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The thermal fatigue resistance of the bottom plate in heat-resistant steel is one of its key properties, as it directly affects the service life and safety of high-temperature industrial equipment such as heat treatment furnaces, car-type furnaces, and petrochemical reactors. The following provides a systematic analysis from four aspects: material properties, thermal fatigue mechanisms, influencing factors, and optimization strategies: 1. Material properties of heat-resistant steel furnace bottom plates. Typical materials include high-alloy heat-resistant steels such as ZG35Cr24Ni7SiN, ZG40Cr9Si2, and ZG30Cr18Mn12Si2N. These materials form a dense oxide film (such as Cr₂O₃) through elements like high chromium (Cr), nickel (Ni), and silicon (Si), which effectively protects them against high-temperature oxidation and corrosion ； At the same time, the austenitic matrix provides high plasticity/toughness, enabling it to absorb thermal fatigue stresses and delay crack propagation. For example, ZG30Cr18Mn12Si2N maintains a strength of ≥100 MPa at 1000°C, exhibits excellent creep resistance, and its service life can reach 3–5 years (compared to only 1–2 years for ordinary materials). Special design: Materials such as Ni7N heat-resistant steel reduce their thermal expansion coefficient through the synergistic effect of nickel and nitrogen, thereby enhancing their resistance to thermal fatigue ； ZG40Cr25Ni20Si2 utilizes titanium/niobium carbides to strengthen the grain boundaries and prevent intergranular corrosion. 2. Mechanism of thermal fatigue: Thermal fatigue is caused by the repeated action of thermal stresses induced by temperature cycles. The key mechanisms include: accumulation of thermal stresses – temperature gradients (such as rapid cooling and heating resulting from the opening and closing of furnace doors) generate thermal stresses within the material; when these stresses exceed the yield strength, plastic deformation occurs, and cracks form as a result of cumulative effects over time. Microstructural degradation: Grain boundary sliding, carbide precipitation, or oxidation film spalling at high temperatures can exacerbate stress concentration; for example, H13 steel fails after 300 cycles when the depth of surface cracks reaches 0.5 mm. Crack propagation pattern: Cracks generally propagate vertically from the surface inward, forming a network of cracks; accompanied by oxide filling, this ultimately leads to the fracture of the material. 3. Key influencing factors: Temperature parameter: The magnitude of temperature changes (ΔT) and their frequency have a direct impact on the level of thermal stress (σ_th = E·α·ΔT, where E is the elastic modulus and α is the coefficient of thermal expansion). High frequency (such as frequent opening and closing of car furnaces) or large ΔT (such as rapid cooling from 1000°C to room temperature) will accelerate damage. Material properties: Differences in thermal expansion coefficients (e.g., at composite material interfaces), grain uniformity (materials with fine grains exhibit better fatigue resistance), carbide distribution (carbides at grain boundaries can easily trigger cracks), and oxidation resistance (stability of the oxide layer) are key factors. Design factors: geometric constraints (such as stress concentration caused by structural discontinuities), surface condition (rough surfaces are prone to crack initiation), and welding/processing techniques (such as residual stresses) affect the thermal fatigue life. 4. Strategies for improving resistance to thermal fatigue Material optimization: Composition design – increasing the Cr and Ni contents to enhance oxidation resistance ； Vanadium/titanium/niobium is added to form fine carbides, thereby enhancing high-temperature strength ； For instance, by optimizing its composition, the fatigue life of the SUH616 heat-resistant steel has been significantly improved. Heat treatment process: Quenching (1020–1050°C) + Tempering (580–600°C, multiple times) to refine grain structure and reduce residual stress ； After this process, the hardness of H13 steel remains at 47-49 HRC, and its resistance to thermal fatigue is improved. Surface modification: Shot peening is used to generate compressive surface stress, polishing helps reduce surface defects, and the application of CrAlN coatings (which increase wear resistance by 7 times at 600°C) or nitriding treatment enhances the surface’s fatigue resistance. Structural design: Avoid stress concentration (such as through rounded transitions), use composite structures (such as heat-resistant steel + ceramic coating), and optimize the temperature distribution in the cooling system. Process control: Control the heating/cooling rate and reduce the frequency of temperature fluctuations ； Precision casting methods (such as centrifugal casting) are used to improve the density of castings and reduce internal defects. 5. Practical application example: ZG30Cr18Mn12Si2N furnace bottom plate – In car-type furnaces, the synergistic effect of Mn and N reduces the thermal expansion coefficient, resulting in better resistance to thermal fatigue compared to traditional ZG40Cr25Ni20. This material reduces costs by 20% and extends the service life by 2–3 years. Ni7N heat-resistant steel: It can operate for extended periods at high temperatures of 1000°C, features a stable oxide film, and possesses excellent toughness at low temperatures, making it suitable for use in extreme environments such as petrochemical reactors. In summary, the thermal fatigue resistance of furnace bottom plates made of heat-resistant steel must be improved through a multi-faceted approach involving material selection, process optimization, structural design, and surface modification. Targeted solutions should be developed based on specific operating conditions (temperature, load, and medium) to ensure long-term stable operation under high-temperature environments.