Key points for melting and spheroidizing treatment of ductile iron

1、 Basic conditions for production

The chemical composition of ductile iron castings must be strictly controlled, otherwise it is difficult to meet customer requirements. Strictly control the process from the selection of raw materials, formulation of smelting process, temperature control, and pre furnace spheroidization incubation treatment. Below, we will summarize and introduce the key points of ductile iron smelting and spheroidization treatment!

1.1 Smelting equipment

The quality requirements of cast ferrite ductile iron for the original molten iron are: stable chemical composition, in compliance with design requirements; Good metallurgical quality, cleanliness, and no oxidation phenomenon; High tapping temperature (usually 1500-1550 ℃).

1.2 Styling equipment

During the solidification of ductile iron, graphite precipitates and expands greatly, which can be utilized in production. Pay attention to the rigidity and hardness of the mold to avoid mold expansion, and obtain a dense, shrinkage free, and macroscopic shrinkage free casting.

2、 Control of raw materials

2.1 Cast pig iron

Pig iron should be high in carbon, low in silicon, low in phosphorus, and low in sulfur. Less interfering elements. But if it contains interfering elements, it can lead to poor spheroidization. Try to avoid or minimize the use of pig iron with large graphite flakes and more slag pores. When selecting pig iron, the silicon content (grade) of the pig iron should be determined based on the requirements of the ductile iron matrix and the amount of furnace feed. Secondly, the manganese content is limited (divided into groups) based on toughness requirements and heat treatment. The less phosphorus and sulfur content in pig iron, the better. The classification of phosphorus and sulfur, as well as the quantification of different types of pig iron, vary greatly. Pig iron used for ductile iron has special phosphorus (P ≤ 0.05%) and special sulfur (S ≤ 0.02%). Steel making pig iron has a special type of sulfur (S ≤ 0.02%), but phosphorus is generally higher. Cast pig iron does not have special phosphorus and special sulfur, with phosphorus and sulfur values of ≤ 0.06% and ≤ 0.03%, respectively.

2.2 Return to furnace iron

The use of ductile iron sprues and scrap castings produced from recycled iron is prohibited, and the use of waste cast iron parts purchased from the market is prohibited.

2.3 Scrap steel

Scrap steel is mainly used to adjust carbon content or as the main raw material for producing ductile iron by increasing carbon content in scrap steel. It should be rust free carbon steel with clear composition, and the use of alloy steel or scrap steel with unknown origin should be avoided.

2.4 Spheroidizing agents and inoculants

Spheroidizing agents require stable and uniform composition. Its selection should be based on the different melting equipment, sulfur content of molten iron, and the temperature at which molten iron is discharged from the furnace. When using a cupola for production, due to the relatively low temperature of the molten iron, high sulfur content, and poor purity of the molten iron, it is necessary to choose a spheroidizing agent with high rare earth and magnesium content, usually Mg7-9%, Re5%, or 7% spheroidizing agent; When producing electric furnaces, due to the required temperature of molten iron, the metallurgical quality of molten iron is good, and the purity is high. Therefore, spheroidizing agents with low rare earth and magnesium content should be selected, such as those containing Mg6-8% and Re2-4%, and those containing Mg5-7% and Rel-3% for thin and small parts.

Inoculant is an important iron alloy material in the production process of ductile iron. Adding inoculant after spheroidization treatment can not only eliminate cementite, but also improve spheroidization rate, refine graphite balls, and make them round and evenly distributed. Practice has proven that the effectiveness of pregnancy is closely related to the composition of the inoculant. At present, 75 silicon iron is widely used in China, and some units use long-acting composite inoculants containing elements such as barium, strontium, calcium, aluminum, bismuth, etc., such as silicon barium, silicon strontium, silicon calcium, etc.

3、 Furnace front control

3.1 Selection of chemical composition

As is well known, the structure and properties of ductile iron are closely related to its chemical composition. The composition design of ductile iron mainly consists of carbon and silicon, while others such as manganese, phosphorus, and sulfur are controlled by raw materials.

