1.Charge
The quality of furnace charge mainly depends on the content of trace elements in raw materials. The trace elements in cast iron mainly come from pig iron and scrap steel. Pig iron itself contains coarse graphite flakes with the melting point of graphite above. During heavy flame, the graphite cannot be completely dissolved. The unmelted graphite becomes the core of graphite crystallization in the crystallization process, making the graphite coarse again. The coarse graphite flakes have the function of splitting the matrix, which greatly reduces the strength of cast iron. Therefore, the tensile strength of gray cast iron will decrease significantly with the increase of pig iron. The experiments of Liu Youping and others in Nanchang diesel engine plant show that when pig iron is added, the tensile strength is relatively low, as shown in the table.
| Sample No. | Pig iron | Return material | Steel scrap | σb/Mpa | HB |
| 1 | 20 | 68.6 | 11.1 | 276 | 217 |
| 1 | 20 | 58.6 | 21.4 | 296 | 229 |
| 1 | 20 | 54.3 | 25.7 | 312 | 229 |
| 1 | 20 | 52.9 | 27.1 | 325 | 231 |
| 2 | 30 | 54.3 | 15.7 | 245 | 207 |
| 2 | 22.8 | 54.3 | 22.9 | 294 | 229 |
| 2 | 21.4 | 54.3 | 24,3 | 302 | 229 |
| 2 | 20 | 54.3 | 25.7 | 312 | 229 |
| 3 | 30 | 54.3 | 15.7 | 245 | 207 |
| 3 | 30 | 52.8 | 17.2 | 257 | 217 |
| 3 | 30 | 50.0 | 20.0 | 264 | 229 |
| 4 | 21.4 | 54.3 | 24.3 | 302 | 229 |
| 4 | 17.1 | 58.6 | 24.3 | 319 | 232 |
In foreign countries, more scrap steel is added before the melting of gray cast iron, so that there are more low-carbon micro areas in the molten iron. The pre eutectic austenite pre storage core formed by this makes the pre eutectic austenite easier to form and develop, so as to refine the pearlite at the boundary of eutectic group, and then improve the tensile strength of cast iron.
2.Melting temperature
In order to obtain high-quality molten iron, firstly, the tapping temperature should be high, which is conducive to the refinement and precipitation of graphite and improve the flow performance of molten iron, so as to obtain perfect castings. At the same time, it improves the mechanical properties of gray iron castings, makes the matrix structure more and more fine, improves the tensile strength, reduces the Poisson’s ratio, and improves the quality coefficient Qi of molten iron (the ratio of maturity RG to hardening Hg).
At present, the most widely used smelting method in China is cupola melting, and the key to cupola melting is to improve the temperature of molten iron. Foreign cupola basically adopts hot air supply, and some also add oxygen. The quality of coke is good, and the smelting temperature of molten iron is high, which can reach above 1550 ° C; Compared with foreign countries, the melting technology of domestic cupola is relatively backward, mostly using cold air supply, paying no attention to the quality of coke, and the melting temperature of molten iron is low, generally about 1450 ° C.