Anti whitemouth phenomenon
On the section of iron castings, the opposite phenomenon of normal section appears. In the central part of the casting or in the part of slow cooling (hot spot), grey mouth structure originally appeared, but white mouth structure or hemp mouth structure appeared; however, in the outer layer of the casting or in the part with fast cooling, white mouth structure or hemp mouth structure originally appeared; however, in the outer layer of the casting or in the part with fast cooling, white mouth structure or hemp mouth structure originally appeared Oral tissue or maxillofacial tissue, but gray oral tissue appears.
In a broad sense, both gray cast iron and nodular iron parts will have anti white mouth phenomenon. However, gray cast iron usually appears in the production of Hypereutectic piston rings. In the production of ductile iron, especially in China, more rare earth and silicon are often added. Therefore, the phenomenon of anti white mouth often appears in the large section ductile iron and hot spot parts which are slowly cooled.
It is difficult to machine and sharpen the tool wear because of the phenomenon of anti white mouth in the casting. In addition, in the production of anti white mouth phenomenon, there are often shrinkage porosity, the matrix structure contains more carbonization, which leads to the mechanical properties of this part reduced, especially the plasticity.
In recent years, the analysis of the composition in the micro region of nodular iron shows that the composition segregation does exist, especially in the slow cooling large section nodular iron. The results of electron probe microanalysis of the boundary distribution around the graphite ball and the eutectic group in nodular iron are shown in the figure. It can be seen from the figure that the elements rich in manganese, chromium, molybdenum, vanadium, titanium and rare earth at the boundary of the eutectic group, as well as the low melting point elements such as phosphorus, sulfur, tin, antimony, bismuth and arsenic, which are enriched at the boundary of the eutectic group are called positive segregation, while the elements such as silicon, copper, nickel and aluminum that promote the graphitization are enriched inside and around the graphite ball, which is called the enrichment Mg = 0.058%
In addition, in the absence of alloy elements, negative segregation of silicon and positive segregation of manganese can also be found in nodular cast iron. For example, in large nodular cast iron rolls, the mass fraction of manganese in the riser is 2.015%, while in the roll with faster cooling speed, the mass fraction of manganese is only 0.6%
The more silicon content in nodular cast iron, the more serious silicon segregation. According to the analysis of Fe-C-Si phase diagram, under the condition of close equilibrium, the first crystallized and solidified phase is high silicon phase, so it is easy to form grey mouth structure. The second crystallized and solidified phase is low silicon phase, so it is easy to form white mouth structure. Therefore, grey mouth structure can appear in the fast cooling part, while white mouth structure (or hemp mouth structure) can appear in the slow cooling part
At the same time of negative segregation of silicon, there will be positive segregation of carbide forming elements at the boundary of eutectic group. Especially, due to the rare earth elements in spheroidizing agent, they are easy to form carbide, so there is enrichment of rare earth elements in the slow cooling part of casting, and the formation of carbide, which leads to the increase of white mouth tendency and the aggravation of anti white mouth phenomenon. In addition, in the center of slow cooling, there will also be breeding decline, which also leads to the aggravation of anti baikou.
Prevention of anti whitemouth
(1) The amount of silicon in the control nodular iron shall not be too high. Even for the ferritic nodular iron, the maximum mass fraction of silicon shall not exceed 2.8%; for the nodular iron with cast smooth matrix, the maximum mass fraction of silicon shall not exceed 2.4%.
(2) Control the manganese content in nodular iron. Even for cast smooth ductile iron, the maximum mass fraction of manganese shall not exceed 0.8%; for ductile iron with ferrite matrix, the maximum mass fraction of manganese shall not exceed 0.3%
(3) In order to control the residual rare earth content in nodular iron, it is advisable to control the mass fraction of rare earth below 0.03% by using a nodulizer with lower rare earth content.
(4) Improve the inoculation technology, improve the inoculation effect, adopt late inoculation, make the nodular iron appear as many graphite balls as possible in one crystallization.
(5) In the casting process design, try to eliminate the difference of the cooling speed of each part of the casting, and adopt the cold iron process for the thick part.
(6) In the smelting process, it is necessary to prevent the bottom coke from being too low and the air supply from being too large, which will lead to serious element burning loss and too high FeO content in the molten iron.
(7) By adopting high temperature annealing process, the phenomenon of anti white mouth can be eliminated locally. However, in order to completely eliminate the micro segregation of silicon, manganese, rare earth and various alloy elements, high temperature homogenization annealing is impossible.