MC-type Carbides Research Articles

Characterization of (Nb,Ti,Mo)C Precipitates in an Ultrahigh Strength Martensitic Steel

A study on ultrahigh strength steel plate subjected to novel thermo-mechanical control process was presented. The mechanical properties examination showed that the investigated steel exhibited excellent combination of ultra-high strength (2200 MPa) and toughness (26 J). The microstructure of the experimental steel was observed by scanning electron microscope and transmission electron microscope. Desired martensitic lath with width of about 180–230 nm was obtained. Nanostructured carbide precipitates with sizes of 20–50 nm, which contained Nb, Ti and Mo, were observed in the lath martensitic microstructure, and confirmed to be MC-type carbides with B1 structure by means of selected area electron diffraction. The compositional characteristics revealed by energy dispersive X-ray spectrometer mapping implied that the carbide forming elements Nb, Ti and Mo distributed in the precipitates evenly. Three-dimensional atom probe tomography reconstruction further indicated that Mo incorporated into the precipitates without enrichment in the carbide-matrix interface and probably substituted for Nb and Ti to form the (Nb,Ti,Mo)C carbides.

Thermal analysis of indefinite chill cast iron modified with ferrovanadium and ferrotungsten

Melt modification is a method commonly used when enhancing mechanical and microstructural properties of various alloys. The addition of heterogeneous nucleation sites generally refines the given microstructure and enhances the toughness of the material. Ferroalloys based on high-melting elements require a longer incubation time in the melt which makes them liable candidates for modification agents. A research on the effect of melt modification was conducted on an indefinite chill cast iron alloy commonly used for working layers in rolls. The goal of the research was to determine the effect of ferrovanadium and ferrotungsten melt treatment with the use of thermal analysis along with thermodynamic modelling using the CALPHAD approach. The results show a decrease in the measured liquidus temperature of the modified alloy along with a possible second eutectic reaction hidden inside the main graphite/cementite eutectic reaction. Scanning electron microscopy revealed small and evenly distributed MC type carbides rich in V with W and other carbide-forming elements. The results presented in this work enable a better understanding of the impact melt modification has on the development of microstructure and subsequently the improvement in mechanical properties of the alloy.

Effect of high magnetic field on carbide precipitation in W6Mo5Cr4V3 high-speed steel during low-temperature tempering

Abstract The effect of a high magnetic field on carbide precipitation in W6Mo5Cr4V3 high-speed steel during low-temperature tempering was investigated. The applied high magnetic field promoted the precipitation of M 6C-type carbides at boundaries and in the grain interior, but maximum spheroidization and refinement occurred for those carbides precipitated at boundaries. Compared with M 6C-type carbides, the effect of high magnetic field on the precipitation behavior of MC-type carbides is much weaker. The high magnetic field hindered M 2C-type carbide precipitation by affecting the Gibbs free energy and increased the microhardness of W6Mo5Cr4V3 high-speed steel at low tempering temperature.

Precipitation behavior of γ′ precipitates in the fusion zone of TIG welded ATI 718Plus®

The precipitation behavior of the main strengthening phase, γ′ precipitates, in ATI 718Plus® superalloy after Tungsten Inert Gas (TIG) welding and postweld heat treatments has been studied. In contrast to electron beam welding, where no γ′ precipitates are reported to form in the as-welded condition, analytical transmission electron microscopy study in this work revealed the formation of γ′ precipitates after the TIG welding, albeit in a non-uniform distribution manner. This is attributable to a more extensive elemental microsegregation that occurred into the interdendritic liquid and slower cooling rate during the TIG welding, which also induced the formation of interdendritic Nb-rich Laves phase particles and MC-type carbides. Theoretical calculations were performed to study the influence of Nb microsegregation, on both the kinetics and extent of γ′ precipitation, and the results agree with experimental observations. It is found that the precipitation kinetics, and not the extent of γ′ precipitate formation in the fusion zone, during postweld heat treatments is affected by the micro-segregation of Nb that produced Laves phase particles during the weld solidification.

