Carbide Formation and Dissolution in Biomedical Co-Cr-Mo Alloys with Different Carbon Contents during Solution Treatment

The microstructures of as-cast and heat-treated biomedical Co-Cr-Mo (ASTM F75) alloys with four different carbon contents were investigated. The as-cast alloys were solution treated at 1473 to 1548 K for 0 to 43.2 ks. The precipitates in the matrix were electrolytically extracted from the as-cast and heat-treated alloys. An M₂₃C₆ type carbide and an intermetallic σ phase (Co(Cr,Mo)) were detected as precipitates in the as-cast Co-28Cr-6Mo-0.12C alloy; an M₂₃C₆ type carbide, a σ phase, an η phase (M₆C-M₁₂C type carbide), and a π phase (M₂T₃X type carbide with a β-manganese structure) were detected in the as-cast Co-28Cr-6Mo-0.15C alloy; and an M₂₃C₆ type carbide and an η phase were detected in the as-cast Co-28Cr-6Mo-0.25C and Co-28Cr-6Mo-0.35C alloys. After solution treatment, complete precipitate dissolution occurred in all four alloys. Under incomplete precipitate dissolution conditions, the phase and shape of precipitates depended on the heat-treatment conditions and the carbon content in the alloys. The π phase was detected in the alloys with carbon contents of 0.15, 0.25, and 0.35 mass pct after heat treatment at high temperature such as 1548 K for a short holding time of less than 1.8 ks. The presence of the π phase in the Co-Cr-Mo alloys has been revealed in this study for the first time.     

Phase chemistry and precipitation reactions in maraging steels: Part IV. Discussion and conclusions

This article summarizes our studies of phase chemistry and precipitation reactions in a variety of maraging steels. The roles of different phases and alloying elements are investigated by comparing the behavior of different steels. The phases considered are Ni₃Ti, Fe₇Mo₆ μ phase, Fe₂Mo Laves phase, ω phase, Ti₆Si₇Ni₁₆ G phase, “Z phase,” austenite, and α matrix. The alloying elements discussed are Ti, AI, Mo, Si, Mn, Ni, Cr, and Co. By comparing the aging behavior of both commercial steels and model alloys, a major role of Co is confirmed to be the lowering of the matrix solubility of Mo. Of the two main hardening elements in maraging steels (namely, Ti and Mo), Ti is much more active than Mo in the very early stage of precipitation. The main Mo-rich precipitate found in this work was Fe₇Mo₆μ phase instead of Laves phase. The precipitation of Mo is modified by the presence of Ti. ω phase appears only in Ti-free alloys, especially when aged at a low temperature. The quantity of Ni-containing precipitates and the presence of Cr in the steels change the austenite reversion behavior. Other phases, such as G phase and “Z phase,” contribute to age hardening in different types of maraging alloys. Formerly Graduate Student with the Department of Materials, Oxford University.     

Influence of heat treatments on microstructure,mechanical properties,and corrosion resistance of weld alloy 625

The effects of heat treatments of the industrial type (eight-hour hold times at temperatures between 600 °C and 1000 °C) on the structural, mechanical, and corrosion resistance characteristics of weld alloy 625 have been studied. During the heat treatment, the mean concentration ratios of Nb, Mo, Si, Cr, Ni, and Fe elements between the interdendritic spaces and dendrite cores show little evolution up to 850 °C. Beyond that temperature, this ratio approximates 1, and the composition heterogeneity has practically disappeared at 1000 °C. An eight-hour heat treatment at temperatures between 650 °C and 750 °C results in increased mechanical strength values and reduced ductility and impact strength linked to the precipitation of body-centered tetragonal metastable intermetallic γ″ Ni₃Nb phase in the interdendritic spaces. An eight-hour treatment in the temperature range between 750 °C and 950 °C has catastrophic effects on all mechanical characteristics in relation with the precipitation, in the interdendritic spaces, of the stable orthorhombic intermetallic δ Ni₃(Nb, Mo, Cr, Fe, Ti) phase. At 1000 °C, the ductility and impact strength are restored. However, the higher the heat treatment temperature, the weaker the mechanical strength. Heat treatments have no effect on the pitting resistance of weld alloy 625 in sea water. The comparison of the results of this study on weld alloy 625 with those previously obtained on forged metal 625 shows that heat treatments below 650 °C and above 1000 °C are the sole treatments to avoid embrittlement and impairment of the corrosion resistance characteristics of alloy 625.     

