During the manufacturing process of iron ring cylindrical spacers, the distribution of internal stress directly affects their dimensional stability, mechanical properties, and service life. Especially under complex loads or high-temperature, high-pressure conditions, uncontrolled internal stress can lead to gasket deformation, cracking, or even seal failure. Heat treatment, as the core means of controlling internal stress, requires precise control of heating, cooling, and phase transformation processes to achieve a reasonable distribution and balance of internal stress. The following analysis focuses on the principles of heat treatment, process selection, and key control points.
Internal stress primarily originates from uneven deformation within the material, including thermal stress and structural stress. Thermal stress is caused by temperature gradients, such as the difference in cooling rates between the gasket surface and core, leading to shrinkage differences. Structural stress is related to volume changes during phase transformation, such as the volume expansion when austenite transforms into martensite. Due to its symmetrical shape, the iron ring cylindrical spacer exhibits a relatively uniform distribution of thermal stress; however, differences in wall thickness or localized work hardening can exacerbate stress concentration. The goal of heat treatment is to achieve a more uniform distribution of internal stress through phase transformation control and stress release, while simultaneously preventing residual stress from exceeding the material's yield strength.
Annealing is a fundamental method for eliminating internal stress, especially suitable for eliminating work hardening or residual welding stress. For iron ring cylindrical spacers, low-temperature annealing or stress-relief annealing can be used. The spacer is heated to a temperature below Ac1 (typically 500-650℃), held at that temperature for a certain time, and then slowly cooled. During this process, atomic diffusion is enhanced, dislocations rearrange, and internal stress is gradually released through microscopic plastic deformation. Care must be taken to control the heating and cooling rates to avoid generating new stress due to thermal shock. For precision spacers, dimensional re-inspection is necessary after annealing to ensure that the deformation is within acceptable limits.
Quenching obtains a high-hardness martensitic structure through rapid cooling, but it is prone to cracking or deformation due to phase transformation stress. For iron ring cylindrical spacers, to improve surface hardness, staged quenching or isothermal quenching can be used: Staged quenching first heats the spacer to the austenitizing temperature, then quenches it in a medium-temperature salt bath (e.g., 250-350℃) and holds it at that temperature to reduce the temperature gradient between the surface and the core, before air cooling to room temperature; Isothermal quenching involves quenching the spacer in the bainite transformation temperature range (e.g., 280-350℃) and holding it at that temperature to obtain a lower bainite structure, which combines high strength and toughness. Both processes can significantly reduce quenching stress and decrease the risk of cracking.
Tempering is a crucial step in eliminating quenching stress and adjusting the microstructure and properties. Iron ring cylindrical spacers must be tempered immediately after quenching, with low-temperature, medium-temperature, or high-temperature tempering selected based on performance requirements. Low-temperature tempering (150-250℃) is mainly used to reduce brittleness while retaining high hardness; medium-temperature tempering (350-500℃) can achieve high elastic limit and yield strength; high-temperature tempering (500-650℃) transforms the microstructure into tempered sorbite, balancing strength and toughness. During tempering, retained austenite decomposes into martensite or carbides, and internal stress is further released through phase transformation and atomic diffusion. Strict control of tempering time and temperature uniformity is necessary to avoid localized overheating or underheating.
For complex structures or high-precision iron ring cylindrical spacers, vibration aging or cryogenic treatment can be used to assist in controlling internal stress. Vibration aging uses mechanical vibration to superimpose internal residual stress and applied dynamic stress, causing microscopic plastic deformation when the yield strength is reached, thereby releasing stress. Cryogenic treatment cools the spacer to -196℃ (liquid nitrogen temperature), further transforming retained austenite into martensite, while shrinkage stress and microstructure stress cancel each other out. These two processes can be combined with heat treatment to significantly improve the dimensional stability of the gasket.
Controlling the internal stress distribution of the iron ring cylindrical spacer through heat treatment must be integrated throughout the entire manufacturing process. From pre-heat treatment of the blank to eliminate casting or forging stress, to stress-relieving annealing after machining, and finally to final heat treatment to optimize microstructure and properties, each step requires a tailored approach based on material characteristics and usage requirements. By rationally selecting annealing, quenching, tempering, and auxiliary processes, precise control of internal stress can be achieved, ensuring the gasket's long-term stable operation under complex conditions.
