Formation Mechanism and Control of Flaw Detection Defects in Nuclear Power Steel
Executive Summary
In pressurized water reactor nuclear power plants, nuclear island equipment operates under extreme conditions of high temperature, high pressure, and high radiation. These demanding environments require pressure boundary materials with exceptional comprehensive performance characteristics. FUSHUN SPECIAL STEEL has conducted extensive research into the formation mechanisms of flaw detection defects in 18MnD5 steel, a material widely utilized in nuclear island equipment manufacturing due to its superior strength, excellent toughness, and cost-effectiveness.
Large-scale cast and forged components require extended cooling and solidification periods within steel ingot molds. During this critical phase, high-melting-point large-sized inclusions continuously aggregate and grow, ultimately leading to components that fail ultrasonic flaw detection quality standards. This comprehensive technical analysis examines the root causes of these defects and presents innovative control measures developed by FUSHUN SPECIAL STEEL’s research team.
Literature Review and Industry Context
Previous Research Findings
Extensive research conducted by Wang Tao revealed the presence of Al₂O₃ inclusions in individual large defects detected through ultrasonic testing of mold-cast 18MnD5 steel. His process design and production analysis provided crucial insights into the relationship between inclusion formation and manufacturing parameters. Subsequently, Wang Shuang’s investigation of mold-cast 18MnD5 steel plates identified non-metallic inclusions concentrated in the plate thickness center as the primary cause of flaw detection failures.
Further research by Zhu Shuai involved anatomical sampling analysis of flaw detection defect locations in mold-cast 16Mn steel thick plate forgings. Results demonstrated that large-sized foreign inclusions, originating from ladle slag and heating agents, were responsible for flaw detection failures. Shi Ruxing’s comprehensive study of hollow forging flaw detection defects in mold-cast 16Mn steel identified large-sized inclusions of various compositions, including MnO-SiO₂-Al₂O₃-CaO, Al₂O₃-MgO-ZrO₂-CaO, and SiO₂-Al₂O₃-ZrO₂ systems.
Inclusion Classification and Characteristics
The majority of flaw detection defects are attributed to large-sized inclusions, with inclusion types determined by specific steel grades and their respective smelting processes. Aluminum-based inclusions represent the predominant inclusion type in aluminum-deoxidized steels. Additionally, steel-slag and refractory material interactions generate magnesium-aluminum spinel inclusions during the steelmaking process.
These inclusion categories possess relatively high melting points and poor plasticity characteristics. Under mold casting production conditions, they tend to aggregate more readily, resulting in products that fail flaw detection standards. Calcium treatment offers an effective method for modifying high-melting-point inclusions, transforming them into low-melting-point liquid inclusions that can be more easily removed from the steel matrix.
Experimental Methodology and Materials
Material Specifications and Chemical Composition
The experimental material utilized in this comprehensive study was 18MnD5 steel, manufactured according to stringent nuclear grade specifications. FUSHUN SPECIAL STEEL’s production process follows the “Electric Arc Furnace (EAF) + Ladle Furnace (LF) + Vacuum Degassing (VD) + Return to LF Heating + Vacuum Casting (VC)” route, representing state-of-the-art steelmaking technology for nuclear applications.
Chemical Composition Requirements for 18MnD5 Steel (wt%)
Production Process Flow
The FUSHUN SPECIAL STEEL production process begins with electric arc furnace rough refining for scrap steel melting. During EAF tapping, aluminum ingots are added for precipitation deoxidation. The LF refining process incorporates aluminum powder and calcium-silicon powder for diffusion deoxidation. Following LF station departure, the molten steel is transferred to the VD position for vacuum treatment, with aluminum-iron additions in the later VD stage to meet aluminum content requirements.
After vacuum breaking, the steel returns to the LF position for heating and soft blowing operations. Upon completion of refining, vacuum casting is performed using the upward casting method. After casting completion, the vacuum chamber upper cover is closed, with no protective slag added to the molten steel top surface, only heating agent coverage. Subsequently, the molten steel cools in a 200-ton steel ingot mold within the vacuum chamber for approximately 40 hours before demolding, followed by heat treatment and forging processes to obtain the final product.
Ultrasonic Flaw Detection and Defect Characterization
Recent Product Analysis Findings
Recent batch analysis conducted by FUSHUN SPECIAL STEEL revealed multiple dense defects in the upper portion of cast ingots following ultrasonic flaw detection. These defects were located approximately 200-800 mm from the ingot top and 0-52 mm from the outer wall. Comprehensive re-examination of product tops and outer walls using ultrasonic flaw detection identified regions with significant defect concentrations.