Orientation relationships between M2C carbide and the austenite matrix in an Fe-Mn-AI-Mo-C alloy

M₂C carbides were observed to precipitate within the austenite matrix of an Fe-24.6Mn-6.6Al-3. IMo-1.0C alloy after quenching from 1200 °C and aging at 750 °C. By means of transmission electron microscopy (TEM) and diffraction techniques, the orientation relationships between M₂C (p) and the austenite (γ) matrix were determined to be: (0001)p//(111)γ, (11– 0)p// (1 0)γ, ( 100)p//(11 )γ. M₂C carbide has been reported by many researchers to precipitate from the ferrite matrix or along austenite/ferrite boundaries in alloy steels containing Mo. However, little information concerning the formation of M₂C in the austenite matrix has been provided. This investigation presents the first evidence for the existence of M₂C carbide wholly within the austenite matrix and its relationship to the austenite. The energy-dispersive spectrometry (EDS) analyses were performed on M₂C carbides, and the results indicate that the solubility of the M₂C carbide for foreign atoms other than Mo is very limited.     

Interphase boundary precipitation in liquid phase sintered W-Ni-Fe and W-Ni-Cu alloys

The microstructure of liquid-phase sintered, tungsten-based heavy alloys comprises a continuous network of spheroidal tungsten single crystals embedded in a ductile, fcc matrix phase, and the integrity of the tungsten-matrix interphase boundaries established during processing is of major importance in determining the resultant mechanical properties. A serious potential source of embrittlement in these systems involves the precipitation of a brittle third phase along these boundaries. In the present work the techniques of selected area and convergent beam electron diffraction, energy dispersive X-ray microanalysis, and scanning Auger electron spectroscopy have been used to identify the embrittling interphase boundary precipitate formed in a commercial W-4.5 wt pct Ni-4.5 wt pct Fe alloy. The interphase boundary precipitation of an intermetallic phase in a W-7.2 wt pct Ni-2.4 wt pct Cu alloy under controlled conditions of heat treatment has also been confirmed. The precipitate phase observed in the W-Ni-Fe alloy in the as-sintered furnace-cooled condition has been found to be an eta carbide with a diamond cubic crystal structure (space group Fd3m,a ₀ = 1.092 ± 0.005 nm) and a tentative composition of the form (Ni,Fe)₆W₆C, where the Ni:Fe atom ratio is approximately 2:3. Neither the carbide nor any evidence of an intermetallic phase was observed in the as-sintered, furnace-cooled W-Ni-Cu alloy, but a continuous interphase boundary film of intermetallic precipitate could be induced in specimens solution treated at 1350°C, water quenched, and aged isothermally in the temperature range 600 to 900°C. Selected area electron diffraction indicated that the phase was isomorphous with the intermetallic Ni₄W of the binary Ni-W system. This paper is based on a presentation delivered at the symposium “Activated and Liquid Phase Sintering of Refractory Metals and Their Compounds” held at the annual meeting of the AIME in Atlanta, Georgia on March 9, 1983, under the sponsorship of the TMS Refractory Metals Committee of AIME. Formerly with Department of Mechanical and Industrial Engineering and Materials Research Laboratory, University of Illinois at Urbana-Champaign     

Carbides in high-speed steels containing silicon

The effects of silicon additions up to 3.5 wt pct on the as-cast carbides, as-quenched carbides, and as-tempered carbides of high-speed steels W3Mo2Cr4V, W6Mo5Cr4V2, and W9Mo3Cr4V were investigated. In order to further understand these effects, a Fe-16Mo-0.9C alloy was also studied. The results show that a critical content of silicon exists for the effects of silicon on the types and amount of eutectic carbides in the high-speed steels, which is about 3, 2, and 1 wt pct for W3Mo2Cr4V, W6Mo5Cr4V2, and W9Mo3Cr4V, respectively. When the silicon content exceeds the critical value, the M₂C eutectic carbide almost disappears in the tested high-speed steels. Silicon additions were found to raise the precipitate temperature of primary MC carbide in the melt of high-speed steels that contained d-ferrite, and hence increased the size of primary MC carbide. The precipitate temperature of primary MC carbide in the high-speed steels without d-ferrite, however, was almost not affected by the addition of silicon. It is found that silicon additions increase the amount of undis-solved M₆C carbide very obviously. The higher the tungsten content in the high-speed steels, the more apparent is the effect of silicon additions on the undissolved M₆C carbides. The amount of MC and M₂C temper precipitates is decreased in the W6Mo5Cr4V and W9Mo3Cr4V steels by the addition of silicon, but in the W3Mo2Cr4V steel, it rises to about 2.3 wt pct.     

