The development of aligned precipitates during internal carbonitridation of Fe–Ni–Cr alloys

Abstract

An austenitic Fe–25Cr–19Ni stainless steel alloy was carbonitrided at 1,000°C in an atmosphere with a_c=1 and P(N₂)=0.9atm to form two discrete precipitation zones. Local equilibrium between the precipitates and the austenite matrix carbon activity was achieved throughout the reaction zone. Small, globular Cr₇C₃ particles were formed immediately beneath the surface. High aspect ratio Cr₂₃C₆ lamellar plates were formed deeper in the precipitation zone and were found to have a cube–cube orientation relationship with the austenite matrix. The inward growth of these carbides was facilitated by the formation of an austenite/depleted austenite grain boundary at the precipitation front, which transformed the austenite to a more appropriate orientation and accelerated the segregation of chromium to the carbide tips.     

Laser in-situ synthesis of mixed carbide coating on steel

A 2.5 KW Nd:YAG laser was employed to modify the surface of a AISI 1010 steel deposited with a precursor powder mixture of Fe, Ti, Cr and C. In-situ formation of TiC and chromium carbides [M₇C₃ (M = Fe, Cr) and Cr₇C₃] was observed as function of laser processing power at constant scan speed. Although TiC was present in all the samples, the chromium carbides were absent in samples processed at certain laser powers. Corresponding to this behavior, variation in mechanical properties of the coating was observed. The hardness and wear properties of the samples without chromium carbides was inferior in comparison to samples with both TiC and chromium carbides.     

Improved surface properties of D2 steel by laser surface alloying

The wear and the high-temperature oxidation resistance of the D2 steel (Fe-1.5 C-12 Cr-0.95 Mo-0.9 V-0.3 Mn) were increased by laser surface alloying after coating the surface with SiC or Cr₃C₂ powder. The surface alloys exhibit two microstructures: hypoeutectic and hypereutectic, respectively, all containing iron solid solutions and iron-chromium carbides, (Fe,Cr)₇C₃. The oxidation resistance of these alloys was measured in isothermal and cyclic conditions, and was shown to increase with silicon or chromium additions, particularly due to the formation of a chromia scale with excellent behaviour during thermal shoks. The surface alloy obtained with Cr₃C₂ also has shown a better resistance to wear due to its hypereutectic microstructure.     

Microstructures in a resistance spot welded high strength dual phase steel

The microstructure of a high strength dual phase steel resistance spot welded with tempering-pulse technology is characterized in this paper. In the fusion zone, there is a needle-like microstructure identified as acicular or side plate ferrite that has a cube-on-cube orientation relationship with respect to the surrounding martensite. In contrast to the microstructures produced by the lower cooling rate arc or laser welding techniques, the nucleation of this fine intragranular ferrite takes place independent of inclusions. Further, a leaf-like microstructure within the martensitic matrix is found to contain primitive orthorhombic Cr₃C₂ and face-centered cubic CrC chromium carbides, rather than Cr₂₃C₆ or Cr₇C₃ as is commonly observed in steel alloys. The formation histories of both the ferrite phase and the chromium carbides are analyzed.     

Relationship between microstructure,hardness and corrosion resistance in 20 wt.%Cr, 27 wt.%Cr and 36 wt.%Cr high chromium cast irons