Gas cutting sampling was performed on areas showing the highest defect density to obtain defect specimens. Water immersion ultrasonic scanning microscopy provided precise defect location determination, followed by scanning electron microscopy and energy-dispersive spectroscopy analysis to identify defect types and characteristics.
Comprehensive Sampling Strategy
To investigate flaw detection defect formation mechanisms, FUSHUN SPECIAL STEEL implemented comprehensive full-process sampling analysis throughout the entire smelting process. Seven strategic sampling positions were established: EAF pre-tapping (Sample ①), LF station entry (Sample ②), LF 40 minutes post-entry (Sample ③), LF station departure (Sample ④), VD post-vacuum breaking with aluminum addition (Sample ⑤), mid-casting stage (Sample ⑥), and casting completion (Sample ⑦).
Chemical composition analysis was performed on all collected steel samples, with results providing crucial insights into elemental evolution throughout the production process. Automated inclusion scanning analysis revealed inclusion evolution patterns, while Factsage 8.1 software enabled corresponding thermodynamic calculations to elucidate defect formation mechanisms and develop improvement measures.
Experimental Results and Analysis
Defect Type Identification
Ultrasonic flaw detection re-examination of specimens revealed precise defect locations, with white spot regions indicating defect positions and black regions representing non-defect areas. The defect overhead perspective view demonstrated numerous dense defects, while the side perspective view at approximately 2,868 μm from the specimen upper surface showed individual defect characteristics with the upper white bright band representing the sample surface.
Defect coordinate position analysis revealed that most defects concentrated between 0.10-0.40 mm below the upper surface. Cross-sectional and longitudinal sectional specimens extracted from high-defect-density regions showed no cracks or other obvious defects at defect locations, indicating inclusion-type defects. Metallographic specimens were prepared through cutting and polishing processes for comprehensive analysis.
Inclusion Analysis Results
Particle X automated inclusion scanning system analysis of defect locations utilized a minimum scanning size setting of 20 μm for comprehensive inclusion information statistics. Cross-sectional scanning identified 52 large-sized inclusions greater than 20 μm, while longitudinal sectional scanning revealed 49 large-sized inclusions exceeding 20 μm dimensions.
Projection of scanned inclusion information onto CaO-MgO-Al₂O₃-SiO₂ quaternary phase diagrams showed inclusion distribution points concentrated near the Al₂O₃ side, indicating aluminum oxide-rich high-melting-point inclusions. Inclusion sizes ranged from 20-200 μm, with most concentrated between 20-80 μm. Since metallographic specimens represent only cross-sectional views in specific directions, actual inclusion sizes exceed detected dimensions.
Full-Process Inclusion Evolution
Research into aluminum oxide-rich large-sized inclusion formation mechanisms involved comprehensive full-process sampling and inclusion evolution analysis using Particle X automated inclusion scanning with a minimum scanning size of 2 μm. Seven process samples revealed distinct inclusion morphology and size characteristics at each production stage.
Pre-EAF tapping typical inclusions were circular calcium aluminate inclusions. LF station entry showed irregular aluminum oxide inclusions. LF 40 minutes post-entry and station departure revealed nearly circular calcium-magnesium-aluminum inclusions. Post-VD vacuum breaking with increased calcium content produced circular calcium-magnesium-aluminum inclusions. Mid-casting and post-casting stages generated block-shaped magnesium-aluminum spinel inclusions.
Thermodynamic Analysis and Formation Mechanisms
Large-Sized Inclusion Formation Conditions
FUSHUN SPECIAL STEEL utilized FactSage 8.1 software Phase Diagram module for comprehensive thermodynamic analysis of aluminum oxide-rich large-sized inclusion formation mechanisms. Temperature settings of 1600°C with steel composition (mass fraction) of w([C])=0.2%, w([Mn])=1.5%, w([Ni])=0.77%, w([Mo])=0.51% enabled calculation of 18MnD5 steel Mg-Al-O equilibrium phase diagrams under varying [Ca] content conditions.
Phase diagram analysis divided regions into four distinct areas, with each generation region area varying according to steel [O] content changes. Phase diagrams without [Ca] showed MgO-Al₂O₃ inclusion generation regions occupying the largest proportion, with [Mg] mass fraction of 0.0001% sufficient for MgO-Al₂O₃ inclusion generation. Steel [Mg] originates from reactions between molten steel, refractory materials, and furnace slag during smelting processes.
Calcium Content Influence on Phase Formation
Theoretical calculations by Qiao Tong and colleagues demonstrated that during LF refining stages, steel [Mg] primarily originates from refractory material erosion, while during VD stages, [Mg] mainly derives from slag-to-steel transfer. Increasing [Ca] mass fraction to 0.0001% and 0.0003% showed CaO-MgO-Al₂O₃ liquid inclusion generation regions replacing original Al₂O₃ inclusion generation regions.
