First, pre-furnace inspection

Pre-inspection is an indispensable part in the production process of ductile iron casting furnace, which is directly related to the quality of ductile iron casting. Timely and accurate judgment of the nodularization of liquid iron can quickly take measures to control the quality of ductile iron. Misjudgment before the furnace will cause a large number of castings to be scrapped and waste modeling hours, so the timely and accurate judgment of the nodulization situation before the furnace is much more important than the post-furnace inspection. There are several methods commonly used in actual production to check the spheroidization of iron liquid before the furnace.

1 Surface crust and flame judgment method

Rare earth magnesium ductile iron has low magnesium content, and rare earth elements, liquid iron surface and pure magnesium ductile iron is not the same, the surface is not so much oxide, the flame is not so much and powerful. However, when 1/3 of the iron liquid is added, the liquid surface will escape the magnesium light and white flame, like a candle flame. According to the number and height of the flame to judge the nodularization situation and magnesium residual amount, the higher the flame, the more powerful, indicating good nodularization. Especially in the pouring, the fire can be seen from the iron flow, such as the height of 25~ 50mm, that is, the spheroidization is good; If the flame is lower than 15mm, the spheroidization is poor. From the surface of the treated ductile iron liquid iron, an oxide film is formed, and there are silver white rolling highlights, that is, good spheroidization. However, if the oxide film is too thick, it means that the liquid iron temperature is low.

2 Triangle test block method

At present, it is a common method to judge spheroidization by observing test blocks. The test blocks used by various factories have many shapes and sizes, and most factories use triangular test blocks. The cross-sectional area of the triangular test block is 12.5mm (bottom) ×50mm(height). Some factories use round test blocks, such as 15mm, 25mm and 30mm. After the iron is processed, the iron is removed from the surface 200mm below the surface with a sampling spoon and poured into a test block. The test block with good spheroidization is round outside, much larger than gray cast iron, and clean and bright in appearance. Usually, the triangular test block with vertical pouring has shrinkage on both sides, and the top surface or sides of the block with horizontal pouring has shrinkage on both sides. The test block has good spheroidization after cooling and breaking, and the test block is silver-white or silver-gray porcelain fracture with clear white mouth at the tip and loose in the middle. If the fracture is silver-white and has a radial pattern, the amount of ball agent added is high, more carbides are produced, and the brittle sound of "patting" is issued when the test block is entered, the test piece is broken by tapping, and the newly struck mouth is very strong calcium carbide smell. Therefore, it is best to carry out floating silicon inoculation when pouring, if the fracture is silver gray and there are uniformly distributed small black spots, the fracture is colored crystal, the spheroidization fails.

1) The triangle test block with good spheroidization will immediately have calcium carbide odor escape after being interrupted by quenching water.

2) The white hole depth is related to the type, type and liquid iron composition of ductile iron, so there is no data given in the table, and the white hole depth of rare earth magnesium ductile iron is not very obvious.

3. Judgment of pouring process

(1) After pouring the liquid iron into the cast, such as the pouring cup has been concave downward, and the surface is very smooth, indicating good spheroidization; If there is a hard shell on the top of the pouring cup, it is also concave, indicating that the liquid iron temperature is low.

(2) During the pouring process, the round flat iron beans splashed on the surface of the sand mold have pitted spots (pits), indicating good spheroidization.

4 Black edge identification method

After the ball is treated, a thick test block of about 10mm is poured with the sample spoon, cooled to dark red, after quenching, if there is a black edge on the surface of the test block, it means poor spheroidization, and the thicker the black edge, the worse the spheroidization, resulting in recession, at this time, if the temperature of liquid iron is high, the alloy can be added. It can also be seen from the test block: if the test block is rounded around, and the middle is concave, and some have wrinkles, it also indicates that the spheroidization is good; If there is a wheat-like point on the surface of the test block, it means that the liquid iron oxygenation is easy to decay. In this case, the furnace should be modified.

5 Observe the solidification of liquid iron

After the nodulization treatment is completed, a small amount of iron liquid is taken out and poured into the diameter of 30mm cylindrical metal type. The phenomenon of iron liquid gushing from the surface during solidification is observed, and the nodulization of iron liquid is judged according to the amount of iron liquid gushing. Good nodulated liquid iron, during the solidification showed a great graphite expansion force, the surface of liquid iron at the beginning of solidification decreased, the surface of the crust after a small amount of liquid iron from the case; The amount of surface gushing of poorly spheroidized liquid iron is less.

6. Rapid metallographic observation before the furnace

(1) The post-metallographic inspection mainly observes the microstructure such as graphite morphology and characteristic metal matrix structure and inclusions, etc. The amount of broken parts can be changed into a cast quality and a solid base by metallographic analysis.

(2) The chemical composition analysis of rare earth magnesium ductile iron can also be said to be a means of detection, if the problem is found, the batching personnel can make appropriate adjustments to avoid major quality problems. In addition, there are mechanical properties testing and ductile iron non-destructive testing.

The detection of ductile iron should change the traditional detection method of "post-furnace analysis" into "real-time online detection." As a large and medium-sized enterprise, the use of advanced testing methods at home and abroad to accurately judge the nodularization situation is a reliable guarantee for the improvement of casting quality. However, for small casting enterprises, it is still of practical significance to use a certain characteristic of ductile iron to judge spheroidization indirectly.