Cr23C6 Carbides Research Articles

Novel core-layer-shell structured nano-composite precipitates formed in selective laser melted reduced activation ferritic-martensitic steel

A novel type of core-layer-shell structured nano-composite precipitates was identified for the first time in a typical reduced activation ferritic-martensitic steel fabricated via selective laser melting (SLM). Comprehensive characterization of morphology, composition and structure was achieved for them using high-resolution electron microscopy. Their formation mechanism is closely related to specific processing features of SLM: the core consists of a triclinic SiO2 nanoparticle (~10nm in diameter), resulting from the combination of Si and O in the powder melting stage; the middle layer corresponds to an MC-type carbide (~10-nm-thick) formed due to preferential nucleation at the SiO2/matrix interface during rapid cooling; the shell is an orthorhombic Cr7C3 carbide, which arises from thermally activated C and Cr diffused continuously to the MC/matrix interface due to the in-situ heat treatment during cyclic SLM scanning. This unique type of precipitate is believed to exert a significant influence on the mechanical properties of RAFM steels.

Effect of Sintering Conditions on the Microstructure of an FeCrMnNiCo High-Entropy Alloy

We investigated the microstructure of an FeCrMnNiCo alloy fabricated by spark plasma sintering under different sintering temperatures (1000–1100°C) and times (1–600 s). All sintered alloys consisted of a single face-centered cubic phase. As the sintering time or temperature increased, the grains of the sintered alloys became partially coarse. The formation of Cr7C3 carbide occurred on the surface of the sintered alloys due to carbon diffusion from the graphite crucible. The depth of the layer containing Cr7C3 carbides increased to ~110 μm under severe sintering conditions (1100°C, 60 s). A molten zone was observed on the surface of the alloys sintered at higher temperatures (>1060°C) due to severe carbon diffusion that reduced the melting point of the alloy. The porosity of the sintered alloys decreased with increasing time at 1000°C, but increased at higher temperatures above 1060°C due to melting-induced porosity formation.

Strengthening mechanism and martensite transformation behavior in grain-refined low-Ni austenitic stainless steel

Low-Ni (less than 3 wt%) austenitic stainless steels with different C contents were designed and heat treated to utilize both the grain refinement and carbide strengthening, and their effects were systematically investigated to elucidate the mechanism of tensile property improvement. Grain refinement to 2–3 μm with homogeneously distributed Cr23C6 carbides resulted in excellent combination of yield strength (YS) and tensile elongation (YS × Tensile elongation of nearly 30,000 MPa %) without yield point elongation. The strengthening due to grain refinement was complemented by precipitation hardening. Grain refinement inhibited deformation-induced martensite transformation (DIMT) to decrease the strain hardening rate, while fine Cr23C6 carbides uniformly distributed within austenite grains promoted DIMT and increased strain hardening rate by depleting Cr and C concentrations. Two competing effects of grain refinement and Cr23C6 carbide formation on DIMT resulted in the increase of strain hardening rate as the latter was more dominant on austenite stability. Strengthening mechanisms were clearly explained by quantifying the contributions from solid solution, grain refinement and precipitation hardening. Stacking fault energy and driving force for γ → α’ varied with C contents and annealing treatment, indicating that the optimum combination of C addition and thermomechanical treatment can yield the high performance low-Ni austenitic stainless steels.

Fe-based metallic glass coatings with suppressed cracks and enhanced wear resistance prepared by extreme high-speed laser cladding

Fe-based metallic glass (MG) coatings draw great attentions due to their excellent wear resistance. The recently developed extreme high-speed laser cladding (EHLC) provides a promising method for their fabrication but limited by the strong tendency to crack. Besides that, the deep insight into the structure evolution and the failure mechanism of the coating is still lacked. Thus, the understanding of the crack origin and the prevention of crack are of great importance. In the present work, Fe-Mo-Cr-Y-C-B MG coatings with adjustable glassy phase content were successfully fabricated by EHLC. The microstructure characterization of the coatings reveals that the cracks is main caused by the hard and brittle nanocrystalline phases precipitations of monoclinic Mo12Fe22C10 and (Fe, Cr)23C6 carbides in the heat affected zone. Therefore, the cracking of the coatings can be effectively reduced by increasing the glassy phase content. Subsequently, the hardness distribution, wear performance and wear mechanism of the coatings were investigated. The results showed that increasing the glassy phase content can effectively achieve low wear rate. The prepared Fe-Mo-Cr-Y-C-B MG coatings exhibit abrasion loss as low as 3.54·10−6 mm3 (N m)−1, which is an order of magnitude lower than that of the 45# steel substrate. The wear of the coatings is primarily attributed to the fatigue wear accompanied with slighting oxidative wear. By suppressing the brittle precipitated crystalline phases in the coating, the fatigue wear can be reduced and the wear resistance of the coatings can be improved. The present work provides insights into the crack prevention and were performance improvement of the Fe-based MG coatings prepared by EHLC.

