How to quickly determine whether defects occur in ductile iron production?
1. Pre-furnace inspection Pre-furnace inspection of ductile iron is an indispensable part of its production process, and it is directly related to the quality of ductile iron castings. By promptly and accurately judging the spheroidization of molten iron, measures can be quickly taken to control the quality of ductile iron. Misjudgment before the furnace will cause a large number of castings to be scrapped and waste molding hours. Therefore, timely and accurate judgment of the spheroidization situation before the furnace is much more important than post-furnace inspection. The methods commonly used in actual production to check the spheroidization of molten iron before the furnace include the following.
1 Method for judging surface crusting and flames Rare earth magnesium ductile iron contains lower magnesium and rare earth elements. The surface of the molten iron is different from that of pure magnesium ductile iron. There is not so much oxide scale on the surface, and the flames are not as numerous and powerful. However, when 1/3 of the molten iron is added, magnesium light and white flames will escape from the liquid surface, shaped like a candle flame. Judgment of the spheroidization condition and residual amount of magnesium is based on the number and height of the flames. The higher and more powerful the flames are, indicating good spheroidization. Especially during pouring, fire can be seen from the molten iron flow. If the height reaches 25~50mm, the spheroidization is good; if the flame is lower than 15mm, the spheroidization is poor. Judging from the surface of the treated ductile iron molten iron, if an oxide film is formed and there are silver-white rolling bright spots, it means that the spheroidization is good. However, if the oxide film is too thick, it means that the temperature of the molten iron is low.
2 Triangular test block method At present, it is a commonly used method to judge the spheroidization condition by observing the test block. The test blocks used by various factories come in many shapes and sizes, and most factories use triangular test blocks. The cross-sectional area of the triangular test block is 12.5mm (bottom) Take out the molten iron 200mm below the surface and pour it into a test block and cool it until it is dark red before quenching it with water. The test block with good spheroidization has a rounded outside and is much larger than gray cast iron. It has a clean and bright appearance and is usually poured vertically. There are shrinkage on both sides of the triangular test block, and there is shrinkage on the top or both sides of the horizontally poured block. The test block spheroidizes well after being cooled and broken. The test block has a silvery white or silvery gray porcelain-like fracture, with a clear white tip and a middle If the fracture surface is silvery white and has radial patterns, the amount of surface ball agent added is too high and more carbides will be produced. At this time, the test piece will make a "slapping" brittle sound when it is inserted, and the test piece will break with a light tap, and The freshly struck mouth has a strong smell of calcium carbide. Therefore, it is best to inoculate floating silicon during pouring. If the fracture surface is silver-gray with evenly distributed small black spots, and the fracture surface shows colored crystals, the spheroidization of the triangular sample has failed. Two ways to identify quality
Note: 1) The smell of calcium carbide will escape immediately after the triangular test piece with good spheroidization is broken by quenching water. 2) The whitening depth is related to the type and type of ductile iron and the composition of molten iron. Therefore, no data is given in the table. The whitening depth of rare earth magnesium ductile iron is not very obvious.
3 Judgment of the pouring process (1) After the ductile iron molten iron is poured into the mold, if the sprue cup has been recessed downwards and the surface is very smooth, it indicates good spheroidization; if there is a hard shell on the upper part of the sprue cup, it is also necessary to Shrinkage indicates that the temperature of the molten iron is low. (2) During the pouring process, the round flat iron beans splashed on the surface of the sand mold have pits (pits), indicating good spheroidization.
4. After the black edge identification method is completed, use a sample spoon to pour a test block with a thickness of about 10mm, and cool it to dark red. After quenching and breaking, if a black edge is found on the upper surface of the test block, it means poor spheroidization and black color. The thicker the edge, the worse the spheroidization and recession. At this time, if the temperature of the molten iron is high, additional alloy can be added. It can also be seen from the test block: if the test block is surrounded by an arc shape, with a depression in the middle, and some have wrinkles, it also means that the spheroidization is good; if there are wheat-like points on the upper surface of the test block, it means that Liquid iron is oxidized and prone to decay. In this case, the furnace should be modified.
5. Observe the solidification condition of the molten iron. After the spheroidization treatment is completed, take out a small amount of molten iron and pour it into a Ф30mm cylindrical metal mold. Observe the phenomenon of molten iron gushing out from the surface during solidification, and judge the spheroidization of the molten iron based on the amount of molten iron gushing out. Condition. The molten iron with good spheroidization shows a large graphite expansion force during solidification. The surface of the molten iron drops somewhat at the beginning of solidification, and a small amount of molten iron gushes out from the surface after the surface is crusted; while the molten iron with poor spheroidization The surface gushing quantity is small.
6. Rapid metallographic observation in front of the furnace. The various methods mentioned above all use certain characteristics of ductile iron to indirectly judge the spheroidization. However, various production conditions change greatly, and the above methods have limitations. Rapid metallographic observation in front of the furnace can Avoid the interference of many factors and directly observe the spherification situation.
2. Post-furnace metallographic inspection (1) Post-furnace metallographic inspection mainly observes microstructure such as graphite morphology and sub-characteristic metal matrix structure inclusions, etc. Through metallographic analysis, the number of breakable parts can be changed to improve the casting quality and solid foundation. (2) The chemical composition analysis of rare earth magnesium ductile iron can also be said to be a detection method. If problems are found, the batching personnel can make appropriate adjustments to avoid major quality problems. In addition, there are mechanical properties testing and non-destructive testing of ductile iron castings.
3. Conclusion The detection of ductile iron needs to change the traditional detection method of "post-furnace analysis" into "real-time online detection." As a large and medium-sized enterprise, using advanced detection methods at home and abroad to accurately determine the spheroidization situation is a reliable guarantee for improving the quality of castings. . However, for small casting companies, it is still of practical significance to use certain characteristics of ductile iron to indirectly judge the spheroidization situation.