Transitions in carbide morphology in a ternary Fe-C-W steel

A ternary steel of composition Fe-0.20C-6.3W (wt pct) was examined with the purpose of characterizing the carbide morphologies that arise during the diffusional decomposition of austenite. Interphase precipitation of M₆C with ferrite in the 700 °C to 800 °C temperature range was observed, as previously reported. At higher temperatures (800 °C to 920 °C), precipitation of M₆C in austenite in a nodular fashion was also observed, never before seen in this alloy. The carbides inside such nodules exhibited a highly degenerate, quasilamellar morphology, and the evidence suggested a discontinuous precipitation mechanism. Comparison with a calculated phase diagram showed that this quasilamellar carbide precipitation reaction dominated both in the stable and metastable ferrite + austenite + carbide three-phase field, since full equilibrium, for a variety of reasons, is not obtained at the “short” reaction times over which this kinetically competitive reaction dominates the microstructural evolution. The formation of carbide-free ferrite preceding the onset of carbide precipitation (found in both cases) is seen to be instrumental in the occurrence of the quasilamellar carbide precipitation reaction, and a simple diffusion model is advanced in support of this view. Finally, the austenite inside these nodules was seen to revert to ferrite at long times, indicating that full equilibrium is not attained at the initial stages of austenite decomposition, in accord with expectation. This article is based on a presentation made in the symposium “Kinetically Determined Particle Shapes and the Dynamics of Solid:Solid Interfaces,” presented at the October 1996 Fall meeting of TMS/ASM in Cincinnati, Ohio, under the auspices of the ASM Phase Transformations Committee.     

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.     

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.     

The microstructure of an Fe-Mn-Si-Cr-Ni stainless steel shape memory alloy

The microstructure and phase stability of the Fe-15Mn-7Si-9Cr-5Ni stainless steel shape memory alloy in the temperature range of 600 °C to 1200 °C was investigated using optical and transmission electron microscopy, X-ray diffractometry (XRD), differential scanning calorimetry (DSC), and chemical analysis techniques. The microstructural studies show that an austenite single-phase field exists in the temperature range of 1000 °C to 1100 °C, above 1100 °C, there exists a three-phase field consisting of austenite, δ-ferrite, and the (Fe,Mn)₃Si intermetallic phase; within the temperature range of 700 °C to 1000 °C, a two-phase field consisting of austenite and the Fe₅Ni₃Si₂ type intermetallic phase exists; and below 700 °C, there exists a single austenite phase field. Apart from these equilibrium phases, the austenite grains show the presence of athermal ɛ martensite. The athermal α′ martensite has also been observed for the first time in these stainless steel shape memory alloys and is produced through the γ-ɛ-α′ transformation sequence.     

Microstructural stability of titanium-modified type 316 and type 321 stainless steel

Optical metallography, transmission electron microscopy, and X-ray diffraction from bulk extracted residues were used to investigate the microstructural stability in the temperature range 450°C to 950°C of a titanium-modified type 316 stainless steel and to compare this steel to a type 321 heat. The effect of cold deformation prior to aging was also investigated. Compared to standard type 316 stainless steel, the nucleation of M₂₃C₆ was delayed and its growth retarded in the titanium modified alloy due to early formation of TiC and Ti₄C₂S₂ which reduced the carbon content in the matrix. Precipitation of the intermetallic σ and χ phases was faster in the titanium modified alloy. The type 321 material formed both M₂₃C₆ and the intermetallic phases less rapidly than either standard or titanium-modified type 316 steels. The relative tendencies toward intermetallic compound formation in various austentic stainless steels are discussed in terms of an “effective equivalent Cr content” remaining in the austenitic matrix after carbide precipitation. Cold work accelerated the precipitation rate of M₂₃C₆ and σ, but it suppressed χ formation due to preferential early σ formation. Early sigma formation was often associated with recrystallization of the cold worked matrix. Mechanisms accounting for this behavior are discussed.     

