The chilling effect of the cold iron will change the local cooling rate of the steel casting, so that the solidification rate of the part where the cold iron is placed in the steel casting is greater than that of its adjacent parts, so as to form an artificial “V” shape solidification in the section where the cold iron is placed. When the span of the two risers is large, the solidification speed between the risers is slow, which is easy to form shrinkage and other defects. Placing cold iron in the middle can form a chilling zone with large temperature difference, that is, artificial end zone, accelerate the solidification speed of this zone and prolong the feeding area of the riser. Therefore, using cold iron can reduce the number of risers and improve the yield of steel castings.
The joint of steel casting has more heat storage than the adjacent parts in the solidification process, and the solidification speed is slow. The adjacent parts cooled faster will form metal skeleton during solidification, resulting in linear shrinkage, resulting in hot crack defects and reducing the strength of steel castings. Placing cold iron at the joint of steel castings can accelerate the cooling of the joint section, balance the solidification speed between the joint and adjacent parts, and reduce the risk of hot crack defects at the joint of steel castings.
Due to the chilling effect of the cold iron, the filled liquid steel solidifies rapidly on the cold iron surface and forms a crystal skeleton. At this time, the adjacent liquid steel on the sand mold surface forms a paste area due to the chilling effect. However, when the cold iron is too large, the stress generated by the crystal skeleton will exceed the crack resistance of the paste area, resulting in hot cracking of steel castings. Therefore, in order to avoid hot cracking caused by cold iron, small cold iron should be used and the cold iron should be chamfered at the same time.
However, air is mixed in the tiny holes on the surface of the cold iron, which will cause pores during casting of steel castings. Therefore, the selected cold iron surface shall be smooth and free of voids. In addition, the rust layer on the surface of the cold iron will react with the carbon in the molten metal to generate CO gas, resulting in air holes in the steel castings. The rust on the surface should be polished off before the cold iron is used. The reaction between the rust layer and the carbon in the molten steel will also reduce the carbon content in the local area of the steel casting, resulting in the phenomenon of casting and welding between the cold iron and the steel casting. In order to prevent casting and welding, high-carbon cast steel should be used as the cold iron as far as possible.