Effect of carbon concentration on precipitation behavior of M<Subscript>23</Subscript>C<Subscript>6</Subscript> carbides and MX carbonitrides in martensitic 9Cr steel during heat treatment

The distributions and precipitated amounts of M₂₃C₆ carbides and MX-type carbonitrides with decreasing carbon content from 0.16 to 0.002 mass pct in 9Cr-3W steel, which is used as a heat-resistant steel, has been investigated. The microstructures of the steels are observed to be martensite. Distributions of precipitates differ greatly among the steels depending on carbon concentration. In the steels containing carbon at levels above 0.05 pct, M₂₃C₆ carbides precipitate along boundaries and fine MX carbonitrides precipitate mainly in the matrix after tempering. In 0.002 pct C steel, there are no M₂₃C₆ carbide precipitates, and instead, fine MX with sizes of 2 to 20 nm precipitate densely along boundaries. In 0.02 pct C steel, a small amount of M₂₃C₆ carbides precipitate, but the sizes are quite large and the main precipitates along boundaries are MX, as with 0.002 pct C steel. A combination of the removal of any carbide whose size is much larger than that of MX-type nitrides, and the fine distributions of MX-type nitrides along boundaries, is significantly effective for the stabilization of a variety of boundaries in the martensitic 9Cr steel.     

Influence of tempering temperature on stability of carbide phases in 2.6cr-0.7mo-0.3v steel with various carbon content

The present work evaluates the influence of the bulk carbon content (0.1, 0.006, and 0.005 wt pct) and tempering temperature (823, 853, and 913 K) on stability, chemical composition, and size of carbide particles in 540 ks tempered states of 2.6Cr-0.7Mo-0.3V steel. The scanning transmission electron microscopy/energy-dispersive X-ray spectroscopy (STEM/EDXS) and electron diffraction methods were used to analyze the carbide particles. A characteristic energy-dispersive X-ray (EDX) spectrum can be attributed to each of the identified carbides. The MC carbide is stable in all experimental states. The phase stability of Fe-Cr-rich carbides increased in the order ε, Fe₃C → M₃C → M₇C₃, with tempering temperature increasing. In steels with higher carbon content tempered at low temperature, M₂₃C₆ carbide was also noted. The Mo₂C and M₆C carbides were not observed. It was shown that the decrease of the bulk carbon content has the same influence on the carbide phases stability as the increase of the bulk vanadium content at the unchanged Cr, Mo, C bulk contents and tempering temperature. Similarly, the decrease of tempering temperature has the same influence on the carbide phases stability as the decrease of the bulk Cr content at the unchanged V, Mo, and C bulk contents.     

Structures and tempering behavior of rapidly solidified high-carbon iron alloys

High-carbon iron alloys containing carbide formers of chromium and molybdenum were rapidly solidified by means of a single roller method. In the alloy containing a high level of both chromium and molybdenum (10Cr-5Mo) and a critical carbon content of about 4 pct, the metastable phases,ε phase and austenite, are retained after solidification. Theε phase could contain a large amount of carbon in solid solution so that during tempering at about 900 K, it decomposes to very fine ferrite and carbide, which bring about an enhanced hardness of 1300 DPN. Even after tempering at a high temperature around 1100 K, the hardness hardly deteriorates due to a remarkable dispersion of fine M₃C and M₇C₃ carbides. Thus, coaddition of chromium and molybdenum is effective in obtaining high hardness. Formerly Graduate Student, Kyushu Institute of Technology     

Studies of Carbides in a Rapidly Solidified High-Speed Steel

Rapid solidification by electron beam surface melting of a Mo-base high-speed steel (M7) has produced microstructural features different from those observed in the conventionally processed material. As a result of rapid solidification, the volume percent of the carbide phases formed has decreased sharply and has resulted in the formation of M₂C and M₂₃C₆ carbide phases, while in the conventionally processed material, M₆C and MC carbides were present. Microanalysis of the extracted carbides formed by electron beam melting has yielded an intriguing finding. M₂₃C₆ is found to be unusually rich in molybdenum, tungsten, and vanadium; the concentration of (Mo + W), for instance, is approximately 60 wt pct. The corresponding values for Fe and Cr are surprisingly low (6 wt pct Cr and 1 wt pct Fe). This is in marked contrast with carbides encountered in the conventionally processed high-speed steel, where Cr and Fe are the major constituents. The shift in composition of the carbide phases could be attributed to the accelerated evaporation of chromium during surface melting as compared to the evaporation of Mo, W, and V. formerly Research Associate, University of Connecticut     

