Open AccessReview 12Cr2Mo1V Steel for Free-Forged Hydrogenation Reactor Shells: Defect Control, Microstructural Evolution, and Service Performance—A Review 1 Chongqing Key Laboratory of Advanced Mold Intelligent Manufacturing, School of Materials Science and Engineering, Chongqing University, Chongqing 400044, China 2 School of Materials Science and Engineering, Jiamusi University, Jiamusi 154007, China 3 School of Mechanical and Electric Engineering, Sanming University, Sanming 365004, China 4 Luoyang Zhongzhong Casting and Forging Co., Ltd., Luoyang 471033, China 5 China National Erzhong Group Co., Ltd., Deyang 618000, China * Author to whom correspondence should be addressed. Materials 2026, 19(12), 2464; https://doi.org/10.3390/ma19122464 (registering DOI) Submission received: 14 May 2026 / Revised: 29 May 2026 / Accepted: 5 June 2026 / Published: 9 June 2026 Abstract Hydrogenation reactor shells are safety-critical thick-section pressure-bearing components in petrochemical hydroprocessing equipment. Long-term exposure to elevated temperature, high pressure, and hydrogen-bearing media requires not only adequate strength, but also toughness, tempering stability, hydrogen-damage resistance, and through-thickness property uniformity. 12Cr2Mo1V steel, a Chinese Cr-Mo-V reactor steel closely related to vanadium-modified 2.25Cr-1Mo-0.25V steels, is widely used for large-shell forgings because its alloy design supports bainitic transformation, carbide stability, and elevated-temperature performance. This review critically synthesizes studies on 12Cr2Mo1V shell forgings, related Cr-Mo-V reactor steels, and heavy free-forged products. The discussion is organized around alloy design, ingot-derived defect inheritance, defect closure during free forging, bainite–grain–carbide evolution during forging and heat treatment, and the resulting strength, toughness, and hydrogen-service performance. Particular emphasis is placed on the process–defect–microstructure–property linkage in super-thick sections. The review shows that free forging is not merely a forming route, but a decisive metallurgical operation for densification, strain penetration, and precursor-structure conditioning. Future work should integrate casting, free forging, and heat treatment with multiscale characterization and data-enhanced predictive quality control. To further reduce descriptive comparison, this review summarizes standardized quantitative indicators for evaluating forging-route design, heat-treatment response, and prediction-method reliability. As summarized in , a typical route begins with steelmaking and secondary refining, followed by ingot or hollow-ingot casting, cropping and conditioning, high-temperature soaking, primary free forging for upsetting, cogging, drawing, and defect closure, piercing or hollowing, mandrel-assisted shell expansion, heat treatment, machining, nondestructive inspection, and final qualification. Free forging therefore acts simultaneously as a geometrical forming process and as a metallurgical operation for densification, strain penetration, and precursor-structure conditioning. The present review focuses on 12Cr2Mo1V steel for hydrogenation reactor shell sections produced by free forging while drawing on the broader literature on metallurgically related 2.25Cr-1Mo-0.25V reactor steels and heavy free-forged products. It provides a critical synthesis of published results rather than new experimental data. The discussion is organized around five connected themes: material characteristics, defect mitigation during free forging, microstructural evolution during forging and heat treatment, mechanical and hydrogen-service performance, and future development directions. The objective is to clarify the process–defect–microstructure–property relationships that govern the internal quality and reliability of heavy-wall shell forgings. A concise literature map is provided in Table 1, and the overall review logic is summarized in . To define the review scope transparently, the literature survey combined targeted keyword searches with citation tracking in major publisher databases and academic search platforms. The principal search terms included 12Cr2Mo1V, 12Cr2Mo1VR, 2.25Cr-1Mo-0.25V, Cr-Mo-V reactor steel, hydrogenation reactor shell, heavy-wall forging, free forging/open-die forging, void closure, macrosegregation, bainite, retained austenite, carbide evolution, hydrogen embrittlement, high-temperature hydrogen attack, digital twin, and data-driven quality prediction. Foundational metallurgy and forging studies were retained when they clarified mechanisms transferable to heavy Cr-Mo-V forgings, whereas studies published from approximately 2000 to 2026 were prioritized for alloy-specific, processing-specific, and