Study on microstructure and properties of centrifugal casting 35Cr45NiNb+MA furnace tubes during service

In the present study, the microstructure evolution of 35Cr45NiNb+MA ethylene cracking furnace tubes during service and its effect on the properties were investigated. According to the results, in the early stage of service, the skeletal M₇C₃ and vermicular NbC were transformed into blocky M₂₃C₆ and G phase (Ni₁₆Nb₆Si₇), respectively, accompanied with many dispersed M₂₃C₆ secondary carbides. With the extension of service time, M₂₃C₆ carbides on the grain boundaries were transformed into M₇C₃ with high stacking fault structure and coarsened, and the blocky G phase was transformed into granular NbC. The yield strength and ultimate tensile strength of the aging furnace tubes decreased by 9%-18%, yet the elongation after fracture decreased significantly from 10.0%-14.0% to 3.0%-5.0%. The hardness of the carburized zone of the carburized tubes increased by 10%-17%, and the rupture time decreased by 45%-75% under the test condition of 1100℃ and 16MPa. Finally, the evolution map was summarized.     

Effect of thermal exposure on the stability of carbides in Ni-Cr-W based superalloy

The microstructure evolutions of Ni-Cr-W based superalloy during thermal exposure have been investigated systematically. M₆C carbides in the alloy decompose into M₂₃C₆ carbides at temperatures from 650 to 1000 °C due to its high content of Cr. The M₆C carbides decompose dramatically from 800 to 900 °C. At temperatures up to 1000 °C, a few M₂₃C₆ carbides form on the surface of M₆C carbides. The decomposition behavior of primary M₆C can be explained by the following reaction: M₆C → M₂₃C₆ + Me (W, Ni, Cr, Mo). At temperatures below 900 °C, coarse lamellar M₂₃C₆ carbides precipitate at the grain boundaries. The carbide lamellae line almost perpendicular to the grain boundaries. While the temperature is above 1000 °C, discrete M₂₃C₆ carbides precipitate at the grain boundaries. Moreover, there are lots of small M₂₃C₆ particles precipitated around M₆C carbides from 650 to 1000 °C.     

Creep resistance of Fe–Ni–Cr heat resistant alloys for reformer tube applications

Relations between microstructure, phase transformations and creep resistance of austenitic Fe–Ni–Cr alloys are investigated. As-cast alloys with different silicon contents and an ex-service tube are submitted to laboratory agings to trigger specific phase transformations, and subsequently creep-tested at 950°C under stresses of 24–48?MPa. As-cast microstructures contain interdendritic chromium-rich M₇C₃ carbides with niobium-rich MC carbides. After aging at 950°C, primary M₇C₃ carbides transform into chromium-rich M₂₃C₆ carbides, associated to a loss in creep strength. The G phase present in the ex-service alloy is reversed into MC carbides by a heat treatment at 1100°C, associated to a slight decrease in creep resistance. Besides, the addition of silicon is highly detrimental to creep strength. Results can be used for alloy design.     

Refining effect of nitrogen on M7C3 carbides in hypereutectic Fe–25Cr–4C–0.5Ti–0.5Nb hardface coatings

Hypereutectic Fe–Cr–C–Ti–Nb coatings with N additives were developed by surface-hardening welding (hardfacing). The experimental results showed that the primary M₇C₃ carbides were refined by the N additives in the coatings. Based on the micro-morphologies of M₇C₃ and (Ti,Nb)(C,N), the (Ti,Nb)(C,N) was present inside the primary M₇C₃ carbides, and they were tightly combined. The mismatch between the (010) crystal plane of M₇C₃ and the (110) crystal plane of (Ti,Nb)(C,N) was 6.15%, which indicated that (Ti,Nb)(C,N) was moderately effective as a heterogeneous nucleus of M₇C₃ carbides. Therefore, the preferentially precipitated (Ti,Nb)(C,N) in the Fe–Cr–C–Ti–Nb coating was the heterogeneous nucleus of the primary M₇C₃ carbides and thereby refined the primary M₇C₃ carbides .     

Evolution of M23C6 phase in HR3C steel aged at 650 °C

The evolution of the morphology, composition, and particle size of the M₂₃C₆ phase in HR3C steel aged at 650 °C was analysed, by means of metallographic microscopy, scanning electron microscopy, and transmission electron microscopy. The results showed that the Cr/Fe ratio in the M₂₃C₆ phase, in the form of irregular square and long strip, increased with the ageing time. Fe was gradually replaced by Cr in the M₂₃C₆ lattice. The M₂₃C₆ phase gradually evolved and then became stable after ageing for 2000 h.     

