M23C6 Carbides Research Articles

Hydrogen Diffusion and Trapping in Low-Alloy Tempered Martensitic Steels

Structural characterization of ten low-alloy tempered martensitic steels of varied composition (C, Cr, Mo, Mn, and V contents) and tempering temperature was performed to question the impact of microstructural features on hydrogen state. Thermal desorption spectroscopy and electrochemical permeation data for each alloy were acquired and interpreted in view of hydrogen diffusion/trapping models. This large database provided precise information regarding solubility, diffusion coefficient, activation energies for diffusion and trapping, hydrogen distribution into lattice, and reversible and irreversible trap sites. The results reveal a tendency for the apparent diffusion coefficient to decrease with increasing yield strength, mainly related to the density of trap sites rather than lattice diffusion. Estimates of trapping at dislocation core could explain the irreversible trapping in the six steels with sub-surface hydrogen concentration smaller than 1.5 wppm. For the four steels with higher solubility, it was calculated the superabundant vacancies concentration necessary to justify the amount of trapping sites. The steel with the highest Mo and V contents presented superior solubility of trapped hydrogen which was related to its precipitation of few nanometers in size. It was considered irreversible trapping at carbon vacancies as well as reversible trapping at elastic strain fields around the detected MC carbides.

Variation of liquation cracking susceptibility for over–aged 247LC superalloy repair weld heat–affected zone

In this study, the liquation cracking temperature range (LCTR) of over-aged 247LC superalloy repair welds was quantitatively evaluated using spot-Varestraint testing, especially for application in the repair weld heat-affected zone (HAZ). The metallurgical mechanism of the evaluated liquation cracking susceptibility and variation of the LCTRs of the over-aged 247LC superalloys was clarified by analyzing the microstructural characteristics at the liquation cracking surface, and using thermodynamic calculations. From the spot-Varestraint tests of the over-aged materials, the LCTR was quantitatively evaluated to be 410 and 620 K for the 500 and 1000 h over-aged specimens, respectively. These results demonstrate that the over-aged 247LC superalloys face significant and serious liquation cracking susceptibility in repair weld HAZ. Based on local element distributions at the liquation cracking surface, analyzed by transmission electron microscopy and the Thermo-Calc calculations, the underlying metallurgical mechanism of high LCTRs of overaged materials could be attributed to the precipitation of fine MC and M6C carbides during the over-aging treatment. The carbides promote local liquation at the interface between the gamma phase at temperatures lower than the equilibrium solidus (1530 K) of the 247LC. The estimated liquation initiation temperatures at the MC/γ and γ/M6C interfaces were 1125 and 1420 K, respectively.

A Comparison of the Microstructure, Mechanical Properties, and Corrosion Resistance of the K213 Superalloy after Conventional Casting and Selective Laser Melting

K213 superalloy was fabricated by conventional casting and selective laser melting (SLM). The microstructures of the two samples were examined, and the mechanical properties and corrosion resistance of these two kinds of K213 alloy were comparatively studied. The results show that segregation of Ti occurs at the grain boundaries of the as-cast alloy, resulting in the formation of MC carbide. Many microcracks were formed in the SLM sample. Premature fracture of the as-cast alloy is caused by the precipitation of the harmful phase (Ti, Mo, Nb)C (MC). The MC carbides and microcracks in the as-cast and SLM alloys, respectively, induce tensile fracture. In comparison, the strength of the SLM sample is greater, while the elongation of the as-cast sample is greater. The oxidation resistance of the SLM sample is better at a high temperature of 800 °C. This is due to the relatively uniform composition and microstructure of the SLM alloy. However, the corrosion rate of the SLM alloy is accelerated during the electrochemical immersion corrosion process due to the existence of microcracks.

Microstructure, retained austenite, and mechanical properties evaluation of Vanadis 60-HfC-Ta0.6Nb0.4C steel matrix composite by vacuum sintering, sub-zero, and heat treatments

This study added different ratios of HfC and Ta60Nb40C powders to Vanadis 60 high-speed steel powders, and then, the Vanadis 60 composite was sintered from 1215 to 1260 °C by vacuum sintering for 1 h. The experimental results reveal that the optimal sintering temperature and the additive amount for Vanadis 60 composites were 1230 °C and 0.5 mass% HfC-0.5 mass% Ta60Nb40C powders, respectively. Moreover, the optimally sintered Vanadis 60 composite possessed an apparent porosity of 0.37%, the transverse rupture strength (TRS) value was 2212.1 MPa, and the hardness value was 80.3 HRA. In addition, it underwent sub-zero plus heat treatments, where the TRS and hardness values were significantly enhanced to 2451.7 MPa and 86.0 HRA, respectively, after quenching, sub-zero, and tempering treatments. The research results of TEM and EBSD show that MC, M6C, M7C3, and M23C6-type carbides obviously appeared in the composite after vacuum sintering, heat treatment, and sub-zero treatments.

