Our company recently signed a significant contract to supply 35CrMo steel spindles for a client with extremely high performance standards. Despite multiple rounds of quenching and tempering based on previous heat treatment experiences, the required mechanical properties were still not met. Through practical production trials, we challenged conventional thinking, made bold improvements to the heat treatment process, and introduced a direct water quenching method that successfully met all the customer's specifications.
**1. Spindle Manufacturing Process and Technical Requirements**
The manufacturing process for the spindle includes: steelmaking → forging → rough machining → heat treatment (quenching and tempering) → semi-finishing. The main shaft is 5,585 mm long, and its chemical composition requirements are detailed in Table 1, while the mechanical properties are listed in Table 2.
**2. Original Heat Treatment Process and Result Analysis**
(1) **Original Heat Treatment Process**
Based on prior experience, the 35CrMo steel shaft forgings typically undergo a water quench followed by oil cooling. The main parameters of the original heat treatment process are shown in Table 3.
After following this process, the mechanical properties of the forgings, as presented in Table 4, did not meet the required technical specifications.
(2) **Analysis of Non-Conforming Chemical Composition**
Chemical analysis of the material, as shown in Table 5, revealed segregation in the bottom portion of the forging. This led to poor hardenability during quenching, resulting in the formation of free ferrite and pearlite in the core, which reduced the mechanical properties. For 35CrMo steel, it's crucial to consider the segregation when designing the heat treatment process, ensuring that the positive segregation zones are not over-quenched, while the negative segregation areas receive sufficient quenching intensity.
**3. Process Improvement**
To achieve the desired mechanical properties, we changed the quenching mode from the traditional water-air-water intermittent cooling to a more efficient approach. Typically, the decision to use water quenching depends on the carbon equivalent (CE) and carbon content of the workpiece.
If the CE of the positive segregation zone is ≤0.75% and the carbon content (wC) is ≤0.31%, the forging can be safely water-cooled. If wC ranges from 0.32% to 0.36%, water cooling is still acceptable. In this case, the main shaft has a CE of 0.518% and a wC of 0.37% in the positive segregation zone, making it suitable for water quenching. Additionally, the simple geometry of the workpiece further supports this approach.
Given the higher carbon content in the positive segregation zone, water quenching was chosen to minimize thermal stress and prevent cracking. During the cooling process, we used a water-air-water intermittent cooling strategy. The air cooling phase was slowed down to ensure the workpiece temperature does not exceed 500°C, reducing internal temperature differences and allowing residual heat to temper the surface layer, thereby improving its toughness and ductility.
In the early stage, the high cooling intensity of water was fully utilized, but the cooling rate was gradually reduced in the later stages. The final temperature of the forging was strictly controlled to prevent cracking. The improved heat treatment parameters are summarized in Table 6.
Table 7 shows the final mechanical properties of the spindle after applying the improved heat treatment process. All performance metrics now meet the technical requirements, laying a solid foundation for future heat treatment of similar products.
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