When the ship lift at Xiangjiang Three Gorges is overloaded, the safety load is transferred to the key components of the lift through the nut column. The overall forging process, production dimensions, and weight of the equipment far exceed basic products, and the technical requirements are high. Therefore, controlling the heat treatment process is challenging. To achieve this goal, based on process research and trial production, combined with the company's actual manufacturing process and equipment, a heat treatment process specification for the nut column is formulated. Companies work together to continuously improve process parameters, achieving mass production and jointly completing the Three Gorges ship lift project.
I. Introduction of Workpiece and Issues to Address
Material: G35CrNiMo6-6+QT1
Dimensions: 4980mm × 2112mm × 589mm
Technical Standards: σs ≥ 650MPa, σb ≥ 800MPa
1. Due to the large size and unique shape of the Three Gorges nut column, special molds are required to complete the heating and cooling process.
2. Since the raw material for the nut column is a newly developed material, a heat treatment plan must be formulated according to its technical standards, and process parameters must be standardized based on testing.
3. Due to the unique shape of the nut column, significant deformation can occur during the heating and cooling process. Adjusting the deformation using molds and processes within the process capacity range is a critical issue to be addressed. Deformation should be controlled within ≤15mm.
II. Analysis of Heat Treatment Process
To address deformation issues and overall quality management from the aspects of material, dimensions, and heat treatment technical standards of the nut column, the specific process analysis is as follows:
1. Since the product is a stainless steel casting, the part structure is uneven and prone to component shrinkage. To achieve comprehensive mechanical properties after tempering, a quenching process is performed before heat treatment to further improve the coarse structure of the casting, optimize it, and eliminate the crystallization imbalance caused by different cross-sectional dimensions.
2. The part's dimensions are large (4980mm × 2112mm × 589mm). Due to its unique shape and different main working surfaces, processing stress is significant, and deformation is easily caused during heating and cooling. Therefore, the residual amount before heat treatment must be controlled.
3. The part can be placed horizontally or laterally to ensure the heating and cooling effects during heat treatment:
1) Horizontal placement
2) Lateral placement
4. To meet the final quality requirements of the part, the hardness value after heat treatment is appropriately increased, and longitudinal and transverse inspections are conducted to achieve uniform hardness.
5. To control deformation of the final processed part, stress relief is performed after heat treatment to eliminate processing stress.
III. Heat Treatment Process Measures
1. Before heat treatment, provide part drawings, material lists, ultrasonic testing reports, cold drawing processes, furnace concrete test blocks, and verify markings.
2. Measure the residual amount of processing before heat treatment: leave 15mm on each end face.
3. Surface requirements: Each endpoint and thread tip should have a fillet radius ≥ R5mm, the root fillet radius of the path ≥ R10mm. The surface of the part should be free of cracks, casting defects, or visible defects, and magnetic particle inspection should be qualified to reduce stress concentration.
4. Provide the same furnace test block according to standard requirements, with two test blocks accompanying each part for treatment.
5. Part placement: Use horizontal placement during normalizing with the threaded tip facing up, tempering with the threaded tip facing down, supported by 2-5 points of pad iron; use vertical placement during quenching.
6. To improve deformation and stress concentration during heat treatment and ensure the overall mechanical properties of the part, control furnace temperature, heating temperature, holding time, cooling time (workpiece oil temperature 150-200℃), quenching oil temperature (40-80℃).
7. The stress relief temperature should be 20-30℃ lower than the tempering temperature to ensure sufficient release of processing stress.
IV. Solutions for Nut Column Deformation
Measure production deformation in each process according to process conditions and adjust horizontal or vertical placement.
1. For nut columns without grooves on the back, use horizontal placement into the furnace, and perform hot correction based on deformation in the next process to control deformation within the residual range.
2. For nut columns with strip grooves on the back, use horizontal placement for normalizing and vertical placement for tempering. In case of significant deformation, use horizontal placement for tempering to correct deformation during the process. For minor deformation, use vertical placement directly for tempering.
V. Summary
By on-site tracking and measurement, adjusting the combination of horizontal and vertical placement, the deformation of the nut column is controlled within a manageable range. While ensuring quality, cost-saving and time reduction are achieved. Heat treatment experiments were conducted on dozens of nut columns, all of which were successful. The post-treatment qualification rate of the nut columns was 100%, meeting technical requirements and demonstrating good hardness uniformity. This shows that the process parameters of the nut column fully meet its technical requirements.