Thermodynamic properties of carbides in 2.25Cr-1Mo steel at 985 K

Thermodynamic properties of carbides present in 2.25Cr-lMo steel were determined at 985 K by a gas flowing method with fixed CH₄/H₂ gas mixtures and by a silica capsule method with reference alloys. The carbon activity range was from 0.06 to 0.5. Total carbon content, carbide species, and Cr and Mo partitionings between the matrix and carbides were measured as a function of the carbon activity. Both M₆C and M₂₃C₆ carbides were present after 1000 to 3000 hours at the test temperature and in the carbon activity range studied. The amount of M₆C was greater in the low carbon activity range, while M₂₃C₆ carbide became the major carbide with increasing carbon activity. The M6C carbide contained Mo as a major element and Cr and Si as minor elements; approximately 13 pct of the metal constituent was (Cr + Si). The stability of M₆C carbide in this steel is significantly higher than M₆C formed in the Fe-Mo-C system. The M₂₃C₆ carbide contained Cr as a major metal component and Mo as a minor. The M₂₃C₆ carbide is more stable in an extended range of the carbon activity in 2.25Cr-lMo steel than in the Fe-Cr-C system. The presence of Si is apparently low in M₂₃C₆. Thermodynamic parameters were computed for M₆C and M₂₃C₆ carbides using a regular solution model of component carbides, FeC_x, CrC_x, and MoC_x.     

Identification of M5C2 carbides in ex- service 1Cr-0.5Mo steels

Rod-shaped precipitates up to 6μm} long and 0.25μm wide, observed as a common feature within proeutectoid ferrite grains of ex-service lCr-0.5Mo steels, have been characterized using electron microdiffraction, energy-dispersive X-ray spectroscopy, and electron energy loss spectroscopy. The majority of the rods have been identified as M₅C₂ carbides, although some were M₃C. The M₅C₂ carbide, also known as the Hägg orX-carbide, is a monoclinic phase that is not known to have been identified previously in creep-resistant Cr-Mo steels. The M₅C₂ rods appeared to nucleate heterogeneously on M₂C carbides and persist in ferrite regions from which the needlelike M₂C carbides had disappeared. This suggests that the M₅C₂ carbide is more stable thermodynamically than M₂C in lCr-0.5Mo steels under typical service conditions. The metallic element compositions of the rodlike carbides varied, but the average compositions were in the range 48 to 56 at. pct Fe, 32 to 42 at. pet Cr, 8 to 12 at. pct Mn, and about 1 at. pct Mo. The Mn content of the rods varied systematically with exposure temperature and thus might be applied to the estimation of the effective service temperature of lCr-0.5Mo steel components.     

Iron-chromium-carbon system at 870°C

A series of 13 Fe-Cr-C alloys was studied to determine the phases present after the alloys were equilibrated for 1000 h at 870°C (1600°F). The alloys had a nominal composition of 1 pct carbon and 0 to 29 pct chromium. The experimental program employed: a) selective etching plus X-ray diffraction studies of extracted carbides to identify the carbides, b) quantitative metallography plus electrolytic carbide extraction to determine the quantity of carbides, and c) electron microprobe analyses plus chemical analyses of extracted carbides to determine the carbide compositions. Microprobe traverses through the carbide particles and the adjoining matrices showed that equilibrium had been established. The In and In₆ carbides were all stoichiometric with respect to car bon—in contrast to most prior diagrams which show the M₃C and M₇C₃ forming a pseudobinary system.     

Carbide precipitation,grain boundary segregation,and temper embrittlement in NiCrMoV rotor steels

