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)     

Microstructure evolution of TiC cermets with ferritic AISI 430L steel binder

From the outlook of healthcare, economic importance and supply risk, utilisation of raw materials like tungsten, cobalt and nickel should be reduced or replaced with other metals. Nontoxic titanium carbide and iron are the top-of-the-line solution for displacing these materials. Our focus was on conventionally fabricated titanium carbide-based cermets with a chromium ferritic steel binder. To study microstructural evolution, specimens were sintered at different temperatures (600–1500°C). We used a scanning electron microscopy, X-ray diffraction and differential scanning calorimetry to analyse the microstructure and phase formation of the cermets. Our results showed that during the solid and liquid phase sintering of the TiC–FeCr cermet, chromium ferrous complex carbides M₇C₃ are formed and as a result, chromium content in the binder phase is decreased. Alloying TiC–FeCr cermets with strong carbide formers improves the structural homogeneity of the cermets. Also, mechanical characteristics (hardness, fracture toughness) were evaluated.     

Effects of alloying additions and austenitizing treatments on secondary hardening and fracture behavior for martensitic steels containing both Mo and W

The effects of alloying additions and austenitizing treatments on secondary hardening and fracture behavior of martensitic steels containing both Mo and W were investigated. The secondary hardening response and properties of these steels are dependent on the composition and distribution of the carbides formed during aging (tempering) of the martensite, as modified by alloying additions and austenitizing treatments. The precipitates responsible for secondary hardening are M₂C carbides formed during the dissolution of the cementite (M₃C). The Mo-W steel showed moderately strong secondary hardening and delayed overaging due to the combined effects of Mo and W. The addition of Cr removed secondary hardening by the stabilization of cementite, which inhibited the formation of M₂C carbides. The elements Co and Ni, particularly in combination, strongly increased secondary hardening. Additions of Ni promoted the dissolution of cementite and provided carbon for the formation of M₂C carbide, while Co increased the nucleation rate of M₂C carbide. Fracture behavior is interpreted in terms of the presence of impurities and coarse cementite at the grain boundaries and the variation in matrix strength associated with the formation of M₂C carbides. For the Mo-W-Cr-Co-Ni steel, the double-austenitizing at the relatively low temperatures of 899 to 816 °C accelerated the aging kinetics because the ratio of Cr/(Mo + W) increased in the matrix due to the presence of undissolved carbides containing considerably larger concentrations of (Mo + W). The undissolved carbides reduced the impact toughness for aging temperatures up to 510 °C, prior to the large decrease in hardness that occurred on aging at higher temperatures.     

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.     

On the bainite structure in Cr-Mo-V rotor steel

The morphology of continuously cooled and isothermally transformed bainite structures formed in a Cr-Mo-V rotor steel has been studied using transmission electron microscopy. The samples were austenitised at 955°C for an hour followed by air cooling to room temperature. The isothermal transformation reaction was carried out at 450°C for up to 100 000 s. The microconstituents observed are predominantly lower bainite with very small amount of upper bainite and martensite (formed from untransformed austenite due to water quenching). Analysis of the selected area diffraction patterns confirm that the carbide in bainite is orthorhombic cementite and the orientation relationship between ferrite and cementite is consistent with that of Bagaryatskii. The carbide particles in isothermally transformed bainite are coarser than those of continuously cooled bainite. Tempering one hour at 670°C of continuously cooled steel samples exhibited formation of fine spheroidal MC type carbides. In addition tempering leads to the enrichment of prior austenite grain boundaries by cementite particles. Tempering ten hours at 670°C exhibited microstructures almost identical to those observed in one hour tempering.     

Study on the Interaction between Rare Earth and Carbon in High Carbon Steel

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Effect of Prior Cold Work on the Martensite Transformation in SAE 52100

Numerous publications refer to the phase transformations and properties of SAE 52100 steel, and this paper concerns itself with the effect of prior cold deformation on the martensitic hardening response. TheA _c1 and A_c3 temperatures are lowered due to cold work as is theM _s with a resultant increase in the retained austenite content for a given hardening cycle. Significantly, the prior cold deformation results in a refinement of the austenite grain size. The low angle dislocation cells produced by the cold deformation recover during the heating to the austenitizing temperature to form fine ferrite subgrains. The intersections of the fine ferrite subgrains with the spheroidal carbides in the soft annealed microstructures are preferential sites for nucleation of austenite. This results in finer austėnite grains, which produces accelerated carbide dissolution and austenite alloy enrichment compared to un worked, soft annealed structures. The mechanism for the accelerated austenitization is significant in predicting heat treatment response from published phase transformation data for SAE 52100 steel.     