Influence of secondary carbides precipitation and transformation on the secondary hardening of laser melted high chromium steel

The influence of secondary carbides precipitation and transformation on the secondary hardening of laser melted high chromium steels was analyzed by means of scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The microstructure of laser melted high chromium steel is composed of austenite with supersaturated carbon and alloy elements and granular interdendritic carbides of type M₂₃C₆. Secondary hardening of the laser melted layer begins at 450 °C after tempering, and the hardness reaches a peak of 672HV at 560 °C and then decreases gradually. After tempering at 560 °C, a large amount of lamellar martensite was formed in the laser melted layer with a small quantity of thin lamellar M₃C cementite due to the martensitic decomposition. The stripy carbides precipitating at the grain boundaries were determined to be complex hexagonal M₇C₃ carbides and face centered cubic M₂₃C₆ carbides. In addition, the granular M₂₃C₆ carbides and fine rod-like shaped M₇C₃ carbides coexisted within the dendrites. As a result, the combined effects of martensitic transformation, ultrafine carbide precipitations, and dislocation strengthening result in the secondary hardening of the laser melted layer when the samples were tempered at 560 °C.     

Precipitation in 9Cr–1Mo steel after creep deformation

The carbides present after creep testing a 9Cr–1Mo steel at 566 °C over a range of stress levels giving rupture times of up to 7300 h have been characterized and identified using a transmission electron microscopy, energy-dispersive X-ray spectroscopy and electron diffraction. The initial carbide precipitates present were M₇C_3, (NbV)C and VC and it was determined that M₆C carbide precipitates were present in all specimens after elevated temperature exposure for greater than approximately 1700 h. No precipitation of M₂₃C₆ was detected. The evolutionary sequence from the initially present carbides during high temperature exposure involved the formation of the stable M₆C carbide directly, without the intermediate formation of M₂₃C₆, as is reported to occur in other Cr–Mo steels.     

Formation of the reversed austenite during intercritical tempering in a Fe-13%Cr-4%Ni-Mo martensitic stainless steel

Formation of the reversed austenite obtained by intercritical tempering has been studied via transmission electron microscopy (TEM) in a Fe-13%Cr-4%Ni-Mo low carbon martensitic stainless steel. It is found that the precipitation of M₂₃C₆ carbides along the martensite lath boundaries will result in Ni-enrichment in the adjacent region. The reversed austenite forms with the Ni-enrichment region as the nucleation sites, keeps a cube-cube orientation relationship with the M₂₃C₆ carbides and bears the Kurdjumov-Sachs (K-S) relationship with the martensite. Moreover, the reversed austenite formed inside the martensite laths is also confirmed. The mechanism for formation of the reversed austenite is discussed in detail.     

Precipitation and thermal instability of M23C6 carbide in cast Ni-base superalloy K452

During long-term thermal exposure M₂₃C₆ dendritically precipitates from the supersaturated γ matrix and then changes in shape from flower-like dendrite to irregular block to regular polyhedron. The reduction in the interfacial energy is responsible for the morphological transition, but the stable morphology of the carbide is principally determined by the anisotropy of the interfacial energy between the carbide and the γ' in the M₂₃C₆-γ' cell. In addition, M₂₃C₆ has a negligible influence on the tensile property of the thermally exposed alloy even though it can block the mobile dislocations and stacking faults to some extent.     

The morphology and microtexture of M7C3 carbides in Fe-Cr-C and Fe-Cr-C-Si alloys of near eutectic composition

The microtexture of M₇C₃ carbides in undercooled 40 g samples of hyper- and hypo-eutectic Fe-Cr-C alloys was determined by electron back scatter diffraction. In the hyper-eutectic alloy the carbides were monocrystalline, while those in the hypo-eutectic alloy were polycrystalline. While in the former the preferred growth direction of the M₇C₃ carbides was [0001], in the hypo-eutectic alloy there was a relatively weak texture near [10¯11]. There was no evidence for the presence of growth twins in either the M₇C₃ carbide rods or in the branching mechanism in the joint between the carbide rods of the hypo-eutectic sample. The morphologies of the M₇C₃ carbides resulting from undercooling were used to explain the microstructure of hardfacing Fe-Cr-C weld deposits applied by the manual metal arc process. The effect of silicon additions on the morphology of M₇C₃ carbides in Fe-Cr-C-Si alloys is explained in terms of the effect of silicon on undercooling.     

Microstructural evolution in bainite,martensite, and δ ferrite of low activation Cr–2W ferritic steels

Abstract

The microstructural evolution in (2–15)Cr–2W–0·1C (wt-%) firritic steels after quenching, tempering, and subsequent prolonged aging was investigated, using mainly transmission electron microscopy. The steels examined were low induced radioactivation ferritic steels for fusion reactor structures. With increasing Cr concentration, the matrix phase changed from bainite to martensite and a dual phase of martensite and δ ferrite. During tempering, homogeneous precipitation of fine W₂C rich carbides occurred in bainite and martensite, causing secondary hardening between 673 and 823 K. With increasing tempering temperature, dislocation density decreased and carbides had a tendency to precipitate preferentially along interfaces such as bainite or martensite subgrain boundaries. During aging at high temperature, carbides increased in size and carbide reaction from W₂C and M₆C to stable M₂₃C₆ occurred. No carbide formed in δ ferrite. The precipitation sequence of carbides was analogous to that in conventional Cr–Mo steels.