service-performance evidence. The inclusion criteria emphasized peer-reviewed papers and technically traceable industrial studies that addressed at least one link in the process–defect–microstructure–property chain: alloy design, ingot-derived defects, deformation-path-dependent defect closure, bainite/M-A/retained-austenite/carbide evolution, through-thickness mechanical performance, hydrogen-service degradation, or predictive quality control. General papers on bainite, recrystallization, hot working, and hydrogen trapping were used only as mechanistic support. The available literature is strongest for Chinese engineering applications, international Cr-Mo-V reactor steels, European and East Asian forging/metallurgy studies, and open-access numerical modeling. Manufacturer-centered heavy-forging experience was considered where available, but was interpreted cautiously when processing data, sampling positions, or validation details were incomplete. Because the reviewed studies differ in component scale, sampling position, heat-treatment route, and validation depth, their evidence was interpreted using standardized comparison indicators, including core effective strain, void-closure evidence, surface–core property scatter, heat-treatment sensitivity, and model-validation error, rather than by qualitative claims alone. 2. Material Characteristics of 12Cr2Mo1V Steel 2.1. Chemical Composition and Alloy Design 2.2. Low-Carbon Bainitic Microstructure 2.3. Material Advantages for Heavy-Wall Shell Forgings 3. Free Forging and Defect Control 3.1. Manufacturing Route of Shell Forgings 3.2. Internal Defects in Heavy-Section Forgings Because hydrogenation reactor shell forgings are generally produced from heavy ingots or large cast stocks, their internal quality is strongly conditioned by casting-derived defects inherited at the starting stage. The most important defects include shrinkage cavities, centerline shrinkage porosity, microporosity, macrosegregation, and non-metallic inclusions; large cast ingots may also retain pronounced as-cast structural heterogeneity before sufficient breakdown deformation is imposed. Reviews on large steel ingots emphasize that macrosegregation and shrinkage porosity are among the principal factors limiting the homogenization of large cast and forged products, while investigations of industrial-scale Cr-Mo ingots show that such defects often coexist within the same starting stock rather than appearing as isolated imperfections [ 9, 10, 11, 12, 13, 14, 15]. Macrosegregation is especially important because it represents chemical heterogeneity at the ingot scale and changes the local composition inherited by subsequent forging and heat treatment. As illustrated in , segregation-ratio maps for C, Mn, and Cr under low and high filling rates provide an example of how ingot-scale positive and negative macrosegregation can persist as inherited heterogeneity after casting and subsequent processing [ 8, 69]. In a 12 MT Cr-Mo steel ingot, experimental mapping revealed positive segregation near the top region, conical negative segregation near the bottom, and a solute-enriched zone between the center and the ingot wall. Such heterogeneity is difficult to eliminate completely, even by prolonged homogenization, and can degrade mechanical performance, structural uniformity, and machinability of the final component [ 9, 10, 11, 12, 13, 14]. 3.3. Defect Closure During Free Forging 4. Microstructural Evolution During Forging and Heat Treatment 4.1. Microstructural Evolution During Hot Deformation During free forging, the initial cast structure is progressively broken down and replaced by a thermomechanically conditioned austenitic structure that serves as the precursor for subsequent phase transformation and tempering. The essential metallurgical processes include dissolution or redistribution of inherited precipitates, dislocation multiplication and rearrangement, dynamic recovery, dynamic recrystallization (DRX), and grain-boundary migration. As illustrated in , compression-type bulk-forming studies show that die friction and contact pressure can generate nonuniform material flow, dead-metal zones, and localized strain fields, which are important when interpreting hot-compression data and forging simulations [ 48]. Related hot-deformation studies on Cr-Mo steels commonly employ controlled routes involving heating, short holding, hot compression, and rapid quenching, thereby providing a thermomechanical basis for analyzing constitutive behavior and recrystallization response [ 12, 37, 49]. Experimental results obtained over 900–1200 °C and 0.01–5 s −1 show that both flow behavior and the final deformed microstructure are highly sensitive to deformation temperature