Knowledge

1 Introduction

K418 (international designation IN713C) is a nickel-based precipitation-hardened equiaxed grain cast superalloy specifically designed for high-stress and corrosive environments below 900°C. As a core material for China's aero-engine and energy equipment, it achieves a balance among high-temperature strength, thermal fatigue resistance, and environmental tolerance through γ' phase strengthening and high-chromium corrosion resistance design. It is particularly suitable for environments containing corrosive media such as sulfur and chlorine. This report systematically analyzes the composition design principles, performance advantages, manufacturing techniques, and future challenges of K418 alloy.

 

2 Basic Component Design

2.1 Chemical Composition and Element Functions

K418 is based on nickel (55% - 65%) and achieves multi-level strengthening through multi-element alloying.

 

Composition design features:

High chromium and low cobalt: Cr content reaches 14%, significantly enhancing thermal corrosion resistance, and without the expensive cobalt element (compared to 8% - 9% Co in K438), reducing costs;

Optimized γ' phase: Al/Ti ratio is controlled at 5:1 - 6:1, ensuring high-temperature stability of the γ' phase (dissolution temperature approximately 1200°C)

 

3 Microstructure and Mechanical Properties

3.1 Microstructure Characteristics

As-cast microstructure: Composed of γ dendrites, (γ + γ') eutectic (accounting for 10% - 15%), MC type carbides (TiC/NbC), and grain boundary borides (M₃B₂), with γ' phase size between dendrites reaching 0.5 - 1.0 μm.

Heat-treated microstructure: After solution treatment at 1180°C for 4 hours followed by aging at 850°C for 24 hours, the γ' phase is refined to 0.2 - 0.5 μm and becomes cubic, carbides partially spheroidize, and segregation reduces.

 

3.2 Mechanical Properties and High-Temperature Behavior

 

Core advantages:

1. Thermal corrosion resistance: In the molten salt environment containing Na₂SO₄/V₂O₅, due to the protective effect of the rich Cr oxide film, the corrosion rate of this material is only 60% of that of K465. This indicates that it has better tolerance in high-temperature corrosive environments and can effectively resist corrosion, thereby extending its service life.

 

2. Thermal Fatigue Resistance: This material can withstand the start-stop cycles of an aircraft engine (with temperature variations exceeding 600℃), and the rate of crack propagation is lower than that of similar cast alloys. This indicates that it possesses excellent fatigue resistance in the face of frequent temperature changes, effectively preventing the formation and propagation of cracks.

 

4 Manufacturing Processes and Technological Innovation

4.1 Precision Casting and Directional Solidification

Double vacuum melting (VIM + ESR): Control oxygen content ≤ 50 ppm, reduce inclusions, and enhance purity;

Directional solidification: Cooling rate 3 - 10℃/min, form columnar crystal structure, and increase longitudinal creep strength by 30%.

4.2 Breakthroughs in Additive Manufacturing Technology

Crack suppression process: BJT developed dedicated parameters for SLM (laser power 220 - 280W, scanning speed 800 - 1250 mm/s, layer thickness 30 - 60μm), eliminating microcracks through substrate preheating (200℃) and oxygen content control (<0.1%).

Performance matching: After solution treatment at 1180℃ and aging at 930℃, the tensile strength of SLM formed parts reached 95% of that of traditional castings, but the elongation was relatively low (about 3%).

4.3 Remanufacturing and Repair Technology

Pulsed laser remanufacturing: Applied to the repair of volume damage on impellers, the hardness of the formed layer reaches 900 - 1400 HV (20% higher than the base material), and the dynamic balance test meets the requirements of 13,500 r/min operation conditions.

Welding of dissimilar materials: When laser welding with 42CrMo steel, the problem of weld under-melting is solved through energy distribution optimization, and the joint strength exceeds that of the base material.

 

5 Technical Challenges and Future Trends

5.1 Existing Bottlenecks

Temperature Limit: When the temperature exceeds 950℃, the coarsening of the γ' phase accelerates (the coarsening rate increases by 5 times), and the creep strength drops sharply.

Additive Manufacturing Defects: The elongation of SLM formed parts is insufficient (elongation ≤ 3%), and cracks are prone to occur at high-angle grain boundaries.

Long-Term Stability: After aging at 800℃, the σ phase precipitates, which may induce brittleness.

5.2 Innovation Directions

Materials Genome Engineering: Develop low cobalt high tungsten variants (such as K418-W), reducing costs by 12% while maintaining corrosion resistance;

Composite Manufacturing Technology:

Diffusion bonding of ceramic matrix composite (CMC) blade bodies with K418 tenons, raising the temperature tolerance to 1400°C;

Gradient thermal barrier coatings (TBC): Surface layer of Al₂O₃-Cr₂O₃ + YSZ ceramic top layer, reducing the base temperature by 150°C;

Green recycling technology: Electrolytic refining return material, oxygen content < 15 ppm, the proportion of aviation casting return material increased to 60%.

 

6 Conclusion

K418 alloy, with its high-chromium corrosion-resistant design and γ' phase strengthening advantages, has become the benchmark material for sulfur-containing corrosive environments below 900°C. Its successful application in aero-engine blades, gas turbine guide vanes, and other fields has verified the material's reliability under extreme conditions. Future development needs to break through the ductility bottleneck of additive manufacturing and further expand the temperature limit through component gradientization (such as surface Cr-rich layers) and composite structure design. With the integration of green recycling technology and digital twin quality control systems, K418 will continue to play a core role in emerging fields such as hypersonic vehicles and hydrogen energy equipment.

 

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