Quality influencing factors and control of large section ductile iron castings

In order to solve various defects and problems of large section ductile iron castings, scholars at home and abroad have done a lot of research on the factors influencing large section ductile iron and quality control measures

Alex Escobar studied the effect of pouring temperature on the solidification of nodular cast iron. By pouring two wedge-shaped castings, five temperature measuring points were selected for temperature measurement and metallographic analysis. It was proposed that increasing pouring temperature would increase the size of graphite ball, eutectic reaction temperature and recrystallization starting temperature would increase, which delayed the cooling of castings, and eventually led to the deterioration of microstructure and properties of nodular cast iron. Thomas Borsato studied the effect of solidification time on the microstructure, mechanical properties and fatigue properties of Ferritic Ductile iron. He found that with the increase of solidification time, the microstructure became rough and the number and size of defects increased; the slower the cooling rate, the worse the tensile properties and fatigue properties were obtained; in the thicker region, with the increase of solidification time The amount of feldspar graphite ball decreases, the size of graphite ball increases, and graphite begins to denature, resulting in massive graphite. Massive graphite accelerates the generation and expansion of cracks, changes the fracture mode of ductile iron, and increases the brittle transition temperature of castings.

Thomas mottitschkafm studied the effect of graphite ball size on fracture mechanics and fatigue of ductile iron, and found that graphite morphology plays an important role in failure mechanics. The results show that the fatigue property of ductile iron is improved with the decrease of graphite ball size, and the smaller graphite ball has higher crack initiation resistance and crack propagation resistance.

By studying the influence of graphite ball size and graphite ball number on the properties of nodular cast iron, Matteo benedet found that with the increase of graphite ball average diameter, the fatigue strength, tensile strength and total elongation decreased, while the fatigue crack growth threshold and fracture toughness increased; with the decrease of graphite ball number, the tensile and fatigue properties and initial fracture properties decreased, and the fracture toughness increased Near threshold fatigue crack propagation.

The influence of casting defects on the fatigue crack and fatigue limit of ductile iron was studied. A large number of systematic samples were taken from the frame of heavy-duty press. It was pointed out that most of the fatigue failure began around the voids. The porosity and shrinkage seriously affected the fatigue performance of ductile iron, especially the high cycle fatigue state.

In the study of microstructure and mechanical properties of qt400-18, rip 0sai proposed that Mn and P contents, melt metallurgical quality, rare earth and inoculation ability have important effects on tensile strength, yield limit and ductility of qt400-18. The interference elements can be counteracted by adding rare earth elements into the molten iron after magnesium spheroidization

The effect of anti spheroidization of silicon on elongation decreases significantly and tensile strength and yield strength increase moderately. The composition of qt400-18 is recommended: C3.5% – 3.7%, si2.4% – 2.5%, Mn < 0.15%, P < 0.025%, s < 0.01%, re0.04% – 0.05%.

Lekakh studied the effect of silicon on the low temperature properties of nodular cast iron, and proposed that the segregation of elements and the change of graphite microstructure in the solidification process of nodular cast iron will greatly affect the impact properties and brittle transition temperature of nodular cast iron. Under the same casting condition, with the increase of silicon content, the graphite ball size increases, compared with the large size graphite, the small size graphite has higher bremsstrahlung; under the same silicon content, the larger graphite ball size and graphite ball spacing will significantly increase the impact toughness and reduce the brittle transition temperature.

The effect of rare earth and bismuth inoculants on the microstructure and mechanical properties of large section ductile iron was studied by rferrom. By detecting the amount of bismuth microaggregate and graphite ball in the center of casting, it was proposed that the mass graphite can be greatly reduced by using the inoculants containing bismuth and rare earth in the flow inoculation.

Matteo benedet pointed out that the long solidification time leads to the degeneration and coarseness of graphite balls. The addition of sb can avoid the occurrence of massive graphite under the condition of slow cooling, but can produce large explosive graphite.

Lena Magnusson aberg studied the effects of different rare earth elements (LA, y, CE) and FeSi alloys containing Zr, Sr, Bi and Mg on the microstructure of large section ductile iron. The impact spheroidizing treatment and stream inoculation were adopted. The combination of mgfesi spheroidizing agent without rare earth and inoculant containing Zr and SR was proposed The center of the obtained large section ductile iron can be spheroidized completely. You Ming – in the study of 100 ton nodular cast iron grinding process, it is considered that the simulation software can predict the location of defects and reduce or eliminate casting defects.

Lgnaszak studied the graphite denaturation after inoculation treatment. The graphite denaturation of castings with different wall thickness after inoculation treatment with different inoculants was studied by thermal analysis method. It was pointed out that the existence time of artificial nuclei was short, and the selection of suitable inoculants and inoculation methods could prolong the spheroidization decay time and the spheroidization effect of Mg, prolong the existence time of artificial nuclei and reduce fragments The formation of amorphous graphite.

Zhao Yifeng proposed that qualified castings could be obtained by reasonable design of casting process, guarantee of sand mold rigidity, full use of graphite self feeding, strict control of liquid iron composition and reasonable spheroidization inoculation treatment. Diao Xiaogang pointed out that reasonable selection of raw and auxiliary materials, proper spheroidization and inoculation process are the guarantee of casting quality.

In order to ensure the microstructure and properties of large section ductile iron castings and improve the casting quality, the following measures can be taken:

(1) The casting process is designed reasonably, and the simulation software is used to simulate and analyze the casting process, so as to predict the location of defects and reduce the cost of R & D and production;

(2) Reasonable design of casting production process;

(3) By selecting suitable mold material and cooling measures, the solidification speed of castings can be accelerated and the solidification time can be shortened, so that the required microstructure and properties can be obtained.

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