Precipitation of Second Phases in High-Interstitial-Alloyed Austenitic Steel

The precipitation reaction of an austenitic stainless steel containing N + C was investigated using transmission electron microscopy. The main precipitate formed during isothermal aging at 1123 K (850 °C) was M₂₃C₆ carbide, and its morphology gradually changed in a sequence of intergranular (along grain boundary) → cellular (or discontinuous) → intragranular (within grain interior) form with aging time. Irrespective of different morphologies, the M₂₃C₆ was consistently related to austenite matrix in accordance with the cube-on-cube orientation relationship. Based on the analysis of electron diffraction, two variants of intragranular M₂₃C₆ were identified, and they were related to each other by twin relation. Prolonged aging produced other types of precipitates—the rod-shaped Cr₂N and the coarse irregular intermetallic sigma phase. The similarities and differences in precipitation behavior between N only and N + C alloyed austenitic stainless steels are briefly discussed.     

Effects of mischmetal addition on phase transformation and as-cast microstructure characteristics of M2 high-speed steel

The influence of mischmetal (Ce-La) addition on phase transformation and as-cast microstructure characteristics of M2 high-speed steel (HSS) was investigated using Thermo-Calc software, differential scanning calorimetry, X-ray diffractometry and scanning electron microscopy with energy dispersive spectrometry. The results showed that the measured phase transition points of M2 HSS were broadly consistent with the theoretical results. After mischmetal addition, the liquidus peak temperature, the peak temperature of the eutectic precipitation of M₆C and MC were all increased, especially for the M₆C which was affected significantly and increased about 31 °C. The contents of Mo and V in the eutectic carbide decreased and that of Fe increased, while in the matrix, the Mo, V and Cr contents all increased slightly. Furthermore, the microstructure of as-cast dendrite and ledeburite were refined, the total eutectic carbide content decreased and distributed into a discontinuous network, the lamellar spacing of M₂C was reduced and the lamellae became thinner.     

Redistribution of the substitutional elements during σ and χ phase formation in a duplex stainless steel

While the kinetics of intermetallic phase formation in duplex stainless steel have been extensively studied for a wide range of compositions, relatively little research has been done on the mechanisms by which the phases nucleate and grow. In this research the emphasis was on the diffusional growth of the sigma (σ) and chi (χ) phases. Intermetallic phase precipitation in a standard DSS type EN 1.4462 was studied in detail at two temperatures: at 850 °C where large amounts of σ phase are formed, and at 650 °C where χ is the main intermetallic phase. The redistribution of the substitutional alloying elements Cr, Mo, Ni, Mn and Si between the ferrite, the austenite and the intermetallic phases was followed by means of scanning electron microscopy (SEM) and energy dispersive X‐ray spectroscopy (EDX). Mn and Si showed no distinct partitioning between the several phases. The diffusion paths of the other elements were found to be determined by two phenomena. On the one hand, the ferrite to austenite transformation causes Mo and Cr to enrich in the ferritic phase and Ni to enrich in the austenite. On the other hand, Mo (σ, χ) and Cr (σ) are removed from the ferrite by intermetallic phase precipitation. At 650 °C, strong Mo enrichments appear at the α‐γ phase boundaries indicating that grain boundary diffusion could be an important mechanism for Mo to diffuse to the growing χ phase.     

Secondary carbide precipitation in a directionally solified cobalt-base superalloy

