the production technology improvement of heavy vermicular iron castings, including the improvement of vermicularizing treatment equipment, the composition of cored-wire for vermicularizing treatment, the wire-feeding speed, the amount of wire added, the alloying process, the pouring process, and the investigation of the vermicularity degeneration process in thick sections. Tests were conducted on an engine cylinder block, and special materials for vermicular iron were selected to control the content of some trace elements and ensure the machining properties. The experimental results show that the vermicularity of the thick section (main bolt holes, camshaft holes) reaches 80%, the vermicularity of the thin section is 85%, the volume fraction of pearlite is 85% – 95%, the tensile strength is 480 – 510 MPa, the elongation is 1.5% – 3%, and the hardness is 200 – 215 HB, all of which meet the requirements of the RuT450 material standard.
Heavy-section vermicular iron castings have a slow cooling rate and a small degree of supercooling. After inoculation, the molten iron forms a small number of nuclei at a very small degree of supercooling, preventing the molten iron from cooling to a temperature at which more nuclei could have formed, resulting in coarse graphite in the castings and low casting performance. Due to the long time between the completion of vermicularizing and pouring, it is easy to cause vermicularity degeneration, making stable production more difficult. Our company has achieved stable production of more than 30 types of engine cylinder blocks, cylinder heads, and other castings by using the wire-feeding method to produce vermicular iron, strictly controlling each process link, changing the casting production process from extensive to refined management, and supplemented by advanced thermal analyzer technology and practical ultrasonic technology. With the high-quality development of the enterprise, some high-end engine materials prefer vermicular iron, especially the cylinder blocks and cylinder heads of large high-end engines (M55, WH20, WH25, etc.) have selected vermicular iron materials.
1. Production Technology Improvement of Heavy-section Castings
1.1 Improvement of Vermicularizing Treatment Equipment
When our company produces vermicular iron engine cylinder blocks and cylinder heads, the amount of molten iron treated per ladle is generally within 2 tons, and the wire-feeding treatment station uses a forklift to transport the treatment ladle in and out. With the development needs of heavy-section vermicular iron cylinder blocks, the amount of molten iron for vermicularizing treatment has increased to 5 tons or more, exceeding the carrying capacity of the original forklift transportation. After demonstration, the wire-feeding treatment station was changed to a mode where the treatment ladle is transported in and out by a flatbed truck. The changed wire-feeding treatment station can cooperate with ladle trays of different heights and can realize the versatility of various treatment ladles of different heights. While suitable for the vermicularizing treatment of large-tonnage vermicular iron castings, it can also meet the molten iron treatment requirements of existing small and medium-sized castings.
1.2 Development of the Composition of Cored-wire for Vermicularizing Treatment
The cored-wire for vermicularizing treatment is the key material used in the wire-feeding process to treat the molten iron, and the composition of the core material directly affects the vermicularizing treatment effect and the performance of the castings. The original cored-wire used in our company has a magnesium (Mg) content of 4% – 5%, which has obvious advantages when treating molten iron of 2 tons or less, but when producing heavy-section castings, quality fluctuations are prone to occur, specifically manifested as a long vermicularizing treatment time, a large temperature drop, a low absorption rate in the later stage of vermicularizing, resulting in difficult control of the residual magnesium (Mg) content, and easy occurrence of unqualified vermicularity. During the product development process, based on the vermicularizing treatment theory of heavy-section castings and the actual field application, multiple explorations and experimental verifications were conducted on the components and content of the cored-wire for vermicularizing treatment of heavy-section castings, and a special cored-wire for large vermicular iron castings with a magnesium (Mg) content of 8% – 11% was developed, and the chemical composition is shown in Table 1.
1.3 Development of Vermicularizing Treatment Process
1.3.1 Development of Wire-feeding Speed
The wire-feeding speed is a key factor in determining whether the wire-feeding treatment process can achieve the desired results. The determination of the wire-feeding speed needs to comprehensively evaluate factors such as the treatment station, the ladle structure, the depth of the treated molten iron, the treatment temperature, the composition of the cored-wire, the type and thickness of the cored-wire steel sheath, and the quality of the cored-wire production. When our company produces heavy-section vermicular iron castings, it matches a dedicated wire-feeding treatment station, specific cored-wire specifications, a dedicated ladle, and a treatment temperature, and optimizes the design of the corresponding orthogonal contrast test scheme. Through three methods: surface observation of the melting situation of the cored-wire steel sheath, spectral comparison of the composition difference between the molten iron on the surface of the ladle and the molten iron at the bottom of the ladle, and sampling thermal analysis to compare the vermicularizing state of the molten iron at different parts of the ladle, the wire-feeding speed for vermicularizing treatment of heavy-section vermicular iron castings suitable for our company’s actual situation was developed: the vermicularizing wire speed is 45 – 55 m/min, and the inoculating wire speed is 21 – 25 m/min.
Table 1 Chemical Composition of the Special Cored-wire for Heavy Vermicular Iron Castings (wt%)
| Element | Mg | Si | RE | Ca | Al | MgO | Fe |
|---|---|---|---|---|---|---|---|
| Content | 8 – 11 | 42 – 45 | 3 – 6 | 2 – 5 | 0.5 – 1 | 1 | Balance |
1.3.2 Development of the Amount of Wire Added
The amount of wire added directly affects the total amount of vermicularizing agent and inoculant added to the molten iron, and determines the level of vermicularity. In order to develop a wire-feeding process suitable for large vermicular iron castings, different wire-feeding amount process tests were conducted, and vermicular iron wedge test blocks with a vermicularity of 70% – 95% were trial-produced. It was deduced that under this process, the absorption rate of Mg is 50% – 70%, and the absorption rate of rare earth (RE) is 75% – 90%. To obtain a vermicularity of more than 80% in the bearing seat area of the cylinder block, according to the chemical composition setting of the original molten iron, the wire-feeding amount for producing heavy-section diesel engine cylinder blocks is 50 – 80 m for the vermicularizing wire and 20 – 30 m for the inoculating wire.