The microstructures, hardness and corrosion behavior of high chromium cast irons with 20, 27 and 36 wt.%Cr have been compared. The matrix in as-cast 20 wt.%Cr, 27 wt.%Cr and 36 wt.%Cr high chromium cast irons is pearlite, austenite and ferrite, respectively. The eutectic carbide in all cases is M₇C₃ with stoichiometry as (Cr_3.37, Fe_3.63)C₃, (Cr_4.75, Fe_2.25)C₃ and (Cr_5.55, Fe_1.45)C₃, respectively. After destabilization at 1000 °C for 4 h followed by forced air cooling, the microstructure of heat-treatable 20 wt.%Cr and 27 wt.%Cr high chromium cast irons consisted of precipitated secondary carbides within a martensite matrix, with the eutectic carbides remaining unchanged. The type of the secondary carbide is M₇C₃ in 20 wt.%Cr iron, whereas both M₂₃C₆ and M₇C₃ secondary carbides are present in the 27 wt.%Cr high chromium cast iron. The size and volume fraction of the secondary carbides in 20 wt.%Cr high chromium cast iron were higher than for 27 wt.%Cr high chromium cast iron. The hardness of heat-treated 20 wt.%Cr high chromium cast iron was higher than that of heat-treated 27 wt.%Cr high chromium cast iron. Anodic polarisation tests showed that a passive film can form faster in the 27 wt.%Cr high chromium cast iron than in the 20 wt.%Cr high chromium cast iron, and the ferritic matrix in 36 wt.%Cr high chromium cast iron was the most corrosion resistant in that it exhibited a wider passive range and lower current density than the pearlitic or austenitic/martensitic matrices in 20 wt.%Cr and 27 wt.%Cr high chromium cast irons. For both the 20 wt.%Cr and the 27 wt.%Cr high chromium cast irons, destabilization heat treatment gave a slight improvement in corrosion resistance.     

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.     

Influence of ion implantation on the microstructure of oxide scales formed on a 20Cr/25Ni/Nb-stabilized stainless steel in carbon dioxide at 825° C

Specimens of a stainless steel (20%Cr, 25%Ni stabilized with niobium and also containing 0.9% Mn and 0.6% Si) implanted with lanthanum to a dose of 10¹⁷ ion cm^–2 , together with unimplanted specimens, have been oxidized in carbon dioxide at 825° C for times up to 9735 h. Transverse sections through the oxide scales formed on the respective specimens have been studied by analytical electron microscopy. After this exposure the scale on the unimplanted 20/25/Nb stainless steel consists of an outer, large-grained, spinel layer, a middle fine-grained Cr₂O₃ layer and an inner, discontinuous silicon rich, niobium and chromium bearing, amorphous layer. The main effects of the lanthanum implantation are to improve oxidation resistance and maintain scale adherence during thermal cycling. The microstructural changes in the scale formed on the lanthanum implanted 20/25/Nb steel include finer Cr₂O₃ oxide grains and an intermediate region between the outer spinel and inner Cr₂O₃ layers comprised of both oxides. The lanthanum concentrates in this region and appears to act as a marker due to its low diffusivity. Mechanisms of scale development on the 20/25/Nb stainless Red and the influence of lanthanum implantation are discussed.     

Quantitative microanalysis of carbide/carbide interfaces in WC–Co-base cemented carbides

Abstract

Carbide/carbide boundaries in WC–Co-base cemented carbides containing 6–20 wt-%Co were studied with two high resolution microanalytical techniques: atom probe field ion microscopy and analytical transmission electron microscopy. All boundaries studied, i.e. WC/WC boundaries and, in materials containing cubic carbides (γ-phase), WC/γ and γ/γ boundaries, were found to contain about half a monolayer of cobalt, localized to a zone of monolayer thickness. The carbide/carbide boundaries may thus be described as grain (phase) boundaries to which cobalt has segregated. The carbide skeleton model for WC–Co is thereby confirmed. In WC–Co materials which contain Cr₃C₂ as a grain growth inhibitor, chromium segregates to WC grain boundaries.

MST/354     

Furnace-melted surface layers from carbide hard alloy powder compounds

No technological difficulties were encountered in the processing of pseudo-hard alloys in the form of powder compounds of conventional nickel-based hard alloys with carbides. The alloy greatly influences the resulting structures of the surface layers. Under some processing conditions tungsten carbide is completely dissolved by the molten alloy matrix. Hard phases based on chromium carbide form on cooling. The induced chromium carbide, Cr₃C₂, retains its structure while absorbing large amounts of iron into its matrix. It can be concluded that not only alloying properties but also to a great extent structural criteria determine the stability of the applied supplementary hard phases.     