Effects of Cr and V additions on the microstructure and properties of high-vanadium wear-resistant alloy steel

The effects of chromium and vanadium additions on the microstructure, hardness and wear resistance of high-vanadium alloy steel (containing 5–10 wt-% V and 2–10 wt-% Cr) were studied by means of optical microscopy, scanning electron microscope (SEM), X-ray diffraction (XRD), energy dispersive spectrometer (EDS), Vickers hardness and Rockwell-hardness tester & M-200 ring block wear tester. Researching results showed that the solidification structure of high-vanadium wear-resistant alloy steel was mainly consisted α-Fe (martensite), vanadium carbide (VC), M₃C and M₇C₃. Vanadium is mainly distributed over VC, and certain amount of vanadium exists in the matrix and M₇C₃ type eutectic carbide. Chromium is mainly distributed over the M₇C₃, and the matrix also contains a small quantity of chromium. It is found that the content of VC increases with the increase of vanadium content when carbon and chromium contents are constant. The change of micro- and macro-hardness was not obvious with the increase of vanadium content. The content of M₇C₃ type eutectic carbides increases gradually with the increase of chromium content when carbon and vanadium contents are constant. The micro- and macro-hardness increases with the increase of chromium content. The increase of vanadium content brings to the increase of wear resistance of alloy steel when carbon and chromium contents are constant. The change of chromium content had no obvious effect on wear resistance of high-vanadium alloy steel when carbon and vanadium contents. The increase of vanadium content brings to the increase of wear resistance of alloy steel when carbon and chromium contents are constant. The wear resistance of as-cast high-vanadium alloy steel is the best when the content of vanadium and chromium is 10 wt-% and 5 wt-% respectively.     

Control of Carbide Precipitation During Electroslag Remelting-Continuous Rapid Solidification of GCr15 Steel

The effect of the electroslag remelting-continuous rapid solidification (ESR-CRS) process on element segregation and carbide precipitation in GCr15 bearing steel was investigated. The results showed that a microstructure with fewer primary carbides and less segregation can be achieved via the ESR-CRS process. In the specimen subjected to ESR, the morphology of primary carbide changed from angular to lumplike. After the ESR-CRS process, the dimension of primary carbide and the mass fraction of Cr in primary carbide decreased. With the increase of cooling intensity during the ESR-CRS process, the microstructure of ESR ingot became more refined and uniform and the size of large primary carbides in ESR ingot gradually decreased. Suppressing the formation of grain boundary cementite and primary carbide during ESR of GCr15 steel is beneficial to inhibiting the presence of large secondary carbide formation after annealing. With the increase of cooling intensity, the mass fraction of carbides in ESR ingot decreased and the mass fraction of Cr in carbides increased, whereas the types of carbides did not change; all the M₃C, M₃C₂, and M₇C₃ exist before and after ESR-CRS. Al₂O₃ inclusions promoted the formation of Ti(N,C) by serving as preferred heterogeneous nucleation sites, whereas the formation of Ti(N,C) was suppressed through the refinement of Al₂O₃ inclusions by increasing the cooling intensity.     

Tempering of 2.25 Pct Cr-1 Pct Mo Low Carbon Steels

The effect of carbon level on the tempering behavior at 700°C of 2.25 pct Cr-1 pct Mo steels having typical weld metal compositions has been investigated using analytical electron microscopy and X-ray diffraction techniques. The morphology, crystallography and chemistry, of each of the various types of carbides observed, has been established. It has been shown that each carbide type can be readily identified in terms of the relative heights of the EPMA spectra peaks for iron, chromium, molybdenum, and silicon. A decrease in the carbon level of the steel increases the rate at which the carbide precipitation reactions proceed, and also influences the final product. Of the carbides detected, M₂₃C₆ and M₇C₃ were found to be chromium-based, and their compositions were independent of both the carbon level of the steel and the tempering time. The molybdenum-based carbides, M₂C and M₆C, however, showed an increase in their molybdenum contents as the tempering time was increased. The rate of this increase became greater as the carbon content of the steel was lowered.     

