Analysis and Countermeasures of Common Defects in Ductile Iron Castings

Ductile iron is an important casting metal material developed in recent 40 years in China. The tensile strength, plasticity and toughness of nodular cast iron are higher than those of other cast iron due to the small stress concentration caused by spherical graphite and the small splitting effect on the matrix. Compared with steel with corresponding structure, the plasticity is lower than that of steel, the fatigue strength is close to that of ordinary medium carbon steel, the yield ratio can reach 0.7~0.8, almost twice that of ordinary carbon steel, and the cost is lower than that of steel, so its application is increasingly widespread.

Of course, nodular cast iron is not perfect. In addition to general casting defects, it will also produce some special defects, such as shrinkage porosity, slag inclusion, subcutaneous porosity, poor spheroidization and recession. These defects affect the properties of castings and increase the rejection rate of castings. In order to prevent the occurrence of these defects, it is necessary to analyze them, summarize various influencing factors, and propose preventive measures, so as to effectively reduce the occurrence of defects and improve the mechanical properties and production benefits of castings. This paper will discuss the main common defects of ductile iron castings: shrinkage cavity, porosity, slag inclusion, subcutaneous porosity, graphite floating, poor spheroidization and spheroidization recession.

1 Shrinkage porosity

1.1 Influencing factors

(1) Carbon equivalent: increase carbon content, increase graphitization expansion, and reduce shrinkage cavity and porosity. In addition, increasing carbon equivalent can also improve the fluidity of ductile iron, which is conducive to feeding. The empirical formula for producing high-quality castings is C%+1/7Si%>39%. However, when the carbon equivalent is increased, other defects such as graphite floating shall not occur in the casting.

(2) Phosphorus: The phosphorus content in molten iron is too high, which expands the solidification range. At the same time, the low melting point phosphorus eutectic can not be replenished during the final solidification, which weakens the casting shell, so there is a tendency to increase shrinkage cavity and porosity. Generally, the phosphorus content controlled by the factory is less than 0.08%.

(3) Rare earth and magnesium: too much residual rare earth will deteriorate the shape of graphite and reduce the spheroidization rate, so the content of rare earth should not be too high. However, magnesium is an element that strongly stabilizes carbides and hinders graphitization. It can be seen that the amount of residual magnesium and residual rare earth will increase the white mouth tendency of ductile iron and reduce the expansion of graphite. Therefore, when their content is high, they will also increase the tendency of shrinkage cavity and porosity.

(4) Wall thickness: when the casting surface forms a hard shell, the higher the temperature of the internal molten metal, the greater the liquid shrinkage, and the volume of shrinkage cavity and porosity not only increases in absolute value, but also increases in relative value. In addition, if the wall thickness changes too suddenly, the isolated thick section cannot be fed, which increases the tendency of shrinkage cavity and porosity.

(5) Temperature: The high pouring temperature is conducive to feeding, but too high will increase the liquid shrinkage, which is unfavorable for eliminating shrinkage cavity and porosity. Therefore, the pouring temperature should be reasonably selected according to the specific situation, generally 1300~1350 ℃.

(6) Compactness of sand mold: if the compactness of sand mold is too low or uneven, the cavity will expand under the action of metal static pressure or expansion force after pouring, resulting in insufficient feeding of the original metal and shrinkage of the casting.

(7) Gating, riser and chill: if the gating system, riser and chill are set improperly, the sequential solidification of molten metal cannot be guaranteed; In addition, the number, size and connection with the casting of the riser will affect the feeding effect of the riser.

1.2 Preventive measures

(1) Control the composition of molten iron: maintain high carbon equivalent (>39%); Reduce the phosphorus content as much as possible (<0.8%); Reduce the amount of residual magnesium (<0.7%); The rare earth magnesium alloy was used to treat, and the residual amount of rare earth oxide was controlled at 0.02% ～ 0.04%.

(2) The process design shall ensure that the castings can be continuously replenished with high-temperature liquid metal from the riser during solidification. The size and number of risers shall be appropriate to achieve sequential solidification.

