How to Prepare Materials that Maintain Fine-Grained Structure at High Temperatures

The key to the preparation of materials to maintain a fine grained structure at high temperatures is to introduce fine distributed second phase atoms to pin grain boundaries. Processing and heat treatment must be controlled so as to prevent anomalous grain growth during high temperature service.

1. Metallurgical and Compositional Design: Introducing Microalloying Elements

Ti, Nb, V, Al: Adding these elements during smelting creates very stable carbides (e.g., TiC, NbC, VC) or nitrides (e.g., AlN) that precipitate during solidification and heating, anchoring grain boundaries.

W, Mo: Improve red hardness and temper softening resistance of steel. They have stable carbides at high temperature which prevent grain boundary movement.

Practical Recommendation: Use microalloyed or naturally fine-grained steels, such as H13+Ti and Al-Killed H13, provided the content of critical elements is within the specified limits (e.g., Ti ≥ 0.015%, Al ≥ 0.02%).

2. Powder Metallurgy Process: Obtaining an Ultrafine Original Microstructure

PM-H13, ASP-23: The use of the gas atomisation powder preparation + hot isostatic pressing (HIP) moulding avoids segregation in traditional ingots, and the microstructure is homogenous with an original grain size of 8-10.

Advantages: Dispersed carbide dispersion, strong isotropy and far better resistance to grain development than conventional cast and forged steel.

Applications: High precision injection moulds in medical and optical applications, improvement of fatigue life over 30%.

3. Grain Size Refinement: Thermomechanical Processing Control

regulated Rolling and Cooling (TMCP): During forging or rolling, the temperature of deformation and cooling rate are regulated, which enables numerous deformations in the austenite region and promotes dynamic recrystallisation and grain size refinement.

Isothermal Annealing + Rapid Cooling: Avoid slow furnace cooling, employ air cooling or wind cooling, decrease the length of high-temperature residence, inhibit grain aggregation and development.

Contraindications: Slow cooling or extended holding at high temperatures is not permitted to avoid secondary recrystallisation resulting in aberrant coarsening.

4. Design of Surface and Composite Structure (Leading-Edge Direction)

Ceramic Matrix Composites (CMCs): e.g. SiC/SiC, Al2O3/ZrO2, which do not have grain growth problems, can operate at temperatures beyond 1400°C and are appropriate as nozzle liners;

Metal-Ceramic Composite Structures Use of ceramic cores or coatings in a metal matrix for improved localised high temperature stability.