Precipitation Of M23C6 Carbides Research Articles

Enhancing the strength-ductility synergy in hot-forged Fe50Mn30Co10Cr10 high entropy alloy (HEA) through carbon additions

In this work, we systematically investigated the effect of carbon additions on the microstructure and room temperature mechanical properties of hot-forged Fe50Mn30Co10Cr10 HEA. The different carbon contents were introduced to Fe50-xMn30Co10Cr10Cx (x = 1, 2.5, 6 at.%, hereinafter, referred as C1, C2.5 and C6, respectively) HEAs to enhance the strength-ductility synergy by triggering the multiple strengthening mechanisms. The results showed that the grain sizes were decreased with increasing of carbon content, while a high fraction of mechanical twins ∼45% were generated in C2.5 HEA, and their fractions were ∼35% in C1 and C6 HEAs. Moreover, the discrete nano-sized M23C6 carbides in C2.5 HEA were transformed into continuous mico-sized M23C6 carbides along the grain boundaries in C6 HEA. Therefore, C2.5 HEA possessed high yield strength of 584 MPa, tensile strength of 1.03 GPa and outstanding ductility of 68%, i-e., breaking the strength-ductility paradox. A highest yield strength of∼662 MPa and tensile strength of more than 1 GPa were observed for C6HEA with refine grains, although the ductility was only ∼10%, which indicated that the massive carbon addition could effectively enhanced the strength of HEA but at the cost of ductility due to the presence of coarse M23C6 carbide precipitation. The enhanced mechanical properties of studied HEAs are mainly ascribed to the contribution of multiple strengthening mechanisms including precipitation, grain boundary, dislocation and interstitial solid solution strengthening.

Effects of Chemical Composition and Solidification Rate on the Solidification Behavior of High-Cr White Irons

The effects of chemical composition and solidification rate on the solidification behavior of high-Cr white irons were investigated through directional solidification. Increasing the solidification rate in hypoeutectic alloys caused finer dendrite-arm spacing, as expected. The eutectic structure, which formed in the interdendritic region, was comprised of M7C3 and austenite; however, secondary dendrite arms of hypoeutectic alloys contained a few M7C3 particles that solidified prior to the eutectic structure. The transition from cellular to dendritic solidification occurred at a solidification rate between 50 µm/s and 100 µm/s in a near-eutectic alloy. In the near-eutectic alloy with cellular solidification, a directionally arrayed in-situ composite of M7C3/austenite formed within the cell. Speckle-like features appeared in the intercellular region due to M23C6 carbide precipitation during subsequent cooling after freezing. Like dendrite-arm spacing in hypoeutectic alloys, the inter-speckle spacing and the inter-fiber spacing became finer with an increasing solidification rate in the cellular solidification range.

Embrittlement Mechanisms of HR3C Pipe Steel at Room Temperature in Ultra-Supercritical Unit

HR3C steel is an austenitic high-temperature-resistant steel. Because of its good strength and high-temperature performance, it has been widely used in ultra-supercritical power plant boilers. With the increasingly frequent start-up and shutdown of thermal power units, leakages of HR3C steel pipes have occasionally occurred due to the embrittlement of HR3C pipe steel after a long service duration. In this study, the embrittlement mechanisms of HR3C pipe steel are investigated systematically. The mechanical properties of the pipe steel after running for 70,000 h in an ultra-supercritical unit were determined. As a comparison, the pipe steel supplied in the same batch was aged at 700 degrees Celsius for 500 h. The mechanical properties and the micro-precipitation of the aged counterparts were also determined for comparison. The results show that the embrittlement of HR3C pipe steel in service for 70,000 h is obvious. The average impact absorption is only 5.5 J, which is a decrease of 96.7%. It is found that embrittlement of HR3C steel also occurs after 500 h of aging at 700 °C, and the average value of impact absorption energy decreases by 70.4%. The comparison experiment between the in-service pipe steel and the aged pipe steel shows that in the rapid decline stage of the impact toughness of HR3C steel, the M23C6 carbide in the microstructure has a continuous chain distribution in the grain boundary. There were no other precipitated phases observed. The rapid precipitation and aggregation of M23C6 carbides leads to the initial embrittlement of HR3C steel at room temperature. The CRFe-type σ phase was found in the transmission electron microscope (TEM) image of the steel pipe after 70 thousand hours of use. The precipitation of the σ phase further induces the embrittlement of HR3C pipe steel after a long service duration.