The effect of carbides on fracture toughness of steels of ferritic matrix

The authors evaluated the effect of the volume fraction and the dispersion rate of cementite on fracture toughness of ferrite. The investigations were performed at -196°C on five types of carbon steels containing 0.028–1.22% of C in which cementite was coagulated at 700°C for 1–8 h from the quenched state. It was determined that the fracture toughness of steel increases very strongly up to the content of carbides of about 7% by volume. At the same time, hardness and strength of these steels grow. First of all, this is the result of size reduction of ferrite grains by fine carbides. These carbides, distributed almost exclusively on grain boundaries, can only participate in the transmission of the crack to the neighbouring grain. At larger contents of carbides, their dispersion rate decreases while their number in the grain volume grows. Fine carbides from inside of the grains set the path of easy cracking on the boundaries with the ferritic matrix while the coarse carbides crack in front of the fracture. As a result, the steel fracture toughness decreases. The fracture development by means of carbides is less harmful than on the carbide/matrix boundaries.     

Cerium carbide in steel

Cerium carbide was found in steel. It has been investigated and identified by means of metallography, electron microprobe, Auger spectra, X-ray diffraction technique and electrolytic separation. The results obtained can be summarized as follows: When the amount of cerium is about 4 times the total amount of oxygen and sulfur, cerium carbide (CeC₂) of a dark grey to black colour forms in steel. It is distributed along the grain boundaries, and is rearranged in working direction after rolling or forging. CeC₂ in steel contains a certain amount of iron, oxygen and some sulfur, also chromium if it is available in the matrix. Cerium carbides are unstable, oxidize in air and decompose in water. It is difficult to separate them from the matrix by the electrolytic method. When steel is held at high temperatures, CeC₂ will change into Fe₄Ce₄C₇ of a light yellow colour. It is much more stable than CeC₂. When steel containing CeC₂ is heated in an oxidizing atmosphere, the CeC₂ will easily be changed into cerium dioxide. This Ce-oxide contains carbon. As soon as Ce-carbide forms in steel, the carbon needed for cementite is decreased. Therefore the pearlite content in steel will be correspondingly decreased.     

Phase instability of manganese-iron carbides

The stability of carbon saturated manganese-iron alloys was studied by means of simulated decrepitation tests, and it was found that the product must contain a minimum of about 5 wt pct iron to be stable during storage. By means of several experimental techniques it was shown that the structure of the carbide phase present in carbon saturated ferromanganese determines whether the alloy is stable. Below the critical iron content of about 5 wt pct, the carbide phase is Hāgg carbide (MnFe)₅C₂, whereas above about 5 wt pct iron the carbide phase is cementite (MnFe)₃C. The role of iron is to stabilize the cementite phase. Experiments with the synthetic manganese carbides, Mn₅C₂ and Mn₃C, showed that the former reacts readily with water whereas the latter is stable.     

The microstructural response of mill-annealed and solution-annealed INCONEL 600 to heat treatment

Samples of INCONEL* 600 were examined in the mill-annealed and solution-annealed states, and after isothermal annealing at 400 °C and 650 °C. The corrosion behavior of the samples was examined, analytical electron microscopy was used to determine the microstructures present and the chemistry of grain boundaries, and Auger electron spectroscopy was used to measure grain boundary segregation. Samples of different alloys in the mill-annealed state were found to have quite different microstructures, with Cr-rich M₇C₃ carbides occurring either along grain boundaries or in intragranular sheets. The corrosion behavior of the samples correlated well with the occurrence of grain boundary chromium depletion. Solution annealing at 1190 °C caused dissolution of all carbides, whereas at 1100 °C the carbides either dissolved or the grain boundaries moved away from the carbides, depending upon alloy carbon content. Low-temperature annealing at 400 °C had little effect on millannealed or fully solutionized samples, but in samples with intragranular carbides present, the grain boundaries moved until intersecting or adjacent to the carbides. Isothermal annealing at 650 °C caused carbide nucleation and growth at grain boundaries in fully solutionized samples. Chromium depletion at grain boundaries accompanied carbide precipitation, with a minimum chromium level of 6 wt pct achieved after 5 hours. Healing was found to occur after 100 hours. Solution-annealed samples with intragranular carbides present had more rapid corrosion kinetics since the grain boundaries moved back to the existing carbides. Thermodynamic analysis of the chromium-depletion process showed good agreement with experimental measurements. The Auger results found only boron present at grain boundaries in the mill-annealed state. Aged samples had boron, nitrogen, and phosphorus present, with phosphorus and nitrogen segregating to the greatest extent. The kinetics of phosphorus segregation are much slower at 400 °C compared with 650 °C.     