and strain rate. These findings confirm that hot deformation in 12Cr2Mo1V steel is not merely a geometric shaping stage but a microstructural renewal process that strongly influences the austenitic precursor state and the subsequent evolution of final microstructure and properties [ 49, 70, 71, 72]. For reactor shell forgings, the key requirement of hot deformation is not only sufficient shape change but also adequate internal structural renewal. Recent studies on related Cr-Mo steels show that higher deformation temperature and lower strain rate generally promote DRX and reduce geometrically necessary dislocation (GND) density, whereas unfavorable parameter combinations cause incomplete recrystallization, subgrain accumulation, and persistent structural heterogeneity. As illustrated in , finite-element deformation simulations show that effective-strain distributions remain nonuniform even under controlled compression conditions, directly affecting local recrystallization and final microstructural uniformity [ 49]. The same study identified an optimal hot-working window of approximately 1170–1200 °C and 0.01–0.1 s −1. In contrast, 900 °C/5 s −1 produced elongated prior-austenite grains and adiabatic shear bands, and 1100 °C/5 s −1 still produced mixed grain sizes because recrystallization remained incomplete. These findings are relevant to heavy shell forgings because process design must ensure that the core is deformed under conditions favorable for austenite renewal and homogenization. 4.2. Bainite, Grain, and Carbide Evolution 4.3. Heat-Treatment Regulation of Final Microstructure Unintended exposure to intercritical temperatures is another important concern. A recent open-access study on 2.25Cr-1Mo pressure-vessel steel showed that intercritical quenching and tempering produced a dual-phase microstructure containing ferrite and tempered martensite, together with Cr-Mo-enriched carbides, and this condition led to marked losses in yield strength, ultimate tensile strength, and toughness compared with conventional quench-and-temper treatment. Although this work was performed on 2.25Cr-1Mo rather than the vanadium-modified grade, its implication is directly relevant here: localized intercritical thermal excursions during fabrication, repair, or complex thermal cycling can generate unfavorable phase mixtures that degrade the final property balance instead of improving it [ 27, 64]. 5. Mechanical Properties and Service Performance 5.1. Strength and Toughness To strengthen the quantitative comparison requested for heavy-wall forgings, the key evidence should be organized by sampling location, heat-treatment state, and directly measurable property indicators rather than by average strength alone. Table 3 summarizes the most relevant indicators for comparing surface and core regions, PWHT sensitivity, retained-austenite decomposition, and defect-closure effectiveness. The table is intended as a compact framework for interpreting available data and for standardizing future reports on 12Cr2Mo1V and related Cr-Mo-V shell forgings. 5.2. High-Temperature and Hydrogen-Service Performance 5.3. Structure–Property Relationships 6. Discussion, Challenges, and Perspectives 6.1. Internal Quality and Microstructural Uniformity 6.2. Process Optimization of Forging and Heat Treatment 6.3. Quantitative Comparison of Process and Prediction Approaches A useful quantitative comparison of technological approaches should be based on common indicators rather than on qualitative claims of improvement. For forging-route comparison, the most relevant indicators include core effective strain, hydrostatic-compressive stress state, void-closure index, surface–core hardness difference, grain-size gradient, Charpy-impact retention, DBTT shift, and property scatter along the wall thickness. Classical free forging is advantageous in flexibility and equipment availability, whereas multi-axis deformation and radial-forging-type routes may improve strain-path diversity and circumferential uniformity when component size and tooling allow. Heat-treatment approaches should be compared using strength retention, impact-energy retention, carbide-coarsening index, retained-austenite decomposition degree, and surface–core property difference after tempering or PWHT. For this reason, each approach should be judged by a minimum evidence set consisting of one process parameter group, one internal-quality or microstructural metric, and one location-resolved mechanical-property or validation indicator. To make the comparison less conceptual and more reproducible, Table 4 converts the process, heat-treatment, and prediction discussions into a standardized evidence matrix. The table separates the variables that should be reported, the quantitative indicators that allow cross-study comparison, and the interpretation limits that should be considered when transferring conclusions to super-thick 12Cr2Mo1V shell forgings. This standardized matrix also clarifies the limits of the available evidence. Studies that report only final average strength or a nominal forging ratio are useful for qualitative discussion, but they are not fully comparable unless they also provide sampling position, local strain or void-closure evidence, microstructural descriptors, and validation error for any predictive model. This distinction helps prevent overgeneralization from individual case studies to industrial super-thick shell forgings. 