What are the difficulties in the production of ductile iron

ductile iron? How to control more points?

ductile iron has excellent mechanical properties. Generally, before pouring, a small amount of spheroidizing agent (usually magnesium, rare earth magnesium alloy or rare earth alloy containing cerium) and inoculant (usually ferrosilicon) are added to the molten iron to form spheroidal graphite after solidification. The strength and toughness of this cast iron is higher than other cast iron, sometimes can replace cast steel and malleable cast iron, in the machinery manufacturing industry has been widely used.

1. ductile iron parts

such castings due to the thick section of the slow cooling, metal liquid solidification time is long, the casting is prone to internal shrinkage.

In the production of ferrite ball ink cast iron, in order to obtain high tensile strength, yield strength and elongation, in the past to carry out ferrite heat treatment, heat treatment temperature is based on the existence of free carburizing body or pearlite in the cast tissue, and the use of 900-950 degrees C high temperature heat treatment. However, the high production cost, complex process and long production cycle bring great difficulties to the production organization and delivery time, which requires that the ferrite matrix must be obtained in the cast state. Therefore, the difficulties in producing this material are mainly in the following aspects:

a. The casting shall be subjected to radiographic inspection in the designated area, and how to solve the internal shrinkage of the casting;
B. How to ensure that more than 90% of the ferrite matrix is obtained in the cast state;
c. How to make the material have sufficient tensile strength and yield strength;
d. How to obtain sufficient elongation (>18%) to obtain the specified elongation after alloying treatment;
c. Use the optimal alloying process.

2.

1. Control of chemical composition

1) Selection of C, Si, CE

due to the weakening effect of spherical graphite on the matrix is very small, the amount of graphite in ductile iron has no significant effect on the mechanical properties, and when the carbon content changes in the range of 3.2%~ 3.8%, there is no obvious effect on the mechanical properties. Therefore, when determining the carbon and silicon content in the process, the main consideration is to ensure the casting performance, and the carbon equivalent is selected around the eutectic composition. Liquid iron with eutectic composition has the best flow performance, the tendency to form concentrated shrinkage holes is large, and the density of the casting tissue is high. However, when the carbon equivalent is too high, it is easy to produce graphite floating, at the same time, it has an impact on the spheroidization to a certain extent, mainly in the high amount of residual Mg required. Increase the number of inclusions in cast iron and reduce the performance of cast iron.

the effect of increasing ferrite in silicon ductile iron is greater than that of gray cast iron, the silicon content directly affects the amount of ferrite in the ductile iron matrix. Silicon has a great influence on the performance of ductile iron, which is mainly manifested in the solid solution strengthening effect of silicon on the matrix, and silicon can refine graphite and improve the roundness of graphite balls. Therefore, the increase of silicon content in ductile iron greatly improves the strength index and reduces the toughness. Nodular cast iron after spheroidization treatment of molten iron has a greater tendency to crystallize and form a white mouth, silicon can reduce this tendency. However, the amount of silicon control is too high, large section ductile iron to promote the generation of broken block graphite, reduce the mechanical properties of castings. The data show that the silicon in the ductile iron is added in the way of inoculation to improve the performance to a certain extent.

According to the above analysis, from the point of view of improving the casting performance, the carbon equivalent of iron water is best selected near the eutectic point, at this time the liquidity of iron water is the best, the tendency of concentrated shrinkage is larger, easy to fill and so on. However, too high carbon equivalent will cause graphite to float, and the thickness of the graphite floating layer will increase with the increase of carbon equivalent. It should be pointed out that too high carbon equivalent is the main reason for graphite floating, but it is not the only reason. Casting size, wall thickness, and pouring temperature are also some important factors.

the relationship between carbon equivalent, casting wall thickness and graphite floating, it is obvious that the carbon equivalent of thin casting wall can be selected to be higher, and graphite floating will not occur. On the contrary, the carbon equivalent of thick castings should be selected to be lower. In short, the upper limit of carbon equivalent is based on the principle of no graphite floating, and the lower limit is based on the principle of no cementite, ensuring that globalization is completed. Under such a premise, carbon equivalent should be increased as much as possible in order to obtain dense castings.