Effect of heat treatment on the microstructure of a Ni–Fe based superalloy for advanced ultra-supercritical power plant applications

The effect of heat treatment on the microstructure and microhardness of a Ni–Fe based superalloy for 700°C advanced ultra-supercritical coal-fired power plants was investigated. Results showed that the main phases in the alloy were γ, γ′, MC and M23C6, and no harmful phase was observed in the alloy. M23C6-type carbides discretely distributed nearby grain boundaries as the alloy was aged at above 840°C. The microhardness decreased with increasing aging temperature. The coarsening of γ′ led to the increment of microhardness at 780°C and 810°C for a short aging time, and a significant decrease in microhardness after aging at 840°C. The aging temperature had more significant role on the microstructure than holding time. Therefore, to obtain optimum strengthening effect for this alloy, the aging temperature should not exceed 810°C.

Microstructure and Room Temperature Mechanical Properties of a New Corrosion-Resistant Alloy GH945 for Oil and Gas Applications

The microstructure and mechanical properties of a new corrosion-resistant alloy GH945 with high strength for oil and gas production were studied in this paper. The results showed that the main precipitations of GH945 alloy after solution heat treatment and two-stage age hardening treatment were γ′ and γ′′ phase, MC-type carbide, σ phase and M23C6-type carbide. The increase of primary aging temperature not only promoted the precipitation of σ phase and film of continuous M23C6 carbide on the grain boundary, but also led to the coarsening of γ′ and γ′′ phase, which resulted in the decrease of yield strength and impact toughness. Alloy with high Nb content had high strength but low impact toughness. The following heat treatment was preferably used to obtain the combination of high strength and excellent ductility and toughness for GH945 alloy: 1010 °C/70 min, WQ+ 735 °C/9 h + FC to 615 °C/18 h, AC.

Effect of molybdenum addition on the precipitation of carbides in the austenite matrix of titanium micro-alloyed steels

The present work aims to reveal the effect of Mo on the precipitation behavior of MC-type (M = Ti and Mo) carbides in the austenite matrix of titanium micro-alloyed steel. The precipitation start-time–temperature curve was determined by a double-pass compression test on a Gleeble simulator, and the elemental mass fraction of MC-type carbides was measured by a physical–chemical phase analysis after a single-pass rolling test. The results shows that 0.2 wt% Mo accelerates the precipitation kinetics of MC-type carbides. During the initial stage of precipitation, Mo tends to distribute in the outer region of precipitates by replacing Ti despite of the high solubility of MoC in austenite. The replacement of Ti by Mo in TiC lattice leads to two opposite effects: First, it restrains MC precipitation due to the higher Gibbs free energy of (Ti, Mo)C relative to TiC; Second, it promotes MC precipitation by decreasing the interfacial chemical energy of MC/austenite system. The second effect is more pronounced during the initial stage of precipitation when MC precipitates are relatively small and hence MC precipitation is accelerated by Mo addition. Compared to TiC, (Ti, Mo)C with stronger coarsening resistance suppresses austenite recovery and recrystallization more effectively, which favors maintaining the deformation microstructures at high temperatures.

Application of titanium machining chips in welding consumables for wear-resistant hardfacing

Equipment of sugar cane plants and mineral extraction are submitted to severe abrasive wear conditions. Welded hardfacings are usually applied to repair this kind of damage, where commercial chromium/carbon-rich welding consumables have usually been employed. In the present work we investigated the microstructure of experimental hardfacings made by addition of residues (chips) collected from the machining of ASTM F67 (unalloyed Ti, grade 4) alloy. Mixtures with different carbide-formers (Cr/Nb ferro-alloys) were also tested. Two layers of ‘pure’ chips (Ti), chips plus Fe–Cr (Ti–Cr), and chips plus Fe–Nb (Ti–Nb) were applied on low-carbon steel specimens by the GTAW/TIG process. The microstructure of hardfacing layers was observed by optical and scanning electron microscopy (SEM) equipped with EDS microanalysis. The microstructural characterization has determined that carbide distributions change significantly with the chemical nature of the hardfacing. SEM observations coupled with EDS microanalysis have confirmed the formation of complex carbides within the metal weld, whose stoichiometry was determined by X-ray diffraction (XRD) analysis. Mixed carbides of MC type and some cementite have been found. As a result it was suggested that use of ASTM F67 chips as carbide formers for composition of welding consumables can contribute to improved wear resistance of hardfacings, if compared with traditional chromium-based hardfacings.