6.4. Future Research Directions Future research on 12Cr2Mo1V heavy-wall shell forgings should move beyond qualitative interpretation toward a quantitative and predictive metallurgical framework. A first priority is explicit modeling of through-thickness gradients in grain size, bainite morphology, carbide state, and hydrogen-related behavior. Recent studies on 12Cr2Mo1V large cylindrical forgings show that industrial production data can be combined with machine-learning models to predict grain size as well as room-temperature and high-temperature tensile properties, demonstrating that process–microstructure–property relationships in this alloy system can be quantified more effectively than by trial-and-error approaches alone [ 48, 49]. A second priority is to deepen the study of process coupling across scales. For very large shell sections, the key scientific issue is no longer a single deformation or heat-treatment parameter, but the chain relationship among casting-derived heterogeneity, local strain penetration during forging, prior-austenite evolution, transformation behavior, carbide precipitation, and final through-thickness properties. Future research should therefore target integrated models that connect defect inheritance, deformation path, and heat-treatment response instead of optimizing these stages separately. As summarized in , a forward-looking predictive-quality-control roadmap should include two interconnected streams: a process and prediction stream involving ingot-quality design, forging simulation, transformation/precipitation modeling, and plant-data-based machine learning; and a digital-enabler stream involving multiscale characterization, digital-twin logic, and uncertainty-aware decision support. In summary, the performance of 12Cr2Mo1V hydrogenation reactor shells produced by free forging is governed by the coupled relationship among alloy design, inherited ingot quality, thermomechanical processing, microstructural evolution, and hydrogen-related service degradation. Three conclusions can be drawn from this review. (1) 12Cr2Mo1V steel is metallurgically suitable for heavy-wall hydrogen-service shells because its Cr-Mo-V alloy design supports bainitic transformation, carbide stability, tempering resistance, and elevated-temperature performance. However, these advantages are realized only when forging and heat treatment jointly control bainitic morphology, carbide evolution, and hydrogen-trapping behavior across the full section thickness. (2) Free forging is not only an appropriate forming route for large reactor shell forgings but also the decisive operation for defect closure, strain penetration, centerline consolidation, and precursor-structure formation. Nevertheless, segregation-related heterogeneity and thickness-dependent microstructural gradients remain persistent challenges in very large sections. (3) Future progress should move from empirical process control toward predictive quality assurance. Integrated casting–free-forging–heat-treatment design, multiscale characterization, mechanism-based modeling, and data-driven tools should be combined to control through-thickness quality, reduce property scatter, and improve the reliability of 12Cr2Mo1V hydrogenation reactor shell forgings. In particular, future studies should establish standardized surface–core sampling schemes, quantitative bainite/carbide/retained-austenite descriptors, and digital-twin-assisted quality prediction so that different forging routes and heat-treatment strategies can be compared on a measurable basis. Comparable numerical indicators, including core effective strain, void-closure index, grain-size gradient, surface–core property scatter, DBTT shift, and model-validation error, should be reported consistently so that individual studies can be transformed into cumulative engineering evidence. Author Contributions Conceptualization, H.W. and G.Q.; methodology, H.W.; investigation, H.W.; resources, L.G., Y.Z., X.L., Y.L. and H.S.; writing—original draft preparation, H.W.; writing—review and