Smelting process steel composition position point analysis revealed that under zero [Ca] mass fraction conditions, all position points fell within or near MgO-Al₂O₃ inclusion generation regions, indicating MgO-Al₂O₃ inclusions as primary inclusions generated under these conditions. With [Ca] mass fraction increased to 0.0001%, position point distributions remained basically consistent with zero [Ca] conditions, falling within or near MgO-Al₂O₃ inclusion generation regions.
Inclusion Behavior and Removal Mechanisms
Different inclusion types exhibit significant differences in formation behavior, interfacial tension, and removal mechanisms. Aluminum oxide-rich inclusions are cluster-shaped solid high-melting-point large-sized inclusions formed through aluminum-oxygen reactions, possessing high interfacial tension and difficult removal characteristics. Liquid calcium aluminate inclusions, due to increased [Ca] content, transform from cluster-shaped to spherical morphology with significantly reduced interfacial tension and melting points compared to aluminum oxide-rich inclusions.
During the approximately 40-hour solidification process in steel ingot molds, aluminum oxide-rich inclusion particles migrate with steel flow under combined thermal convection, solidification shrinkage, and density difference-induced convection effects. Simultaneously, particles undergo random Brownian motion at microscopic scales due to inherent energy. Before complete steel solidification, inclusions continuously grow through collision and coagulation, forming aluminum oxide-rich large-sized inclusions ultimately captured by solidified shells.
Process Optimization and Control Measures
Calcium Content Optimization Strategy
FUSHUN SPECIAL STEEL’s comprehensive analysis using Factsage 8.1 software Equilib module at 1600°C temperature settings examined calcium content effects on different phase inclusion formation. Initial [Al] mass fraction settings progressed from 0.01% to 0.02%, 0.06%, and 0.07%, with [Mg] mass fraction set at 0.0008% and [O] mass fraction at 0.004%. Additional steel composition settings included w([Si])=0.18%, w([Mn])=1.50%, w([Ni])=0.77%.
Results demonstrated that with [Al] mass fraction at 0.01%, increasing [Ca] content caused inclusion evolution from magnesium-aluminum spinel to liquid inclusions. When [Ca] mass fraction reached 0.003%, magnesium-aluminum spinel completely disappeared, transforming entirely into liquid inclusions. Maximum liquid inclusion content occurred at [Ca] mass fraction of 0.004%, with further [Ca] increases converting liquid inclusions to 2CaO·SiO₂ and CaO inclusions.
Implementation Guidelines for Nuclear Steel Production
18MnD5 steel product specifications require [Al] mass fraction control between 0.010%-0.017% and [Ca] mass fraction below 0.015%. Under [Al] mass fraction conditions of 0.01%, optimal [Ca] mass fraction control ranges between 0.003%-0.004%. This calcium content window enables inclusion type transformation from solid inclusions to liquid inclusions, ultimately achieving flaw detection failure rate reduction objectives.
FUSHUN SPECIAL STEEL’s implementation strategy focuses on precise calcium addition control during the steelmaking process. Calcium treatment processes require appropriate calcium addition quantities, as excessive calcium generates CaO and CaS formations. Optimal calcium content ensures complete transformation of high-melting-point inclusions to low-melting-point liquid inclusions without forming new high-melting-point inclusions.
Recommended Process Parameters for FUSHUN SPECIAL STEEL Production
Research Conclusions and Industrial Applications
Primary Research Findings
First Key Finding:
Aluminum oxide-rich large-sized inclusions represent the primary cause of 18MnD5 steel flaw detection failures. These large-sized inclusions form through gradual evolution during smelting, casting, and mold casting processes. During LF station entry, inclusions primarily consist of aluminum oxide inclusions. At LF station departure, inclusions evolve into CaO-MgO-Al₂O₃(-SiO₂) inclusions. Following casting completion, inclusion aluminum oxide content increases further, evolving into magnesium-aluminum spinel and other aluminum oxide-rich inclusions. During solidification stages, these inclusions further aggregate and merge to form large-sized inclusions.
Second Key Finding:
Thermodynamic condition calculations for inclusion formation demonstrate that under current operating conditions, molten steel generates substantial quantities of magnesium-aluminum spinel and other aluminum oxide-rich inclusions. Inclusion evolution mechanism analysis reveals that increasing molten steel [Ca] content causes Al₂O₃ inclusion generation region disappearance, CaO-MgO-Al₂O₃ liquid inclusion generation region expansion, and MgO-Al₂O₃ inclusion region reduction. These findings indicate that increased steel [Ca] content can modify high-melting-point Al₂O₃ and MgO-Al₂O₃ inclusions into low-melting-point CaO-MgO-Al₂O₃ liquid inclusions.
