely, that is 228nm, 193nm, 77nm respectively. The precipitate enriches in high melting point elements Ti, Nb, Mo, W, Zr and poor in low melting point elements Ni, Cr, Co, Al. Crystal structure analysis reveals that the precipitate is MC, M23C6 or M6C carbide.