For motor products with special requirements or special operating conditions, their shafts should undergo quenching and tempering treatment. Quenching and tempering refers to a dual heat treatment method of quenching followed by high-temperature tempering, the purpose of which is to give the workpiece good comprehensive mechanical properties. High-temperature tempering refers to tempering between 500-650 degrees Celsius. Quenching and tempering can greatly adjust the properties and material composition of steel, resulting in better strength, plasticity, and toughness, and thus good comprehensive mechanical properties.
From the perspective of actual heat treatment processes, most processes require heat treatment of the shaft after rough machining. This is because heat treatment can cause shaft deformation and may also lead to adverse effects such as impacts. Therefore, performing finish machining after heat treatment can avoid some of the unsuitable effects caused by the heat treatment process. Some may ask, why not perform heat treatment in the raw material state? We can say with certainty that any processing company will consider processing cycle and processing costs. Heat treatment in the raw material state will increase energy costs, and after heat treatment, due to changes in material properties, the processing difficulty will be relatively greater, naturally increasing processing costs. For these reasons, choosing to perform heat treatment between rough machining and finish machining is more appropriate.
Knowledge Expansion – Quenching and Tempering of 45 Steel
45 steel is a medium-carbon structural steel with good cold and hot working properties, good mechanical properties, and is widely used due to its low price and wide availability. Its biggest weakness is its low hardenability, making it unsuitable for workpieces with large cross-sectional dimensions or high requirements.
Quenching and tempering is widely used for structural parts that require excellent comprehensive properties, especially those operating under alternating loads, such as automobile shafts and gears, and aero-engine turbine shafts and compressor discs. Structural steel parts requiring induction hardening are typically quenched and tempered before surface hardening to obtain fine and uniform sorbite, which is beneficial for the surface hardened layer and also allows the core to acquire good comprehensive mechanical properties.
Tempering nitrided parts before nitriding improves the machinability of the steel and prepares the microstructure for nitriding. To achieve a high surface finish on measuring tools before quenching, eliminate stress caused by rough machining, reduce quenching deformation, and ensure high and uniform hardness after quenching, tempering can be performed before finishing.
For tool steels that have network carbides or coarse grains after forging, quenching and tempering can be used to eliminate the carbide network and refine the grains, as well as spheroidize the carbides to improve machinability and prepare the microstructure for the final heat treatment.
Quenching is the first step in the process. The heating temperature depends on the steel's composition, while the quenching medium is selected based on the steel's hardenability and the dimensions of the workpiece. After quenching, steel exhibits high internal stress and is brittle, necessitating tempering to relieve stress, increase toughness, and adjust strength. Tempering is the most crucial step in finalizing the mechanical properties of quenched and tempered steel. The curves showing the change in mechanical properties of various steels with tempering temperature, also known as the steel's tempering curves, can serve as a basis for selecting tempering temperatures. For high-temperature tempering of certain alloy quenched and tempered steels, care must be taken to prevent the occurrence of second-type temper brittleness to ensure the steel's performance in use.
Because the purpose of tempering is to obtain comprehensive mechanical properties, the hardness range is relatively wide. However, if the drawing specifies a hardness requirement, the tempering temperature must be adjusted according to the drawing to ensure the required hardness. For example, some shaft parts require high strength, thus requiring high hardness; while some gears and keyway shaft parts, which will undergo milling and broaching after tempering, require lower hardness. Regarding the tempering holding time, it depends on the hardness requirement and the size of the workpiece. We believe that the hardness after tempering depends on the tempering temperature and is not significantly related to the tempering time, but thorough tempering is essential. Generally, the tempering holding time for workpieces is always more than one hour.
Parts subjected to quenching and tempering are mainly structural components of various machines and mechanisms, such as shafts, connecting rods, studs, and gears, and are widely used in the manufacturing industries of machine tools, automobiles, and tractors. Quenching and tempering is especially prevalent for large components in heavy machinery manufacturing. Therefore, quenching and tempering plays a very important role in heat treatment. The required performance of quenched and tempered parts in mechanical products varies depending on their stress conditions. Generally speaking, all quenched and tempered parts should possess excellent comprehensive mechanical properties, namely a proper combination of high strength and high toughness, to ensure the long-term smooth operation of the parts.
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