Detailed explanation of the performance and aging treatment of gray iron castings

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The heat treatment of gray iron castings is one of the important processes in mechanical manufacturing.

The heat treatment of gray iron castings is one of the important processes in mechanical manufacturing. Compared with other processing techniques, heat treatment generally does not change the shape and overall chemical composition of the workpiece, but rather changes the microstructure inside the workpiece or the chemical composition on the surface of the workpiece to endow or improve its performance. The characteristic of gray iron castings is to improve the internal quality of the workpiece. In order to provide the necessary mechanical, physical, and chemical properties for metal workpieces, in addition to selecting materials and various forming processes, heat treatment processes are often essential.

Gray iron castings are cast alloys based on iron and carbon, which mainly exist in the form of flake graphite particles. The performance of gray iron castings depends on the morphology, distribution, and matrix structure of graphite. There are three ways to handle aging: natural, artificial, and vibration.

1. Natural aging treatment: Gray iron castings should be left outdoors for several months or years. Although this treatment method is effective, it takes a long time.

Natural and thermal aging, that is, the workpiece is placed outdoors for a long time. Due to natural temperature changes and other environmental changes, the size of the workpiece becomes increasingly stable. Generally speaking, up to two years. Strike the workpiece with a wooden hammer, vibrate directly with a wind grip, etc. All are applied in actual production. Some people believe that mechanically processing parts is equivalent to accelerating natural aging. This method has been published in special issues abroad for a long time. The basic principle is to use the periodic external force of the exciter - the excitation force causes it to resonate with the workpiece (the exciter produces a vibration frequency that is consistent with the natural frequency of the workpiece).

Thus, considerable vibration energy can be obtained, which can be compared with thermal energy. The alternating initial stress and residual stress in resonance superimpose, driving the workpiece to produce large vibrations and local yielding, causing dislocations in the crystal and micro slip at grain boundaries, causing microplastic deformation, and promoting a large number of dislocations. Some are pinned to impurities, while others gather at grain boundaries, and the other dislocations obtain sufficient energy. It can pass through grain boundaries and enter another grain, causing overall relaxation or homogenization of residual stress in the workpiece, manifested as size stability, improved hardness, corrosion resistance, and fatigue resistance, reduced internal friction of the metal, and improved plasticity. Due to the concentration of dislocations in the grains at grain boundaries and impurities, the internal stress of each grain is uneven, and the microstructural stress increases. So dislocations are in a larger stress field, and slip damping increases, making it difficult for dislocations to slip again.

2. Artificial aging annealing is a better aging treatment method for gray iron castings, but its cost is higher and increases the cost. Artificial aging is the process of heating gray iron castings to 550-650 degrees Celsius for stress relief annealing, which saves time and is easier to remove residual stresses than natural aging.

3. Vibration aging treatment: Use a vibration aging instrument to perform vibration aging treatment on gray iron castings. Removing residual stress is the process of applying mechanical vibration by firmly fixing the exciter to the workpiece, and placing the workpiece on a rubber block or other elastic support to prevent the damping effect of the ground on the vibration.

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