(3) If necessary, cold iron and subsidies shall be used to change the temperature distribution of castings, so as to facilitate sequential solidification.

(4) The pouring temperature shall be 1300 ～ 1350 ℃, and the pouring time of a package of molten iron shall not exceed 25min to avoid spheroidizing recession.

(5) Improve the compactness of sand mold, generally not less than 90; Sand striking shall be uniform, and the moisture content shall not be too high to ensure that the mold has sufficient rigidity.

2 Slag inclusion

2.1 Influencing factors

(1) Silicon: silicon oxide is also the main component of slag inclusion, so the silicon content shall be reduced as much as possible.

(2) Sulfur: The sulfide in molten iron is one of the main reasons for the formation of slag inclusion defects in ductile iron castings. The melting point of sulfide is lower than that of molten iron. During the solidification of molten iron, sulfide will separate from molten iron, increasing the viscosity of molten iron and making it difficult for slag or metal oxide in molten iron to float. Therefore, when the sulfur content in molten iron is too high, the casting is easy to produce slag inclusion. The sulfur content of nodular cast iron should be controlled below 0.06%. When it is between 0.09% and 0.135%, the slag inclusion defect of cast iron will increase sharply.

(3) Rare earth and magnesium: In recent years, it is believed that slag inclusion is mainly caused by the oxidation of magnesium, rare earth and other elements, so the residual magnesium and rare earth should not be too high.

(4) Pouring temperature: when the pouring temperature is too low, the metal oxides in the liquid metal are not easy to float to the surface and remain in the liquid metal due to the high viscosity of the liquid metal; When the temperature is too high, the slag on the liquid metal surface becomes too thin to be removed from the liquid surface, and often flows into the mold with the liquid metal. In actual production, too low pouring temperature is one of the main reasons for slag inclusion. In addition, the selection of pouring temperature should also consider the relationship between carbon and silicon content.

(5) Gating system: The gating system shall be reasonably designed with slag retaining function to enable the molten metal to fill the mold smoothly and avoid splashing and turbulence.

(6) Molding sand: if there is excess sand or paint on the surface of molding sand, they can form slag with the oxide in the molten metal, resulting in slag inclusion; The compactness of sand mold is uneven, and the surface of mold wall with low compactness is easy to be eroded by molten metal and form low melting point compounds, resulting in slag inclusion of castings.

2.2 Preventive measures

(1) Control the composition of molten iron: reduce the sulfur content in molten iron as much as possible (<0.6%), add rare earth alloy (0.1% ～ 0.2%) in an appropriate amount to purify molten iron, and reduce the silicon content and residual magnesium content as much as possible.

(2) Smelting process: the outlet temperature of the molten metal shall be increased as much as possible, and the molten metal shall be properly killed to facilitate the floating and aggregation of non-metallic inclusions. Clean up the slag on the surface of molten iron, and place a covering agent (perlite, plant ash, etc.) on the surface of molten iron to prevent oxidation of molten iron. Select a suitable pouring temperature, preferably not lower than 1350 ℃.

(3) The pouring system shall be equipped with slag collecting bag and slag blocking device (such as slag filter screen) to ensure smooth flow of molten iron, so as to avoid sand washing in the sprue.

(4) The mold compactness shall be uniform and the strength shall be sufficient; Sand in the mold shall be blown clean when the box is closed.

3 Graphite floating

3.1 Influencing factors

(1) Carbon equivalent: The carbon equivalent is too high, so that a large amount of graphite is precipitated from molten iron at high temperature. As the density of graphite is smaller than that of molten iron, the graphite floats to the upper part of the casting driven by magnesium vapor. The higher the carbon equivalent is, the more serious the graphite floats. It should be pointed out that too high carbon equivalent is the main reason for graphite floating, but not the only reason. Casting size and wall thickness are also important factors affecting graphite floating.

(2) Silicon: under the condition of constant carbon equivalent, properly reducing the silicon content will help reduce the tendency of graphite floating.