Development and Application of On‐Line Solution Annealing Process for 304 Austenitic Stainless Steel

After hot rolling, 304 austenitic stainless steel requires a solution annealing treatment to prevent intergranular corrosion and eliminate work hardening effects. Compared to traditional offline processes, on‐line solution annealing offers advantages in terms of cost and time savings. However, both recrystallization behavior and M23C6 carbide precipitation behavior are significantly influenced by the cooling process after rolling, which poses conflicting requirements. This study investigates the precipitation behavior of M23C6 carbides and the recrystallization softening behavior during the continuous cooling process of hot‐rolled samples. The kinetics equations are derived using the Scheil's additivity rule. The temperature profiles in different regions of the plate are studied using finite element analysis. A practical approach for online solution annealing is proposed and applied in industrial testing.

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 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.

Precipitation and Transformation of Carbides during Tempering of 7Cr14 Martensitic Stainless Steel

The precipitation and transformation of carbides in steel are gaining more interest since they are closely related to the properties of steel. Herein, the precipitation and transformation of carbides during tempering of the 7Cr14 martensitic stainless steel subjected to high‐temperature solution treatment (HTST) are investigated. The results indicate that the precipitation of M3C (300–333 °C) and M23C6 (612–645 °C) carbides occurs during tempering of the HTST steel. The effective precipitation activation energies of M3C and M23C6 carbides are 120 ± 7 and 272 ± 13 kJ mol−1, respectively. The precipitation processes of M3C and M23C6 carbides are dominated by C and Cr atomic diffusion, respectively. Moreover, it is found that two transformation processes occur during tempering with increasing temperature, namely, M3C carbides dissolve in the matrix and then M23C6 carbides precipitate along the boundaries of martensitic laths, and the plate‐like M3C carbides decompose into several globular or rod‐like M23C6 carbides that arrange in a chain‐like morphology.

Oxidation behavior and corrosion properties of 2-GPa grade Co-reduced ultrahigh-strength stainless steel

This study investigated the oxidation behavior and corrosion resistance of 2-GPa grade ultrahigh-strength stainless steel. Results revealed that the addition of V to Co-reduced FerriumS53 (7Co1V) accelerated the precipitation of MC carbide during aging, while inhibiting the precipitation of Cr-rich M23C6 carbide. A significant amount of Cr was predicted in the microstructure of the alloys, and this microstructure increased the partitioning of Cr to the alloy surface and provided enhanced resistance against corrosion pitting. These findings suggest that the addition of V into Co-reduced stainless secondary hardening steels can enhance their oxidation and corrosion resistance without compromising their mechanical properties.

Effects of tempering temperature on the microstructure evolution and mechanical properties of 16%Cr–5%Ni super martensitic stainless steel

The effects of tempering temperature on the microstructure evolution and mechanical properties of 16%Cr–5%Ni super martensitic stainless steel (SMSS) were investigated in the present study. Reversed austenite was detected via X-ray diffraction when the as-quenched samples with full martensite were tempered above 500 °C, and its volume fraction attained a maximum of ∼17% at 620 °C. When tempered below 620 °C, all the reversed austenite formed was able to keep stable at room temperature, while partial or even entire reversed austenite formed above 640 °C with Ni content less than 8 wt% transformed back into fresh martensite during cooling, demonstrating that the enrichment of Ni determined the thermal stability of reversed austenite. EDS results revealed that the precipitation of M23C6 carbides caused Ni enrichment in the adjacent regions, promoting the nucleation of reversed austenite, and the growth of reversed austenite was mainly controlled by Ni diffusion. The results of tensile tests and instrumented impact tests indicated that stable reversed austenite effectively improved the ductility and toughness, and offset the deterioration of toughness caused by the M23C6 precipitating at grain boundaries. Tempering at 540 °C achieved an excellent combination of strength and toughness, while the best plasticity of samples was obtained by tempering at 620 °C.