2) Manganese (Mn)

manganese plays a different role in ductile iron than in gray iron. In gray cast iron, manganese can not only strengthen ferrite and stabilize pearlescent body, but also reduce the harmful effect of sulfur. In ductile iron, the spheroidizing elements have a strong desulfurization ability, and manganese no longer has this effect. Because manganese has a serious tendency of positive segregation, it is often enriched in the grain boundary of the eutectic group, which promotes the formation of intergranular carbides and significantly reduces the toughness of ductile iron. For thick and large section ductile iron, the tendency of manganese segregation is more serious. At the same time, the increase of manganese content increases the content of pearlite in the matrix, so the strength index is improved and the toughness is reduced. The manganese content in high toughness ductile iron should be controlled more strictly.

, therefore, the lower Mn is possible in the case of raw materials, the better. The upper limit of manganese control for large castings is Mn<0.3%.

3) Phosphorus:

phosphorus has a serious tendency of segregation in nodular cast iron, and it is easy to form phosphorus eutectic at the grain boundary, which seriously reduces the toughness of nodular cast iron. Phosphorus also increases the shrinkage tendency of ductile iron. When ductile iron is required to have high toughness, phosphorus should be controlled below 0.06%.

4) Sulfur:

sulfur and spheroidizing elements in ductile iron have a strong ability to combine to generate sulfides and sulfur oxides, which not only consume the spheroidizing agent, resulting in unstable spheroidization, but also increase the number of inclusions and accelerate the rate of spheroidization recession. Sulfur involved in smelting from the carburant, process control as far as possible to reduce the sulfur content in raw materials at the same time, take the furnace desulfurization measures.

is treated with Re-Mg alloy, the residual amount of sulfur is less than 0.02% , which has no effect on the spheroidization recession and sulfide slag inclusion. When the S in the original molten iron is more than 0.02% , desulfurization treatment must be used.

5) Molybdenum:

Mo improves the high temperature strength and room temperature strength of the material. Due to the use of Mo, it is easy to form a certain amount of pearlite and carbide, reducing toughness. For nodular cast iron alloyed with Mo, the material specification requires Mo content to be controlled by 0.3~0.7.

6) Magnesium and rare earth content

Magnesium is the main spheroidizing element, and rare earth has desulfurization, neutralization and anti-spheroidizing elements, has a protective effect on Mg, and improves the anti-recession ability of molten iron. However, the rare earth element is a carbide-forming element, and therefore, the residual amount of the rare earth is controlled as much as possible while ensuring good spheroidization. When Re = 0.01~0.04% and Mg = 0.03~0.06%, spheroidization can be guaranteed.

According to the above analysis and calculation, the final chemical composition is as follows:

C:3.3-3.8%;Si:2.2-2.7%;Mn:<0.30%;S<0.02%;Re=0.01~0.04%;Mg=0.03~0.06%,Mo:0.3~0.7%

3. smelting control

1. Selection of raw materials

the production of ferritic ductile iron, it is very necessary to select high purity raw materials. The contents of Si, Mn, S and P in the raw materials should be less (Si<1.0, Mn<0.3, S<0.03, P<0.03), and the contents of Cu, Cr, Mo and other alloying elements should be strictly controlled. Because many trace elements are most sensitive to spheroidization, such as tungsten, antimony, tin, titanium, vanadium and so on. Titanium has a great influence on spheroidization and should be controlled, but high titanium is the characteristic of pig iron in China, which is mainly related to the metallurgical process of pig iron.

2. Desulfurization

the sulfur content of the original iron liquid determines the amount of spheroidizing agent, the higher the sulfur content in the original iron liquid, the more the amount of spheroidizing agent is added, otherwise the casting with good spheroidization cannot be obtained.S content in the raw iron liquid was controlled below 0.02% before the spheroidization treatment.When the sulfur content of the raw iron liquid before the spheroidization treatment is high, it must be desulfurized.

3.Mo alloy treatment:

Mo alloying treatment, the eddy current process is used, and the addition amount is controlled at 0.5-1.0, which is adjusted according to the final Mo content. In order to ensure effective absorption of Mo, the grain size of the alloy should be strictly required.

4. Sphorizing agent and spheroidizing treatment

When producing thick and large section ductile iron parts, in order to improve the anti-recession ability, a certain proportion of heavy rare earth is added to the spheroidizing agent, which can not only ensure the content of Mg for spheroidization, but also increase the heavy rare earth elements with high anti-recession ability, such as yttrium. According to the test and production practice of many domestic factories, it is very ideal to use the compound spheroidizing agent of Re-Mg and yttrium based heavy rare earth as the spheroidizing agent for the production of thick and large section ductile iron parts, and the use of this spheroidizing agent has also achieved good results in the actual production and application process of our company. According to the relevant information, the spheroidization ability of yttrium is second only to magnesium, but its anti-recession ability is much stronger than magnesium, and does not return to sulfur, yttrium can be added in excess, high carbon inoculation is good, will not appear carburizing body. In addition, yttrium and phosphorus can form high melting point inclusions, so that the phosphorus eutectic is reduced and dispersed, thereby further improving the elongation of ductile iron. In the spheroidizing treatment, in order to improve the absorption rate of magnesium, control the reaction speed and improve the spheroidizing effect, the special spheroidizing process is used. control of the spheroidizing process,is mainly controlled on the reaction speed, and the spheroidization reaction time is controlled at about 2 minutes.