Failure mechanism of HR3C austenitic steel during creep at 650°C

AbstractThis paper investigates the intergranular failure and degradation process of HR3C steel during creep. Substantial microscopic information for the morphological and compositional characteristics of intergranular precipitates were revealed here. The results showed that the intergranular M23C6 carbides precipitated progressively, resulting in a formation of Cr‐depleted region, and coexist with the NbCrN phase at the grain boundary. Besides, the dislocation configuration and density of HR3C steel also changed remarkably after creep, which manifests that the increase of lattice debonding extent and irregular atoms array. The intergranular damage of the M23C6 matrix interface was revealed by the HRTEM observation, and the main damage features were the volumetric defect generation as well as the large‐area distortion of atom lattice around the M23C6, which would contribute to the intergranular crack forming at the end of creep stage.

Effect of Mo Content on Microstructure and Mechanical Properties of Laser Melting Deposited Inconel 690 Alloy

Inconel 690 alloy is widely used in nuclear power, petrochemical, aerospace, and other fields due to its excellent high-temperature mechanical properties and corrosion resistance. The Inconel 690 alloy with different Mo content was fabricated by laser melting deposition (LMD). The effects of Mo content on the microstructure and mechanical properties were investigated. The microstructure of as-deposited Inconel 690 is composed of columnar dendrites grown epitaxially, and M23C6 carbides are precipitated in the grain boundaries. With the increase of Mo content, the amount of precipitated carbide increases gradually. At the same time, the grain boundary becomes convoluted. The tensile test at room temperature shows that the high Mo content in the as-deposited Inconel 690 increases the ultimate strength but decreases the ductility. Compared with low Mo content, the alloy with high Mo deposition has better mechanical properties. The present study provides a new method to achieve the preparation of Inconel 690 alloy with excellent integrated mechanical properties.

Microstructure and Mechanical Properties Evaluation of Microalloyed Low‐Carbon Reduced Activation Ferritic/Martensitic Steel

A novel method for reducing the amount of M23C6 carbides, increasing the amount of MX carbonitrides, and decreasing the coarsening rate of M23C6 carbides is put forward to improve the high‐temperature properties of reduced activation ferritic/martensitic (RAFM) steel. Scanning electron microscopy, transmission electron microscopy, X‐ray diffraction, tensile tests, and impact tests are used to systematically investigate the microstructure evolution and mechanical properties of RAFM (RAFM‐0.1C) steel, low‐carbon RAFM (RAFM‐0.04C) steel, and minor boron and nitrogen microalloyed low‐carbon RAFM (RAFM‐CBN) steel. Some δ ferrite develops when C decreases from 0.1 to 0.04 wt% and subsequently disappears after 0.01 wt% N addition. The amount and size of M23C6 carbides and dislocation density decrease with the decrease of C (RAFM‐0.04C), while the amount of MX carbonitrides is 2‐3 times, and dislocation density is ≈2 times higher than that of RAFM‐0.04C steel after 0.01 wt% N addition (RAFM‐CBN). Compared with RAFM‐0.1C steel, the yield strength at room temperature and 550 °C slightly decreases for RAFM‐0.04C steel and considerably increases for RAFM‐CBN steel. The contributions of microalloy on strengthening mechanisms of RAFM steel at room temperature are also systematically revealed by combining the experimental and theoretical data.

Underlying Mechanism for “Loss of Passivation” Effect of a High-Carbon Martensitic Stainless Steel Coating via Laser Cladding

Recently, laser cladding (LC) technology has become a cost-effective and convenient method to protect metal substrate from corrosion by producing metal coating with high corrosion resistance. In order to fully investigate the pitting mechanism for high carbon martensitic stainless steel (HMSS) coating, the microstructure and pitting performance of high-carbon martensitic stainless steel (HMSS) samples, which were produced via laser cladding (LC) and hot isostatic pressing (HIP) were comparatively investigated via electrochemical measurements and electron microscopies. Dendritic and network connected M23C6 carbides are the main precipitates in the HMSS coating, while the M23C6 carbides in HMSS bulk are spherical or elongated in shape. Pitting resistance of the HMSS coating is dramatically deteriorated. The massive and continuously distributed dendritic M23C6 carbides could form a large-area cathode and cause the micro-galvanic corrosion of the HMSS-LC coating matrix, thus can be considered as underlying factor for the “loss of passivation (LOP)” effect of the HMSS coating.