Effect of chromium carbide coating on thermal properties of short graphite fiber/Al composites

A chromium carbide coating was synthesized onto graphite fibers by molten salts method to improve the interfacial bonding and thermal properties of short graphite fiber/Al composites which were fabricated by vacuum pressure infiltration technique. The graphite fiber/Al composites with different thicknesses of chromium carbide coatings were prepared through varying plating times to investigate the influence of chromium carbide layer on the microstructures and thermal properties of the composites. The combined Maxwell–Garnett effective medium approach and acoustic mismatch model schemes were used to theoretically predict thermal conductivities of the composites. The results indicated that the chromium carbide coating formed on graphite fiber surface in molten salts consists mainly of the Cr₇C₃ phase. The Cr₇C₃-coating layer with plating time of 60 min and thickness of 0.5 μm was found to be most effective in improving the interfacial bonding and decreasing the interfacial thermal resistance between graphite fiber and aluminum matrix. The 40 vol% Cr₇C₃-coated graphite fiber/Al composite with Cr₇C₃ thickness of 0.5 μm exhibited 45.4 % enhancement in in-plane thermal conductivity of 221 W m^?1 K^?1 compared to that of uncoated composite, as well as the coefficient of thermal expansion of 9.4 × 10^?6 K^?1, which made it as very interesting material for thermal management applications.     

The role of microstructural features in abrasive wear of a D-2 tool steel

There are three major constituents, i.e. tempered martensite, retained austenite and primary carbides of Cr₇C₃ and Cr₂₃C₆, in the microstructure of a D-2 tool steel. An abrasive wear test with SiC sand paper under two different loads was conducted on specimens having various contents of the above constituents in order to investigate their role in the wear characteristics. It is found that although the wear resistivity seems to vary with changing retained austenite content or hardness, the primary carbides, however, are more likely to be the dominating factor causing the weight loss, especially for wear under a relatively heavy load, From microstructural examination of the worn specimens, cracks and spallation initiated from the primary carbides were observed. Both of the primary carbides and retained austenite were massively removed from the worn surface layer. On the other hand, the role of retained austenite was significant only for the wear test under a light load.     

Effect of thermal aging on pitting corrosion resistance of 16Cr – 5Ni – 1Mo precipitation hardening stainless steel

Abstract

Specimens of precipitation hardening 16-5-1 stainless steel were solution treated at 1050°C for 1 h followed by aging at temperatures in the range 400 – 750°C for various holding times (1 – 16 h). After heat treatment, two types of corrosion test (accelerated and immersion testing) were conducted in 6% ferric chloride solution. The results showed that the pitting corrosion resistance was affected by austenite content, δ ferrite and precipitation of molybdenum and chromium carbides. Three critical temperature ranges were identified, which were related to the phases formed: (a) high corrosion rate at 475°C (δ ferrite and Mo₂ C); (b) low corrosion rate at 550 – 625°C (reversed austenite and Laves phase); (c) intermediate corrosion rate at 750°C (Cr₂₃ C₆ and TiC). The morphology of the pitting was dependent on the form of the δ ferrite and carbides.     

Carbide surface coating of Co-Cr-Mo implant alloys by a microwave plasma-assisted reaction

A technique to grow a hard carbide surface coating on Co-Cr-Mo implant alloys used in artificial joints was developed. The carbide surface coating was applied to as-cast and forged Co-Cr-Mo alloys to improve their wear properties. The surface carbide layers were produced by reactions between the alloy surface and a methane-hydrogen mixed gas by a microwave plasma-assisted surface reaction. The new carbide layers showed brain coral-like surface morphology and appear to consist of mixed phases including Cr₃C₂, Cr₂C, Cr₇ C₃, Cr₂₃C₆, and Co₂C. The Vickers microhardness of thin carbide coatings (3 m thick) was about HV 1100 regardless of the test location. The Vickers microhardness of thick carbide coatings (10 m thick) showed a wide range of hardnesses from HV 1000 to HV 2100. Co-deposition of soot and diamond films occured on a small area of the forged alloy substrates and diamond particles were sparsely dispersed on as-cast alloy substrates. The carbide surface layer has the potential to increase the wear resistance of the Co-Cr-Mo alloy as a wear resistant coating.     

