Inoculation treatment of the thickest large sectio

2022-08-08
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Inoculation treatment of heavy section nodular iron castings with China's entry into WTO and the development of shipbuilding industry, the demand for heavy section nodular iron castings is increasing. In recent years, on the basis of mass production of crankshaft with shaft diameter of mm, the factory has continuously produced 6 pieces for the ship repair Institute ф one thousand six hundred and fifty-one × 415 mm, max. hot spot ф The static bearing body with 331 mm and weight of 5750kg has obtained user satisfaction

nodular iron castings with thick and large cross-section have some problems in production, such as slow cooling rate, graphite distortion in the thick wall center or hot spot of the casting, reduction of the number of balls, coarse structure and graphite floating. Therefore, the mechanical properties of castings are significantly reduced, which brings difficulties to the production of large section castings. Combined with production practice, we believe that we should strengthen control in the following aspects

1. selection of raw materials and design of chemical composition

(1) selection of raw materials

use 14# pig iron of Tianjin Iron Works (c4..4%sil..44%p ≤ 0.06%, s ≤ 0.02%). Cupola and medium frequency furnace are used for duplex smelting, CaC2 is used for desulfurization, RE Mg alloy is used as nodulizer, and the inoculant containing (re%, mg%, Si%) is 75 Si Fe. After spheroidizing agent and inoculation treatment, it can meet the production of thick section ductile iron castings

the effective stroke of the tensile machine shall be at least 880mm to ensure the normal operation of the experiment when scrap steel is added in the batching. Alloy elements, such as Cu, Mo, Sb, etc., play a certain role in improving and stabilizing the grade of thick and large section ductile iron castings

(2) design of chemical composition

raw hot metal c3 8%Sil1.. 4,Mn0.. 4%, s ≤ 0.02%, P ≤ 0.05%

C3 after spheroidization 4%,Si2.. 4%,Mn0.. 8%,S≤0.02%,P≤0.05%

RE0.. 03%,Mg0.. 06%,Cu0.. 8%,Mo0.. 4%

Sb0.. 01%

① mg makes nodular graphite round, which can slow down the spheroidization recession of large section nodular iron. Mg hinders the precipitation of graphite. The amount of residual mg is high, which increases shrinkage and brittleness. Mg is easy to oxidize and forms an oxide film on the surface of molten iron. 2. The operation steps of rubber fatigue testing machine. Sand mold is easy to produce slag inclusions and subcutaneous pores in castings. The amount of residual mg should be controlled under the premise of ensuring spheroidization. However, we consider that the long solidification time of large parts should improve the anti recession ability. The amount of Mg should be higher and controlled at 0 06%。

② re plays the role of spheroidization indirectly by counteracting the harmful effect of interfering elements. Therefore, using pig iron with more impurities can also produce well-developed ductile iron castings through the role of re. However, in thick and large castings, high re retention is likely to cause the increase of fragmented graphite, which is generally controlled at 0 03%。

③ Cu refines graphite and strengthens pearlite to prevent the formation of ferrite. Add 0.5% Cu refines sorbite to thick large section castings to significantly improve as cast mechanical properties and improve the homogeneity of thick section microstructure, generally adding 0 8%。

④ Mo is a stable carbide element, which can improve the creep resistance of thick and large section ductile iron and improve the spheroidization grade. The addition amount is 0 4%: Mo has a similar effect with copper, which can promote spheroidization in the center of large section castings and inhibit graphite floating. The addition amount of cu:mo is 2:1

⑤ produce thick and large cross-section pearlitic ductile iron, add an appropriate amount of sb energy, open the air filter on the oil tank to improve the amount of pearlitic light, and improve the spheroidization condition at the center of the cross-section, so that the ball diameter is small, the number of balls is large, and the as cast performance is improved. The addition of sb also improves the strength and hardness of the matrix. Using sb, the white mouth tendency is significantly lower than that of conventional alloys, and the spheroidization grade is also improved. To produce thick pearlite spheres with large cross-section, adding a certain amount of sb can inhibit the appearance of fragmented graphite

2. spheroidizing agent and spheroidizing treatment

(1) generally, the spheroidizing agent used for spheroidizing agent is mg and RE Mg alloy. Mg treated ductile iron is small and round. Pressure plus magnesium method is widely used. At present, our factory uses RE Mg alloy as nodulizing agent. Practice has found that in the production of thick and large section ductile iron, RE Mg alloy can play the characteristics of Mg nodulizing strength and good re and s degassing effect, which is more suitable for thick and large section ductile iron parts

