Formation and elimination of common defects in gray cast iron

The cylinder block is the framework of the automobile engine, with thin wall and complex structure. In the casting production, it is not only required that the key parts should not have defects, but also undergo various pressure tests. As the gray cast iron cylinder block has many casting process steps and high dimensional accuracy requirements, more sand cores are required to meet various structures and shapes of the casting, and the oil duct core or water jacket core needs to be fixed. Therefore, it is relatively difficult to cast the cylinder block, and the casting process control requirements are very strict.

Defects in the production of gray iron castings are mainly divided into two categories: one is microstructure defects, the other is macro structure defects. The microstructure defects of gray cast iron lead to the poor uniformity of the overall structure of the cylinder block, which makes the performance of the casting change greatly, and it is very easy to fail in the process of use. Macro structural defects of gray cast iron mainly include structural dimensional accuracy deviation and casting defects. Common casting defects include air hole, sand hole, slag hole, sand washing, cold shut, leakage and fire escape. Such defects usually appear in various key parts of the casting and are the main cause of casting scrap.

The structure and properties of grey cast iron can be greatly improved by controlling the quality of molten iron, improving the composition of raw materials and optimizing the proportion of additives. However, the defects of grey cast iron occur from time to time, and the problem of high scrap rate is still an important problem that needs to be solved urgently in the production of engine cylinder block by sand casting.

In view of the typical defects that often appear in the production of gray cast iron cylinder block of the cooperative unit, such as white tendency, air hole, slag inclusion, etc., the causes are analyzed, and targeted countermeasures are put forward to assist the cooperative unit to improve the process, reduce the scrap rate and improve the product quality.

1. The tendency of white cast iron cylinder body is mainly caused by the long-time high-temperature standing of molten iron, which often occurs at the angle between the thin wall and the tip. The appearance of white cast iron structure makes the hardness of this part larger, reaching more than 500hv, which has a very adverse impact on the subsequent processing, and the phenomenon of tool breakage may occur in serious cases.

2. There are many kinds of porosity defects, which are mainly caused by the influence of sand mold moisture and gas entrapment during pouring. In gray cast iron, intrusive and reactive porosity are the main types. The pore defects are dark gray and regular in shape, but sometimes there are winged oxide scales and small inclusions on the inner wall. This defect often occurs in the key parts of castings, which increases the rejection rate of castings.

3. The slag hole (sand hole) defect is mainly formed by the influence of the scum and sand burr not removed from the molten iron. It often appears on the upper mold surface of the casting or the inner surface of the cylinder barrel. Its shape is irregular and its interior is dark gray. Through the analysis of the electron microscope, it can be found that some white and bright substances, mainly Al2O3 A mixture of SiO2 and MnO2 (scum component). The sizes of slag hole defects are different, and the smaller defects will not affect the use after repair, but the larger defects also increase the rejection rate of castings.

4. In view of the above casting defects, it is necessary to improve the smelting process (raw and auxiliary material composition control, molten iron quality control), pouring process (pouring parameter rationality, pouring system rationality, pouring stability control), solidification cooling process (cooling process, unpacking time), sand core composition and molding control, etc, Only in this way can high-quality gray cast iron cylinders with low scrap rate, high dimensional accuracy and good mechanical properties be produced efficiently.