MST/1049     

Study on transformation characteristics of carbides in an 8?% Cr roller steel

The characteristics of carbide transformation in an 8 % Cr roller steel under the conditions of equilibrium, slow heating, and isothermal treatment were investigated using thermodynamic calculation, differential scanning calorimetry, X-ray diffraction, and scanning electron microscopy. The carbides under equilibrium conditions were identified as MC, M₂₃C₆, and M₇C₃ types. The austenite transformation onset and finish temperatures, Ac₁ and Ac₃, were 745 and 780 °C, respectively. M₂₃C₆ decomposition took place at 820–850 °C and M₇C₃ at 1060–1130 °C, under slow heating conditions. Quantitative relationships were obtained between the isothermal conditions and average carbide size and aspect ratio, which increased with increasing holding temperature and time.     

Effect of tempering temperature on the microstructure and properties of ultrahigh-strength stainless steel

The microstructure, precipitation and mechanical properties of Ferrium S53 steel, a secondary hardening ultrahigh-strength stainless steel with 10% Cr developed by QuesTek Innovations LLC, upon tempering were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and tensile and impact tests. Based on these results, the influence of the tempering temperature on the microstructure and properties was discussed. The results show that decomposition occurred when the retained austenite was tempered above 440 °C and that the hardening peak at 482 °C was caused by the joint strengthening of the precipitates and martensite transformation. Due to the high Cr content, the trigonal M₇C₃ carbide precipitated when the steel was tempered at 400 °C, and M₇C₃ and M₂C (5–10 nm in size) coexisted when it was tempered at 482 °C. When the steel was tempered at 630 °C, M₂C and M₂₃C₆ carbides precipitated, and the sizes were greater than 50 nm and 500 nm, respectively, but no M₇C₃ carbide formed. When the tempering temperature was above 540 °C, austenitization and large-size precipitates were the main factors affecting the strength and toughness.     

Chemical stability of carbon nanotubes in the 2024Al matrix

Five volume percent of carbon nanotubes and 2024Al alloy powder were mixed with ball milling method, and then the composite was fabricated at 873 K by hot pressing sintering technique. The microstructure of the composite was investigated using optical microscopy, transmission electron microscopy, and X-ray diffraction. The experimental results showed that carbon nanotubes are reacted and changed into Al₄C₃. Nano-Al₄C₃ phases with needle shape are distributed mainly on Al grain boundaries; meanwhile some of them exist within Al grains. The reaction mechanism of carbon nanotubes-Al is discussed.     

Interface temperature measurement of M2C and M6C eutectic carbides in the Fe–Mo–C system

The High speed cast iron, which is used for hot rolling parts, needs high fracture toughness and wear resistance. To improve these properties, the control of eutectic carbides, M₃C, M₇C₃, M₆C and MC is important by adding elements such as Cr, W, V and Mo.The aim of this study is to estimate which carbide will solidify under certain solidification conditions and compositions. This prediction criterion can be gained by measuring the interface temperature of each carbide in various samples with different solute elements, composition and growth rate.In this report, the solidified temperature of γ+M₂C and γ+M₆C eutectic carbide in the Fe–Mo–C ternary system in the composition range near to the eutectic monovariant line, was measured during the unidirectional solidification process. The relationship between solidified interface temperature and growth rate was obtained. In eutectic solidification along the γ+M₆C monovariant line, a coefficient of undercooling, the k value, was obtained.The authors have already measured the k values of other eutectic carbides, such as γ+M₃C, austenite+M₇C₃, and γ+VC in Fe–Cr–C and Fe–V–C system. The paper also discusses the relationships between these properties of eutectic carbides.     

Interface temperature measurement of M2C and M6C eutectic carbides in the Fe–Mo–C system

The High speed cast iron, which is used for hot rolling parts, needs high fracture toughness and wear resistance. To improve these properties, the control of eutectic carbides, M₃C, M₇C₃,M₆C and MC is important by adding elements such as Cr, W, V and Mo.

The aim of this study is to estimate which carbide will solidify under certain solidification conditions and compositions. This prediction criterion can be gained by measuring the interface temperature of each carbide in various samples with different solute elements, composition and growth rate.

In this report, the solidified temperature of γ + M₂C and γ + M₆C eutectic carbide in the Fe–Mo–C ternary system in the composition range near to the eutectic monovariant line, was measured during the unidirectional solidiication process. The relationship between solidified interface temperature and growth rate was obtained. In eutectic solidification along the γ + M₆C monovariant line, a coefficient of undercooling, the k value, was obtained.

