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By observing the macro and micro characteristics of three common defects, namely poor spheroidization, graphite floating and slagging, Xinyuanzhu Group analyzed the causes of defects in ductile iron metallurgy and summarized the measures to prevent defects, which can effectively control and reduce the generation of scrap and improve the casting qualification rate through several casting experiments.
1、ductile iron metallographic defects generated by the causes
In the production of ductile iron, often due to the processing process and raw materials, will cause various defects in the casting, thus affecting the intrinsic quality of the casting, reducing the mechanical properties of the casting. xinyuanzhu group according to the ductile iron metallographic analysis of the observed poor spheroidization, graphite floating and slag three defects for an analysis, to prevent defects to reduce scrap, improve the casting pass rate.
1.1 poor spheroidization
1.11 macroscopic characteristics of poor spheroidization in the casting section appears sesame-dotted black spots, the closer to the center of the casting, the more dense black spots, as the severity of poor spheroidization increases, the diameter of the black spots also increases, the number is also increasing.
1.12 microscopic characteristics of metallographic observation for the concentrated distribution of thick flake graphite, a very small amount of spherical graphite, and spherical graphite is particularly round, in the thick flake graphite around the ferrite organization, this organization makes the mechanical properties of ductile iron decreased sharply.
1.13 The main reason for the formation of poor spheroidization is caused by the insufficient number of intermediate alloys during the spheroidization reaction
1.131 The amount of alloy added is small.
1.132 The amount of spheroidizing agent is suitable, but the amount of sulfur in the iron is high.
1.133 The iron water is oxidized, resulting in the amount of magnesium burned and caused by insufficient spheroidizing agent content and other reasons.
1.14 Measures to be taken to prevent poor spheroidization in production
1.141 The spheroidizing agent should be added in sufficient quantity.
1.142 In the spheroidization treatment, should try to prevent the magnesium burn loss, improve the absorption rate of spheroidizing agent.
1.143 Strictly control the sulfur content of the original iron, should choose low sulfur pig iron.
1.144 iron temperature should be controlled within the process requirements, the iron temperature is too high will produce too much spheroidizing agent burn loss, the temperature is too low alloy is easy to "die", will produce spheroidization, bad.

2、Graphite floating
2.1 macroscopic characteristics of graphite drift often appear on the upper surface of the casting, the edge of the test block and the bottom of the riser, in the fracture of the upper surface of a layer of obvious boundaries and flat black spots.
2.2 microscopic characteristics of its graphite aggregation, flowering and branch Li-shaped, some completely burst, this organization seriously weakened the mechanical properties of ductile iron, so that the material strength and toughness indicators significantly reduced.
2.3 The main reason for graphite floating is the high carbon and silicon equivalent (carbon equivalent > 4.55 ). When the carbon silicon equivalent exceeds the eutectic composition, due to the high casting temperature, the iron precipitates graphite before solidification, if the liquid state stays for a period of time, the graphite grows and gathers, because the density of graphite is much lighter than that of the iron, the gathered graphite is easy to float, sometimes inclusions are also brought to the upper surface of the casting, thus producing graphite floating on the upper surface of the casting; under the same conditions (the same carbon equivalent), the larger the casting, the higher the casting temperature Under the same conditions (same carbon equivalent), the larger the casting, the higher the casting temperature, the slower the cooling rate, the iron in the casting to maintain high temperature liquid state for a long time, graphite has enough time to float, and thus the more serious graphite floating.
2.4 Measures to be taken to prevent graphite floating
2.41 Strictly control the carbon and silicon equivalent, carbon should not exceed 49.6, silicon should not exceed 39.6, thick castings of carbon and silicon amount should be lower.
2.42 Improve the cooling rate, placing cold iron at the wall thickness, etc.
2.43 Add a small amount of elements that strongly prevent graphitization, such as the addition of molybdenum can prevent graphite floating.
2.44 Strictly control the residual amount of rare earth elements.

3、Clip slag
3.1 macroscopic characteristics of the slag distribution on the upper surface of the casting and the dead center of the casting, slag in the section is intermittent dark black areas or spots, no metallic luster.
3.2 microscopic characteristics of sulfides (MgS, FeS) and oxides (MgO, SiO2, FeO, A1203), in addition to spherical graphite in the slag area, there is a class of flaky graphite, the appearance of slag greatly reduces the mechanical properties of ductile iron, especially elongation and impact toughness decline more obvious.
3.3 Analysis of the reasons for the high sulfur content and residual magnesium in the iron, coupled with the casting temperature is too low, the formation of slag defects.
3.4 measures to prevent slagging can be used to reduce the sulfur content in iron (sulfur content should be controlled at 0.03 ~ 0.04 ) and residual magnesium ≤ 0.05; control enough rare earth amount, increase the casting temperature (≥ 1300 ℃), so that the molten slag easy to float, reduce slagging; spheroid processing must be many times after the slag picking, so as to avoid non-metallic slagging in the iron caused by slagging. In addition, in the casting, reduce the time of the molten iron in the air, so as to avoid the chance of generating secondary slag due to oxidation of the molten iron flow in the casting.

Xinyuanzhu Group specializes in the production of Ductile iron castings, Pig iron gray iron casting, Steel casting, Brass Casting, Malleable Iron Casting, and undertakes batch casting processing of castings. Welcome guests to cooperate with pictures or samples.