This paper presents a study of carbide precipitation, grain boundary segregation, and temper embrittlement in NiCrMoV rotor steels. One of the steels was high purity, one was doped with phosphorus, one was doped with tin, and one was commercial purity. In addition, two NiCrV steels, one high purity and one doped with phosphorus, were examined. Carbide precipitation was studied with analytical electron microscopy. It was found that after one hour of tempering at 600 ‡C only M₃C carbides were precipitated in the NiCrMoV steels. These were very rich in iron. As the tempering time increased, the chromium content of the M₃C carbides increased significantly, but their size did not change. Chromium rich M₇C₃ precipitates began to form after 20 hours of tempering, and after 50 hours of tempering Mo-rich M₂C carbides were precipitated. Also, after 100 hours of tempering, the matrix formed bands rich in M₃C or M₇C₃ and M₂C particles. Tempering occurred more rapidly in the NiCrV steels. Grain boundary segregation was studied with Auger electron spectroscopy. It was found that the amount of phosphorus and tin segregation that occurred during a step-cooling heat treatment after tempering was less if a short time tempering treatment had been used. It will be proposed that this result occurs because the low temperature tempering treatments leave more carbon in the matrix. Carbon then compctes with phosphorus and tin for sites at grain boundaries. This compctition appears to affect phosphorus segregation more than tin segregation. In addition to these two impurity elements, molybdenum and nickel segregated during low temperature aging. The presence of molybdenum in the steel did not appear to affect phosphorus segregation. Finally, it will be shown that all of the steels that contain phosphorus and/or tin exhibit some degree of temper embrittlement when they are aged at 520 ‡C or are given a step-cooling heat treatment. Of the NiCrMoV steels, the phosphorus-doped steel showed the least embrittlement and the commercial purity steel the most. The phosphorus-doped NiCrV steel was also more susceptible to temper embrittlement than the phosphorus-doped NiCrMoV steel. This latter difference was attributed to molybdenum improving grain boundary cohesion. It was also found that as the segregation of phosphorus or tin to the grain boundaries increased, the measured embrittlement and the amount of intergranular fracture increased. However, there was a large amount of scatter in all of these data and the trends were only qualitative. All parts of this study are compared in detail to others in the literature, and general trends that can be discerned from all of these results are presented. Formerly with the University of Pennsylvania, Department of Materials Science, Philadelphia, PA     

Microstructural changes in 1Cr-0.5Mo steel after 20 years of service

Metallographie studies have been conducted on 1Cr-0.5Mo steel “taken from a pressure vessel which had been in service for 20 years in a hydrogenous environment at 524 °C. The original microstructure of the steel, reproduced by reheat treatment of the exposed material, consisted of proeutectoid ferrite and tempered bainite, the carbides being mainly cementite. The service exposure caused precipitation of needle-like M₂C carbides in the ferritic regions and M₇C₃ carbides in the vicinity of the original cementite particles. Chromium and molybdenum moved from solid solution to the carbides during the service exposure with 72 pct and 32 pct of the total chromium and molybdenum contents, respectively, remaining in solid solution after service for 20 years. Formerly with AMAX Materials Research Center (formerly Climax Molybdenum Company of Michigan)     

Overall kinetics and morphology of the products of austenite decomposition in a Fe-0.46 Pct C-5.2 Pct Cr alloy transformed isothermally above the bay

The overall transformation kinetics of austenite isothermal decomposition above the bay of the time-temperature-transformation (TTT) curve and the eutectoid morphology of the resulting products have been studied in a Fe-0.46 pct C-5.2 pct Cr alloy. Classical lamellar pearlite was formed at high temperatures while complex ferrite plus carbide morphologies, sometimes described as spiky pearlite, arborescent structures, or nonclassical decompositions products of austenite in Fe-Cr-C alloys, formed at low temperatures. While X-ray diffraction of extracted carbides and selected area diffraction-transmission electron microscopy (TEM) showed evidence for a mixture of M₃C and M₇C₃ carbides, thermodynamic calculations results obtained only M₇C₃ as the equilibrium carbide at the temperatures studied. A tentative explanation for the arborescent morphology is presented, based on the hypothesis of the existence of a drag force or free energy dissipation term that is locally relaxed by the partition of Cr into the carbides at the reaction front, consequently removing Cr from the interface. This article is based on a presentation made in the “Hillert Symposium on Thermodynamics & Kinetics of Migrating Interfaces in Steels and Other Complex Alloys,” December 2–3, 2004, organized by The Royal Institute of Technology in Stockholm, Sweden.     

