As a connecting component, the comprehensive mechanical properties of double-head screws directly affect the stability and safety of equipment operation. Especially under conditions of alternating loads, vibration, impact, or complex environments, their strength, toughness, wear resistance, and corrosion resistance need to be optimized through heat treatment processes. Currently, a multi-technology synergistic system has been formed for the heat treatment of double-head screws, with quenching and tempering as the core, combined with surface hardening, localized treatment, and chemical heat treatment, which can significantly improve their overall performance.
Quenching and tempering is the core process of double-head screw heat treatment, achieving performance optimization through a combination of quenching and high-temperature tempering. During the quenching stage, the screw needs to be heated to the austenitizing temperature (typically 820-860℃ for medium carbon steel, and slightly lower to 800-840℃ for alloy steel), causing the internal structure of the material to transform into uniform and fine austenite. Subsequently, rapid cooling (such as water cooling or oil cooling) forms a martensitic structure, significantly improving hardness and strength. However, the martensitic structure after quenching is brittle and requires high-temperature tempering (200-400℃) to eliminate internal stress and adjust the structure to tempered sorbite or tempered troostite, improving toughness and plasticity while retaining high hardness. For example, after tempering, the tensile strength of a double-head screw in an automobile engine increased by 30%, and the impact toughness increased by 50%, effectively resisting vibration fatigue fracture.
Surface hardening treatment addresses the contact wear problem of double-head screws by strengthening surface properties through techniques such as carburizing, nitriding, or induction hardening. Carburizing involves heating the screw in a carbon-containing medium to increase the surface carbon concentration, forming a high-hardness carbide layer after quenching, suitable for scenarios subject to high contact stress. Nitriding involves heating in a nitrogen-containing atmosphere to generate a nitride film (such as Fe3N, Fe4N) on the surface, achieving a hardness of over 1000 HV, while also possessing excellent wear resistance and anti-galling properties, commonly used for surface strengthening of double-head screws in precision instruments. Induction hardening utilizes high-frequency current to generate eddy currents on the screw surface, achieving rapid localized heating and hardening. It allows for precise control of the hardened layer depth (typically 1-3mm) and is suitable for localized strengthening of long screws or stepped shaft-type double-head screws.
Local heat treatment processes, tailored to the structural characteristics of double-head screws, balance performance and cost through differentiated treatment. For screws with a large aspect ratio or significant abrupt changes in cross-section, overall heat treatment can easily lead to deformation or cracking. In such cases, localized quenching and tempering can be used, strengthening only critical stress-bearing areas (such as the threaded section or head). For example, a wind turbine double-head screw, through localized induction hardening, achieved a threaded section hardness of HRC45-50, while the shank maintained a toughness of HRC28-32, meeting wear resistance requirements while avoiding overall embrittlement. Furthermore, for irregularly shaped double-head screws, laser hardening can achieve uniform hardening of complex surfaces, avoiding the edge effects of traditional processes.
Chemical heat treatment, by altering the chemical composition and microstructure of the screw surface, imparts special properties. Besides carburizing and nitriding, carbonitriding can simultaneously introduce carbon and nitrogen elements to form a composite hardened layer, possessing both high hardness and corrosion resistance. Sulfur-nitrogen co-diffusion, through the addition of sulfur, further enhances surface friction reduction properties, making it suitable for frequently assembled and disassembled double-head screws. For example, after sulfur-nitrogen co-diffusion treatment, the surface friction coefficient of double-head screws in a certain chemical equipment decreased by 40%, and their wear life was extended by more than 2 times.
Optimization of heat treatment processes needs to be considered in conjunction with the material properties and service conditions of the double-head screws. Medium carbon steel screws are typically treated with quenching and tempering, while alloy steel screws can achieve a precise match between strength and toughness by adjusting the quenching temperature and tempering cycles. Stainless steel screws require control of heating temperature and cooling rate to avoid intergranular corrosion or σ-phase precipitation. Furthermore, post-heat treatment quality inspection (such as hardness testing and metallographic observation) and process parameter monitoring (such as temperature uniformity and cooling rate) are crucial for ensuring stable performance.
The heat treatment process for double head screws needs to be tailored to their materials, structure, and application scenarios. It should comprehensively utilize technologies such as quenching and tempering, surface hardening, localized treatment, and chemical heat treatment. By optimizing the microstructure and property distribution, it can achieve a synergistic improvement in strength, toughness, wear resistance, and corrosion resistance, thus providing a reliable guarantee for the safe operation of the equipment.
