Several Ways and Methods to Improve the Creep Resistance of Refractory Materials
Refractories, as core basic materials in high-temperature industries such as steel, metallurgy, ceramics, and glass, have their creep resistance directly affecting kiln life, production efficiency, and energy consumption control. Creep is the phenomenon of slow deformation of a material over time under high temperature and constant stress. If creep resistance is insufficient, it can easily lead to shell rupture, furnace lining collapse, and other failure issues.
This paper systematically elaborates on scientific methods to enhance the creep resistance of refractory materials from four dimensions: chemical composition optimization, microstructure control, process improvement, and composite technology.
Chemical Composition Optimisation: Enhancing Intrinsic Material Properties at the Source
1. Reduction of Low Melting Point Phases
Low melting point phases (e.g., glassy phases, impurity phases) are the primary pathways for creep. Taking Al₂O₃-SiO₂ system refractory materials as an example, conventional electrofused corundum shells are prone to creep and fracture at 1200–1300 °C due to the presence of sodium feldspar and other low-viscosity glassy phases. Through phase diagram analysis, the proportion of glassy phases can be significantly reduced by selecting materials (such as EC95 fine crystalline corundum) that form high-temperature stable heterogeneous phases (such as secondary mullite) with SiO₂. Experimental results show that the high-temperature creep resistance of EC95 shells sintered at 1550 °C is improved by more than 40% compared to that of ordinary corundum.
2. Introduction of Solid Solution Strengthening Elements
In metal-based refractories, the addition of alloying elements with high solid solubility (e.g., Mg, Zr, Y) can form substitutional or interstitial solid solutions that hinder dislocation movement. For example, the Mg-Al-Zn-Bi-Sn-Sb alloy is strengthened through solid solution mechanisms, promoting the formation of intermetallic compounds at grain boundaries, which effectively inhibit grain boundary sliding. This leads to a 60% reduction in creep rate at 300 °C.
3. Regulation of Grain Boundary Chemical Composition
Grain boundaries are fast diffusion pathways that contribute to creep. The addition of trace elements (such as Si, Al, and Mg) can lead to preferential oxidation at grain boundaries, forming dense protective layers that hinder oxygen diffusion. For instance, adding silicon powder to carbon-containing refractories can generate a SiO₂ film on the surface, increasing the oxidation-resistant temperature from 500 °C to 1200 °C.
Microstructure Control: Building a "Skeleton" for Creep Resistance
1. Grain Size Refinement
Refining grain size increases the number of grain boundaries, which hinders dislocation movement through grain boundary diffusion. Research shows that reducing the corundum grain size from 50 μm to 10 μm can improve medium-temperature creep resistance by 25%. However, it should be noted that excessively fine grain sizes (<1 μm) may weaken grain boundaries, thereby reducing creep resistance.
2. Promotion of Direct Bonding Between Heterogeneous Phases
High-temperature sintering (e.g., above 1500 °C) can induce the formation of secondary mullite, resulting in a corundum–mullite interlaced heterogeneous network structure. Such a structure significantly increases the material's elastic modulus and reduces the creep rate by an order of magnitude. For example, corundum–mullite composites with a 75% mullite content exhibit better high-temperature creep resistance than single-phase corundum materials.
3. Optimisation of Pore Structure
Porosity reduces the effective load-bearing area and provides space for deformation. It can be minimized through appropriate particle size distribution (e.g., a "large–small–large" gradation pattern) and high-pressure molding (pressure ≥100 MPa). Experimental results show that reducing porosity from 20% to 10% can decrease the creep rate by 50%.
Process Improvement: Precise Control of the Preparation Process
1. Increasing Firing Temperature and Holding Time
High-temperature firing promotes grain growth and the bonding of heterogeneous phases. For example, standard corundum requires firing above 1500 °C to form secondary mullite, while EC95 fine crystalline corundum can complete the phase transformation at 1450 °C, significantly shortening the process cycle. The holding time should be adjusted according to the material thickness; typically, for every 10 mm increase in thickness, the holding time should be extended by one hour.
2. Adopting Molten Impregnation Technology
Immersing refractory materials in molten metal or ceramic slurry can fill internal pores and form a dense surface layer. For example, impregnating corundum with ferrosilicon nitride improves creep resistance by 30% compared to unimpregnated samples and enhances volumetric stability by 15%.
3. Optimising the Heat Treatment Regime
Thermal stress cracking can be reduced by using a staged heating process (e.g., 300 °C/h up to 800 °C, followed by 50 °C/h up to 1500 °C). During cooling, the rate must be controlled (≤50 °C/h) to prevent the propagation of microcracks caused by rapid temperature changes.
Composite Technology: Synergistic Enhancement of Creep Resistance
1. Fibre Reinforcement
The introduction of fibres such as silicon carbide (SiC) and alumina (Al₂O₃) creates a "bridging effect" that prevents crack propagation. For example, the flexural strength of corundum composites containing 15 wt% SiC fibres increased from 120 MPa to 280 MPa, and the creep rate was reduced by 70%.
2. Particle Dispersion Strengthening
The uniform distribution of nanoparticles (e.g., ZrO₂, TiO₂) pins grain boundaries and impedes dislocation motion. Experiments show that the high-temperature creep rate of corundum materials with 3 wt% nano ZrO₂ is 85% lower than that of pure corundum.
3. Layered Composite Structure
The design of an alternating "corundum layer–mullite layer" structure utilizes interface debonding to consume fracture energy. For example, the thermal shock resistance of a three-layer composite shell increased from 10 to 30 cycles, and creep deformation was reduced by 40%.
Application Example: Practical Verification in the Directional Solidification Process
In the directional solidification process of aero-engine turbine blades, traditional corundum shells are prone to steel leakage due to insufficient creep resistance. By using EC95 fine crystalline corundum combined with 15 wt% ferrosilicon nitride composite material, after directional solidification testing at 1600 °C, the shell thickness can be reduced by 50%, the operating temperature increased by 200 °C, and no creep rupture was observed. This solution has been successfully applied in the mass production of a certain type of aero-engine blade, resulting in a 12% cost reduction per engine.
Conclusion
Enhancing refractory creep resistance requires optimization through a multidimensional synergy of chemical composition, microstructure, process control, and composite design. In the future, with the development of new technologies such as 3D printing and in-situ reactions, refractory materials will move toward "customized microstructures" and "intelligent creep" behavior, providing more reliable solutions for high-temperature industries.