Atom-scale characterization and 2D/3D modeling of carbides and Fe Co phases in Stellite 6 laser clad coating after aging treatment

This study applied various aging treatment (AT) times at 650 °C in Stellite 6 laser cladding layer, conducted the atom-scale characterization of stacking faults (SFs) and nanoscale M23C6 carbides, described the martensitic transformation of γ → ε, and discussed on the growth mechanism of FeCo and M23C6 phases by 3D/2D modeling. Results show the formation of three distinct regions relative to Fe diffusion, namely, interfacial layer with FeCo and M23C6 phases, Fe diffusion-affected zone with FexCoy distribution at the edge of M23C6 carbides along grain boundary, and Fe diffusion-unaffected zone with abundant carbide precipitation and SF generation. In addition, the formation of M23C6 and FeCo phases conformed to dynamics and thermodynamics. Suzuki effect influence the nanoscale carbides with profile and growing direction of [121¯] and [110], respectively, and martensite transformation occurred on (1¯11¯)γ parallel to (0002)ε as characterized by 2D modeling and HRTEM. The microhardness of surfacing layer was measured in diverse zones, and the results present that AT softened the interfacial layers but increased the hardness of Fe diffusion-affected zones and Fe diffusion-unaffected zones.

Effects of normalizing temperature on microstructure and high-temperature properties of 2.25Cr-1Mo-0.25V bainitic steel

In 2.2Cr-1Mo-0.25V bainitic steels, the effect of the initial microstructure on carbide precipitation has not yet been elucidated. Therefore, in this work, the effect of normalizing temperature on the microstructure evolution and corresponding high-temperature strength change of bainitic steels was investigated by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) using extra-thick hydrogenation reactor steel as the assessed material, and the precipitation strengthening mechanism was discussed. When the normalizing temperature increased from 900 °C to 990 °C, the high-temperature yield strength increased from 451 to 475 MPa and the average absorbed Charpy impact energy at −30 °C decreased from 93 to 41 J. Therefore, the optimal normalizing temperature was 960 °C. The volume fraction of MC carbides with almost no change in average diameter increased from 0.21% to 0.44%, and the dispersed strengthening of carbide was significantly enhanced. The increase in normalizing temperature increased the solid solution of alloying elements within the bainite matrix, which increased the precipitation of MC carbides during the tempering process. In addition, the lamellar M-A constituents in the initial microstructure decomposed to form MC carbides.

Effect of Mo in Ni-30Cr filler metal on the eutectic phases of weld metal

The effects of Mo content on the microstructure and solidification path of gas tungsten arc welding deposited Nb-bearing nickel-based alloy welding wire were studied. The Mo entered the matrix γ phase as a solid solution and was precipitated in the form of the eutectic phases including Laves, σ, and MC carbides. In Ni-30Cr weld metal, Mo plays a leading role in the eutectic σ formation, while a relatively minor role in the eutectic MC carbides and Laves formation compared with Nb. The addition of Mo could reduce the content of Nb in the dendrite arm, interdendritic region, and Laves phase. With the increasing Mo content, the Laves phase changed from a rod-like shape to a blocky shape. When the Mo content exceeded 4%, σ + Laves + γ eutectic structure was formed. When the Mo content was about 5.0%, there were microcracks in the σ and Laves phases, and at the σ/MC interface. To avoid the formation of the σ phase due to the segregation of Mo, the Mo content had to be controlled below 4%.

Evolution of MC carbides in Ni-Mo-Cr-Ti based alloy for applications in molten salt reactors

Evolution of MC carbides in Ni-Mo-Cr-Ti based alloy for applications in molten salt reactors

The impacts of M/A constituents decomposition and complex precipitation on mechanical properties of high-strength weathering steel subjected to tempering treatment