editing, G.Q., L.G., Y.Z., X.L., Y.L. and H.S.; visualization, H.W.; supervision, G.Q.; project administration, G.Q.; funding acquisition, G.Q. All authors have read and agreed to the published version of the manuscript. Funding This research was funded by the National Science and Technology Major Project, grant number 2025ZD1605900. Data Availability Statement No new data were created or analyzed in this review. Data sharing is not applicable to this article. Acknowledgments The authors acknowledge support from the participating institutions and technical discussions related to heavy forging and reactor shell materials. Conflicts of Interest Authors Lin Gao, Yuqing Zhang, and Xiao Liu were employed by the company China National Erzhong Group Co., Ltd. Authors Yichou Lin and Haopeng Shi were employed by the company Luoyang Zhongzhong Casting and Forging Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Abbreviations The following abbreviations are used in this manuscript: API American Petroleum Institute CAE Computer-aided engineering DBTT Ductile-to-brittle transition temperature DRX Dynamic recrystallization EBSD Electron backscatter diffraction FEM Finite element method HTHA High-temperature hydrogen attack M/A Martensite-austenite constituent PWHT Post-weld heat treatment TEM Transmission electron microscopy References Corrected schematic manufacturing route for free-forged hydrogenation reactor shells, covering steelmaking and secondary refining, ingot/hollow-ingot casting, cropping and soaking, breakdown free forging, piercing or hollowing, mandrel-assisted shell expansion, heat treatment, machining and nondestructive testing, and final qualification. The schematic highlights the dual geometrical and metallurgical roles of free forging in defect closure, center consolidation, and precursor-structure conditioning. Corrected schematic manufacturing route for free-forged hydrogenation reactor shells, covering steelmaking and secondary refining, ingot/hollow-ingot casting, cropping and soaking, breakdown free forging, piercing or hollowing, mandrel-assisted shell expansion, heat treatment, machining and nondestructive testing, and final qualification. The schematic highlights the dual geometrical and metallurgical roles of free forging in defect closure, center consolidation, and precursor-structure conditioning. Schematic macrosegregation features in large steel ingots, including hot top segregation, A-segregates, V-segregates, negative base segregation, shrinkage cavity, banding, and inverse segregation. Schematic macrosegregation features in large steel ingots, including hot top segregation, A-segregates, V-segregates, negative base segregation, shrinkage cavity, banding, and inverse segregation. Main formation mechanisms of macrosegregation in large steel ingots, including solute-enriched liquid flow, equiaxed-crystal sedimentation, shrinkage-induced feeding flow, and deformation of the dendritic network. Main formation mechanisms of macrosegregation in large steel ingots, including solute-enriched liquid flow, equiaxed-crystal sedimentation, shrinkage-induced feeding flow, and deformation of the dendritic network. Review framework linking service environment, alloy design, ingot-derived heterogeneity, thermomechanical regulation, final microstructure, and service performance in 12Cr2Mo1V free-forged hydrogenation reactor shell forgings. Review framework linking service environment, alloy design, ingot-derived heterogeneity, thermomechanical regulation, final microstructure, and service performance in 12Cr2Mo1V free-forged hydrogenation reactor shell forgings. Representative as-received microstructure of 2.25Cr-1Mo-0.25V steel in the normalized-and-tempered condition. ( a) Optical metallographic microstructure; ( b) low-magnification SEM image showing granular bainite, lath bainite, and prior-austenite boundaries; ( c) high-magnification SEM image showing fine and coarse carbides at prior-austenite boundaries [ 7]. Representative as-received microstructure of 2.25Cr-1Mo-0.25V steel in the normalized-and-tempered condition. ( a) Optical metallographic microstructure; ( b) low-magnification SEM image showing granular bainite, lath bainite, and prior-austenite boundaries; ( c) high-magnification SEM image showing fine and coarse carbides at prior-austenite boundaries [ 7]. Microstructures of 2.25Cr-1Mo-0.25V steel after simulated post-weld heat treatment (SPWHT). ( a) Optical microstructure after minimum SPWHT; ( b) low-magnification SEM image after minimum SPWHT showing lath bainite and granular bainite; ( c) high-magnification SEM image after minimum SPWHT showing fine and coarse carbides; ( d) optical microstructure after maximum SPWHT; ( e) low-magnification SEM image after maximum SPWHT showing