Carbon Redistribution and Carbide Precipitation in a High-Strength Low-Carbon HSLA-115 Steel Studied on a Nanoscale by Atom Probe Tomography

HSLA-115 is a newly developed Cu-bearing high-strength low-carbon martensitic steel for use in Naval structural applications. This research provides, for the first time, a comprehensive compositional analysis of carbon redistribution and associated complex phase transformations in an isothermal aging study of HSLA-115 at 823 K (550 °C). Specifically, we characterize carbon segregation at lath boundaries, grain-refining niobium carbonitrides, cementite, and secondary hardening M₂C carbides, in addition to copper precipitation, by 3D atom probe tomography (APT). Segregation of carbon (3 to 6 at. pct C) is observed at martensitic lath boundaries in the as-quenched and 0.12-hour aged microstructures. On further aging, carbon redistributes itself forming cementite and M₂C carbides. Niobium carbonitride precipitates do not dissolve during the austenitizing treatment and are inherited in the as-quenched and aged microstructures; these are characterized along with cementite by synchrotron X-ray diffraction and APT. Sub-nanometer-sized M₂C carbide precipitates are observed after the formation of Cu precipitates, co-located with the latter, indicating heterogeneous nucleation of M₂C. The temporal evolution of the composition and morphology of M₂C carbides at 823 K (550 °C) is described using APT; their precipitation kinetics is intertwined with Cu precipitates, affecting the bulk mechanical properties of HSLA-115. Phase compositions determined by APT are compared with computed compositions at thermodynamic equilibrium using ThermoCalc.     

Comparison of the Activation Energies of the Formation of Chromium Carbide Coating on Carburized and Uncarburized AISI 1020 Steel

Chromium carbide coatings deposited by the salt bath method have a lot of technologically interesting characteristics. This method produces hard, wear–resistant, oxidation and corrosion–resistant coating layers on steel substrates. In the present study, the kinetics of chromium carbide formation on carburized and uncarburized AISI 1020 steel substrates has been compared. The presence of the Cr₇C₃ phase on the surface of steel substrates was confirmed by X‐ray diffraction. Cross–sectional observation of optical and SEM images showed that chromium carbide layers formed on the steel substrates were rather compact and smooth. The kinetics of chromium carbide coating by salt bath immersion indicated a parabolic relationship between carbide layer thickness and treatment time. The activation energy of the formation of carbide on the surface of carburized and uncarburized steel was calculated to be 87.9 and 225.6 kJ/mol, respectively. Moreover, an attempt was made to present contour diagrams for predicting the thickness of the chromium carbide layer. In addition, the possibility of establishing and using some mathematical relationships between process parameters and chromium carbide layer thickness was investigated.     

Morphological changes of carbides during creep and their effects on the creep properties of inconel 617 at 1000 °C

Creep tests have been correlated with microstructural changes which occurred during creep of Inconel 617 at 1000 °C, 24.5 MPa. The following results were obtained: 1) Fine intragranular carbides which are precipitated during creep are effective in lowering the creep rate during the early stages of the creep regime (within 300 h). 2) Grain boundary carbides migrate from grain boundaries that are under compressive stress to grain boundaries that are under tensile stress. This is explained in terms of 1 the dissolution of relatively unstable carbides on the compressive boundaries, 2 the diffusion of the solute atoms to the tensile boundaries and 3 the reprecipitation of the carbides at the tensile boundaries. The rate of grain boundary carbide migration depends on grain size. 3) M₂₃C₆ type carbides, having high chromium content, and M₆C type carbides, having high molybdenum content, co-exist on the grain boundaries. M₂₃C₆ type carbides, however, are quantitatively predominant. Furthermore, M₆C occurs less frequently on the tensile boundaries than on the stress free grain boundaries. This is attributed to the difference of the diffusion coefficients of chromium and molybdenum. 4) The grain boundaries on which the carbides have dissolved start to migrate in the steady state creep region. The creep rate gradually increases with the occurrence of grain boundary migration. 5) The steady state creep rate depends not so much on the morphological changes of carbides as on the grain size of the matrix.     