M5C2 Carbides in a High-Chromium Ferritic/Martensitic Steel

The precipitate phases in an 11 pct Cr ferritic/martensitic steel normalized at 1323 K (1050 °C) for 0.5 hour and tempered at 1053 K (780 °C) for 1.5 hours have been investigated. Except for dominant phases, Cr-rich M₂₃C₆ carbide and Nb-rich, Ta-Nb-rich, and V-rich MC carbides, needle-like precipitates with a typical size of 70 to 310 and 10 to 30 nm for the length of the long and short axis of the needles, respectively, were also observed on the extraction carbon replica of the steel. The typical metallic element composition of the needle-like precipitates is about 53-82Fe, 14-26Cr, 0.5-18Ta, 1-6W, and 2-5Co in atomic pct. Through energy dispersive X-ray analysis and electron diffraction along with calculations regarding lattice parameter and interplanar spacing, the needle-like precipitates were identified as a Fe-rich M₅C₂ carbide, which is not known to have been reported previously in high-chromium steels. The M₅C₂ carbide has a base-centered monoclinic crystal structure with the approximate lattice parameters a/b/c = 1.142/0.5186/0.5383 nm and β = 104.68 deg. The formation of the Fe-rich M₅C₂ carbides in the steel has been discussed. The effect of chromium content in matrix and boron addition on the precipitate phases in ferritic/martensitic steels has also been discussed.     

Carbide Precipitation in 2.25 Cr-1 Mo Bainitic Steel: Effect of Heating and Isothermal Tempering Conditions

The effect of the tempering heat treatment, including heating prior to the isothermal step, on carbide precipitation has been determined in a 2.25 Cr-1 Mo bainitic steel for thick-walled applications. The carbides were identified using their amount of metallic elements, morphology, nucleation sites, and diffraction patterns. The evolution of carbide phase fraction, morphology, and composition was investigated using transmission electron microscopy, X-ray diffraction, as well as thermodynamic calculations. Upon heating, retained austenite into the as-quenched material decomposes into ferrite and cementite. M₇C₃ carbides then nucleate at the interface between the cementite and the matrix, triggering the dissolution of cementite. M₂C carbides precipitate separately within the bainitic laths during slow heating. M₂₃C₆ carbides precipitate at the interfaces (lath boundaries or prior austenite grain boundaries) and grow by attracting nearby chromium atoms, which results in the dissolution of M₇C₃ and, depending on the temperature, coarsening, or dissolution of M₂C carbides, respectively.     

Solidification microstructure and M2C carbide decomposition in a spray-formed high-speed steel

The solidified carbide morphology, the decomposition behavior of the M₂C carbide, and the carbide distribution after forging of an Fe-1.28C-6.4W-5.0Mo-3.1V-4.1Cr-7.9Co (wt pct) high-speed steel prepared by spray forming have been investigated. The spray-formed microstructure has been characterized as a discontinuous network of plate-shaped M₂C carbides and a uniform distribution of fine, spherical MC carbides. The metastable M₂C carbides formed during solidification have been fully decomposed into MC and M₆C carbides after sufficient annealing at high temperatures. Initially, the M₆C carbides nucleate at M₂C/austenite interfaces and proceed to grow. In the second stage, the MC carbides form either inside the M₆C carbides or at the interfaces between M₆C carbides. With this increasing degree of decomposition of the M₂C carbide, the carbides become more uniformly distributed through hot forging, which produces a significant increase in ultimate bend strength. The decomposition treatment of M₂C carbide has been found to be most important for obtaining a fine homogeneous carbide distribution after hot forging.     

Correlation of microstructure with the wear resistance and fracture toughness of duocast materials composed of high-chromium white cast iron and low-chromium steel

The objective of this study is to investigate the correlation of microstructure with wear resistance and fracture toughness in duocast materials that consisted of a high-chromium white cast iron and a low-chromium steel as the wear-resistant and ductile parts, respectively. Different shapes, sizes, volume fractions, and distributions of M₇C₃ carbides were employed in the wear-resistant part by changing the amount of chromium and molybdenum. In the alloys containing a large amount of chromium, a number of large hexagonal-shaped primary carbides and fine eutectic carbides were formed. These large primary carbides were so hard and brittle that they easily fractured or fell off from the matrix, thereby deteriorating the wear resistance and fracture toughness. In the alloys containing a smaller amount of chromium, however, a network structure of eutectic carbides having a lower hardness than the primary carbides was developed well along solidification cell boundaries and led to the improvement of both wear resistance and toughness. The addition of molybdenum also helped enhance the wear resistance by forming additional M₂C carbides without losing the fracture toughness. Under the duocasting conditions used in the present study, the appropriate compositions for wear resistance and fracture toughness were 17 to 18 pct chromium and 2 to 3 pct molybdenum.     

Phase equilibria in the iron-molybdenum-1 Pct carbon system between 1143 and 1253 K

A series of Fe−Mo-1 pct C alloys containing 0 to 5 pct molybdenum was used to determine portions of Fe−Mo−C isothermal sections at 1143, 1198, and 1253 K. The identity, compositions, and amounts of the phases present, after homogenizing for either 500 or 1000 h at temperature, were determined by a variety of experimental techniques. The solubility of molybdenum in the austenite of Fe−Mo-1 pct alloys was found to be lower than that previously reported, and a unique metastability of M₆C carbides was found in specimens held for as long as 2500 h at 1143 K.     