Effects of post-weld heat treatment on microstructure and mechanical properties of laser welds in GH3535 superalloy

In this study, the microstructure and mechanical properties of laser welds before and after post-weld heat treatment processes were studied. The results show that the tensile strength of the joints can be increased by 90MPa by a post-weld heat treatment process at 871°C for 6h, exceeding the strength of the original state of the base metal. Besides, elongation of the joints are also increased to 43% by the process, whereas the elongation of as-welded joints are only 22%. In addition, the Charpy impact properties of laser welds almost do not change. Second phase precipitates, which were identified as Mo–Si rich M6C-type carbides by transmission electron diffraction and scanning electron microscope, were observed at solidification grain boundaries and solidification subgrain boundaries. These carbides can pin dislocations during the following tensile deformation, hence are responsible for the strengthening of tensile properties of the joints.

Microstructure and mechanical properties of laser-arc hybrid welding joint of GH909 alloy

In this paper, laser-arc hybrid welding of 10 mm thick low-thermal-expansion superalloy GH909 components was carried out to obtain a joint with good performance. This investigation was conducted using an optical microscope, scanning electron microscope, energy diffraction spectrum and other methodologies. The results showed that weld joints with a desirable wineglass-shaped weld profile can be obtained employing appropriate process parameters. The different grains in between the upper central seam and the bottom seam were associated with the temperature gradient, the pool’s flow and the welding thermal cycle. MC-type carbides and eutectic phases (γ+Laves) were produced at grain boundaries due to the component segregation during the welding process. In addition, γ′ strengthening phase presented in the interior of grains, which kept a coherent relationship with the matrix. The lowest hardness value occurred in the weld center, which indicated that it was the weakest section in the whole joint. The average tensile strength of the joints reached to 632.90MPa, nearly 76.84% of the base metal. The fracture analysis revealed that the fracture mode of the joint was ductile fracture and the main reason for joint failure was as a result of the occurrence of porosities produced in the weld during the welding process.

Weldability of Ni-Based Superalloys Waspaloy® and Haynes® 282® - A Study Performed with Varestraint Testing

There is a need for materials with high strength, oxidation resistance, thermal stability and adequate weldability in order to facilitate the production of large structural jet engine components. Therefore, the weldability of Waspaloy® and Haynes® 282® have been evaluated using the Varestraint weldability test. The experiments reveal that Waspaloy® has a higher susceptibility to hot cracking compared to Haynes® 282®. This conclusion is supported by increased total crack length (10 mm or more) and larger brittle temperature range (approx. 65°C) for Waspaloy® when compared to Haynes® 282® in Varestraint and Gleeble hot ductility tests, respectively. The cracking in Haynes® 282® seems to be connected with a secondary phase which presumably can be associated with Ti-Mo based MC-type carbide observed in the fusion zone. Also, a surrounding segregated area is present near this secondary phase as well as along the grain boundaries. Furthermore, micro-Vickers hardness results revealed more or less the same weld metal hardness (260- 280 HV) but a difference in the base metal hardness. The weld metal hardness of Waspaloy® was lower than that of the base metal hardness, while Haynes® 282® had a higher hardness in comparison.

Origin of Threshold Stresses in a P92-type Steel

The creep behavior of a P92-type steel was examined at 923 K. The threshold shear stress of 51 MPa was revealed by an analysis of steady state creep behavior at applied stress ranging from 118 to 200 MPa. The tempered martensite lath structure stabilized by M23C6-type carbide particles with an average size of about 110 nm and MX-type carbonitrides with a size of 40 nm located within ferritic matrix evolved after tempering. The transient creep at an applied stress of 118 MPa was accompanied by two-fold decrease in internal elastic stress and dislocation density. The threshold stresses matched the values of internal stresses, which were measured by means of the lattice curvature within individual laths, and those corresponded to Orowan stressor detachment stress originated from M(C,N) carbonitrides. A dispersion of M(C,N) carbonitrides played a crucial role in superior creep resistance of P92-type steels because of dispersion hardening and hindering the relief of internal stresses originated from lath boundaries.

