Defects in heat treatment process of large gray iron castings

At present, with the development of marine engine to large-scale, the demand for large cylinder diameter, large tonnage and high-performance engine castings is increasing. In order to meet the market demand for high-performance engines, our factory, as the production base of high-end Large Engine Castings, has also carried out the production and development of high-performance large cylinder diameter castings. In view of the problem of local cracks caused by excessive stress in the recent production of high-grade gray cast iron body castings, the heat treatment process of large gray cast iron body castings is discussed from the aspects of stress relief process selection, heat treatment furnace selection and heat treatment process optimization.

The casting weighs 8640 kg and the outer contour dimension of the blank is 3 m × 1.7 m × 1.3 m, the key wall thickness is 15.5 mm ~ 25 mm, the material is HT300, and the three-dimensional diagram of the casting is shown in Fig. 1. It is modeled with alkaline phenolic resin sand and melted in an electric furnace. The pouring temperature is 1370 ℃ ~ 1380 ℃. After pouring, it is boxed 168 h, and aging heat treatment is carried out after rough cleaning. The heat treatment curve is shown in Fig. 2.

1) Through the heat treatment process test, it is proved that for large castings with complex structure, the most effective process to eliminate stress is to use the heat time effect method for heat treatment. At the same time, the generation of secondary stress should be avoided in the heat treatment process. Gradient heating can be used in the heating stage to ensure the uniformity of the whole temperature of the casting and avoid the generation of heat treatment cracks.

2) Under the condition of ensuring the uniformity of furnace temperature, according to the casting structure and the enterprise’s own situation, gas furnace, electric furnace and gas electric heating furnace can be used.

3) The heat treatment process can not completely remove the casting stress. The solution to the stress problem is mainly from the structural design to avoid the generation of stress.

Defect location
Defect morphology

Local hot cracks occur during casting production, as shown in Figure 3. The hot crack location is fixed at the side observation window. The defects occur after boxing and heat treatment, especially after heat treatment. The analysis causes are: there is a thick and large end face above the area, the solidification of molten iron is slow, the shrinkage tendency is large in the later stage, and the thermal crack caused by excessive tensile stress in the defect area. The defect after heat treatment is the crack that occurs when the stress is released without the support of sand core and sand mold because the heat treatment process can not meet the demand.