uses a compound spheroidizing agent of medium and low Mg, Re spheroidizing agent and yttrium-based heavy rare earth, and the amount of spheroidizing agent is determined according to the amount of residual Mg.

Prevention of spheroidization decline: on the one hand, the cause of spheroidization decline is related to the escape of Mg and RE elements from molten iron, and on the other hand, it is also related to the continuous decline of inoculation. In order to prevent spheroidization decline, the following measures are taken: A. Sufficient spheroidization element content should be maintained in molten iron; C, reduce the sulfur content of the original iron liquid and prevent oxidation of the iron liquid; C, shorten the residence time of molten iron after spheroidization treatment; D, after the molten iron is spheroidized and stripped of slag, in order to prevent Mg and RE elements from escaping, the surface of the molten iron can be covered tightly with a covering agent to isolate the air to reduce the escape of elements.

5. Inoculants and inoculation treatment

spheroidization treatment is the basis of ductile iron production, inoculation treatment is the key to the production of ductile iron, the inoculation effect determines the diameter of the graphite ball, the number of graphite balls and the roundness of the graphite ball, in order to ensure the inoculation effect, the inoculation treatment adopts multi-stage inoculation treatment. The closer the inoculation treatment is to the pouring, the better the inoculation effect is. It takes a certain amount of time from gestation to pouring, and the longer the time, the more severe the gestation decline. In order to prevent or reduce pregnancy decline, the following measures are taken:

A, the use of long-acting inoculant (containing a certain amount of barium, strontium, zirconium or manganese silicon-based inoculant);

B. Adopt multi-stage inoculation treatment (in-bag inoculation, inoculation tank inoculation, instantaneous inoculation at water outlet, etc.);

C, try to shorten the inoculation to pouring time.

the addition of inoculant is controlled at 0.6~1.4%, and the addition of inoculant is too small, which directly causes poor inoculation effect and excessive inoculation, resulting in casting inclusions.

6. Pouring process control

pouring should adopt the principle of fast pouring and smooth injection. In order to improve the uniformity of instantaneous inoculation and prevent slag from entering the cavity, the total capacity of the nozzle basin should be equivalent to the gross weight of the casting. During pouring, the inoculant should be put into the nozzle basin, and the molten iron should be injected into the nozzle at one time, so that the molten iron and the inoculant are fully mixed, the surface scum is stripped off, and the nozzle is blocked for pouring.

4. casting process control principles

1) reasonable casting process is a crucial factor,

2) The solidification time is controlled by the casting process. The principle is to place cold iron at the thick section to adjust the temperature field and accelerate the solidification of molten iron. (Some factories in the same industry adopt forced cooling process, which not only adds water cooling or air cooling and other forced measures to strengthen the solidification of castings and reduce the solidification time under the condition of using cold iron, but the effect is very good. However, there is a certain risk and high technical requirements. In addition, the unpacking temperature of ferrite matrix should be controlled below 600 ℃.)

5. result analysis
According to the above process method, in the production practice can be completely in the cast state to obtain the customer standard requirements, the production of the matrix is completely ferrite, the ball grade is ASTM standard I-II grade castings.

6. Conclusion

Through the analysis of the various factors affecting the cast iron ferrite ductile iron, the control, the performance indicators fully meet the requirements, in the premise of obtaining high tensile strength and yield strength, get high elongation, reasonable control of the spheroidization process, the whole ferrite structure under the cast state, to avoid the follow-up heat treatment.

According to the above-mentioned control technology of ferrite ductile iron, we have been fully verified in the production of ductile iron parts by GE Company in the United States, and achieved good results. The production technology of thick and large section ductile iron has been reported at home and abroad, although their methods are different, but the principles are the same. Namely:

1) strict control of raw materials, reasonable selection of chemical composition, reduce the content of elements that interfere with the ball.

2) Adopt effective recession prevention measures.

3) Choose a reasonable casting process.

4) reasonable choice of spherical inoculation treatment method.