(3) Rare earth: when the content of rare earth is too small, the solubility of carbon in molten iron will be reduced, and a large amount of graphite will be precipitated from molten iron, which will increase the graphite floatation.

(4) Spheroidizing temperature and inoculation temperature: In order to improve the absorption rate of magnesium and rare earth elements, domestic experiments and studies show that the most appropriate temperature of molten iron during spheroidizing treatment is 1380~1450 ℃. In this temperature range, the absorptivity of magnesium and rare earth increases with the increase of temperature.

(5) Pouring temperature: Generally, the higher the pouring temperature, the greater the tendency of graphite floating. This is because the casting is in liquid state for a long time, which is conducive to graphite precipitation. A. P. Druschitz and W.W. Chaput found that if the solidification time was shortened, the floating tendency of graphite decreased with the increase of pouring temperature.

(6) Retention time: The retention time between inoculation and pouring is too long, which provides conditions for graphite precipitation. Generally, this time should be controlled within 10min.

3 2 Prevention measures

(1) Control the composition of molten iron: strictly control the carbon equivalent, which shall not be greater than 46%; The carbon content of molten iron shall not be greater than 40%, and the carbon content of molten iron can be adjusted by scrap; Low silicon (<12%) pig iron is used; Improvement of inoculation treatment and enhancement of inoculation effect can reduce the amount of inoculated ferrosilicon.

(2) Control the addition amount of rare earth: on the premise of ensuring spheroidization, the addition amount should be small.

(3) Improve the structure of the casting to make the wall thickness as uniform as possible and less than 60mm; If there is a large difference in wall thickness and hot spot, chill can be added at the thick wall or hot spot; If the hot spot or thick wall is located at the top of the casting, a riser can be added here.

(4) Strictly control the temperature: it is generally required to spheroidize at 1380~1450 ℃ and pour at 1360~1400 ℃. At the same time, the residence time between molten iron discharging and pouring shall be shortened as much as possible.

(5) If necessary, anti graphitization elements such as molybdenum can be added to improve the solubility of carbon in molten iron, thus reducing graphite precipitation.

4 Subcutaneous stomata

4.1 Influencing factors

(1) Carbon equivalent: Proper increase of silicon content will help reduce subcutaneous stomata. At the same time, when the silicon content remains unchanged, with the increase of carbon content, the number of subcutaneous pores in ductile iron presents a single peak curve, and the peak point is always kept around the eutectic point. Therefore, it is better to select a higher carbon silicon content, so that the carbon equivalent of ductile iron is slightly greater than the eutectic point.

(2) Sulfur: high sulfur will cause defects such as subcutaneous stomata, which is formed due to the generation of H2S gas. When the sulfur content exceeds 0.094%, subcutaneous stomata will occur. The higher the sulfur content, the more serious the situation is.

(3) Rare earth: the addition of rare earth elements in molten iron can deoxidize and desulfurize, improve the surface tension of molten iron, and thus help prevent subcutaneous pores. However, if the content of rare earth is too high, it will increase the content of oxides in molten iron, increase the external core of bubbles, and increase the subcutaneous porosity. The amount of residual rare earth should be controlled below 0.043%.

(4) Magnesium: too high magnesium will increase the hydrogen absorption tendency of molten iron, and a large number of magnesium bubbles and oxides will enter the mold cavity, increasing the external core of the bubbles; In addition, magnesium vapor directly interacts with the water in the sand mold to generate MgO flue gas and hydrogen, and also generates subcutaneous pores. The test shows that subcutaneous pores are easy to appear when the residual magnesium content is greater than 0.05%, and the higher the residual magnesium is, the more serious it is. Therefore, on the basis of ensuring spheroidization, the residual magnesium shall be reduced as much as possible.

