What changes does the special heat treatment process bring to the microstructure of the brake spring?

The special heat treatment process profoundly reshapes the microscopic morphology of the brake spring through multi-stage phase transformation and reorganization. In the quenching process, the high-temperature austenite undergoes a shear transformation under severe cooling conditions, forming a lath martensite network with dense dislocation entanglement, and the dispersed residual austenite fills the lath gaps in the form of a thin film. This structure not only retains high strength but also improves deformation coordination ability. After the introduction of the graded isothermal process, some areas undergo a diffusion transformation, generating lower bainite layers with alternating carbides and ferrites. Its fine carbide array effectively blocks dislocation movement. During the tempering process, the martensite matrix undergoes decomposition and reorganization, precipitating a nano-scale ε carbide strengthening phase, while the residual austenite is partially transformed into secondary martensite, forming a three-dimensional interconnected structure composed of tempered martensite, stable austenite and carbides.

The surface treatment process constructs a gradient nanocrystalline structure on the surface of the material, and the 50-nanometer ultrafine grains on the surface transition to submicron grains in the interior. This gradient organization significantly improves the ability to resist crack propagation. The residual compressive stress layer produced by shot peening can reach a depth of 300 microns. The high-density dislocation network formed by the surface lattice distortion works synergistically with the fine precipitation phase inside to transfer the stress concentration point from the surface to the subsurface. The grain boundary segregation phenomenon caused by the migration of alloy elements is particularly obvious during high-temperature treatment. The enrichment of elements such as chromium and molybdenum at the grain boundaries forms a corrosion-resistant barrier, and the solid solution strengthening effect of silicon inhibits the coarsening of carbides. This multi-scale composite structure enables the material to maintain a strength of 2000MPa while increasing the fracture toughness by about 40%, and extending the fatigue life by two orders of magnitude.