The impact of microstructure evolution on the mechanical properties of a typical 500 MPa-grade weathering steel produced by an identical thermo-mechanical control process and different tempering treatments at 450–650 °C were thoroughly investigated mainly using scanning electron microscopy (SEM) equip with an electron backscatter diffraction (EBSD), high-resolution transmission electron microscopy (HRTEM), and X-ray diffractometer (XRD). Results indicated that the as-rolled steel consists of granular bainitic ferrite (GBF) and a considerable amount of martensite/austenite (M/A) constituents, exhibiting unsatisfied mechanical properties. As the tempering temperature increased from 450 to 550 and 650 °C, the massive twin-type M/A constituents decomposed preferentially into carbides following the sequence of Fe3C→(Cr, Mn, Fe)3C→(Cr, Mn, Fe)3C + (Cr, Mn)23C6, with a small amount of fine lath-type M/A constituents remained. The matrix recovered with the bainitic ferrite laths merged, the dislocation density decreased and the proportion of high-angle grain boundary (HAGB) increased. Nanoscale (Ti, Nb)C and ε-Cu particles also precipitated simultaneously, leading to an increase of yield strength. The strain hardening capacity and hence the tensile strength reduced resulting from the decomposition of M/A constituents. Moreover, the impact toughness was first enhanced by tempering at 450–600 °C with the decreasing twin-type M/A constituents content and increasing HAGBs proportion, and then deteriorated by tempering at 650 °C due to the precipitation of necklace-like M23C6 carbides at the grain boundaries. An optimum mechanical property combination was achieved via tempering at 550–600 °C.

Microstructure evolution and its effect on the stress rupture behavior of 9%Cr-CrMoV dissimilar welded joint

Taking the economic factor into account, 9 %Cr martensite heat-resistant steel is chosen to be joined with CrMoV steel by welding technique to utilize their potential at different temperatures. In the present study, the dependence of rupture time on the applied stress was determined and in accordance with the equation of lg(tr) = 7.224–0.014σ. All the stress rupture specimens were ruptured in the base metal of CrMoV steel (CrMoV-BM), indicating the high welding quality and the slight effect carbon migration on the stress rupture properties. The microstructure characterization revealed that the M23C6, M7C3 and MX type carbides were identified in the CrMoV-BM in the original state. These carbides were the typical precipitates existed in the heat resistant steel. Partially coarsened carbides and Mo-rich M6C phase were detected during the stress rupture test. The micro-voids tended to be initiated in the regions near these partially coarsened carbides and Mo-rich M6C ones along the grain boundaries. Subsequently, the macro-cracks formed via the micro-void convergence and eventually caused the fracture of the ruptured specimens.

Dual-effects of carbon doping on the recrystallization kinetics of the Cantor alloy during mid-temperature annealing

In this study, the comprehensive effects of carbon doping on the recrystallization process and the corresponding mechanical performance of the Cantor alloy during mid-temperature (700 °C) annealing were investigated. In the early stage of recrystallization, i.e., before 7 min annealing, abundant M23C6 carbides form preferentially along deformation bands and hence accelerate recrystallization process by providing dense heterogeneous nucleation sites. The fast consuming of the stored defects by recrystallization in this stage leads to the drastic decrease of strength but the regain of ductility. Detailed characterization on the precipitation behavior further shows that the initial fine carbide (≤10 nm in size) firstly enriches in carbon only, and then gradually grows to perfect M23C6 as controlled by the diffusion of Cr. In the later stage of recrystallization, e.g., after 7 min annealing, these carbides coarsen and Zener-pin grain boundaries from further migration, which in turn suppress the overall recrystallization process. In this stage the mechanical properties vary slightly as a result of the balance between the slowly deceased amount of hard non-recrystallized regions and the slowly increased carbide volume fraction.

Investigating the effects of aging time on corrosion resistance and antibacterial property of newly designed medium-entropy super austenitic stainless steels

In this study, a newly developed medium-entropy austenitic stainless steel (MEASS) was subjected to a heat treatment to examine the effects of different aging times on its strength, corrosion resistance, and antibacterial performance. The properties of the MEASS were compared with those of a commercial stainless steel (UNS S31254). All of the aged MEASS specimens exhibited higher hardness than the solution-treated S31254 and MEASS owing to the precipitation of the Cu-rich phase, as characterized by a transmission electron microscope (TEM) and scanning TEM (STEM). However, as aged at a temperature of 700 °C for longer than 4 h, the aged MEASS specimens were prone to intergranular corrosion. This was owing to the lower integrity of the passive film as a result of the precipitation of the M23C6 carbides and σ phases at the grain boundaries. Although the antibacterial rate increased from 83.9% to 98.1% as the aging time was prolonged from 4 h to 48 h, the preliminary results suggested that the MEASS specimen subjected to the aging treatment for 4 h exhibited an optimal combination of strength, corrosion resistance, and antibacterial performance.