(5) Aluminum: The aluminum in molten iron is the main reason for hydrogen porosity in castings. It is reported that when the residual aluminum content of green cast ductile iron is 0.030% ～ 0.050%, subcutaneous porosity will occur. E. R. Kaczmarek et al. believed that the molten iron reacts with the water in the mold to generate FeO and H2. Due to the deoxidation of aluminum, it also generates Al2O3, which is the core of bubble generation and can absorb certain gases, increasing the tendency of ductile iron to produce subcutaneous pores. However, when reducing the FeO content in the slag, the presence of magnesium makes aluminum appear redundant, so the sensitive content of aluminum has a certain range.

(6) Wall thickness: The subcutaneous pores also have the characteristics of “wall thickness effect”, that is, the pores are generated within a certain wall thickness range, which is actually related to the solidification speed of the casting. When the wall thickness of the casting is large, its solidification time is delayed, which is conducive to the escape of bubbles. Therefore, it is not easy to produce subcutaneous pores when the wall thickness is less than 6mm or more than 25mm.

(7) Pouring temperature: The pouring temperature is similar to the wall thickness effect, but also has a temperature range. At 1285~1304 ℃, the subcutaneous porosity is quite serious. The author further considers that the dangerous temperature is different for different wall thickness, so the pouring temperature should be determined according to the wall thickness of the casting. Of course, increasing the pouring temperature can delay the formation of oxide film, prevent the slag from entering the mold cavity, and prolong the baking time of sand mold to make the water migrate outward.

(8) Moisture content of molding sand: The tendency of the mold to produce subcutaneous porosity decreases in the following order: wet mold, dry mold, sodium silicate mold, shell mold. Si Naichao’s research also proved this, that is, with the increase of molding sand moisture, the tendency of ductile iron to produce subcutaneous porosity increases, while when the molding sand moisture is less than 4.8%, the subcutaneous porosity is close to zero.

(9) Compactness and permeability of molding sand: the permeability of molding sand is too low, resulting in the gas generated by the mold wall can not be discharged out of the mold, but invades into the metal, resulting in porosity of the casting; With the increase of the compactness of molding sand, the tendency of subcutaneous porosity also increases, but when the compactness is quite high, the tendency decreases. This may be due to the high compactness of surface sand, which increases the migration resistance of water to the casting direction. However, if the moisture of molding sand is also high, the possibility of water vapor explosion will increase.

(10) Gating and riser: reasonably design the gating and riser to ensure smooth pouring of molten iron and strong slag retaining function; At the same time, the height of sprue and riser shall be increased appropriately to increase the static pressure of molten metal.

4 2 Prevention measures

(1) The chemical composition of molten iron shall be strictly controlled, so that the carbon equivalent is slightly greater than the eutectic point composition, and the sulfur content is not greater than 0.094%; The residual rare earth is less than 0.043%; The residual magnesium content is not more than 0.05%; The aluminum content is outside the range of 0.03%~0.05%.

(2) Reasonably design the casting structure so that the wall thickness is not less than 25mm; The pouring temperature shall be determined according to the wall thickness, and the thin wall small pieces shall not be less than 1320 ℃; Medium parts shall not be less than 1300 ℃; Large pieces shall not be less than 1280 ℃.

(3) The metal furnace charge, inoculant and tools used shall be dry, and the surface shall be free of rust and oil stain. At the same time, the moisture of molding sand should not be too high, and should be less than 4.8% as far as possible. The content of gas generating substances such as coal powder and heavy oil should be properly controlled to reduce the clay content, and some substances that can increase the permeability, such as wood chips, can be added.

(4) Reasonably design the gating system to make it open. The air outlet can be set at the highest position of the mold cavity. At the same time, the height of the gating and riser should be guaranteed to increase the static pressure of the liquid metal.