First-Principles study of hydrogen solubility and embrittlement of Cr23C6 in nickel-based alloys

This research examines the crucial role played by Cr23C6 carbides in hydrogen trapping and their subsequent impact on the mechanical properties of the material. The hydrogen solution energies at different defect sites within the bulk phase of Cr23C6 and the Ni/Cr23C6 interface were analyzed using first-principles computations. This study underscores the notable vulnerability of nickel-based alloys to hydrogen embrittlement as the carbide content increases. Substantial hydrogen enrichment at the Ni/Cr23C6 interface, particularly at octahedral interstitial sites on the Ni side and C vacancies at the interface, was identified through comprehensive atomistic simulations. This enrichment negatively affects separation at the interface, indicating an increased risk of brittle fracture in the presence of hydrogen. By providing insights into the microscopic processes involved, our results seek to contribute to the development of nickel-based alloys that are more resistant to hydrogen, thereby influencing material selection and treatment in industrial applications prone to hydrogen embrittlement.

Research on the Structural–Phase and Physical–Mechanical Characteristics of the Cr3C2-NiCr Composite Coating Deposited by the HVOF Method on E110 Zirconium Alloy

Composite coatings based on chromium carbide (Cr3C2) and nickel–chromium alloys (NiCr) are widely used due to their unique properties, including high heat resistance, wear resistance and corrosion resistance. This article studies the structural–phase and physical–mechanical characteristics of Cr3C2-NiCr composite coatings applied by high-velocity oxygen fuel to E110 zirconium alloy. The HVOF method was chosen to create coatings with high adhesion to the substrate and excellent performance properties. Analysis of the microstructure of the cross-section showed the thickness of the modified surface layer from 75 to 110 μm, depending on the processing modes. Energy dispersive X-ray spectral analysis revealed the presence of elements Cr, Ni, C and O in the coating composition. Structural–phase analysis confirmed the formation of coatings with a high concentration of Cr3C2 carbide particles and NiCr (nickel–chromium) phases. The resulting composite coatings based on Cr3C2-NiCr had a significantly high microhardness, ranging from HV 1190 to HV 1280, and the friction coefficient varied in a significant range from 0.679 to 0.502 depending on the processing conditions. The maximum adhesion strength was 9.19 MPa per square centimeter.

Thermally Sprayed Coatings for the Protection of Industrial Fan Blades.

This paper presents a study on thermally sprayed coatings. Coatings produced by high-velocity oxygen-fuel spraying HVOF and plasma spraying deposited on the A03590 aluminum casting alloy are tested. The subject of this research concerns coatings based on tungsten carbide WC, chromium carbide Cr3C2, composite coatings NiCrSiB + 2.5%Fe + 2.5%Cr, mixtures of tungsten and chromium powders WC-CrC-Ni, mixtures of carbide powders with the Cr3C2-NiCr + the composite 5% NiCrBSi and WC-Co + 5% NiCrBSi. The aim of this research is to find a coating most resistant to the erosive impact of particles contained in the medium centrifuged by industrial rotors. The suitability of the coating is determined by its high level of microhardness. The hardest coatings are selected from the coatings tested and subjected to abrasion tests against a sand particle impact jet and the centrifugation of a medium with corundum particles. It is found that the most favorable anti-erosion properties are demonstrated by a coating composed of a mixture of tungsten carbide and chromium carbide WC-CrC-Ni powders. It is concluded that the greatest resistance of this coating to the erosive impact of the particle jet results from the synergistic enhancement of the most favorable features of both cermets.

