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As is well known, there are usually four manufacturing processes for stainless steel screws: forming, grinding, cleaning, and heat treatment. So, the first three processes of the screw have already been processed, why do we need a heat treatment process? Is it a stainless steel screw or a high-strength carbon steel bolt? In fact, after the first three steps, there is still some residual stress inside the screw, and the thread tends to extend inward. This can be improved by quenching and tempering during heat treatment; In addition, stainless steel screws are made of low-carbon steel and undergo carburizing treatment. The main purpose of heat treatment is to increase the carbon content on the surface, thereby improving the strength, torsional resistance, tensile resistance, wear resistance and other properties of the surface, that is, to extend the service life of stainless steel screws.
Generally speaking, heat treatment is divided into preliminary heat treatment and final heat treatment. The purpose of screw pre-treatment heat treatment is to improve processing performance, eliminate internal stress, and prepare for the final heat treatment. Heat treatment processes include annealing, normalizing, aging, quenching, and tempering.
1. Annealing and normalizing of stainless steel screws: used for hot working blanks, carbon steel and alloy steel with a carbon content greater than 0.5%, to reduce hardness and facilitate cutting; For carbon steel and alloy steel with a carbon content less than 0.5%, normalizing treatment is used to avoid sticking to the knife when the hardness is too low. Annealing and annealing can refine grain size and achieve uniform structure, preparing for subsequent heat treatment. Annealing and normalizing are usually arranged after blank manufacturing and before rough machining.
2. Screw aging treatment: mainly used to eliminate internal stress generated during rough manufacturing and processing. For parts with general precision, an aging treatment can be arranged before precision machining. For parts with high precision requirements, arrange two or more aging treatments, and simple parts cannot undergo aging treatment. For some precision parts with poor stiffness, in order to eliminate the internal stress generated during machining and stabilize the machining accuracy of the parts, multiple aging treatments are carried out between rough machining and semi precision machining. The shaft components of fasteners are subjected to aging treatment after straightening.
3. Quenching and tempering of stainless steel screws: This process involves high-temperature tempering after quenching. Uniform and fine tempered martensite structure can be obtained, which prepares for surface quenching and nitriding treatment in the next process to reduce deformation. Therefore, quenching and tempering can also be used as preheating treatments. After quenching and tempering, the comprehensive mechanical properties of fasteners are good, and products with lower requirements for hardness and wear resistance can be used as the final heat treatment process.
4. Final heat treatment: The final heat treatment of stainless steel screws is to improve hardness, wear resistance, strength, and related mechanical properties.
5. Quenching: divided into surface quenching and integral quenching. Differently, surface quenching is widely used due to its low deformation, oxidation, and decarburization. It has the advantages of high external strength, good wear resistance, good internal toughness, and strong impact resistance. In order to improve the mechanical properties of surface quenching, preheating treatment must be carried out through heat treatment such as quenching tempering or normalizing
The process is: cutting, forging, normalizing (annealing) - rough machining, quenching and tempering, semi precision machining, surface quenching and precision machining.
6. Carbonization and quenching: Suitable for low carbon steel and low alloy steel, it can increase the carbon content on the surface of the parts, improve the hardness after quenching, while maintaining a certain strength, high toughness, and plasticity in the core. Carbonization is divided into integral carburizing and localized carburizing. When partially carburizing, anti-seepage measures should be taken for non carburizing parts. Generally, copper plating or processed anti-seepage materials are used. The deformation of carburizing and quenching is large, and the carburizing depth is generally between 0.5-2mm. This process is arranged between semi precision machining and precision machining.
The process is: cutting, forging, normalizing, rough machining, semi precision machining, carburizing, quenching, and precision machining
When using the process of increasing the allowance and cutting off the excess carburized layer for partially carburized but non carburized parts, cutting should be carried out before quenching after carburization.
7. Nitriding treatment: It is the process of allowing nitrogen atoms to infiltrate the metal surface, resulting in a layer of nitrogen-containing compounds. Nitriding layer can improve the surface hardness, wear resistance, fatigue strength, and corrosion resistance of parts. Due to the low nitriding temperature, small deformation, and thin nitriding layer (generally less than 0.7mm-41), the nitriding process should be arranged as later as possible. In order to reduce nitriding deformation, stress relief and high-temperature tempering are generally required after cutting.
In summary, carbon steel, high-strength screws, high-strength nuts, and high-strength washers are more suitable for heat treatment.