Influence of solution temperature on microstructure and mechanical properties of DZ411 alloys with different Ta compositions

The influence of solution treatment temperature on microstructure and mechanical properties of DZ411 alloys with different Ta compositions (2.72, 3.10, and 4.00 wt.%) is studied in details. With the increase of Ta composition, the average values of the primary dendrite arm spacing (PDAS) in superalloys before heat treatment are found to increase from 145.9 ± 3.7 to 209.8 ± 7.6 μm, the area fractions of carbides increase from 0.59 ± 0.05% to 0.65 ± 0.05%, the area fractions of γ-γ′ eutectics increase from 4.6 ± 0.05% to 8.1 ± 0.05%, and the cubicity of the γ′ phase also increases. After heat treatment, since the γ′ phase shows more superior high temperature performance in alloys with higher Ta composition, the solution temperature of alloys with higher Ta composition should be increased to promote the dissolution of the coarse γ′ phase. The evolution of carbides is mainly divided into two types: the decomposition of MC carbides: MC + γ → M23C6 + γ′ and the precipitation of fine M23C6 carbides. The increase of cubicity of γ′ phase will increase the interfacial energy of two phases, which can lead to the pile-up of dislocation and improve the microhardness of alloys. The fine M23C6 carbides will hinder the initiation and propagation of cracks, and the interaction of fine carbide particles reduces the stress concentration and improve the fracture strength of alloys. Therefore, the addition of Ta increases the mechanical properties including both tensile properties and microhardness of alloys with higher Ta composition increases more obviously after heat treatment at higher temperature.

Effect of Hot-Rolling on the Microstructure and Impact Toughness of an Advanced 9%Cr Steel

A 9%Cr martensitic steel with Ta and B additions was subjected to thermo-mechanical treatment (TMT) including rolling in the range of metastable austenite at 900–700 °C followed by water quenching and tempering at different temperatures. Applied TMT with tempering at T ≥ 700 °C substantially improved the impact toughness. The application of the TMT with subsequent tempering at 780 °C decreased the ductile–brittle transition temperature from 40 to 15 °C and increased the upper shelf energy from 300 to 380 J/cm2 as compared to the normalized and tempered (NT) condition. The microstructural observations with scanning and transmission electron microscopes showed the precipitation of fine Ta-rich MX carbonitride and M23C6 carbide during TMT and subsequent tempering. The analysis of the cleavage facets and the secondary cracks with electron back-scattered diffraction (EBSD) revealed that the brittle fracture occurred via cleavage cracking along {100} planes across the laths, while the high-angle boundaries of martensite blocks and packets were effective barriers to the crack propagation. The increased impact toughness of the tempered TMT steel sample was attributed to enhanced ductile fracture owing to the uniform dispersion of the precipitates and favorable {332}⟨113⟩ crystallographic texture.

The impacts of M/A constituents decomposition and complex precipitation on mechanical properties of high-strength weathering steel subjected to tempering treatment

The impact of microstructure evolution on the mechanical properties of a typical 500 MPa-grade weathering steel produced by an identical thermo-mechanical control process and different tempering treatments at 450–650 °C were thoroughly investigated mainly using scanning electron microscopy (SEM) equip with an electron backscatter diffraction (EBSD), high-resolution transmission electron microscopy (HRTEM), and X-ray diffractometer (XRD). Results indicated that the as-rolled steel consists of granular bainitic ferrite (GBF) and a considerable amount of martensite/austenite (M/A) constituents, exhibiting unsatisfied mechanical properties. As the tempering temperature increased from 450 to 550 and 650 °C, the massive twin-type M/A constituents decomposed preferentially into carbides following the sequence of Fe3C→(Cr, Mn, Fe)3C→(Cr, Mn, Fe)3C + (Cr, Mn)23C6, with a small amount of fine lath-type M/A constituents remained. The matrix recovered with the bainitic ferrite laths merged, the dislocation density decreased and the proportion of high-angle grain boundary (HAGB) increased. Nanoscale (Ti, Nb)C and ε-Cu particles also precipitated simultaneously, leading to an increase of yield strength. The strain hardening capacity and hence the tensile strength reduced resulting from the decomposition of M/A constituents. Moreover, the impact toughness was first enhanced by tempering at 450–600 °C with the decreasing twin-type M/A constituents content and increasing HAGBs proportion, and then deteriorated by tempering at 650 °C due to the precipitation of necklace-like M23C6 carbides at the grain boundaries. An optimum mechanical property combination was achieved via tempering at 550–600 °C.