5 Spheroidization decline and poor spheroidization

5.1 Influencing factors

(1) Carbon equivalent: When the carbon equivalent of molten iron is too high (especially when the silicon content is also high), graphite spheroidization will be affected. The test shows that for thick wall castings, when the carbon equivalent exceeds the eutectic composition, flowering graphite may be produced. However, increasing the carbon content of molten iron is beneficial to the improvement of magnesium recovery. Therefore, the principle of high carbon and low silicon is mostly adopted in production, and the silicon content is usually controlled at about 2%. In addition, the selection of carbon equivalent is also related to the casting wall thickness: when the wall thickness is 6.5~76 mm, the carbon equivalent is 4.35%~4.7%; When the wall thickness is>76mm, the carbon equivalent is 4.3% ～ 4.35%.

(2) Sulfur: When the sulfur content in molten iron is too high, sulfur will form sulfide with magnesium and rare earth, which will float to the surface of molten iron due to its low density. These sulfide will react with oxygen in the air to form sulfur, which will return to molten iron and repeat the above process, thus reducing the content of magnesium and rare earth. When the sulfur in molten iron is greater than 0.1%, graphite cannot be completely spheroidized even if a large amount of spheroidizing agent is added.

(3) Rare earth and magnesium: When the content of rare earth and magnesium is too low, poor spheroidization or spheroidization decline often occurs. Generally, the amount of nodulizing agent required by the factory is 1.8%~2.2%.

(4) Wall thickness: if the casting wall is too thick, it is easy to produce poor spheroidization and recession defects, mainly because the molten iron is in liquid state for a long time in the mold, and the magnesium vapor floats up, resulting in the reduction of magnesium content; The latent heat of crystallization released by the formation of a large amount of graphite during eutectic makes the austenitic shell re melt, and the graphite extends out of the shell and grows deformed to form non spherical graphite.

(5) Temperature: If the temperature of molten iron is too high, the molten iron will be seriously oxidized. Because magnesium and rare earth are easy to react with the oxide, the content of magnesium and rare earth will be reduced. At the same time, high temperature will also increase the burning loss and evaporation of magnesium; If the temperature of molten iron is too low, the nodulizer cannot be melted and absorbed by molten iron, but floats to the surface of molten iron for combustion or oxidation.

(6) Retention time: The content of magnesium in molten iron decreases with the increase of retention time after inoculation, mainly due to the oxidation and evaporation of sulfur, magnesium and rare earth. Generally, the retention time shall not exceed 20min.

(7) Gating and riser: If the design of the gating and riser is not reasonable, the pouring time will be too long, the molten iron will splash and the air will be drawn in, causing serious oxidation of magnesium and rare earth.

5.2 Preventive measures

(1) Strictly control the composition of molten iron: select appropriate carbon equivalent; The sulfur content in molten iron shall be less than 0.08% (the sulfur content of pig iron shall not be greater than 0.03%, and that of coke shall not be greater than 0.08%). Sodium bicarbonate can be used for desulfurization.

(2) Add enough nodulizing agent, generally 1.8%~2.2%; In addition, attention shall be paid to the quality of nodulizing agent. If the nodulizing agent is used after being broken, the storage time shall not exceed one week. The residual amount of rare earth magnesium in the treated ductile iron molten iron should not be too low, with Mg residue>0.02% and RE residue>0.02%.

(3) Reasonably design the casting structure to avoid excessive wall thickness, or add cold iron at the wall thickness to improve the solidification speed and shorten the liquid time, so as to prevent spheroidization recession and defects.

(4) Pay attention to the processing temperature. The discharging temperature shall be lower than 1460 ℃ to prevent severe burning loss of nodulizer; To prevent oxidation under high temperature, cover the iron plate covered with nodulizer (thickness should be>3mm); The molten iron shall be covered with plant ash after being raked off; When the temperature of molten iron is higher than 1350 ℃, the spheroidizing agent can be added; When the temperature is lower than 1350 ℃, the nodulizer can not be added, nor can the nodular iron castings be poured. Only other molten iron can be added to pour unimportant gray iron castings or core bones.

(5) The molten iron shall be poured timely after being discharged from the furnace, and the retention time shall not exceed 20min.

(6) The gating and riser shall be reasonably designed, and spheroidizing treatment shall be adopted in the mold and on the mold to strengthen inoculation.