Carbothermal reduction process for synthesis of nanosized chromium carbide via metal-organic vapor deposition

Nanosized chromium carbide was synthesized by metal-organic chemical vapor deposition method in a fluidized bed using mixtures of methane/hydrogen ambient as carburization source in the temperature range 700-850 °C. The microstructure and the phase evolution were deciphered by XRD, TEM and XPS analysis. The carburization process involved the sequential deposition of carbon on the outer surface of the Cr₂O₃ powder followed by carbon diffusion into the powder, leading to the formation of metastable Cr₃C_2 − x phase and stable Cr₃C₂. STEM line scan mode was utilized to delineate the resultant composition gradient within the interlayer of the metastable intermediates and the final stable powder species, that were generated during the course of the carburization process. The formation of carbon nanofilms surrounding the carbide crystallites provides the stress and assists the phase transformation from metastable Cr₃C_2 − x to stable Cr₃C₂. XPS spectral analysis revealed that, the chromium ion in stable chromium carbide carries higher valance than that in metastable chromium carbide.     

Secondary carbide precipitation in an 18 wt%Cr-1 wt% Mo white iron

High chromium (18%) white irons solidify with a substantially austenitic matrix supersaturated with chromium and carbon. The austenite is destabilized by a hightemperature heat treatment which precipitates chromium-rich secondary carbides. In the as-cast condition the eutectic M₇Ca₃ carbides are surrounded by a thin layer of martensite and in some instances an adjacent thicker layer of lath martensite. The initial secondary carbide precipitation occurs on sub-grain boundaries during cooling of the as-cast alloy. After a short time (0.25 h) at the destabilization temperature of 1273 K, cuboidal M₂₃C₆ precipitates within the austenite matrix with the cube-cube orientation relationship. After the normal period of 4 h at 1273 K, there is a mixture of M₂₃C₆ and M₇C₃ secondary carbides and the austenite is sufficiently depleted in chromium and carbon to transform substantially to martensite on cooling to room temperature.     

Influence of interfacial carbide layer characteristics on thermal properties of copper–diamond composites

Copper–diamond composites are increasingly being considered for thermal management applications because of their attractive combination of properties, such as high thermal conductivity (λ) and low coefficient of thermal expansion (CTE). In this research, thermal properties of Cu–diamond composites with two different types of interfacial carbides (Cr₃C₂ and SiC) were studied. The interface thermal conductance (h _c) was calculated with Maxwell mean-field and differential effective medium schemes, wherein experimentally measured λ was entered as an input parameter. The λ and h _c of both the Cu–Cr₃C₂–diamond and Cu–SiC–diamond composites are higher than those reported in previous studies for Cu–diamond composites with no interfacial carbides. The value of h _c is intimately related to the morphology and thickness of the interface carbide layer, with the highest h _c being associated with a thin and continuous interface carbide layer. A lower h _c resulting from a thicker Cr₃C₂ layer can provide an alternate explanation for a previously reported trend in λ of Cu–Cr₃C₂–diamond composites with different Cr-contents. The experimentally measured CTE was compared with the Turner and Kerner model predictions. The CTE of both the Cu–Cr₃C₂–diamond and Cu–SiC–diamond composites is lower and better matches the model predictions than the previously reported CTE of Cu–diamond composite with no interfacial carbides. The CTE of Cu–Cr₃C₂–diamond composites agrees better with the Kerner model than the Turner model, which suggests that deformation during temperature excursions involves shear.     

Oxidation of electrodeposited black chrome selective solar absorber films

X-ray photoelectron spectroscopy and Auger electron spectroscopy were used to study the composition and oxidation of electrodeposited black chrome films. The outer layer of the film is Cr₂O₃ and the inner layer is a continuously changing mixture of chromium and Cr₂O₃. Initially, approximately 40 vol.% of the chromium was combined as Cr₂O₃ and the percentage of Cr₂O₃ increased to greater than 60 vol.% after heat treatment for only 136 h at 250°C. After 3600 h at 400°C the percentage of Cr₂O₃ increased to as high as 80 vol.%. The thermal emittance decreased approximately linearly with increasing oxide content whereas the solar absorptance remained constant until the percentage of Cr₂O₃ exceeded approximately 70%. Oxidation was slower when the Cr³⁺ concentration in the plating bath was reduced from 16 to 8 g 1^?1 and when black chrome was deposited on stainless steel rather than on sulfamate nickel.     