Microstructure and mechanical properties of in-situ SiO2-reinforced mechanically alloyed CoCrFeNiMnX (X= 5, 20, 35 at.%) high-entropy alloys

The CoCrFeNiMnX (X = 5, 20, and 35 at.%) alloys, reinforced with 5 at.% SiC powder, were prepared by mechanical alloying and spark plasma sintering. This preparation method resulted in microstructural refinement of the FCC solid solution grains, while the SiC addition reacted with residual oxygen to form homogeneously distributed ultrafine SiO2 particles. Additionally, the alloys contained Cr7C3 carbides, which were significantly larger than the SiO2 particles and enriched with the alloy's elements, including Mn. Among the tested materials, the reinforced CoCrFeNiMn20 alloy exhibited the highest hardness and compressive yield strength (CYS), achieving 392 ± 9 HV30 or 476 ± 3 HVIT1 and 1024 ± 18 MPa, respectively. This alloy also maintained superior CYS (956 ± 35 MPa) even after 100 h of annealing at 800 °C. The reduction in CYS after annealing was smallest for the CoCrFeNiMn5 alloy (35 MPa) and increased with higher Mn content, highlighting Mn's significant role in diffusion-related processes. At elevated temperatures (600 and 800 °C), the CoCrFeNiMn5 alloy had the highest CYS of 190 ± 33 MPa, while the CoCrFeNiMn35 alloy had the lowest at 80 ± 6 MPa. This trend was also observed in wear rate tests, where the CoCrFeNiMn5 alloy had the lowest specific wear rate coefficient of 3.54 ± 0.09 × 10-4 mm³ N⁻1 m⁻1. This was due to matrix softening and oxide lubrication without significant spalling. Thus, the CoCrFeNiMn5 alloy demonstrated superior mechanical properties and wear resistance under various conditions, making it a promising candidate for applications requiring high strength and durability.

Synthesis of chromium carbide powder by vacuum-free electric arc plasma method

Chromium carbide powders were synthesized by vacuum-free electric arc method using pure chromium and carbon powder as starting components. The powders obtained in the experimental series were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), laser diffraction and differential thermal analysis. In the experiments, electrical parameters were recorded, and the thermal field generated in the reaction zone was measured using an infrared camera and tungsten‑rhenium thermocouples. The study results show the feasibility of plasma arc discharge synthesis of chromium carbide compounds Cr3C2 and Cr7C3 within 60 s at current intensity of 200 A. SEM analysis of the resulting powder shows that it consists of acute-angled particles of ∼9 to ∼50 μm in size. An updated design of the discharge circuit employed in the study significantly increases the purity of the final product. Vacuum-free electric arc synthesis of chromium carbide has been studied for the first time.

Microstructure and properties of WC-reinforced AlCoCrFeNiSi high entropy alloy coatings prepared via high-velocity arc spraying

In this research, (AlCoCrFeNiSi)100-x(WC)x (x=0, 20 and 40 wt%) high entropy alloy (HEA) coatings were successfully deposited on the surface of Q235 steel substrate using high-velocity arc spraying method. The study investigated the presence of WC particles on the phase composition, microstructure, tensile bonding strength, and corrosion behavior of the HEA coatings. X-ray diffraction analysis revealed that without WC particles, the predominant phase in the coating was a face-centered cubic (FCC). However, when 20 wt% and 40 wt% of WC particles were added, the phase composition shifted to include FCC, body-centered cubic (BCC), W3C, Cr7C3, and other carbides. The WC particles also reduced the crystalline grain size of the coatings from 14.75 nm to 10.34 nm. The microhardness value increased from 252.5 HV0.1 to a peak of 529.9 HV0.1, as the content of WC increased representing a microhardness improvement factor ranging from 1.4 to 3 times that of the substrate. The tensile bonding strength of the coatings was also enhanced with WC content. The coating with 40 wt% WC particles demonstrated the highest tensile bonding strength of 40.23 MPa. The cohesion, adhesion, and glue failures occurred after the tensile test, with adhesion failure being more prominent in the coatings without WC particles and glue failure being more prominent in the WC-reinforced coatings. Compared to the Q235 steel substrate, the coated samples showed enhanced resistance against corrosion due to the formation of the passive oxide layers. The effective resistance against corrosion was attained when the proportion of WC particles was 20 wt%, displaying the least values for corrosion current density and corrosion rate in 3.5 wt% NaCl solution. Examination through SEM and XPS analysis indicated that the coatings’ structures and the formation of protective passive layers enhanced the corrosion resistance.