(2) spheroidizing treatment we adopt the impulse method for spheroidizing treatment. The RE Mg alloy is broken into fragments with a particle size of mm. The amount of addition depends on the thickness of the casting. The amount of addition of the casting with a thickness of more than 200mm is 1 0%, treatment method: bake the spheroidized hot metal ladle with a capacity greater than 1/2 of the treated hot metal and a dam at the bottom of the ladle into dark red, then put the alloy fragment into the inner side of the iron flow at the back of the dike, tamp it, and then put 0.4% crushed Si Fe with a particle size of mm on it, and then cover it with 30mm thick straw ash or perlite after tamping. In order to delay the reaction time, generally, an iron plate is covered on the straw ash to reduce the reaction temperature of the spheroidizing agent and delay the reaction time, The iron tapping temperature is ℃. If the temperature of molten iron is>1500 ℃, the spheroidization reaction is intense, resulting in molten iron decarburization, which can reach 0.5%. Moreover, a large amount of Mg volatilizes, which increases the consumption of Mg and affects the spheroidization. The iron flow cannot directly rush to the alloy. It is better to start the reaction when the iron water surface overflows over the alloy for more than 200mm. The first rust amount should account for 2/3 of the total amount. The spheroidization reaction takes minutes, and then scrape off the slag on the water surface after it is still (at this time, the iron is very thick, and there is a small flame on the liquid surface), and then rush into the remaining 1/3 of the iron. The purpose is to rush Si Fe for inoculation

3. inoculants and inoculation treatment

inoculants are required to have a strong role in promoting graphitization, and can be maintained for a long time, with high and stable absorption rate, less cooling and low price. We choose 75Si Fe as inoculants

after the hot metal spheroidization treatment, the test piece is white, because mg hinders the precipitation of graphite. At this time, Part C exists in the form of cementite. The inoculation treatment is to promote the nucleation of graphite by adding Si Fe to obtain a wide range of fine graphite balls. The purpose of putting a layer of Si Fe on the spheroidizing agent is to counteract the effect of Mg hindering the precipitation of graphite, and the inoculation effect is not great. Inoculation must be carried out when hot metal is added, and Si Fe must be flushed into it to fully stir and scrape the slag. At this time, the fracture of the test piece is silver gray, and there are depressions on both sides of the test piece

in practice, it is found that a bag of treated ductile iron is poured. After pouring the remaining molten iron for about 20 minutes, it is found that the white mouth is increased, which belongs to the inoculation recession. It is too large, deformed, and the number is reduced. Therefore, when pouring thick section ductile iron parts, considering the inoculation recession, we do not use the method of one-time large inoculation, but use multiple inoculation. Generally, after the second inoculation, we also use floating silicon in the bag for inoculation, and put Si Fe blocks in the nozzle, It melts with pouring, and its weight and blockiness depend on the pouring speed of molten iron weight. For parts with uniform thickness ≥ 200mm, use an iron funnel to sprinkle Si Fe powder with the iron flow, and add inoculant to the mold cavity, but calculate the weight and flow rate

for the poor spheroidization of the center of thick section, the main problem is spheroidization recession. During the second inoculation, a small amount of sb was added to the molten iron to reduce the spheroidization recession and improve the spheroidization of graphite at the center of the section

in practice, it is concluded that under the condition that the composition, temperature and nodulizing agent of molten iron remain unchanged, the nodularization of large section ductile iron pieces inoculated for many times is good, and the graphite balls of the test block poured in combination are more and round, on the contrary, the graphite is less, large and deformed. Therefore, on the premise of ensuring that the final silicon content remains unchanged, the method of multiple inoculation is adopted, and the nucleating agent is continuously added to the molten iron to ensure the quality stability of thick and large face ductile iron parts

castings with the above treatment process achieve the desired purpose. There was no incubation recession, and the Y-shaped samples poured in the later stage were tested for mechanical properties σ b=701N/mm2, δ= 2.3% metallographic observation: the selection of chemical composition of 100mm thick large section ductile iron castings with spheroidization level of 2 and spheroidal diameter of 5 will directly affect the success or failure of the production of thick large section ductile iron castings; Selecting CE correctly, using reg as spheroidizing agent and inoculating for many times can prevent spheroidization from declining. The General Administration of quality supervision, inspection and quarantine and the National Standards Administration approved the issuance of the new standard gb/t16491-2008 electronic universal testing machine in June 2008, with defects such as graphite floating and ball number reduction

(2) add some alloy elements such as copper, molybdenum and antimony to the chemical composition, use a small amount of multiple inoculation to inhibit graphite floating and graphite distortion, so that the middle and bottom of the thick wall of ductile iron still maintains good spherical graphite, obtain spheroidal luster as cast, and the performance reaches the QT index

(3) in order to speed up the cooling speed, the molding adopts the forming cold iron process, and forced fast cooling is necessary. (end)

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