Effects of Pulsed Current and Pulsed Magnetic Field Complex Pretreatment on Martensite Transformation of Cr5 Steel during Continuous Cooling

Carbide precipitation and martensite transformation in Cr5 steel have been observed in situ by high-temperature confocal laser-scanning microscopy. In this way, the influences of pulsed current and pulsed magnetic field complex pretreatment on carbide precipitation and martensite transformation during continuous cooling have been studied. The results show that the electropulsing complex pretreatment promotes the precipitation of M7C3-type carbides at high temperature, increases the start and finish temperatures of martensite transformation, and extends the phase transformation time. Martensite prefers to nucleate in the austenite with less precipitation of carbides due to the chemically homogeneous distribution of solute atoms.

Comparison of three kinds of MC-type carbide modified thick W coatings fabricated by plasma transferred arc surfacing

Abstract Three kinds of W-based composite powders containing NbC, TiC and VC were prefabricated on a carbon steel substrate, respectively. Prefabricated powders were then processed by plasma transferred arc (PTA) surfacing to form thick W–NbC, W–TiC and W–VC coatings, respectively. Microstructures and nano-indentation properties of the three kinds of coatings were investigated and compared. It was observed that, in addition to W and MC-type carbide phases, Fe 7 W 6 phase was formed in the three kinds of coatings because of the dilution effect of Fe from the substrates. The relative contents of Fe 7 W 6 phase in the W–NbC, W–TiC and W–VC coating were 48.1 wt.%, 52.2 wt.% and 36.3 wt.%, respectively. Tungsten grain size in the W–NbC coating was about 2.8 μm, which was smaller than that in the W–TiC coating (about 3.4 μm) and that in the W–VC coating (about 4.7 μm). There was no simple relation between the mechanical response and the nano-indentation hardness H or the effective Young's modulus E * alone; however, this response was strongly dependent on the ratio H / E *. For coatings with high H/E* value, the elastic recovery was also be larger. The best ability to resist elastic strain-to-failure and the highest toughness were obtained in the W–VC coating when compared with that in the W–NbC and W–TiC coatings.

Carbide precipitation and microstructure refinement of Cr-Co-Mo-Ni bearing steel during hot deformation

The dynamic recrystallization and carbides precipitation of the Cr-Co-Mo-Ni bearing steel were investigated by hot compression tests performed at temperatures ranging from 850 °C to 1080 °C with strain rate of 1-20 s-1. The activation energy (Q) for the tested steel is calculated to be around 682.99 kJ/mol at a deformation strain of 0.6. Microstructural analysis by SEM shows that the dynamic recrystallization (DRX) behavior is dependent sensitively on the deformation strain, temperature and strain rate, while an exponential relationship between DRX grain size and Z parameter is obtained from the computational formula. Moreover, the M6C-type carbides (<1 μm) act as the main prohibitor of grain coarsening, and the polynomial regression relationship between them is worked out. With electron backscatter diffraction (EBSD) observation, DRX is the main nucleation mechanism responsible for the formation of new grains during hot compression. In conclusion, the interaction between DRX affected by hot deformation parameters and carbides precipitation determines the ultimate grain size refinement.

Phase stability, electronic and elastic properties of Fe6−xWxC (x = 0−6) from density functional theory

The calculated formation enthalpies of Fe3W3C (W in the 48f site) and Fe2W4C are negative, and they are of higher stability than other (Fe,W)6C phases. The 16d and 32e sites are comfortable for Fe atoms, while the W atom is preferential in the 48f site during the formation process of (Fe,W)6C carbides. An exchange of electrons takes place between the Fe/W and the C atom, and also appears between the Fe and W atom. In M–M bonds, besides metallic characters, a valence character between W–W bonds exists. Besides the nonmetal C atom with negative charges (obtaining electrons), the partial metal atoms of Fe and W in different M6C-type carbides show an electronegative phenomenon. The carbides W6C, Fe5WC, Fe2W4C and Fe3W3C-II likely have high temperature superconductive characters. The hardness of Fe3W3C (16.5 GPa) is the highest in these phases, and that of Fe2W4C (13.7 GPa) is the second highest. The Debye temperature (θD) of Fe3W3C–I (Fe in the 48f site) is lowest (254 K) in these carbides, and that of Fe3W3C-II suddenly changes to 568 K when W is in the 48f site. The change of the atom in the 48f site intensively influences the properties of M6C (M = Fe/W).