Effect of aging time on precipitates and mechanical properties of the electron beam freeform fabricated Ni-Fe-Cr alloys reinforced by γ’ and TiC phases

Utilizing a synergistic multi-phase and multi-scale strengthening strategy, a novel Ni-Fe-Cr alloy reinforced by nano-scale Ni3(Al, Ti)-γ’ and micron-scale TiC phases was designed and prepared by electron beam freeform fabrication with powder-cored wire. In the as-built sample, the long-strip-shaped TiC phases precipitated along the interdendritic regions with an area fraction of 6%, while the precipitation of the γ’ phase was inhibited. Tensile performance exhibited certain anisotropy, with elongation being higher in the vertical direction than in the horizontal direction. To tailor the strengthening phases and enhance mechanical properties, the as-built alloys were subjected to solution and aging treatments with different aging times. After heat treatment, the γ’ phases were adequately precipitated, with their size increasing to over 100 nm, enhancing their strengthening effect. A portion of TiC phases at the grain boundaries were transformed into Cr23C6 carbides through the TiC+γ → Cr23C6 + γ' reaction. Fracture mechanism analysis revealed that the Cr23C6 carbides with a continuous distribution at the grain boundaries significantly compromised the toughness of the alloys. Consequently, after heat treatment, the yield strength of the alloys significantly increased in both the horizontal and vertical directions. Elongation initially increased and then decreased in the horizontal direction, while it continued to decrease in the vertical direction, and the anisotropy in performance was still maintained.

Thermodynamic Diagram Analysis and Smelting of Complex Fe-Cr-Mn-Si Ferroalloy

A state diagram of the Cr-Mn-Si-Fe metallic system, which simulates the compositions of complex chromium-manganese-silicon-containing ferroalloy (Fe-Cr-Mn-Si), has been constructed using thermodynamic-diagram analysis. Theoretical investigations have determined that the system comprises eight elementary tetrahedra. The sum of the relative volumes of the elementary tetrahedra equals one (1.00000), confirming the accuracy of the tetrahedration conducted. Analytical expressions for each tetrahedron have been derived. Calculations have determined the tetrahedron characterizing the phase composition of the ferroalloy. This tetrahedron is identified as the most voluminous phase triangle in the Cr-Mn-Si-Fe metallic system, implying that its significant volume facilitates the smelting process of chromium-manganese-silicon-containing ferroalloy, i.e., there is the possibility of freely regulating the charge compositions to achieve the required alloy grade. Based on the obtained results, an experimental smelting of the Fe-Cr-Mn-Si ferroalloy was conducted using a charge composed of chromium ore, low-grade manganese ore, and coke, resulting in a sample with the following composition, %: 44.07 Cr, 13.97 Fe, 12.48 Mn, 9.48 Si, 4.45 C. The microstructure comprises a complex silicide (Cr, Fe, Mn)3Si and Cr3C2 carbides.

The Influence of Cr Addition on the Microstructure and Mechanical Properties of Fe-25Mn-10Al-1.2C Lightweight Steel

The influence of Cr addition on the microstructure and tensile properties of Fe-25Mn-10Al-1.2C lightweight steel was investigated. The characteristics of the microstructures and deformation behavior were carried out through X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), and room temperature tensile testing. Fe-20Mn-12Al-1.5C steel without Cr exhibited a fully austenitic single phase. With the addition of Cr, the volume fraction of ferrite continuously increased. When the content of Cr exceeded 5 wt%, the precipitation of Cr7C3 carbides was observed. In the steel with 5 wt% Cr, the quantity of κ carbides remarkably decreased, indicating that the addition of 5 wt% Cr significantly inhibited the nucleation of κ-carbides. As the Cr content increases from 0 wt% to 5 wt%, the austenite grain sizes were 8.8 μm and 2.5 μm, respectively, demonstrating that Cr alloying is an effective method of grain refinement. Tensile strength increased slightly while elongation decreased with increasing Cr content. As the Cr content exceeded 5 wt%, the yield strength increased but the elongation drastically decreased. The steel with 2.5 wt% Cr achieved a synergistic improvement in strength and ductility, exhibiting the best tensile performance.

