Ductile Iron Castings

Product Description

Ductile Iron Castings is a grey cast iron with excellent mechanical properties. Generally, before pouring, a small amount of nodularizing agent (usually magnesium, rare earth magnesium alloy or rare earth alloy containing cerium) and inoculant (usually ferrosilicon) are added to the molten iron to solidify the molten iron to form spherical graphite. The strength and toughness of this cast iron are higher than other cast irons, and it can sometimes replace cast steel and malleable cast iron, and has been widely used in the machinery manufacturing industry. Ductile iron was used for industrial production abroad in 1947.

After more than ten years of precipitation, Qinhuangdao Zhongwei Precision Machinery Co., Ltd. has rich experience in the production of various grades of ductile iron, superalloy castings, stainless steel and other castings. We expect manufacturers from all over the world to consult and negotiate business.

Ductile Iron Castings By Country

1. Implementation standards: The company strictly implements ISO9001 & TS 16949 certification.

2. Product material standards: ISO, GB, ASTM, SAE, ISO, EN, DIN, JIS, BS

3. Main processes: sand casting, silica sol investment casting, water glass investment casting,shell casting,deburring, sand blasting, machining, heat treatment, leak testing, surface treatment, etc.

4. Available materials:

And alloy steel, gray iron, cast iron, cast steel, cast aluminum, cast copper, etc. can be customized according to customer requirements.

Production Control Of Ductile Iron Castings

1. Difficulties in the production of ductile iron parts

Due to the thick section of such castings, the cooling is slow, and the solidification time of the metal liquid is long, and shrinkage porosity is easily generated inside the casting.

In the production of ferritic ductile iron, in order to obtain higher tensile strength, yield strength and elongation, ferritic heat treatment was carried out in the past. The heat treatment temperature is based on whether there is free cementite or pearlite in the as-cast structure. , while the high temperature heat treatment of 900-950 ° C is used. 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 as-cast state. Therefore, the difficulties in producing this material mainly include the following aspects:

a. The casting needs to be subjected to regional radiographic inspection, and how to solve the internal shrinkage porosity of the casting;

b. How to ensure that more than 90% of the ferrite matrix is obtained in the as-cast state;

c. How to make the material have sufficient tensile strength and yield strength;

d. How to obtain sufficient elongation (>18%) and obtain the specified elongation after alloying treatment;

e. Alloying process used.

2. Quality control technology for as-cast ferritic ductile iron castings with thick and large sections

(1) Control of chemical composition

1) Selection of C, Si, CE

Since the weakening effect of spheroidal graphite on the matrix is very small, the amount of graphite in the ductile iron has no significant 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. The fluidity of molten iron with eutectic composition has a large tendency to form concentrated shrinkage cavities, and the density of the casting structure is high. However, when the carbon equivalent is too high, it is easy to cause graphite to float, and at the same time, it affects the spheroidization to a certain extent, which is mainly manifested in the high 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 in gray cast iron, so the level of 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 the silicon content in the ductile iron greatly improves the strength index and reduces the toughness. The spheroidized molten iron of ductile iron has a large tendency to crystallize undercooling and form white mouth, and silicon can reduce this tendency. However, the control of silicon content is too high, which promotes the formation of fragmented graphite in the large-section ductile iron and reduces the mechanical properties of the casting. The data shows that silicon in ductile iron is added in the way of inoculation, which improves the performance to a certain extent.

According to the above analysis, from the perspective of improving casting performance, the carbon equivalent of molten iron is selected near the eutectic point. At this time, the fluidity of molten iron, the tendency to concentrate shrinkage cavities is large, and it is easy to feed. 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 flotation, but not the 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 castings can be selected to be higher, and graphite floating will not occur. On the contrary, the carbon equivalent of thick and large castings should be selected 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 absence of cementite to ensure complete globalization. Under this premise, carbon equivalent should be increased as much as possible to obtain dense castings.

2) Manganese (Mn)

Manganese plays a different role in ductile iron than in grey cast iron. In gray cast iron, in addition to strengthening ferrite and stabilizing pearlite, manganese can also reduce the harmful effect of sulfur. In ductile iron, the spheroidizing element has a strong desulfurization ability, and manganese no longer has this effect. Because manganese has a serious positive segregation tendency, it is often enriched at the grain boundaries 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 segregation tendency of manganese is more serious. At the same time, with the increase of manganese content, the content of pearlite in the matrix increases, so the strength index is improved, and the toughness is reduced at the same time. The control of manganese content in high toughness ductile iron should be stricter.

Therefore, the lower the Mn, the better the raw material. The upper limit of manganese control for large castings is Mn<0.3%.

3) Phosphorus:

Phosphorus has a serious segregation tendency in ductile iron, and it is easy to form phosphorus eutectic at the grain boundary, which seriously reduces the toughness of ductile 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:

The sulfur in ductile iron has a strong ability to combine with spheroidizing elements, forming sulfides and sulfur oxides, which not only consumes the spheroidizing agent, resulting in unstable spheroidization, but also increases the number of inclusions and accelerates the spheroidization decline. Sulfur is involved in the recarburizer in the smelting process, while the process control reduces the sulfur content in the raw materials as much as possible, and measures are taken to desulfurize before the furnace.

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

5) Molybdenum:

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

6) Content of magnesium and rare earth

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

According to the above analysis and calculation, the final chemical composition is determined 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. Melting control

(1) Selection of raw materials

In the production of ferrite ductile iron, it is very necessary to select high-purity raw materials, and the content 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% ), the content of some alloying elements such as Cu, Cr, and Mo should be strictly controlled. Because many trace elements are most sensitive to spheroidization recession, such as tungsten, antimony, tin, titanium, vanadium, etc. Titanium has a great influence on spheroidization and should be controlled, but high titanium is the characteristic of pig iron in my country, which is mainly related to the metallurgical process of pig iron.