Titanium nitride laser-beam surface-alloying treatment on carbon tool steel

In order to improve the wear resistance of tool steel, a study of TiN surface-alloying treatment on 1% carbon steel by irradiation with a CO₂ laser beam was performed. Argon and nitrogen were used as shielding gases, and their effects on the formation of the surface-alloyed layer were investigated. The effect of cobalt additions to the TiN powder on the hardness of the alloyed layer was also investigated. When argon was used as shielding gas, the depth of the alloyed layer was increased compared with the depth when nitrogen was used as a shielding gas. A portion of the TiN decomposed into titanium in the argon environment, the nitrogen apparently being lost as a gas. The structure of the surface-alloyed layer was composed of a ferritic phase without martensitic structure even at high cooling rates. When this layer was annealed at 1000 ° C for 3 h, part of the titanium precipitated as TiC particles. The hardness of the annealed alloyed layer increased to about 500 Hv. This increase in hardness was accompanied by the appearance of martensite. When nitrogen was used as shielding gas, decomposition of TiN was suppressed and the hardness of the alloyed layer reached 850 Hv. These layers had a martensitic structure. Thus, nitrogen is preferable to argon as a shielding gas if a martensitic structure is desired in this system. When 5% cobalt was added to the TiN powder, the hardness of the alloyed layer increased to 1100 Hv. This increased hardness is caused by stabilization of the martensitic structure caused by an increase in theM _s temperature.     

The relationship between sensitisation and small punch test behaviour in a high-nitrogen austenitic stainless steel at cryogenic temperatures

Austenitic stainless components used in nuclear fusion reactors must be capable of maintaining reasonable mechanical properties to thermal ageing caused by welding and in‐service. Recently, high‐nitrogen (High‐N) austenitic stainless steels (SS) are receiving increased attention because of their strength advantages, but they have been found to be susceptible to dichromium nitride (Cr₂N) precipitation during thermal exposure at 823–1073 K. The susceptibility to sensitisation at thermal ageing temperature for high‐N austenitic SS is examined using the single‐loop electrochemical potentiokinetic reactivation (EPR) test. High‐N SS were found to be susceptible to sensitisation caused by grain boundary precipitation of Cr₂N, with the degree of sensitisation increasing systematically with ageing time and temperature. In particular, it was found that the precipitates, which effected sensitisation, were changed from carbides (M₂₃C₆) to nitrides (Cr₂N) with increasing ageing time and temperature. The deterioration of mechanical properties associated with thermal ageing in high‐N SS was investigated by a small punch (SP) test using miniature specimens at cryogenic temperatures. Results indicated that the degradation of mechanical properties in this alloy was caused by a decrease of cohesive strength resulting from carbides (Cr₂₃C₆) and nitrides (Cr₂N) precipitated in grain boundaries.     

Synthesis and characterization of nanostructured Cr3C2NiCr

Pre-alloyed Cr₃C₂-25 (Ni20Cr) powder was synthesized by mechanical ball milling in Hexane [H₃(CH₂)₄CH₃]and the variation of powder characteristics with milling time was investigated using SEM, X-ray and TEM. The average powder size drastically decreased with time during the first four hours of milling; then decreased slightly as milling continued up to 20 hours. For milling times in excess of four hours, the particle size approached 5 microns. X-ray diffraction analysis revealed a larger structural change in the NiCr solid solution powder relative to that experienced by the chromium carbide phases. This result indicated that the NiCr solid solution powder was subjected to heavier deformation than the chromium carbide powder. During the initial stages of milling, the brittle chromium carbide powders are fractured into sharp fragments and embedded into the NiCr solid solution powder. As milling continued a NiCr chromium carbide polycrystal composite powder was formed for times up to 20 hours of milling, transforming the sharp carbide fragments into spherical carbide particles. Conventional cold welding and fracturing processes primarily occurred only among the NiCr powder and composite powders. Milling times of up to 20 hours led to the formation of a poly crystal nanocomposite powder system in which chromium carbides, with average size of 15 nm, were uniformly distributed in NiCr matrix.