As critical engine components subject to complex alternating loads, optimizing the toughness of u-bolts requires systematic improvements throughout the entire heat treatment process. The quenching process is crucial for determining the microstructure of u-bolts. Traditional quenching media, such as standard quenching oil, cool too quickly in the high-temperature zone, easily leading to coarsening of the martensite structure and the formation of a brittle phase. Modern processes employ step-quenching technology. By controlling the quenching medium temperature gradient, the workpiece is slowly cooled between 650°C and 400°C, promoting the formation of a composite structure of fine, acicular martensite and retained austenite. This structure, through the retained austenite film between the martensite laminae, effectively blocks crack propagation paths, significantly improving fracture toughness.
Coordinated control of temperature and time during the tempering process plays a crucial role in optimizing toughness. Conventional tempering processes are prone to temper brittleness in the 300-400°C range. The new process employs a two-stage tempering process: the first stage, low-temperature tempering at 250°C, eliminates quenching stresses while retaining high-density dislocations; the second stage, high-temperature tempering at 580-620°C, promotes spheroidal precipitation of carbides and forms a tempered bainite structure. This gradient tempering process increases the impact energy absorption of U-bolts by over 40% while maintaining a yield strength of at least 900 MPa. Localized induction tempering, particularly at the bend of the bolt, eliminates the work-hardened layer caused by cold bending and restores the material's plastic reserve.
Pretreatment prior to heat treatment has a fundamental impact on the final toughness performance. For common materials such as 40Cr and 35CrMo, spheroidizing annealing achieves uniformly distributed carbides in a granular form, maintaining a grain size of 8-9 in the annealed structure. This pretreatment promotes uniform austenitization during subsequent quenching, preventing toughness loss caused by coarse grains. For high-strength u-bolts, an isothermal spheroidizing annealing step—holding at 760°C followed by isothermal cooling to 680°C—can produce a finer spheroidized structure, providing an ideal starting point for subsequent heat treatment.
The choice of cooling medium directly influences the thermal stress distribution during quenching. A new polymer quenchant allows for continuous adjustment of the cooling rate by adjusting its concentration, automatically reducing the cooling rate below the Ms point, effectively reducing the temperature gradient between the workpiece surface and core. U-bolts treated with this medium experience over 60% lower residual stress levels compared to traditional oil quenching and a 25% increase in bending fatigue strength. For large u-bolts, a water-air alternating quenching process is employed. By controlling the ratio of water cooling time to air cooling time, a dynamic balance between microstructural transformation and stress release is achieved.
Upgrades to heat treatment equipment provide hardware support for process optimization. The use of a vacuum high-pressure gas quenching furnace achieves simultaneous cooling of the workpiece surface and core by precisely controlling the quenching pressure (2-6 bar) and gas flow rate (0.5-2 m/s). U-bolts treated in this equipment achieve a microstructure uniformity three levels higher than that achieved in conventional furnaces, with hardness fluctuations within ±1.5 HRC. For mass production, the continuous mesh belt furnace uses computer-controlled atmosphere, temperature, and conveying speed to ensure that each workpiece undergoes a consistent heat treatment process, significantly improving product consistency.
A multi-dimensional testing system is required for process validation. In addition to conventional tensile and impact testing, metallographic analysis, scanning electron microscopy (SEM) fracture observation, and X-ray residual stress measurement should be added. By establishing a microstructure-property-stress correlation model, toughness performance under varying process parameters can be accurately predicted. For U-bolts used in critical applications, rotating bending fatigue testing is also required to verify their fatigue limit under 10⁷ cycles of loading, ensuring that the heat treatment process meets the requirements of actual engine operating conditions.
