Steel quenching process

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The quenching of steel is a heat treatment process in which the steel is first austenitized and then cooled at an appropriate rate, causing the workpiece to undergo a transformation to a martensitic, unstable microstructural structure throughout its cross-section or within a certain area. I. Process flow for quenched workpieces: For ordinary workpieces: quenching → cleaning → tempering → surface treatment such as sandblasting (or shot blasting) → inspection. Shaft parts and easily deformed workpieces: quenching → cleaning → tempering → straightening → stress relief treatment → sandblasting → inspection. II. Preparations before quenching: (1) Verify the quantity, material, and dimensions of the workpieces, and check for any defects that could affect the quality of quenching, such as cracks, dents, missing edges, sharp edges, corners, or rust. (2) Based on the drawings and process documents, specify the specific requirements for quenching, such as hardness and the range of local quenching. (3) According to the quenching requirements, select appropriate tools and fixtures; for some workpieces, apply proper binding, and take corresponding protective measures in areas prone to cracking, such as wrapping them with iron sheet or asbestos rope and plugging holes. (4) For workpieces whose surfaces must not be oxidized or decarburized, they should be heated in a salt bath furnace or a furnace with a pre-evacuated protective atmosphere, or the following protective measures should be taken: a. Coating protection; the following coating formulations are recommended: ① 10% graphite + 90% grease (by mass). ② 100g SiO2 + 5g Al2O3 + 25g NaSiO3 + 40g H2O. The thermal coating thickness is 0.05–0.10 mm; it provides oxidation and decarburization protection when the heating temperature is below 1050°C. ③ 20g SiO2 + 10g Al2O3 + 10g feldspar + 10g Cr2O3 + 10g SiC + 8g KSiO3 + 12–15g H2O; a thermal coating of 0.2–0.30 mm thickness, which provides oxidation and decarburization protection at heating temperatures below 1200°C. b. Place the workpiece in an iron box filled with charcoal or used cast iron shavings, then seal it with a lid. (5) A large number of workpieces must undergo trial quenching on a single-piece or small-batch basis; only after a suitable process has been developed can batch quenching be carried out, with regular spot checks conducted during production. III. Furnace Loading (1) Workpieces of different materials but with the same heating process may be loaded into the same furnace for heating. (2) The workpieces to be loaded into the furnace must be dry, free of oil and other contaminants. (3) When workpieces of different cross-sectional sizes are loaded into the same furnace, the larger ones should be placed at the back of the furnace, and the heat retention time should be calculated separately for large and small workpieces. (4) When loading the furnace, the workpieces must be arranged systematically on the loading rack or furnace bottom plate, and stacked using hooks, pliers, or specialized tools; the workpieces must not be thrown directly into the furnace to avoid damaging them or the furnace lining. (5) Long and slender workpieces must be heated by being vertically hung in a shaft furnace or salt furnace to reduce deformation. (6) When heating workpieces in a box furnace, they are generally arranged in a single layer, with a gap of 10~30 mm between the workpieces. Small items can be stacked appropriately, but the insulation time should be increased accordingly. IV. Heating 1. Heating method (1) For carbon steel and alloy steel workpieces, they can generally be directly placed in a furnace heated to a temperature 20~30°C higher than the specified quenching temperature. (2) High-carbon, high-alloy steels and workpieces with complex shapes should be preheated first. 2. Selection of heating temperature. 3. Calculation of the workpiece heating time: The workpiece in the furnace should be maintained at the specified heating temperature for an appropriate period of time to ensure the necessary structural transformations and diffusion. Heating time refers to the total duration of the heating process, from when the workpiece is placed in the furnace and heating begins until it is removed from the furnace. The heating time is related to factors such as the effective thickness of the workpiece, the type of steel, the method of loading into the furnace, the amount loaded, the loading temperature, the performance of the furnace, and its degree of sealing. 4. Selection of effective thickness (applicable to both annealing and normalizing processes) (1) For round bar-shaped workpieces, it is calculated based on the diameter. (2) Flat workpieces are calculated based on thickness (the upper limit of the thermal insulation coefficient is used). (3) The solid cone is calculated based on the diameter at 1/3 of the height from the larger end.