Chromium partitioning during the austenite-pearlite transformation

Partitioning of chromium between cementite and ferrite during the austenite to pearlite transformation in a eutectoid steel containing 1.29 pct chromium has been studied using analytical electron microscopy. No partitioning occurred at the austenite-pearlite interface below 703°C (the no-partition temperature), while above this temperature chromium partitioned preferentially to cementite at the transformation front. Chromium segregation to cementite occurred at all transformation temperatures after pearlite had formed. Measurements of pearlite growth rate and interlamellar spacing have been made for a range of transformation temperatures, and used to examine the rate controlling process for pearlite growth below the no-partition temperature. Growth rates calculated assuming volume diffusion of carbon to be rate controlling were in reasonable agreement with measured growth rates, although the discrepancies between the rates could be accounted for by the partial involvement of interfacial diffusion. Formerly affiliated.     

Crystallography and Morphology of Carbides in a Low-Cycle Fatigued 1Cr-1Mo-0.25V Steel

The carbide precipitation in 1Cr-1Mo-0.25V steel subjected to low-cycle fatigue (LCF) deformation at room and elevated temperatures was investigated by means of transmission electron microscopy. Based on the electron diffraction analyses, three types of carbides, M₃C-type cementite, M₂C, and MC, were identified in normalized and subsequently tempered specimen. The cyclic deformation at high temperature led to the following changes in morphology and composition of carbides: the spheroidization of cementite, the enhanced precipitation of H-carbide, the formation of M₂C and M₂₃C₆ at lath or prior-austenite grain boundaries, and the enrichment of Mo in most of carbides. Particular attention has been paid to the crystallographic orientation relationship (OR) between the cementite and the ferrite (α) matrix. The combined analyses based on the simulation of diffraction patterns and the trace analyses of habit plane on stereographic projection have shown that most cementite was related to the α matrix in accordance with Bagaryatskii OR, but in some cases, the Isaichev OR also was observed in the lath interior after LCF deformation at elevated temperature. In addition, M₂C obeyed the Burgers–Jack OR, and MC was related to the α by the Baker–Nutting OR.     

Chromium depletion in the vicinity of carbides in sensitized austenitic stainless steels

The analytical electron microscope (AEM) was used to examine the microstructure of type 316LN stainless steel alloys which had been annealed for 50 to 300 hours in the temperature range 600 to 700 °C. The M₂₃C₆ carbide chemistry and distribution are described as a function of heat treatment.X-ray spectroscopy in the AEM revealed significant chromium depletion at grain boundaries in the vicinity of carbides for samples aged at 50 and 100 hours at 650 °C and 100 and 300 hours at 700 °C, with lower grain boundary chromium values observed at 650 °C than at 700 °C. The width of the chromium depleted zone normal to the grain boundaries increased with increasing annealing time and/or temperature. Measurements of chromium concentration along the grain boundaries away from a carbide were made after aging at 700 °C for 100 hours, and the chromium level rose steadily until the bulk value was reached at a distance of ~3μm from the carbide. The width of the chromium depleted zone normal to the boundaries in the same sample was an order of magnitude less. Some molybdenum depletion was also found at the grain boundaries, and the Mo-depletion profiles were in form and extent similar to the chromium results. Simple thermodynamic models were used to calculate the equilibrium value of chromium at the carbide-matrix interface, and the chromium distribution along and normal to the grain boundaries. The results of these models agreed well with the AEM results, and the agreement can be improved by considering the effect of electron probe configuration on the AEM measurements. The calculated thermodynamic data and the AEM results were related to the corrosion behavior of the alloys. The occurrence of severe asymmetries in some concentration profiles normal to the grain boundaries, which increased with increasing annealing temperature or time, was shown to be due to boundary movement during the discontinuous precipitation of M₂₃C₆ carbides.