“Pouring and solidification process” is a key process that has a significant impact on the quality of steel castings. More than 80% of casting defects are produced in this process. For example:
(1) during pouring, due to the scouring of molten steel on the pouring system and mold cavity and the insufficient floating of residual sand in the pouring system and mold cavity, the defects of “pores and sand holes” are caused.
(2) during pouring, due to the thermal radiation of molten steel on the cavity surface (especially the upper surface of the cavity), the sand will fall off the surface. If it cannot float up sufficiently, it will also lead to the defects of “pores and sand holes”.
(3) at the end of pouring, if the steel flow is not controlled properly, the steel slag will enter the mold cavity with the steel flow, resulting in slag inclusion.
(4) during solidification, if the feeding channel is blocked, there will undoubtedly be shrinkage and porosity defects.
(5) most hot cracks are formed at the end of solidification, and some are formed just after solidification.
Therefore, controlling the “pouring and solidification process” will greatly reduce the probability of casting defects.
Preventive measures for casting defects during “pouring and solidification process”:
(1) remove the sundries left in the pouring system and mold cavity before pouring.
(2) make the pouring system with “refractory pouring brick pipe” (prevent sand from entering the joint of brick pipe).
(3) improve the pouring speed, and maximize the pouring speed. Why? Here is a brief introduction to some basic characteristics of casting sand.
Because the volume of quartz sand will expand after being radiated by heat during pouring. The expanded surface quartz sand is separated from the inner layer and falls into liquid steel, resulting in sand holes and pores. A considerable number of peers do not pay enough attention to or understand this process. A colleague once asked me, “before pouring, the cavity is treated very clean, but why are there still many sand holes in the casting?”
Quartz (i.e. quartz sand) is a cubic crystal system at room temperature α Quartz, with a density of 2.659/cm3, can be transformed into hexagonal helioid quartz with an increase of temperature, and the density decreases to 2.269/c. At room temperature. When the temperature of one quartz rises to 870 ℃, its volume increases by 17.26%, and when the temperature reaches more than 370 ℃, its volume expands sharply. Knowing this characteristic of quartz sand, it is not difficult to understand the main causes of sand holes.
Compared with quartz sand, other types of foundry sand (such as chrome ore sand, staggered sand, limestone sand, etc.) have their remarkable characteristics: some do not have phase transformation expansion, and some have very small phase transformation expansion. Although it has also been applied in a certain range in casting production, the high-temperature pulverization of limestone sand makes the castings shrink and sink, seriously affects the surface quality of castings, and limits its application range; Chrome ore and wrong sand are expensive due to resource constraints, so they are only used in key parts. In the actual production of castings, quartz sand is still the main, as is the case for large and medium-sized steel castings.