What are the manufacturing processes for blade steel?

　　The manufacturing process of blade steel must focus on the core objectives of high purity, fine-grained microstructure, and a balanced combination of strength and toughness, covering the entire process—from raw material smelting to finished-product inspection. While process details vary slightly depending on the specific application (such as steam turbines, gas turbines, and wind turbines), the core process flow is as follows:

　　1. Raw material smelting: Laying the foundation for purity

　　Blade steels have extremely high requirements for impurity content (S, P, O, H) and must be produced using high-precision smelting processes to avoid internal defects.

• Primary refining in an electric arc furnace (EAF): melting scrap steel/ferroalloys and making initial adjustments to carbon and alloy element content.
• Secondary Steelmaking (LF/VD/VOD)

　　LF refining: desulfurization, adjustment of chemical composition, and enhancement of molten steel homogeneity;

　　VD/VOD Vacuum Degassing: Removes harmful gases such as H, O, and N from molten steel, reducing the risk of porosity and looseness.

• Electroslag Remelting (ESR): A critical process for high-end blade steels (such as those used in aerospace and gas turbines), which further refines the steel through electroslag remelting, eliminates segregation, and produces a uniform and dense ingot microstructure.

　　2. Ingot Casting and Heating: Controlling the Initial Grain Structure

• Ingot Casting: The refined molten steel is cast into ingots that meet the requirements for forging. The casting speed and cooling rate are carefully controlled to prevent the formation of coarse columnar crystals.
• Homogenizing Annealing: The steel ingot is heated to 1100–1200℃ and held at this temperature to eliminate casting stresses and compositional segregation, thereby providing a uniform microstructure for subsequent forging.

　　3. Forging and Forming: Grain Refinement + Streamline Optimization

　　Forging is a key step in controlling the performance of blade steel, aimed at breaking up coarse as-cast grains and forming metal flow lines aligned with the direction of applied stress.

• Process Type

　　Isothermal Forging: A specialized process for high-end blades (such as those used in aero-engines and gas turbines), which involves slow deformation at a constant temperature to achieve uniformly fine grains.

　　Forging/Precision Forging: Using a die to press the forging blank into an approximate blade shape, ensuring dimensional accuracy.

　　Roll forging / Ring rolling: Suitable for long-axis components such as wind turbine blade spindles, enabling large-deformation machining.

• Key process points: Strictly control the forging temperature (initial forging temperature: 1050–1150℃; final forging temperature ≥850℃) to prevent cracking caused by low-temperature forging. Employ multi-pass forging to ensure a deformation degree of ≥60% and thoroughly refine the grain structure.

　　4. Heat Treatment: Controlling the Microstructure to Achieve Tailored Performance

　　Depending on the type and application of blade steel, different heat treatment processes are employed, with the core principle being a balance between strength and toughness.

　　5. Surface treatment: Enhances wear resistance, corrosion resistance, and fatigue resistance.

　　Failure of blades often begins at the surface, and surface treatment is key to extending their service life.

• Shot peening: High-speed steel shot is used to impact the blade surface, introducing residual compressive stress that counteracts tensile stress during operation, thereby increasing fatigue life by a factor of 2 to 5.
• Coating protection

　　Turbine blades: Coated with a WC-Co wear-resistant layer to resist erosion caused by steam and water droplets;

　　Gas turbine blades: Coated with an MCrAlY metallic coating plus a ceramic thermal barrier coating to enhance high-temperature oxidation resistance.

　　Offshore wind turbine blades: coated with an anti-corrosion layer to resist salt spray corrosion;

• Nitriding: Low-temperature gas nitriding forms a dense nitrided layer on the surface, enhancing hardness and wear resistance.

　　6. Precision Machining and Forming

　　Rough machining: Turning and milling are used to remove material and obtain the basic profile of the blade.

　　Finishing: CNC milling, grinding, and polishing to ensure the blade surface accuracy (tolerance ≤ 0.05 mm) and surface finish (Ra ≤ 0.8 μm).

　　Special machining: The complex surfaces of aero-engine blades are machined using electrochemical machining and laser machining to ensure precision.

　　7. Quality Inspection: Comprehensive process control to eliminate defects.

　　As the core rotating component, blades are subject to extremely stringent inspection standards:

　　Nondestructive Testing: Ultrasonic Testing (UT) for detecting internal defects, Magnetic Particle/ Penetrant Testing (MT/PT) for identifying surface cracks, and Radiographic Testing (RT) for inspecting weld defects.

　　Mechanical Property Testing: Tensile Test (measuring strength and ductility), Impact Test (measuring toughness), Fatigue Test (measuring resistance to cyclic loading);

　　Metallographic Examination: Observe grain size and microstructure to ensure the absence of abnormal microstructures (such as coarse martensite or network carbides).

　　Dimensional Inspection: A three-coordinate measuring machine is used to verify the surface accuracy and ensure compliance with design requirements.