The authors have already measured the k values of other eutectic carbides, such as γ + M₃C, austenite + M₇C₃, and γ + VC in Fe–Cr–C and Fe–V–C system. The paper also discusses the relationships between these properties of eutectic carbides.     

Precipitate design for creep strengthening of 9% Cr tempered martensitic steel for ultra-supercritical power plants

Abstract

It is crucial for the carbon concentration of 9% Cr steel to be reduced to a very low level, so as to promote the formation of MX nitrides rich in vanadium as very fine and thermally stable particles to enable prolonged periods of exposure at elevated temperatures and also to eliminate Cr-rich carbides M₂₃C₆. Sub-boundary hardening, which is inversely proportional to the width of laths and blocks, is shown to be the most important strengthening mechanism for creep and is enhanced by the fine dispersion of precipitates along boundaries. The suppression of particle coarsening during creep and the maintenance of a homogeneous distribution of M₂₃C₆ carbides near prior austenite grain boundaries, which precipitate during tempering and are less fine, are effective for preventing the long-term degradation of creep strength and for improving long-term creep strength. This can be achieved by the addition of boron. The steels considered in this paper exhibit higher creep strength at 650 °C than existing high-strength steels used for thick section boiler components.     

Formation of lamellar M23C6 on and near twin boundaries in austenitic stainless steels

Thin foil electron microscopy studies were made on the precipitation of lamellar M₂₃C₆ during aging at 973 K and 1073 K in water-quenched specimens of two austenitic stainless steels. After the precipitation on incoherent twin boundaries M₂₃C₆ formed on coherent twin boundaries and in the regions adjacent to incoherent twin boundaries. These precipitates showed lamellar morphology and were aligned in a specific manner with respect to the twin boundaries. Such lamellar precipitates were absent in the specimens which were isothermally treated at 1073 K after being transferred from the solution treatment temperature. The lamellar morphology of M₂₃C₆ is suggested to be developed by the influence of residual specific stress field around twin boundaries resulted from quenching.     

Fe-Cr-C hardfacing alloys for high-temperature applications

The microstructure and phase chemistry of a Fe-34Cr-4.5C wt% hardfacing alloy has been investigated using transmission electron microscopy and microanalytical techniques. The microstructure is found to consist of large primary M₇C₃. carbides in a eutectic mixture of austenite and more M₇C₃. The results indicate that the microstructure of the undiluted alloy becomes configurationally frozen at a temperature of about 1150° C during deposition by the manual metal arc welding technique. This allows the metastable austenite phase to contain a large chromium concentration ( 16 to 17 wt %), thus imparting good corrosion and oxidation resistance. Experimental data on the partitioning of chromium, manganese and silicon between the carbide phases are discussed in the context of the high-temperature stability of the alloy.     

Effects of Mo on microstructure of as-cast 28 wt.% Cr–2.6 wt.% C–(0–10) wt.% Mo irons

Microstructures of as-cast 28 wt.% Cr–2.6 wt.% C irons containing (0–10) wt.% Mo with the Cr/C ratio of about 10 were studied and related to hardness. The experimental irons were cast into dry sand molds. Microstructural investigation was performed by light microscopy, X-ray diffractometry, scanning electron microscopy, transmission electron microscopy and energy-dispersive X-ray spectrometry. It was found that the iron with about 10 wt.% Mo was eutectic/peritectic, whereas the others with less Mo content were hypoeutectic. The matrix in all irons was austenite, partly transformed to martensite during cooling. Mo addition promoted the formation of M₂₃C₆ and M₆C. At 1 wt.% Mo, multiple eutectic carbides including M₇C₃, M₂₃C₆ and M₆C were observed. M₂₃C₆ existed as a transition zone between eutectic M₇C₃ and M₆C, indicating a carbide transition as M₇C₃(M_2.3C) → M₂₃C₆(M_3.8C) → M₆C. At 6 wt.% Mo, multiple eutectic carbides including M₇C₃ and M₂₃C₆ were observed together with fine cellular/lamellar M₆C aggregates. In the iron with 10 wt.% Mo, only eutectic/peritectic M₂₃C₆ and M₆C were found without M₇C₃. Mo distribution to all carbides has been determined to be increased from ca. 0.4 to 0.7 in mass fraction as the Mo content in the irons was increased. On the other hand, Cr distribution to all carbides is quite constant as ca. 0.6 in mass fraction. Mo addition increased Vickers macro-hardness of the irons from 495 up to 674 HV₃₀. High Mo content as solid-solution in the matrix and the formation of M₆C or M₂₃C₆ aggregates were the main reasons for hardness increase, indicating potentially improved wear performance of the irons with Mo addition.