High-strength fastener heat treatment technology upgrade
Heat treatment plays a vital role in the production of high-strength fasteners, especially in ensuring their internal quality. To manufacture high-quality fasteners, advanced heat treatment technologies and equipment are essential.
1. Heat Treatment Status of High-Strength Fasteners at Dongfeng Motor
Dongfeng Motor produces high-strength fasteners with four performance grades: 8.8, 9.8, 10.9, and 12.9. These fasteners undergo conditioning to meet specific mechanical properties, such as tensile strength and yield ratio. The quenching and tempering process is critical in achieving these properties, and it requires strict control over raw materials, furnace temperature, atmosphere, quenching medium, and quality inspection.
1.1 Materials
The main materials used for high-strength fasteners at Dongfeng Motor include ML35, 35K, ML40Cr, and ML35CrMo steel, with 75% of ML35CrMo being imported. Common material issues include segregation, decarburization, and cracking. Before cold forming, the materials need to be spheroidized and annealed in an oil furnace, which lacks a protective atmosphere and often leads to decarburization.
1.2 Heat Treatment Furnaces
High-strength fasteners of grade 12.9 are tempered on a cast furnace line, where computer-controlled temperature and atmosphere (using Rx gas) help manage oxidation and decarburization effectively, resulting in better quality. However, 8.8-grade fasteners are processed in a vibration hearth furnace, which has poor sealing, inconsistent temperature distribution, and limited atmosphere control. This leads to issues like decarburization, uneven heating, and surface damage due to part collisions.
1.3 Quenching Mediums
Dongfeng Motor uses two types of quenching media: alkaline water for ML35 and ordinary mechanical oil for medium carbon alloy steels. Alkaline water can cause quench cracking, while mechanical oil may result in uneven hardness or non-compliance, requiring multiple tempering cycles in production.
1.4 Quality Inspection
After heat treatment, key tests include hardness, tensile strength, metallographic structure, and RX gas analysis. For 12.9-grade fasteners, 100% magnetic particle inspection is conducted to detect cracks. However, some critical aspects such as cooling performance of the quenching medium, quantitative metallography, internal defects, and hardness grading are not fully addressed.
2. Heat Treatment Technology in Foreign Countries
2.1 Materials
In Japan, Kobe Steel has advanced spheroidizing annealing technology, providing pre-spheroidized steel that eliminates the need for re-annealing and avoids decarburization. In Taiwan and Russia, induction heating is used before spheroidizing in a protective atmosphere, which improves material quality. In Western countries, electric heating hood furnaces with controlled atmospheres are widely used, ensuring reliable spheroidizing without decarburization.
2.2 Heat Treatment Furnaces
Mesh belt or cast chain heat treatment lines are commonly used abroad. These systems feature excellent sealing, advanced atmosphere control, and computerized temperature regulation. They also include fault alarms and automatic operations from feeding to tempering, ensuring consistent and high-quality heat treatment outcomes.
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