Carbides in M-50 high speed steel

The carbides in M-50 high speed tool steel were studied in detail. The dissolution of carbides as a function of austenitizing temperature, and their precipitation as a function of tempering temperature were characterized by X-ray diffraction and microchemical analysis. The carbides in the annealed steel are M₂₃C₆, M₆C, M₂C, and MC. Upon austenitizing, with increasing temperatures, the carbides dissolve in the order: M₂₃C₆, metastable M₂C, M₆C, and MC. The residual carbides in the heat treated steel are MC and stable M₂C. The solvus temperatures of M₂₃C₆ and M₆C were determined. Upon tempering the hardened steel, with increasing tempering temperatures, carbides precipitate in the order: M₂₃C₆, metastable M₂C, MC, and M₆C. It is shown that the composition of the precipitated metastable M₂C is different from that of the residual stable M₂C and it varies with the tempering temperature.     

Retained Austenite Decomposition and Carbide Formation During Tempering a Hot-Work Tool Steel X38CrMoV5-1 Studied by Dilatometry and Atom Probe Tomography

The microstructural development of a hot-work tool steel X38CrMoV5-1 during continuous heating to tempering temperature has been investigated with the focus on the decomposition of retained austenite (Stage II) and carbide formation (Stages III and IV). Investigations have been carried out after heating to 673.15?K, 773.15?K, 883.15?K (400?°C, 500?°C, 610?°C) and after a dwell time of 600?seconds at 883.15?K (610?°C). Dilatometry and atom probe tomography were used to identify tempering reactions. A distinctive reaction takes place between 723.15?K and 823.15?K (450?°C and 550?°C) which is determined to be the formation of M₃C from transition carbides. Stage II could be evidenced with the atom probe results and indirectly with dilatometry, indicating the formation of new martensite during cooling. Retained austenite decomposition starts with the precipitation of alloy carbides formed from nanometric interlath retained austenite films which are laminary arranged and cause a reduction of the carbon content within the retained austenite. Preceding enrichment of substitutes at the matrix/carbide interface in the early stages of Cr₇C₃ alloy carbide formation could be visualised on the basis of coarse M₃C carbides within the matrix. Atom probe tomography has been found to be very useful to complement dilatational experiments in order to characterise and identify microstructural changes.     

Influence of Martensitic Structure and Carbide Precipitation Behavior on Mechanical Properties of 25CrMoNbB Alloy Steels with Mo Contents of 0.25 and 0.5 Pct

In this article, the effects of Mo contents of 0.25 and 0.50 pct on the martensitic structure and carbide precipitation behavior of the experimental steels were investigated and their effects on strength, toughness, and fatigue strength were studied. The results showed that the martensitic blocks and laths were refined and the dislocation density increased with the addition of Mo contents of 0.25 and 0.50 pct. Meanwhile, the amount of carbides increased and the size of carbides decreased in tempered steels. The refinement of carbides is due to the increment of nucleation sites resulting from martensitic structure refinement and the dislocation density increment. Besides, the improvement of thermal stability of M₂₃C₆ carbides enriched with Mo also contributes to carbide refinement. With the addition of 0.50 pct Mo, the strength was improved and the toughness did not deteriorate compared with the steel 0.25 pct Mo. Meanwhile, the fatigue strength was also significantly improved with the addition of 0.50 pct Mo.

     

The effect of titanium on creep strength in 2.25 Pct Cr-1 Pct Mo steels

The effect of a low level of titanium on the microstructure and creep properties of 2.25 pct Cr-1 pct Mo steels has been examined as a function of carbon content and austenitizing temperature. The addition of 0.04 wt pct titanium resulted in a dramatic increase in creep strength at 565 °C, and this was found to be associated with the presence in the microstructure of very small (50 to 100 Å) titanium-bearing precipitates based upon both TiC and Mo₂C. The variation of the minimum creep rate with carbon content and austenitizing treatment was explained in terms of the solubility of TiC in austenite. The titanium-bearing carbides have an important effect on microstructural stability and on the maintenance of creep strength, but it is also apparent that solid solution strengthening by molybdenum can make a significant contribution to creep strength at low carbon levels (0.02 wt pct).