lath bainite and granular bainite; ( f) high-magnification SEM image after maximum SPWHT showing coarse carbides, carbide chains, and precipitate clusters [ 7]. Microstructures of 2.25Cr-1Mo-0.25V steel after simulated post-weld heat treatment (SPWHT). ( a) Optical microstructure after minimum SPWHT; ( b) low-magnification SEM image after minimum SPWHT showing lath bainite and granular bainite; ( c) high-magnification SEM image after minimum SPWHT showing fine and coarse carbides; ( d) optical microstructure after maximum SPWHT; ( e) low-magnification SEM image after maximum SPWHT showing lath bainite and granular bainite; ( f) high-magnification SEM image after maximum SPWHT showing coarse carbides, carbide chains, and precipitate clusters [ 7]. Redrawn casting assembly for a 12 MT steel ingot, showing the mold, hot top, refractory/insulation system, riser, and exothermic cap at the starting-stock stage before free forging. Adapted and modified from Ref. [ 14]. Redrawn casting assembly for a 12 MT steel ingot, showing the mold, hot top, refractory/insulation system, riser, and exothermic cap at the starting-stock stage before free forging. Adapted and modified from Ref. [ 14]. Sectioning, macro-etching, and chemical-mapping workflow for a 12 MT cast steel ingot. ( a) As-cast ingot showing the hot top region, top-bottom direction, and central longitudinal sectioning position; ( b) sampling layout, with the left side used for macro-etching and the right side divided into a grid for segregation mapping; ( c) longitudinal macro-etched section showing the center-to-surface sampling direction; ( d) sectioned samples for characterizing macrosegregation along height and radial directions; and ( e) chemical-mapping blocks from the hot top region. Adapted and modified from Ref. [ 14]. Sectioning, macro-etching, and chemical-mapping workflow for a 12 MT cast steel ingot. ( a) As-cast ingot showing the hot top region, top-bottom direction, and central longitudinal sectioning position; ( b) sampling layout, with the left side used for macro-etching and the right side divided into a grid for segregation mapping; ( c) longitudinal macro-etched section showing the center-to-surface sampling direction; ( d) sectioned samples for characterizing macrosegregation along height and radial directions; and ( e) chemical-mapping blocks from the hot top region. Adapted and modified from Ref. [ 14]. Chemical segregation-ratio patterns in low-filling-rate (LFR) and high-filling-rate (HFR) steel ingots. ( a) C distribution under LFR; ( b) C distribution under HFR; ( c) Mn distribution under LFR; ( d) Mn distribution under HFR; ( e) Cr distribution under LFR; ( f) Cr distribution under HFR [ 8]. Chemical segregation-ratio patterns in low-filling-rate (LFR) and high-filling-rate (HFR) steel ingots. ( a) C distribution under LFR; ( b) C distribution under HFR; ( c) Mn distribution under LFR; ( d) Mn distribution under HFR; ( e) Cr distribution under LFR; ( f) Cr distribution under HFR [ 8]. Contact-pressure and friction effects during compression-type bulk forming. ( a) Material flow under no-friction and frictional compression; ( b) friction-induced material flow, macroscopic shear bands, and dead-metal-zone formation; ( c) representative simulated strain distribution in a compressed specimen [ 48]. Contact-pressure and friction effects during compression-type bulk forming. ( a) Material flow under no-friction and frictional compression; ( b) friction-induced material flow, macroscopic shear bands, and dead-metal-zone formation; ( c) representative simulated strain distribution in a compressed specimen [ 48]. Effective-strain distributions of 25CrMo4 steel during deformation at different temperatures. ( a) 950 °C; ( b) 1000 °C; ( c) 1050 °C; ( d) 1100 °C [ 49]. Effective-strain distributions of 25CrMo4 steel during deformation at different temperatures. ( a) 950 °C; ( b) 1000 °C; ( c) 1050 °C; ( d) 1100 °C [ 49]. Optical microstructures of 2.25Cr-1Mo-0.25V steel. ( a) Raw steel showing acicular ferrite, quasi-polygonal ferrite, and lath ferrite features; ( b) annealed steel showing a carbide-containing transformed matrix after annealing [ 26]. Optical microstructures of 2.25Cr-1Mo-0.25V steel. ( a) Raw steel showing acicular ferrite, quasi-polygonal ferrite, and lath ferrite features; ( b) annealed steel showing a carbide-containing transformed matrix after annealing [ 26]. Through-thickness microstructural evolution in heavy-wall shell forgings, showing surface, mid-thickness, and core pathways controlled by cooling-rate differences, bainite morphology, M/A constituents, and carbide-related risks. Through-thickness microstructural evolution in heavy-wall shell forgings, showing surface, mid-thickness, and core