3.1.1 Carbon and silicon

The production of as cast ferrite ductile iron adopts a high carbon, low silicon, and high inoculation method. Carbon and silicon elements have a significant impact on the properties of ductile iron. It is usually considered comprehensively based on carbon equivalent. The selection of carbon equivalent mainly focuses on improving casting performance, eliminating casting defects, obtaining sound castings and high mechanical properties. Our company requires a ferrite matrix structure content greater than 80% for as cast ferrite ductile iron, and does not allow the presence of cementite. Therefore, when designing carbon equivalent, the criterion should be that the casting does not have graphite floating, does not produce white spots, and ensures spheroidization. The carbon equivalent should be controlled between 4.3-4.8%, with 3.3-3.9% carbon, 1.2-1.6% silicon content in the original molten iron, and 2.6-3.0% final silicon content. But the silicon content should not be too high, because silicon strengthens the ferrite matrix, making it brittle and reducing its plasticity and toughness.

3.1.2 Manganese

Manganese is an element that promotes the formation of pearlite. It solidly dissolves in ferrite to increase strength and reduce toughness, which is unfavorable for the production of as cast ferrite ductile iron. Therefore, it is required to keep manganese as low as possible, control the manganese content to be less than 0.4%, and thin and small parts to be less than 0.2%.

3.1.3 Phosphorus and sulfur

Phosphorus and sulfur are both harmful elements, and high phosphorus easily forms phosphorus eutectic at the grain boundaries of the matrix, reducing the elongation of the material and increasing its brittleness. High sulfur content can lead to low residual spheroidization elements, resulting in poor spheroidization, as well as casting defects such as subcutaneous pores and slag inclusions in castings. We control the phosphorus content to be below 0.06%, the sulfur content of the original molten iron before spheroidization treatment is below 0.04%, and after treatment it is less than 0.02%. 3.1.4 Residual magnesium and rare earth elements in cast iron should have a certain amount of residual magnesium and rare earth elements to ensure graphite spheroidization. Magnesium mainly plays a spheroidizing role in molten iron, while rare earths mainly serve as degassing, desulfurization, and neutralizing interfering elements, playing a role in purifying molten iron. Magnesium and rare earths are also strongly formed carbide elements. If the residual amount is too high, it will deteriorate the graphite morphology, increase the tendency of casting white spots and casting defects such as slag inclusion, shrinkage, and subcutaneous pores. Therefore, in the production of as cast ferrite ductile iron parts, while ensuring good spheroidization, the residual amounts of magnesium and rare earth should be minimized as much as possible, which is particularly important for thin and small parts. Usually, the residual magnesium content is controlled between 0.03-0.05%, and the residual rare earth content is controlled between 0.02-0.03%.

3.2 Spheroidization and incubation treatment

To ensure successful spheroidization, attention should be paid to the following points:

1) The structure of the molten iron ladle should meet the process requirements. Generally, the ratio of the height of the inner cavity to the inner diameter of the molten iron ladle is 1.0-1.2, which is conducive to the absorption of magnesium and rare earth elements. The bottom of the package should be lined with pits or dams.

2) Crush the spheroidizing agent into appropriate particle sizes. Particles that are too small are prone to floating and oxidation; If it is too large, the dissolution rate will be too slow, the reaction time will be prolonged, and the temperature of the molten iron will decrease significantly. The size of the particle size depends on the amount of iron processed each time. The spheroidizing agent should be broken on the same day and used on the same day. Do not leave it for too long or get damp.

3) The amount of spheroidizing agent added should be determined based on the smelting process, tapping temperature, and sulfur content of the raw iron.

4) During electric furnace smelting, the slag should be scraped clean before tapping.