(2) Desulfurization

The sulfur content of the original molten iron determines the addition amount of the nodularizing agent. The higher the sulfur content in the original molten iron, the more the nodularizing agent is added, otherwise the casting with good nodularization cannot be obtained. Before spheroidizing treatment, the S content in the original molten iron was controlled below 0.02%. Desulfurization treatment must be carried out when the sulfur content of the molten iron before spheroidizing treatment is high.

(3) Mo alloy treatment:

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

(4) Spheroidizing 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 nodularizing agent, which can not only ensure the content of Mg that plays a role in nodularization, but also increase the anti-recession ability. of heavy rare earth elements, such as yttrium, etc. According to the test and production practice of many domestic factories, it is very ideal to use the composite nodulizer of Re-Mg and yttrium-based heavy rare earth as the nodulizer for the production of thick and large-section ductile iron parts. The actual production application process has also achieved good results. According to the relevant data, the spheroidizing ability of yttrium is second only to that of magnesium, but its anti-recession ability is much stronger than that of magnesium, and it does not return to sulfur, yttrium can be added in excess, and cementite will not appear when high carbon is inoculated well. In addition, yttrium and phosphorus can form high-melting inclusions, which reduce and disperse the phosphorus eutectic, 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, a special spheroidizing process is adopted. The control of the spheroidization treatment is mainly to control the reaction speed, and the spheroidization reaction time is controlled to be about 2 minutes.

For this purpose, a composite spheroidizer of medium and low Mg, Re spheroidizer and yttrium-based heavy rare earth is used, and the addition amount of the spheroidizer is determined according to the residual Mg amount.

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

(5) Inoculant and inoculation treatment

Nodularization treatment is the basis of ductile iron production, and inoculation treatment is the key to ductile iron production. The inoculation effect determines the diameter of graphite balls, the number of graphite balls and the roundness of graphite balls. In order to ensure the inoculation effect, multi-stage inoculation is adopted for inoculation treatment. deal with. The closer the inoculation treatment is to pouring, the better the inoculation effect. It takes a certain time from inoculation to pouring, and the longer the time, the more severe the inoculation decline. To prevent or reduce fertility decline, use the following measures:

A. Use long-acting inoculants (silicon-based inoculants containing a certain amount of barium, strontium, zirconium or manganese);

B. Adopt multi-stage inoculation treatment (incubation in the bag, inoculation in the inoculation groove, instantaneous inoculation in the nozzle, etc.);

C. Try to shorten the time from inoculation to pouring.

The amount of inoculant added is controlled at 0.6~1.4%. Too little inoculant will directly cause poor inoculation effect, and excessive inoculum will lead to inclusions in castings.

(6) Pouring process control

The pouring should adopt the principle of fast pouring and smooth pouring. 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 equal to the gross weight of the casting. When pouring, put the inoculant into the nozzle basin, and inject the molten iron into the nozzle at one time, so that the molten iron and the inoculant are mixed together. Mix well, scrape off the surface scum, and put out the nozzle plug for pouring.

4. The control principle of casting process

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 and large section to adjust the temperature field to accelerate the solidification of molten iron. (Some factories in the same industry use the forced cooling process, which means adding forced measures such as water cooling or air cooling under the condition of using cold iron to strengthen the solidification of castings and reduce the solidification time. The effect is very good. However, there are certain risks and technical requirements. High. In addition, in order to obtain the ferrite matrix, the unboxing temperature should be controlled below 600 °C.)

Post Casting Process

1. Heat treatment: annealing, carbonization, tempering, quenching, normalizing, surface tempering

2. Processing equipment: CNC, WEDM, lathe, milling machine, drilling machine, grinder, etc.;

3. Surface treatment: powder spraying, chrome plating, painting, sandblasting, nickel plating, galvanizing, blackening, polishing, bluing, etc.

Moulds and Inspection Fixtures

1. Mold service life: usually semi-permanent. (except for lost foam)

2. Mold delivery time: 10-25 days, (according to product structure and product size).

3. Tooling and mold maintenance: Zhongwei is responsible for precision parts.

Quality Control

1. Quality control: the defective rate is less than 0.1%.

2. Samples and trial run will be 100% inspected during production and before shipment, sample inspection for mass production according to ISDO standards or customer requirements

3. Testing equipment: flaw detection, spectrum analyzer, golden image analyzer, three-coordinate measuring machine, hardness testing equipment, tensile testing machine;

4. Provide after-sales service.

5. The quality can be traced back.

Application

● Pressure pipes and fittings: Many industrial countries use ductile iron as the material for making pipes and fittings, because it is more resistant to pressure than ordinary cast iron pipes during transportation, and it is also more convenient and quicker during construction, so choose it. It is sensible to be the material for pressure piping.

● Automotive applications: Ductile iron is mainly used in generators, gears, bushings, brakes and special devices in the automotive industry. Just like the well-known Ford company, almost all crankshaft parts are made of ductile iron.

● Agricultural machinery and construction applications: Generally, agricultural machinery requires a long life, and the various components formed of ductile iron can achieve this. In addition, bulldozers and cranes used in some construction projects or road paving are also applied to ductile iron.

● Valve manufacturing: Ductile iron is also mainly used in valve manufacturing. Ductile iron plays a great role in transporting liquids such as acid, alkali and salt.