pathways controlled by cooling-rate differences, bainite morphology, M/A constituents, and carbide-related risks. Heat-treatment regulation of the bainite–grain–carbide system and its linkage with mechanical response, transformation behavior, precipitation state, and service performance. Heat-treatment regulation of the bainite–grain–carbide system and its linkage with mechanical response, transformation behavior, precipitation state, and service performance. Redrawn and modified hydrogen-diffusion model for analyzing through-thickness hydrogen concentration evolution in 2.25Cr-1Mo-0.25V steel. Adapted and modified from Ref. [ 21]. Redrawn and modified hydrogen-diffusion model for analyzing through-thickness hydrogen concentration evolution in 2.25Cr-1Mo-0.25V steel. Adapted and modified from Ref. [ 21]. Representative fracture morphologies of 2.25Cr-1Mo-0.25V base-metal and weld-zone specimens under hydrogen-free testing conditions. ( a) Base-metal fracture surface showing dimples, voids, and second-phase particles; ( b) weld-zone fracture surface showing dimples, secondary cracks, and second-phase particles [ 45]. Representative fracture morphologies of 2.25Cr-1Mo-0.25V base-metal and weld-zone specimens under hydrogen-free testing conditions. ( a) Base-metal fracture surface showing dimples, voids, and second-phase particles; ( b) weld-zone fracture surface showing dimples, secondary cracks, and second-phase particles [ 45]. Packing-line evolution during steel-ingot solidification at different heat-transfer coefficients. ( a) 100 W m −2 K −1; ( b) 200 W m −2 K −1; ( c) 300 W m −2 K −1; ( d) 400 W m −2 K −1 [ 74]. Packing-line evolution during steel-ingot solidification at different heat-transfer coefficients. ( a) 100 W m −2 K −1; ( b) 200 W m −2 K −1; ( c) 300 W m −2 K −1; ( d) 400 W m −2 K −1 [ 74]. Roadmap for predictive quality control of 12Cr2Mo1V shell forgings through casting–free-forging–heat-treatment integration, multiscale characterization, digital-twin logic, and data-enhanced modeling. Roadmap for predictive quality control of 12Cr2Mo1V shell forgings through casting–free-forging–heat-treatment integration, multiscale characterization, digital-twin logic, and data-enhanced modeling. Table 1. Condensed literature framework used to define the review scope and evidence chain. Table 1. Condensed literature framework used to define the review scope and evidence chain. Review Domain Key Scope Role in This Review Service/alloy basis Hydrogen-service requirements; 12Cr2Mo1V/12Cr2Mo1VR and related 2.25Cr-1Mo-0.25V steels Defines the material-selection logic and reliability requirements of thick shell forgings. Ingot defects and free forging Macrosegregation, shrinkage porosity, inclusions, upsetting, cogging, drawing, and void closure Links casting inheritance and deformation path with internal densification. Bainite–carbide evolution Granular/lath bainite, M/A constituents, retained austenite, carbide precipitation, tempering, and PWHT Explains strength, toughness, tempering stability, and hydrogen trapping. Through-thickness performance Hardness, tensile properties, Charpy toughness/DBTT, hydrogen embrittlement, HTHA, fatigue, and creep Connects microstructural gradients and defect state with section-wide service reliability. Characterization and prediction EBSD, TEM, chemical mapping, 3D reconstruction, FEM, machine learning, and digital-twin monitoring Provides tools for quantitative comparison and predictive quality control. Table 2. Simplified process-control framework for heavy-wall shell forgings. Table 2. Simplified process-control framework for heavy-wall shell forgings. Stage Key Variables Metallurgical Role Main Risk If Uncontrolled Ingot casting Hot top design; filling rate; cooling path Sets the initial segregation and shrinkage state Centerline looseness and chemistry gradients Breakdown/upsetting Reduction per pass; die geometry; bite ratio Promotes core strain penetration and hydrostatic compression Incomplete internal void closure Cogging/shell forming Pass schedule; feed/rotation; temperature control Maintains densification while forming the shell Underworked core despite dimensional accuracy Cooling/normalizing/quenching Section size; cooling rate; thermal path Establishes bainite morphology and surface–core gradients Coarse granular bainite or unstable M/A constituents Tempering/PWHT Time–temperature history Regulates carbide evolution and strength–toughness balance Carbide coarsening or clustered precipitation Service exposure Hydrogen pressure; temperature; exposure time Controls hydrogen diffusion, trapping, and HTHA susceptibility Embrittlement or carbide destabilization Table 3. Compact indicators for comparing through-thickness property uniformity in