Microstructure development of modified 9Cr-1Mo steel during laser powder bed fusion and heat treatment

Herein, the effect of laser powder bed fusion (LPBF) parameters on the microstructure development of modified 9Cr-1Mo steel and the relationship between the as-built and heat-treated microstructures were characterized using electron backscattered diffraction (EBSD). The optimal energy density for the steel was determined to be 100–150 J/mm3. The microstructure of the as-built steel was found to be mainly composed of coarse and columnar δ-ferrite grains and fine martensite grains. δ-ferrite was formed through rapid solidification, while martensite was formed in the heat-affected zone during LPBF. A higher energy density and scan strategy with a 67 deg. rotation in each layer were found to be suitable for obtaining a higher area fraction of martensite because they can provide a larger heat-affected zone compared with a lower energy density and a 90 deg. rotation. Tempering reduced the hardness via recovery of martensite and enhanced the precipitation of M23C6 carbide at martensite, while the microstructural morphology in the as-built condition was maintained. The typical as-built microstructure disappeared after the normalizing treatment, while the size of the prior austenite grains was affected by the as-built microstructure. In-situ EBSD observation of the phase transformation behavior during normalization revealed that martensite became fine prior austenite grains, while δ-ferrite became coarse prior austenite grains. From these results, the effects of energy density and scan strategy on microstructure development were identified not only for LPBF but also during the normalizing and tempering heat treatments.

Pitting Performance of Cold- and Hot-Rolled Nickel-Saving High-Strength Metastable Austenitic Stainless Steel

Nowadays, nickel-saving metastable austenitic stainless steel (MASS) has become the right solution to meeting the growing requirement of higher strength, better corrosion resistance and more cost saving for the automobile industry. Better understanding of the pitting mechanism of the MASS after either cold- or hot-rolled can offer guidance for the producing of high-performance automobile steel. In the current work, for uncovering the pitting mechanism of the cold- and hot-rolled MASS, the microstructural evolution and pitting performance of nickel-saving metastable austenitic stainless (MASS) steel after cold- (CR) and hot-rolling (HR) were researched via electron microscopy technique and electrochemical methods. Austenite composites the main phase of the MASS. Small amounts of martensite film were proven to form in the MASS. The precipitation of Cr-rich M23C6 carbides was observed in the CR-MASS, while no carbides existed in the HR-MASS. The pitting resistance of the HR-MASS was better than the CR-MASS, which could be attributed to the fact that the stable pits in CR-MASS were initiated near the carbides, whereas the MnS inclusion would serve as the initiation sites for stable pits in HR-MASS. Findings in this work will provide a guidance for developing new generation MASS for automobile industry.

Microstructure evolution and mechanical properties of 16Cr–25Ni superaustenitic stainless steel weld metal during 600 °C aging

The effect of high temperature aging on the microstructure evolution and mechanical properties of 16Cr–25Ni superaustenitic stainless steel weld metal was investigated at 600 °C for up to 5000 h. Interdendritic micro-size M6C carbide, nano-size M6C and M23C6 carbides were identified in the 200 h aged weld metal, which appeared in clusters and distributed dispersedly. With the aging time increased to 1000 h, the interdendritic region was filled with newly formed precipitates, which were identified as nano-size M12C and M23C6 carbides. Compared with M6C carbide, M12C carbide was preferred to precipitate for lower Gibbs free energy, and the pre-existed M6C carbide had transformed to M12C carbide through the release of C atoms to the matrix. As the aging time further increased to 5000 h, the M12C carbide kept stable with the M23C6 carbide. Due to the precipitation and coarsen of M12C and M23C6 carbides that consumed massive solute atoms from matrix, which weakened the precipitation strengthening effect and suppressed the dynamic strain aging, the tensile properties kept superior stability during aging. The evolution of impact toughness was also closely related to the interdendritic precipitates. As the interdendritic region was occupied by precipitates since the aging time increased from 200 h to 1000 h, the impact toughness rapidly decreased from 76 J to 39 J, and then slightly decreased to 33 J with the aging time increased to 5000 h.