A directionally solidified cobalt-base alloy, DZ40M, was solidified with a columnar grained austenitic matrix with a 〈001〉 preferential orientation and primary carbides of chromium-rich M₇C₃ and MC at grain boundaries and interdendritically. Secondary carbides in DZ40M alloy are chromium-rich M₂₃C₆ and tungsten-rich M₆C. The M₂₃C₆ carbide has a cube-cube orientation relationship with the austenitic matrix. Initial precipitation of secondary carbide, M₂₃C₆, occurred on dislocations in the austenitic matrix of the as-cast DZ40M alloy during cooling. Aging treatment (100 to 1000 hours at 850°C) produced a profusive precipitation of the M₂₃C₆ carbide mainly around the primary carbides. In the interior of grains, M₂₃C₆ precipitated preferentially on dislocations and stacking faults. Subsequently, M₂₃C₆ grew into laths near the primary carbides and coalesced into chains. The precipitation behavior of M₂₃C₆ can be explained by the following reaction: 23M+6C→M₂₃C₆. The primary carbides are a carbon reservoir for the precipitation of M₂₃C₆. The M₆C carbide was found only on the surface of the primary M₇C₃ carbide adjacent to tungsten-rich MC in the aged condition. The precipitation of the tungsten-rich M₆C is atributed to the tungsten segregation, which resulted from decomposition of the tungsten-rich MC and good lattice match between M₆C and M₇C₃. The inhomogeneity of secondary precipitation is due to the uneven distribution of alloying elements.     

Coarsening resistance of M2C carbides in secondary hardening steels: Part I. Theoretical model for multicomponent coarsening kinetics

The development of very high-strength levels in many alloy steels is achieved by a secondary hardening reaction. In high Co-Ni steels containing the strong carbide-forming elements Mo, Cr, and W, secondary hardening is accomplished by the precipitation of fine-scale M₂C alloy carbides. Coarsening resistance of the M₂C precipitates depends on the alloy content of these elements, and there should be an addition to the alloy of these carbide-forming elements which optimizes the M₂C coarsening resistance. Current Lifshitz-Slyozov-Wagner (LSW) theory^[2,3] cannot properly be used to describe, the coarsening behavior of multicomponent carbides, which involves concentrations and diffusivities of two or more solutes and nonspherical carbide morphologies. A model is introduced for the coarsening resistance of multicomponent carbides. This model treats the coarsening of shape-preserving particle and is applicable to rodlike particles.     

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.     

Phase transformation and stabilization of a high strength austenite

An investigation of the phase transformation and the austenite stabilization in a high strength austenite has been made. An Fe-29Ni-4.3Ti austenite age-hardened byγ′(Ni₃Ti) precipitates showed a further increase of strength after martensitic and reverse martensitic phase transformations. The stability of ausaged austenite as well as ausaged and transformation-strengthened austenite was improved significantly through an isothermal treatment at 500°C. TheM _s temperature of the strengthened austenite was restored to nearly that of annealed austenite while the austenite was hardened toR _c 41 through precipitation and phase transformations. The observed austenite stabilization is attributed to the formation of G.P. zones or short-range order of less than ∼10? size. Formerly with University of California, Berkeley     

The partitioning of alloying elements in vacuum arc remelted,Pd-modified PH 13-8 Mo alloys

The partitioning of alloying elements in as-solidified PH 13-8 Mo stainless steel containing up to 1.02 wt pct Pd has been investigated. The as-solidified structure is composed of two major phases, martensite and ferrite. Electron probe microanalysis reveals that Mo, Cr, and Al partition to the ferrite phase while Fe, Ni, Mn, and Pd partition to the martensite (prior austenite) during solidification and cooling from the solidus. In addition to bulk segregation between phases, precipitation of the intermetallic, PdAI, in the retained ferrite is observed. Precipitation of the normal hardening phase, β-NiAl, is also observed in the retained ferrite. Partition ratios of the various alloying elements are determined and are compared with those observed previously in duplex Fe-Cr-Ni stainless steel solidification structures. The martensite start temperature (M_s) was observed to decrease with increasing Pd concentration.     

Age hardening and mechanical properties of a 2400 MPa grade cobalt-free maraging steel

The age hardening kinetics in the temperature range of 713 to 813 K of a 2400 MPa grade cobalt-free maraging steel (Fe-(18.8 ∼ 19.1) pct Ni-(4.4 ∼ 5.4) pct Mo-2.6 pct Ti, wt pct) has been studied. Study of microstructure and mechanical properties showed that a high number of Ni₃Ti and Fe₂(Mo,Ti) precipitates were formed during the ageing process, which resulted in high strength and relatively low fracture toughness. Ni₃Ti was the main precipitation phase. Fractography has shown ductile failure of tensile and fracture toughness specimens. Thermodynamic calculations showed that the equilibrium phases are Ni₃Ti, Fe₂(Mo,Ti), ferrite, and austenite.