heavy Cr-Mo-V reactor-steel forgings. Table 3. Compact indicators for comparing through-thickness property uniformity in heavy Cr-Mo-V reactor-steel forgings. Comparison Target Recommended Indicators Interpretation for Shell-Forging Quality PWHT sensitivity Hardness, Charpy energy, yield/tensile strength, carbide size, and bainite-lath coarsening Thermal exposure can degrade toughness and strength through coarsening and carbide redistribution. Surface–core heterogeneity Location-resolved hardness, tensile/impact data, DBTT, bainite type, M/A morphology, and retained-austenite fraction Surface data alone cannot represent core performance; central sampling is essential. Defect/strain state Local effective strain, hydrostatic compression, void-closure index, residual porosity, and segregation intensity Mechanical scatter should be interpreted together with deformation and defect-closure maps. Reporting standard Sampling coordinates, heat-treatment history, mechanical data, and quantitative bainite-carbide descriptors Enables comparable assessment of forging routes, heat treatments, and predictive models. Table 4. Standardized comparative criteria for forging-route, heat-treatment, and prediction strategies relevant to 12Cr2Mo1V and related Cr-Mo-V heavy-wall reactor-steel forgings. Table 4. Standardized comparative criteria for forging-route, heat-treatment, and prediction strategies relevant to 12Cr2Mo1V and related Cr-Mo-V heavy-wall reactor-steel forgings. Approach or Technology Minimum Variables and Numerical Metrics That Should Be Reported Interpretation for Cross-Study Comparison Conventional free forging/breakdown cogging Provides the most direct evidence for densification and core working, but results remain strongly dependent on ingot size, die geometry, pass schedule, and initial defect morphology. Multiaxial deformation Number of loading directions, cumulative strain, temperature window, strain-path sequence, core effective-strain distribution, anisotropy index, and grain-size gradient. Useful for comparing strain-path diversity and internal uniformity; transferability requires similar workpiece geometry and comparable thermal histories. Radial forging/mandrel-assisted shell expansion Radial reduction, feed per stroke, mandrel size, wall-thickness strain distribution, circumferential property scatter, NDT results, and dimensional deviation. Can support circumferential uniformity and dimensional repeatability, but conclusions for ultra-large reactor shells must consider tooling rigidity, mandrel design, and size limits. Normalizing, quenching, tempering, and PWHT Allows comparison of heat-treatment sensitivity and surface–core property stability; data should be separated by sampling location rather than averaged over the section. FEM/CAE process simulation Appropriate for comparing mechanism-based process feasibility, but reliability depends on boundary-condition calibration and experimental validation. Surrogate and machine-learning prediction Useful for plant-scale prediction and optimization, but vulnerable to data shift when alloy chemistry, ingot quality, press route, or heat-treatment practice changes. Digital-twin-assisted quality control Most suitable for closed-loop quality assurance, but it requires traceable data architecture, uncertainty management, and engineer-supervised model updating. Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content. © 2026 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license. Share and Cite Wang, H.; Quan, G.; Lin, Y.; Gao, L.; Zhang, Y.; Liu, X.; Shi, H. 12Cr2Mo1V Steel for Free-Forged Hydrogenation Reactor Shells: Defect Control, Microstructural Evolution, and Service Performance—A Review. Materials 2026, 19, 2464. https://doi.org/10.3390/ma19122464 Wang H, Quan G, Lin Y, Gao L, Zhang Y, Liu X, Shi H. 12Cr2Mo1V Steel for Free-Forged Hydrogenation Reactor Shells: Defect Control, Microstructural Evolution, and Service Performance—A Review. Materials. 2026; 19(12):2464. https://doi.org/10.3390/ma19122464 Wang, Haitao, Guozheng Quan, Yichou Lin, Lin Gao, Yuqing Zhang, Xiao Liu, and Haopeng Shi. 2026. "12Cr2Mo1V Steel for Free-Forged Hydrogenation Reactor Shells: Defect Control, Microstructural Evolution, and Service Performance—A Review" Materials 19, no. 12: 2464. https://doi.org/10.3390/ma19122464 Wang, H., Quan, G., Lin, Y., Gao, L., Zhang, Y., Liu, X., & Shi, H. (2026). 12Cr2Mo1V Steel for Free-Forged Hydrogenation Reactor Shells: Defect Control, Microstructural Evolution, and Service Performance—A Review. Materials, 19(12), 2464. https://doi.org/10.3390/ma19122464 Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here. Article Metrics Article metric data becomes available approximately 24 hours after publication online.