Three-Body Abrasive Wear-Resistance Characteristics of a 27Cr-Based 3V-3Mo-3W-3Co Multicomponent White Cast Iron with Different Ti Additions

A multicomponent white cast iron containing 5 wt.% each of Cr, V, Mo, W, and Co (MWCI) is known to have excellent wear-resistance properties due to the precipitation of some very hard carbides, such as MC, M2C, and M7C3. However, it seems possible to improve the wear resistance of MWCI by increasing the carbide volume fraction (CVF). Thus, 27 wt.% Cr based on 3 wt.% each of V, W, Mo, and Co was simultaneously added into the white cast iron. To avoid the tendency of carbides to crack due to high M7C3 precipitation levels, titanium (0–2 wt.% Ti) was also added. A rubber wheel abrasive machine test according to the ASTM G65 standard with two different abrasive particle sizes (average: 75 and 300 μm) was used to evaluate the wear characteristics of the alloy. The results show that the wear resistance of these new alloys (0Ti, 1Ti, and 2Ti) is lower than that of MWCI in small silica sand, owing to the lower hardness. However, a different condition is present in large silica sand, for which the abrasive wear resistance of MWCI is lower than that of the 0Ti and 1Ti specimens. In addition, TiC precipitation effectively refined the size of M7C3 carbides and reduced their cracking tendency. Thus, the wear resistance of 1Ti is comparable to that of 0Ti, although it has a lower hardness factor. However, the wear resistance of the alloy significantly decreased following the addition of Ti by more than 1 wt.% due to the lower hardness and CVF. Therefore, it can be said that the abrasive wear characteristics of the alloy are not only affected by the hardness, but also by the micro-structural constituents (type, size, and volume fraction of carbides) and silica sand size.

Effect of carbon on microstructure and mechanical properties of Co-free medium-entropy alloys

Effect of carbon content on the microstructure and mechanical properties of a Co-free C x (CrFeNi)100−x (x = 0, 0.5, 1, 1.5, 3, 4.5, and 6 at.-%) medium-entropy alloys were systematically investigated. The increase of carbon content promoted the phase structure of alloys changing from a single FCC phase to an FCC + M23C6 carbides dual-phase structure, accompanied by an apparent increase in yield strength and an uneven decrease in plasticity. The improvement of yield strength was attributed to the interstitial solid solution strengthening of carbon element and the precipitation strengthening of carbides. The decrease of plasticity was related to the formation of coarse carbides. The C1.5(CrFeNi)98.5 alloy exhibited an enhanced yielding strength of 265 MPa while maintained a considerable elongation of 38%.

Discussion of the Segregation and Low Hardness of Large-Diameter M3 High-Speed Steel Produced by Spray Forming.

As an advanced near-net-shape processing method in which directly preformed, semi-finished products are created from liquid metals, spray forming has become popular in the development and application of new materials and is supporting industrialization. However, as investigated in this work, the problems of segregation and low hardness exist in the actual industrialization process, particularly for large-diameter M3 high-speed steel. It was here found that the annual ring segregation morphologies were mostly distributed from the edge to 1/2R, with a large number of stripes primarily enriched in C, Mo, and Cr elements, and the degree of segregation was mild. The ring segregation was located at the 1/2R position, where the main elemental enrichments were C, W, Mo, Cr, and V, and the segregation degree was severe. The formation of segregation during deposition is described based on an equilibrium solidification model. A slow cooling rate and heat dissipation from the surface to the inside were judged to be the main factors causing segregation and changes in the carbide morphology. In terms of hardness, with the increase in the quenching temperature to 1230 °C, the tempering hardness increased significantly. The analysis shows that a faster cooling rate in the atomization stage caused the solidified droplets to exhibit rapid solidification characteristics, and there was a higher proportion of MC carbide in the deposited billet. MC carbides cannot be fully dissolved using the conventional heat treatment process, which decreases the C, Cr, Mo, and V contents in the solution and, thus, reduces the secondary hardening capability. The findings show that, when the spray forming process is used to prepare large-diameter materials, it should not be considered a rapid solidification technology simply because of its atomization stage. Moreover, more attention should be paid to the influence of microstructure transformation during atomization and deposition.

The effect of tempering conditions on carbides in P110 casing steels

The effect of tempering conditions on the carbides in P110 casing steels with two different Cr compositions was studied. Local Mo and Cr concentrations varied from the nominal compositions by ≈± 0.05 wt-% Mo and ≈± 0.07 wt-% Cr. The size of the fine (<20 nm) MC carbides remained relatively unaffected by tempering. Fe-based carbides exhibited a large increase in mean log-normal size from 65 to 102 nm when tempered at 650 and 715°C for 45 min, respectively. This rapid carbide coarsening is attributed to the dissolution of M23C6 carbides at 715°C. Associated with this significant increase in carbide size was an increase in the rate of hardness reduction with increasing Hollmon-Jaffe parameter for temperatures above 650°C.