The Microstructure, Surface Topography and Wear Resistance of Cold-Sprayed (Cr3C2-25(Ni20Cr))-(Ni-graphite) Composite Coatings Modified by Diode Laser Treatment

Cold-sprayed composite coatings have several advantages; however, some properties, such as hardness and abrasion resistance, are lower than those in plasma- or HVOF-sprayed deposits. This work showed that the use of surface diode laser processing allowed the development of (Cr3C2-25(Ni20Cr))-(Ni-graphite) cermet coatings with good adhesion to the steel substrate, and increased properties in the near-surface zone, below which the properties of cold-sprayed coatings were retained. Studies of the microstructure in the micro/nanoscale of the laser-treated coatings showed strong grain refinement after surface treatment. Cr7C3 carbide of various shapes and sizes was formed in the structure; while, a several hundred nanometre layer of Cr2O3 oxide appeared on the coating surface. The changes occurring in the microstructure have resulted in increased mechanical and tribological properties of the laser-treated zone of deposits.

Effect of grain size and segregation on the cryogenic toughness mechanism in heat-affected zone of high manganese steel

High manganese steels have been nominated as desirable materials used for various cryogenic applications due to their excellent cryogenic toughness caused by twinning. High manganese steel welded joints were obtained through gas metal arc welding, and the microstructural differences at three subzones of heat-affected zone (HAZ) were utilized. The aim was to investigate the effect of grain size and Mn/C/Cu segregation on the cryogenic toughness mechanism of high manganese steel welded joints. The results indicate that the average impact absorbed energies at −196 °C for 1.0, 3.0, and 5.0 mm from the fusion line (denoted by FL + 1, FL + 3, and FL + 5) is 118.6 J, 102.4 J, and 117.2 J, respectively. Compared with base metal, the embrittlement occurred in HAZ. Large grains improve toughness, while small grain boundaries, segregation, and precipitation in HAZ deteriorate toughness. Larger grain size facilitates the primary twins with thinner inter-twin spacing and twin thickness, and the high density of these primary twins inhibits the activation of secondary twins. This, in turn, enhances cryogenic impact toughness. Although the stacking fault energy (SFE) throughout the welding joint lays within the range for the twinning-induced plasticity effect, the enrichment of Mn/C/Cu slightly increases the local SFE and critical resolved shear stress for twining within the segregation zone. This phenomenon makes the activation of primary twins more challenging. Furthermore, the C segregation at grain boundaries leads to the precipitation of fine Cr23C6 carbides. These carbides serve as crack initiation points during impact tests, contributing to degradation in cryogenic impact toughness.

Carbide Precipitation Behavior of High‐Alloy Steel During Solidification and Cooling Process

The precipitation, growth, and transformation of carbides rich in Cr and Mo during solidification and cooling process of high‐alloy steel for cold working dies are investigated. The Fe–C pseudobinary phase diagram of high‐alloy steel is calculated by Thermo‐Calc software to ascertain the critical phase transformation temperatures. The temperatures of phase transformation during the solidification process are determined by a differential scanning calorimeter. Then the microstructure of the samples at these characteristic temperatures is observed in situ by high‐temperature confocal microscope. The results show that the characteristic temperatures for phase transformation and carbide precipitation are 1380, 1315, 1140, and 980 °C, respectively. At 1315 °C, thick rod‐like or island‐like M7C3 carbides enriched in Cr and Fe are formed. At 1140 °C, secondary M7C3, adhering to the primary eutectic M7C3, grows rapidly in a rod‐like morphology and maintains a similar composition. The secondary M6C carbides of segregated Mo elements precipitate in fan‐shaped clusters near M7C3 through the dissolution of M7C3 and the conversion of M2C. Thermodynamic calculation further indicates that the eutectic Cr7C3 precipitates at 1213 °C, and secondary Cr7C3 remains under conditions favorable for precipitation below the solidus, preceding the precipitation of secondary carbides Cr23C6 and Mo2C.

Interface bonding characteristics and thermal damage behavior of brazed diamond with Nb-added Ni[sbnd]Cr filler alloys

NiCr alloy is one of the important filler for brazed diamond tools because of its high hardness, high wear resistance and good wettability to diamond. However, the high brazing temperature and catalytic action of Ni element contained in alloy tend to cause the thermal damage of diamond and interface failure of brazed samples. In this work, the noval NiCr filler alloys added with different contents of Nb are prepared, and the microstructures, mechanical and wear resistance properties of brazed layers used these alloys are investigated. On this basis, the interfacial bonding characteristics and thermal damage behaviors of diamond brazed with Nb-added NiCr filler alloys are further studied. The results show that the addition of Nb element improves the hardness and wear resistance of NiCr filler layer, and the filler alloys exhibit good wettability and climbing effect to diamond. The refined Cr3C2 and Cr7C3 carbides are formed on the surface of diamond, which effectively enhances the interface bonding strength between filler alloy and diamond. Meanwhile, the addition of Nb element consumes some Ni and generates NbNi compound, which weakens, to some extent, the graphitization catalysis and chemical erosion of Ni to diamond. When the addition amounts of Nb are 1–1.2 wt%, the brazed diamond samples present excellent interfacial bonding strength and mechanical properties.

