After heat treatment, the balance between surface hardness and residual stress in processing parts directly impacts their performance and reliability. This process requires a comprehensive integration of material properties, heat treatment processes, and subsequent processing methods, achieving a synergistic optimization of hardness enhancement and stress relief through multi-dimensional control to ensure part stability under complex operating conditions.
Quenching is a key step in improving the surface hardness of processed parts. Rapid cooling creates a high-hardness martensite structure on the material surface, significantly enhancing wear resistance and fatigue resistance. However, differences in cooling rates between the surface and the interior during quenching can lead to temperature gradients, which in turn induce residual stresses. If these stresses are not effectively controlled, they can cause part deformation, cracking, or dimensional deviations, particularly for precision parts with complex shapes and uneven wall thicknesses. Therefore, optimizing the quenching process requires a balance between cooling rate and uniformity. For example, step-quenching or austempering can be used to control the temperature and flow rate of the coolant to minimize the impact of temperature gradients on residual stresses.
Tempering is a critical step in balancing hardness and residual stress. After quenching, parts retain high residual tensile stresses. Tempering can reduce these stresses and stabilize the microstructure. The choice of tempering temperature depends on the material's properties and application requirements. Low-temperature tempering (150-250°C) retains high hardness and is suitable for precision parts requiring high wear resistance. Medium-temperature tempering (350-500°C) reduces hardness while improving toughness and is suitable for parts subject to impact loads. High-temperature tempering (500-650°C) fully relieves stress but may result in a significant decrease in hardness. Therefore, the tempering process requires experimental determination of optimal temperature and time parameters to ensure that hardness requirements are met while residual stresses are kept within a safe range.
Surface hardening techniques can further enhance the surface properties of processed parts. Shot peening involves the impact of high-velocity projectiles on the surface, causing plastic deformation and forming a residual compressive stress layer. This compressive stress partially offsets tensile stress, reducing the risk of cracking while also increasing surface hardness and fatigue strength. Rolling hardening uses rollers to apply pressure to the surface, causing the metal to undergo cold work hardening, forming a hard layer and compressive stress. Both methods are suitable for precision parts, but process parameters must be selected based on part shape and precision requirements. For example, excessive shot peening intensity may increase surface roughness, while excessive rolling pressure may cause dimensional deviations. Parameter optimization requires process testing.
The impact of machining methods on residual stress requires special attention. During precision machining, cutting parameters such as cutting speed, feed rate, and depth of cut alter the surface stress state. High cutting speeds may cause tensile surface stress due to thermal effects, while low cutting speeds may cause compressive stress due to cold work hardening. Therefore, cutting parameters must be adjusted to control stress levels while maintaining surface quality. Furthermore, using small cuts and multiple passes can reduce stress concentration caused by individual cuts and improve process stability. For high-precision parts, ultra-precision cutting or grinding can be combined to further reduce surface stress.
Residual stress testing and adjustment are crucial for ensuring a balanced balance. Non-destructive testing techniques such as X-ray diffraction and the blind hole method can quantitatively assess the distribution of residual stress on the surface of processed parts. If excessive stress is detected during testing, adjustments can be made using methods such as local heating, vibration aging, or mechanical stretching. For example, vibration aging effectively relieves stress by stimulating resonance in the part and realigning the internal grains. Local heating adjusts stress distribution through the principle of thermal expansion and contraction. These methods must be selected based on the structural characteristics of the part to avoid secondary effects on precision.
Material selection and pretreatment are the foundation of balance control. Different materials respond significantly to heat treatment. For example, high-carbon steel exhibits high hardness but high residual stress after quenching, while alloy steel can improve hardenability and stress distribution through the addition of elements. Therefore, material selection for processing parts requires a comprehensive consideration of both performance and process feasibility. Furthermore, pretreatments such as forging and normalizing can refine grains, homogenize the structure, and reduce stress concentration during heat treatment, laying the foundation for subsequent balance control.
Balancing the surface hardness and residual stress of processing parts after heat treatment requires a comprehensive approach throughout the entire process: process design, parameter optimization, strengthening treatment, machining, stress testing, and material selection. Through coordinated control of these multiple links, an optimal match between hardness and stress can be achieved, ensuring stable performance under complex operating conditions and meeting the requirements of high precision and long life.