Unsteady state precipitation of M23C6 carbides during thermal cycling in reduced activation steel manufactured by laser melting deposition

Unsteady state precipitation of M23C6 carbides during thermal cycling in reduced activation steel manufactured by laser melting deposition

Study on the mechanism of carbide precipitation by surface quenching treatment on GCr15 bearing rings based on the phase-field method

In this paper, the thermodynamics database describing Gibbs free energy of FCC phase and M23C6 phase based on Fe-Cr-C ternary alloy was coupled into phase field simulation, and the phase field model was developed on the mesoscopic scale which simulates precipitation, growth and coarsening of M23C6 carbides in FCC austenitic matrix by considering the elastic energy. As the boundary condition of imported phase field calculation, the heat treatment COSMAP software was applied to obtain the macro numerical simulation results of quenching. The phase composition, element distribution, and the evolution including the nucleation, growth and coarsening was simulated by phase field method for the M23C6 carbides in quenching process of GCr15 bearing ring. By means of comparison between the calculation results of phase field simulation and the morphology, quantity and distribution of carbides observed by SEM experiment, this paper present the mechanism on precipitation of micro M23C6 carbides influenced by quenching process parameters, and the experiment verifies the feasibility and accuracy of phase field simulation.

Effect of twin-related boundaries distribution on carbide precipitation and intergranular corrosion behavior in nuclear-grade higher carbon austenitic stainless steel

Based on different character distribution of twin-related boundaries (Σ3n, 1 ≤ n ≤ 3) introduced by special thermomechanical processing, the effect on carbide precipitation and the resulting intergranular corrosion (IGC) behavior was investigated in depth in nuclear-grade 316H stainless steel. It was worked out that the Σ3 twin boundaries with single distribution was difficult to suppress the IGC percolation along random high-angle grain boundaries, while the interconnected Σ3 twin boundaries could fully block the percolation channel and thus greatly improve the IGC resistance. Among which, the precipitation of M23C6 carbides at different boundaries plays key role to affect their ability of resisting IGC attack.

Effect of Carbon on Stress-Relief Cracking Susceptibility of T23 Steel

The coarse-grained heat-affected zone (CGHAZ) samples were prepared from T23 steel with three different carbon contents on a thermal simulator, then isothermal slow strain rate tensile tests were carried out at 500° ~ 750°C, and the effect of carbon content on the stress-relief cracking (SRC) sensitivity of T23 steel was evaluated according to fracture ductility. The microstructure evolution of the CGHAZs in the process of SRC was observed by optical microscopy, structural equation modeling energy-dispersive spectroscopy, and transmission electron microscopy energy-dispersive spectroscopy/selected area electron diffraction to reveal the mechanism of SRC. Finally, the feasibility of adjusting carbon content to prevent SRC was discussed. The results showed the classical theory of intragranular precipitation hardening cannot explain the cause of SRC, and a new viewpoint was put forward that the precipitation of M23C6 at the grain boundary was the main reason for SRC. The precipitation of M23C6 carbide cannot only provide a preferred site for void nucleation but also leads to the depletion of alloy elements in the matrix near the grain boundary. The interaction of these two aspects leads to the direct weakening of grain boundaries. Moreover, the SRC of T23 steel could be inhibited when the carbon content was determined to be close to the ASME standard lower limit of 0.04 wt-%. In summary, reducing the carbon content can inhibit the SRC sensitivity of T23 steel, which is not due to reducing the relative weakening of the grain boundary by reducing the precipitation hardening in the grain interior but to inhibiting the direct weakening of the grain boundary by reducing the precipitation of M23C6 at the grain boundary.

Phase Transformations of 5Cr-0.5Mo-0.1C Steel after Heat Treatment and Isothermal Exposure

This study consists of the experimental and numerical analysis of the phase transformations of 5Cr-0.5Mo.0.1C steel after heat treatment. The microstructure of the as-received steel comprised ferrite and bainite, which is in agreement with the microconstituents predicted by the Calphad-calculated TTT diagram. Calphad-based precipitation calculations show that the cooling stage of normalizing treatment did not cause carbide formation. In contrast, tempering at 700 °C for 15 min promotes the intergranular precipitation of Fe3C, M7C3 and M23C6 carbides, which is consistent with experimental results. Aging at 600 °C for short periods caused the precipitation of both M7C3 and M23C6 carbides; however, M23C6 is the dominant phase after prolonged aging. This is in agreement with experimental results. A rapid decrease in the steel hardness was observed after short aging, which is attributable to bainite transformation. Further reduction in hardness is associated with the diffusion-controlled coarsening of M23C6 carbide.