The role of the preparation route on microstructure and mechanical properties of AlCoCrFeNi high entropy alloy

Nearly equiatomic AlCoCrFeNi alloy samples were prepared by induction melting and mechanical alloying (MA) combined with spark plasma sintering (SPS). The cast sample showed a dendritic microstructure composed of spinodally decomposed nanometric constituents of the B2 and BCC phases. The spark plasma sintered sample exhibited an ultrafine-grained microstructure of B2 phase and FCC solid solution and Cr23C6 carbides. The MA + SPS sample was strengthened by compressive stress-strain test up to a yield strength of 2029 ± 5 MPa, resulting significantly higher compared to 1366 ± 32 MPa of the cast sample. In addition to the higher compressive yield strength, the sintered sample exhibited a hardness of more than 130 HV higher compared to the cast alloy. On the other hand, the cast alloy showed high plastic deformation (29%) and significantly high ultimate compressive strength of 3072 ± 122 MPa. Together with these mechanical characteristics, the MA + SPS sample showed good thermal stability while preserving the mechanical properties even after annealing at 800 °C. This was not the case with the cast sample, in which ductility and ultimate strength significantly decreased upon annealing at 800 °C. A substantial yield strength reduction of both MA + SPS and cast samples was recorded when tested at 800 °C. Nevertheless, stress-strain curve trends were observed to be quite different between the two samples, thus suggesting dissimilar deformation mechanisms under high-temperature compression.

Microstructural Analysis on Grain Boundary of Boron– and Zirconium–Containing Wrought Nickel-Based Superalloys

Trace elements such as boron (B) and zirconium (Zr) can increase creep resistance in nickel-based superalloys. This study investigates the change of microstructures on the grain boundary (GB) in phase-controlled nickel-based superalloys through the addition of trace elements. The basis alloy without B and Zr has distributed micrometer-sized (Nb, Ti)C and Cr23C6 carbides at the GBs. Zr is detected alongside Nb and Ti within certain (Nb, Ti)C carbides and its addition increases the fraction of (Nb, Ti)C or (Nb, Ti, Zr)C carbides. B affects the formation of precipitates constructed by nanometer-sized precipitates, which are Cr23C6 carbides, Cr23(C, B)6 boro-carbides, and Cr-rich borides, surrounded by γ’ phases. This film structure, which includes nanometer-sized precipitates surrounded by γ’ phases, forms more continuously with the addition of B and Zr. It is constructed with precipitates of (Nb, Ti)C carbides and Cr23(C, B)6 boro-carbides surrounded by γ’ phases. Numerous nanometer-sized precipitates (i.e., (Nb, Ti)C and Cr23(C, B)6) are distributed alternately within the film structure. The effect of the addition of B and Zr is such that nucleation sites of each precipitate are formed simultaneously and alternately along the GBs. The experimental results were discussed by correlating them with the predicted fraction of stable phases depending on the temperatures of these alloys, using the JMatPro program.

Degradation of a Cr-Mo steel by carbide precipitation over long-term service

Microstructure of the 40-year service-exposed 1Cr-0.5Mo steel extracted from pressure vessels through the boat sampling technique was characterized using x-ray diffraction, field-emission scanning electron microscopy, energy dispersive spectrometry, electron backscatter diffraction, focused ion beam, and transmission electron microscopy. The microstructure degradation during long-term service was identified. Statistical crystal plasticity models were developed to correlate the microstructure and mechanical properties quantitatively. The steel is found significantly strengthened but slightly embrittled. Nanoscale Mo2C carbides have nucleated within the ferrite matrix over the long-term service, resulting in significant strengthening of the matrix. The coarser grain boundary Cr23C6 carbides, on the other hand, act as the damage accumulation sites and lead to the degradation.