During the flange shaft surface quenching process, the key to avoiding uneven hardness between the flange and the journal lies in precisely addressing the differences in heating, cooling, and microstructure responses caused by their structural differences. This requires systematic control throughout the entire process. Flanges are typically thicker and have a wider surface area, while the journal is smaller in diameter and longer. This structural difference directly leads to different rates of heat absorption, transfer, and dissipation. Without targeted process optimization, hardness variations are highly likely to occur. Therefore, a differentiated control strategy must be established starting with the heating process.
The key to the heating process is to achieve uniform austenitization conditions for the flange and the journal. For the flange area, a wraparound inductor can be used, covering the end faces and sides. This expands the heating contact area and allows heat to penetrate more evenly into the flange, avoiding incomplete austenitization due to insufficient heating in certain areas. For the journal area, a ring-shaped inductor, conforming to its circumference, ensures consistent surface heating rate and depth, preventing overheating due to excessive heat concentration. Temperature monitoring is also required to track surface temperature changes on both sides in real time. Based on the monitoring results, the sensor's power output and movement speed are fine-tuned to ensure synchronized heating of the flange and journal, laying the foundation for subsequent uniform hardness.
The cooling process must be controlled to match the different heat dissipation characteristics of the two. The flange, with its large size and high heat capacity, dissipates heat more slowly during cooling. Insufficient cooling rates can lead to incomplete microstructural transformation, which in turn affects hardness. The journal, with its small size and rapid heat dissipation, cools too quickly, potentially causing localized stress concentrations and excessively high hardness due to rapid microstructural transformation. To address this, multi-directional spray cooling can be used on the flange to ensure uniform coverage of the end and side surfaces, ensuring consistent cooling rates across the entire structure. For the journal, a combination of immersion quenching and spray cooling is employed: a short immersion quenching cycle is used to rapidly cool the surface, followed by a slow, low-pressure spray cooling cycle. This ensures that the required hardness is met while balancing cooling rates and microstructural stability to avoid exacerbating hardness variations due to cooling variations.
Pre-quenching pretreatment is crucial for reducing hardness variations. First, the flange shaft must undergo quenching and tempering to achieve a uniform sorbite structure throughout. This eliminates structural defects such as carbide accumulation and uneven grain size that may have occurred during the forging process. Left untreated, these defects can lead to local variations in hardenability during quenching, which in turn can cause hardness fluctuations. The transition area between the flange and the journal must be rolled or shot peened to eliminate residual stress from machining. This prevents stress release during heating, which can lead to localized temperature fluctuations and affect hardness development. Furthermore, the flange shaft surface must be thoroughly cleaned of impurities such as scale and oil. These impurities can form an insulating layer during heating, resulting in inadequate local heating and the potential for uneven hardness.
The uniformity of the material itself must also be carefully controlled. The composition of the steel used must be verified to ensure a uniform distribution of carbon and alloying elements to avoid variations in hardenability due to local compositional variations. Low carbon content in the flange or insufficient alloying elements in the journal can lead to different hardnesses even with the same quenching process. The steel should also be inspected for internal defects such as porosity and inclusions. These internal defects can affect heat transfer and structural transformation, leading to significant hardness differences between the defective area and the surrounding area. Therefore, flaw detection testing is necessary during the pretreatment stage to eliminate inherent material problems.
Post-quenching tempering is a key step in further stabilizing hardness. An isothermal tempering method should be employed, allowing the flange and shaft journal to be tempered at the same temperature to ensure simultaneous elimination of quenching stresses and stabilize the martensitic structure. During the tempering process, sufficient holding time should be ensured to ensure adequate heat transfer within the flange. This prevents incomplete tempering due to the flange's thickness, resulting in a hardness difference between the surface and core. It also prevents the shaft journal from experiencing excessively high or unstable hardness due to insufficient tempering time. Through integrated tempering, the hardness of the flange and shaft journal is aligned, while also improving overall mechanical properties and reducing the risk of deformation during subsequent use.
Quality inspection and dynamic process adjustments are the final line of defense for ensuring uniform hardness. After quenching, multiple inspection points are selected along the circumference of the flange end face, and uniform sampling is performed along the length of the journal surface. Hardness testing is then performed to confirm the consistency of the hardness of the two. At the same time, the microstructure of both is observed using a metallographic microscope to confirm that uniform tempered martensite is formed, with no abnormal structures such as undissolved ferrite or over-tempered structures. If local hardness deviations are found, the process needs to be promptly reviewed. If the flange hardness is low, the power output during heating can be adjusted or the heating time can be extended. If the journal hardness is high, the tempering temperature can be appropriately increased or the tempering time can be extended. Through targeted adjustments and continuous process optimization, the overall hardness of the flange shaft can be uniform.