Mechanical properties and electronic structures of M23C6 (M = Fe, Cr, Mn)-type multicomponent carbides

The mechanical and electronic properties of M23C6- (M = Fe, Cr, Mn) and M23C6-type multicomponent carbides are investigated systematically by first-principles calculations. The values of cohesive energy and formation enthalpy exhibited thermodynamically stable structures of these carbides. The stress–strain method and Voigt-Reuss-Hill approximation were used to calculate the elastic constants and moduli, respectively. The mechanical properties of the doped Fe or Mo in M23C6 compounds are superior to the pure phases of Cr23C6, Mn23C6 and Fe23C6. Mechanical anisotropy of these compounds was illustrated from the anisotropic index and different shapes of the surface contour by Young's modulus. Moreover, the total density of states of M23C6 compounds indicated that the bonding behaviors of M23C6 compounds were combinations of metallic and covalent bonds.

Evolution of Minor Phases in a 9PctCr Steel: Effect of Tempering Temperature and Relation with Hydrogen Trapping

The evolution of minor phases in ASTM A335 P91 steel has been studied on specimens submitted to different thermal treatments including a tempering step. Particular emphasis has been put on the tempering temperature range 573 K to 873 K (300 °C to 600 °C), which has not been yet intensively studied. The techniques used in this investigation were X-ray diffraction with synchrotron light, scanning electron microscopy with field emission gun and transmission electron microscopy. In the low tempering temperature range [573 K to 673 K (300 °C to 400 °C)], retained austenite, Fe3C and MX precipitates are observed. In the high tempering temperature range [773 K to 1053 K (500 °C to 780 °C)], M23C6-type carbides, MX-type carbonitrides and M2X precipitates are observed. The effect of the microstructure on hydrogen trapping is analyzed. The distorted matrix around the M2X and MX particles provides the most important trap sites in the P91 steel.

Structural characterization of M7C3-type carbide precipitated in the aging treated 100Mn13 steel

The distribution and the microstructure of carbides and pearlite, orientation relationships of pearlitic component phase in 100Mn13 steel by solution treatment at 1050 °C and aging treatment at 525 °C were investigated by optical microscope and transmission electron microscope. The results show that there is a large number of carbides and pearlite forming within the austenite grain and at the grain boundaries. The carbides in the pearlite of 100Mn13 steel have two kinds of morphologies, namely flaky and short rod. The type of carbides is M7C3. The carbides with different morphologies in the pearlite have different crystallographic orientation relationships with the ferrite. The orientation relationships between the flaky carbides and the ferrite are , , while the orientation relationships between the short-rod carbides and the ferrite are , . The short-rod carbides distribute directionally and orderly on the ferrite, which is similar to the morphology of the interphase precipitate. Therefore, the class-interphase precipitate mechanism is proposed in this study.

纳米级碳化物及小角界面密度对Fe-C-Mo- M ( M =Nb、V或Ti)系钢耐火性的影响

通过高Nb、V或Ti(~0.1%),低Mo(≤0.2%)微合金化设计,在经TMCP工艺后用恒载荷拉伸实验测定了Fe-C-M-Mo(M=Nb、V或Ti)系合金钢的失效温度。用EBSD分析了TMCP后样品中的界面密度,用TEM观测了恒载拉伸实验后样品中的纳米析出相。结果表明:在Fe-C-V/Nb钢中添加约0.2%Mo使其在280 MPa恒载荷拉伸升温过程中的失效温度提高约40℃。小角度界面为MC型析出相形核析出提供了有利位置,加速了MC相的析出,在升温过程中细小弥散的MC相在小角度界面形核析出起到了良好的高温沉淀强化作用,提高了耐火钢的失效温度。含Mo的Ti-Mo钢具有较高的小角度界面密度,导致其中MC型析出相析出较快,因此具有最高的失效温度,Nb-Mo钢次之,V-Mo钢因小角度界面密度最小使其在高温下MC相析出的动力学减缓,因此失效温度最低。