Detailed Explanation of the Properties and Ageing Treatment of Gray Iron Castings

Heat treatment of gray iron castings is one of the crucial processes in mechanical manufacturing. Compared to other machining techniques, heat treatment typically does not alter the shape or overall chemical composition of the workpiece; instead, it modifies the microstructure within the material or adjusts the surface chemistry, thereby enhancing or improving the workpiece's functional performance. A key advantage of heat treating gray iron castings lies in its ability to elevate the intrinsic quality of the component. To ensure that metal parts achieve the desired mechanical, physical, and chemical properties, heat treatment processes—alongside the proper selection of materials and various forming techniques—are often indispensable.

　　Gray iron castings are casting alloys based on iron and carbon, with the latter primarily existing in the form of flaky graphite particles. The performance of gray iron castings depends on the morphology, distribution, and matrix structure of the graphite. Ageing treatments can be carried out in three ways: natural, artificial, and vibratory.

　　1. Natural aging treatment involves leaving gray cast iron parts exposed outdoors for several months or even years. While this method is effective, it takes a long time.

　　Natural and thermal aging involves leaving the workpiece exposed outdoors for an extended period. Due to natural temperature fluctuations and other environmental changes, the dimensions of the workpiece gradually stabilize—typically within a maximum of two years. Techniques such as tapping the workpiece with a wooden hammer or directly vibrating it using air grips are commonly applied in actual production. Some argue that mechanically applying stress to the workpiece essentially accelerates the natural aging process. This method has already been extensively documented in specialized publications abroad. Its underlying principle relies on using the periodic external force generated by an exciter—known as the "excitation force"—to induce resonance between the exciter and the workpiece (where the exciter produces vibrations at a frequency matching the workpiece's natural resonant frequency).

Thus, considerable vibrational energy is generated—comparable to thermal energy—by the superposition of alternating and residual stresses during resonance. This energy drives the workpiece into significant vibrations and localized yielding, triggering microslip along dislocations and grain boundaries within the crystal structure, ultimately leading to microplastic deformation. As a result, a large number of dislocations are produced: some become pinned at impurities, while others accumulate at grain boundaries. Meanwhile, certain dislocations acquire enough energy to traverse these barriers and enter adjacent grains, helping to relieve or homogenize the overall residual stresses in the material. This process manifests as enhanced dimensional stability, improved hardness, better corrosion resistance, and increased fatigue strength, while simultaneously reducing internal material damping and boosting ductility. However, because dislocations tend to concentrate at grain boundaries and impurities, stress distribution within individual grains becomes uneven, intensifying microscopic stress levels. Consequently, these dislocations find themselves immersed in a stronger stress field, which significantly increases their resistance to slip. As a result, further dislocation movement becomes much more difficult.

2. Artificial aging annealing is a superior aging treatment method for gray iron castings, though it comes with higher costs that increase overall expenses. In artificial aging, the gray iron castings are heated to 550–650°C for stress-relief annealing, which saves time compared to natural aging and more effectively eliminates residual stresses.

　3. Vibration Aging Treatment: Use a vibration aging machine to perform vibration aging treatment on gray iron castings. To eliminate residual stresses, securely fix the exciter directly onto the workpiece and apply mechanical vibrations. The workpiece should be placed on rubber blocks or other elastic supports to prevent damping effects from the ground during vibration.