Our company has recently signed a significant contract to supply 35CrMo steel spindles to a major client, with extremely high performance requirements. Despite multiple rounds of quenching and tempering based on previous heat treatment experiences, the required mechanical properties were still not met. Through practical production, 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 route for the spindle includes: steelmaking → forging → rough machining → quenching and heat treatment → semi-finishing. The main shaft is 5585mm long, and its chemical composition and mechanical properties are detailed in Table 1 and Table 2, respectively.
[Image: Chemical composition of the main shaft]
2. **Original Heat Treatment Process and Result Analysis**
(1) **Original Heat Treatment Process**
Based on prior experience, the forging deformation in the sampled area was significant, with shallow depth. Therefore, a water-quenching followed by oil cooling process was typically used. The key parameters of the original heat treatment process are listed in Table 3.
[Image: Original heat treatment parameters]
After applying this process, the mechanical properties of the forged parts did not meet the required standards, as shown in Table 4.
[Image: Mechanical properties after original process]
(2) **Analysis of Non-Conforming Results**
Chemical composition analysis revealed segregation at the bottom 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 essential to consider compositional segregation when designing the heat treatment process, ensuring that the positive segregation zones are not over-quenched while maintaining sufficient intensity for the negative segregation areas.
[Image: Chemical composition segregation]
3. **Process Improvement**
To meet the mechanical property requirements, we changed the quenching mode from intermittent water-air-water cooling to direct water quenching. The decision to use water quenching is typically based on the carbon equivalent (CE) and carbon content (wC) of the workpiece.
When CE ≤ 0.75% and wC ≤ 0.31% in the positive segregation zone, water quenching is safe. If wC ranges between 0.32% and 0.36%, it can also be water quenched. In this case, the main shaft has a CE of 0.518% and a wC of 0.37% in the positive segregation zone. Given its simple shape, water quenching is feasible.
Considering the high carbon content in the positive segregation zone, we opted for water quenching to reduce thermal stress and prevent cracking. We adopted a water-air-water intermittent cooling approach, with slow air cooling to ensure the workpiece temperature does not exceed 500°C. This helps reduce temperature differences across different sections and utilizes residual heat for tempering, improving surface plasticity and toughness. In the early stage, the high cooling intensity of water is maximized, while in the later stages, the cooling is slowed down, and the final temperature of the forging is strictly controlled to prevent cracking.
The improved heat treatment parameters are summarized in Table 6.
[Image: Improved heat treatment parameters]
Table 7 shows the final mechanical properties of the spindle after applying the new process. All performance indicators meet the technical requirements, laying a solid foundation for future heat treatment of similar products.
[Image: Final mechanical properties]
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