5) The spheroidization treatment method should be reasonable. We use the flushing method, where the spheroidizing agent is placed in the pit and should be compacted tightly, covering it with inoculated silicon iron and cast iron chips. Do not add spheroidizing agents when the iron ladle is hot and red, to avoid oxidation and loss of spheroidizing ability. During iron tapping, the iron water flow cannot directly rush into the spheroidizing agent. The iron water flow cannot be too large or too small, and cannot be interrupted. The amount of iron produced should be as accurate as possible, with no tolerance of 5%. After the spheroidization reaction is completed, the slag collector should be added as soon as possible, stirred to remove the slag, and subsequent incubation treatment should be carried out.

The main function of inoculation treatment is to eliminate the tendency of white spots caused by spheroidizing elements, promote graphite precipitation, improve spheroidization rate, refine graphite balls, and make them evenly distributed. Different inoculants, dosages, methods, and times of inoculation have a significant impact on the mechanical properties of ductile iron. At present, domestic enterprises adopt different inoculants and methods, but they all emphasize the need for instantaneous inoculation process to enhance inoculation effect and ensure that the molten iron is in a good inoculation state for a long time until pouring is completed. The recommended method is intra package inoculation (first time)+molten iron ladle floating silicon inoculation (second time)+sub package inoculation (third time). Practice has shown that it is feasible and the inoculation effect is good.

3.3.3 Pre furnace inspection

The judgment of spheroidization in front of the furnace can be determined using intelligent instruments such as the furnace front triangular test block method, furnace front rapid metallographic method, and spectral analysis.

1) Triangle block method. The commonly used methods are vertical pouring or horizontal pouring. When the surface of the sample turns dark red, remove it and quench it downwards into water, breaking and observing the fracture surface. If there are indentations on the fracture surface and both sides, and looseness in the center, the fracture surface is rugged, serrated, silver gray (silver white), dense, with small grains. There is no white or<1-2mm white at the tip, and there is a sound of steel tapping and a smell of calcium carbide, indicating good spheroidization and incubation. If there are small black spots on the fracture surface, the spheroidization is not very good. If the fracture surface is black, the grain size is coarse, or there are many black spots, forming a pockmarked shape and making a dull sound when knocked, it is considered non spheroidization.

2) Rapid metallographic method in front of the furnace. The sample is φ 20 x 30mm (depending on the size and thickness of the casting). Metallographic observation shows that the spheroidization should be above 2, indicating good spheroidization. Below 2 indicates poor spheroidization.

3) Spectral analyzer, which can determine the chemical composition of molten iron within 2 minutes, is timely, accurate, and has small analysis errors.

4、 Casting process control

4.1  The casting process design of ductile iron parts should be conducive to promoting the formation of ferrite and obtaining sound castings.

1) After spheroidization treatment, ductile iron has a lot of thick slag, and the pouring system should consider skimming, slag blocking devices, and process measures;

2) Ductile iron is prone to oxidation. If turbulence and splashing occur during pouring, secondary oxidation slag will be generated. Therefore, when designing the pouring system, it is necessary to consider the stable horizontal entry of iron into the mold cavity;

3) During the solidification of ductile iron, graphite precipitates and the expansion is large. The mold must have sufficient rigidity to obtain castings without shrinkage cavities. In terms of process design, the implementation of non riser casting can also be considered;

4) Ductile iron has the characteristics of Congee solidification. The effective feeding distance of the riser is short. More risers can be set, or the riser can be matched with the cold iron to eliminate shrinkage cavity and porosity.

Given the above characteristics, when designing the casting process of as cast ferrite ductile iron, the gating system generally adopts a semi closed or open type.

5) The cooling rate of castings in the mold varies with different opening times. Different unboxing times will result in different matrix structures. To obtain a microstructure mainly composed of ferrite matrix in the as cast state, in addition to sufficient sand intake in the mold, the casting should have sufficient insulation and slow cooling time in the mold. At least the casting should be cooled to below 650 ℃ in the mold before being unpacked.