Third Key Finding:
Thermodynamic calculations examining [Ca] content effects on different phase inclusion formation demonstrate that according to steel grade composition content requirements, under [Al] mass fraction conditions of 0.01%, [Ca] mass fraction control within the 0.003%-0.004% range increases liquid inclusion generation quantities, reduces aluminum oxide-rich inclusion generation, and subsequently decreases flaw detection failure rates.
FUSHUN SPECIAL STEEL Industrial Implementation
Production Process Improvements
FUSHUN SPECIAL STEEL has successfully implemented comprehensive production process improvements based on these research findings. The company’s advanced steelmaking facilities now incorporate precise calcium addition control systems enabling real-time monitoring and adjustment of calcium content throughout the production process. These improvements have resulted in significant reductions in flaw detection failure rates and enhanced product quality consistency.
Implementation of optimized calcium treatment processes has enabled FUSHUN SPECIAL STEEL to achieve superior nuclear grade steel quality standards. The company’s commitment to continuous improvement and technological advancement ensures that nuclear power industry requirements are consistently met or exceeded. Advanced process control systems monitor inclusion formation and evolution throughout the entire production chain, enabling proactive quality management.
Quality Assurance and Testing Protocols
FUSHUN SPECIAL STEEL maintains rigorous quality assurance protocols incorporating advanced ultrasonic flaw detection equipment and comprehensive inclusion analysis capabilities. The company’s testing facilities utilize state-of-the-art scanning electron microscopy, energy-dispersive spectroscopy, and automated inclusion scanning systems to ensure product quality verification at multiple production stages.
Comprehensive metallographic analysis capabilities enable detailed inclusion characterization and formation mechanism understanding. FUSHUN SPECIAL STEEL’s laboratory facilities support full-process sampling and analysis, providing crucial data for continuous process optimization and quality improvement initiatives. These capabilities ensure that nuclear grade steel products consistently meet the most demanding industry specifications.
Future Research and Development Initiatives
FUSHUN SPECIAL STEEL continues investing in research and development initiatives focused on advanced inclusion control technologies and nuclear grade steel quality enhancement. The company’s research partnerships with leading academic institutions and industry organizations ensure access to cutting-edge metallurgical science developments and emerging technologies.
Future development plans include implementation of artificial intelligence-based process control systems, advanced inclusion prediction models, and real-time quality monitoring technologies. These initiatives will further enhance FUSHUN SPECIAL STEEL’s capability to produce consistently high-quality nuclear grade steel products while reducing production costs and environmental impact.
Technical Specifications and Performance Parameters
Nuclear Grade Steel Requirements
Nuclear grade 18MnD5 steel produced by FUSHUN SPECIAL STEEL meets stringent international standards for nuclear power plant applications. The steel exhibits exceptional mechanical properties including high tensile strength, superior impact toughness, and excellent fracture resistance under extreme operating conditions. These characteristics ensure reliable performance in pressurized water reactor environments subject to high temperature, pressure, and radiation exposure.
Comprehensive mechanical testing protocols verify that FUSHUN SPECIAL STEEL’s 18MnD5 products consistently achieve required yield strength, ultimate tensile strength, elongation, and impact energy values. Advanced metallurgical testing confirms uniform microstructure, controlled inclusion content, and minimal segregation patterns throughout large-scale cast and forged components.
Mechanical Properties of FUSHUN SPECIAL STEEL 18MnD5 Nuclear Grade Steel
Conclusions and Future Outlook
This comprehensive research conducted by FUSHUN SPECIAL STEEL has successfully identified the root causes of flaw detection defects in nuclear grade 18MnD5 steel and developed effective control measures. The study demonstrates that aluminum oxide-rich large-sized inclusions represent the primary cause of ultrasonic flaw detection failures, with these inclusions forming through complex evolution processes during steelmaking, casting, and solidification stages.
The thermodynamic analysis and experimental validation confirm that precise calcium content control provides an effective method for inclusion modification, transforming high-melting-point solid inclusions into low-melting-point liquid inclusions that can be more readily removed from the steel matrix. FUSHUN SPECIAL STEEL’s implementation of optimized calcium treatment processes has resulted in significant improvements in product quality and reduced flaw detection failure rates.
Future research initiatives will focus on advanced process control technologies, real-time inclusion monitoring systems, and artificial intelligence-based quality prediction models. These developments will further enhance FUSHUN SPECIAL STEEL’s capability to produce consistently high-quality nuclear grade steel products meeting the most demanding industry specifications while maintaining cost-effectiveness and environmental sustainability.
FUSHUN SPECIAL STEEL remains committed to advancing metallurgical science and technology, ensuring continued leadership in nuclear grade steel production and contributing to the safe and reliable operation of nuclear power facilities worldwide.