1.3.3 Development of Alloying Process
Currently, the wall thickness of the bearing seat area of the cylinder block reaches 80 – 120 mm, and the cooling rate is slow and the degree of supercooling is low. To meet the performance requirements of the heavy-section vermicular iron castings, a Cu and Sn alloying process was developed. The order and amount of alloy addition were explored and tested. Cu and Sn are low-melting-point alloys. According to the previous test results, directly adding the alloy to the bottom of the ladle can ensure the absorption rate. To ensure the performance of the vermicular iron, the determined addition amounts are: 0.5% – 0.9% for copper (Cu) and 0.05% – 0.09% for tin (Sn).
1.4 Development and Application of Pouring Method
1.4.1 Quantitative Ladle
To ensure the vermicularizing effect, according to the quality of castings such as the 12M55, WH20, and WH25 cylinder blocks, dedicated ladles (with tonnages of 5 tons, 6 tons, and 8 tons respectively) were selected, and the ladle diameter/height ratio was also appropriately adjusted.
1.4.2 Design of Quantitative Pouring Basin
To meet the quality requirements of high-end castings, a quantitative pouring basin was used to pour the castings. The specific process flow is as follows: the entire ladle of molten iron is first poured into the quantitative pouring basin, the molten iron is left to stand for 1 – 2 minutes and then the temperature is measured. After the temperature is qualified, the plug is pulled to allow the molten iron to flow smoothly into the mold cavity. The use of a quantitative ladle can reduce defects such as slag inclusion and porosity in the castings and improve the surface quality of the castings.
1.5 Investigation of the Vermicularity Degeneration Process in Heavy Sections
The production process of heavy-section vermicular iron castings is long, and the time from the completion of vermicularizing to pouring is twice or more than that of small and medium-sized vermicular iron castings. It usually takes 13 – 15 minutes to complete the pouring, and a vermicularity degeneration test is required to find out the speed of vermicularity degeneration. To study the degeneration of the vermicularity at different times after the completion of vermicularizing treatment, the sulfur (S) content in the original molten iron is controlled at ≤0.015% in the test, and the waiting time from the end of vermicularizing treatment to pouring is controlled at 15 minutes. The first test block is poured starting from 3 minutes after the completion of vermicularizing, and samples are taken every 3 minutes, a total of 5 times.
Through the vermicularity degeneration test, it can be seen that the vermicularity increases by about 5% (from 85% to 90%) from the end of vermicularizing treatment to 15 minutes after pouring, and there is no obvious degeneration phenomenon of the vermicularity. The vermicularity does not change significantly from 3 minutes, 6 minutes, and 9 minutes after vermicularizing, and the vermicularity remains at 85%. The vermicularity starts to degenerate at 12 minutes after vermicularizing, and the vermicularity is 90%. After 15 minutes, the vermicularity is 90%. This shows that in the actual production process, controlling the waiting time from the end of vermicularizing treatment to the end of pouring within 15 minutes can ensure the stability of the vermicularity.
2. Control Requirements for Special Materials for Vermicular Iron
To control the content of some trace elements (such as Ti, Pb, etc.) in the vermicular iron and ensure the machining properties, it is best to use special main raw materials for the vermicular iron. The specific standards are as follows:
(1) For high-purity pig iron, it is best to choose pig iron with low S, low P, low Ti, stable, high C, and low Si.
(2) For the special scrap steel for vermicular iron, it is recommended to use low-Ti carbon steel (such as Q235, Q255, etc.) with regular shapes, uniform thickness, and relatively stable types.
(3) The return charge for vermicular iron needs to be shot blasted and broken.
(4) A graphitizing and low-S carburizer should be selected.
3. Test Methods and Results
3.1 Equipment Used in the Test
An 8 t ABP medium-frequency induction furnace, a domestic vermicularizing treatment station, an OCC thermal analyzer, an imported direct reading spectrometer, and a C-S analyzer were used to detect the composition of the original molten iron and the molten iron after vermicularizing treatment. A metallographic microscope was used to observe the microstructure of the test blocks, and a tensile testing machine and a hardness tester were used to measure the tensile strength and hardness of the test blocks.
3.2 Controlled Data during the Test Process
The material grade is RuT450, and the chemical composition of the castings is shown in Table 2, and the wire-feeding parameters for vermicularizing and inoculating treatments are controlled as shown in Table 3.
3.3 Test Results
According to the different wall thicknesses of the castings, the corresponding sizes of the attached test blocks were designed, and the test results of the metallographic structure and mechanical properties are shown in Table 4, all of which meet the technical requirements.
To ensure the performance of the castings, the metallography at different locations was detected, and the vermicularity in the thick position (main bolt holes, camshaft holes) is 80%, and the vermicularity in the thin position is 85% .
4. Conclusion
Through tackling the technical difficulties of vermicularizing equipment, cored-wire composition, vermicularizing treatment process, and vermicularity degeneration one by one, and selecting special materials for vermicular iron, a breakthrough in the production of heavy-section vermicular iron castings has been achieved.
Table 2 Chemical Composition of the Castings (wt%)
| Si | Mn | Cu | Sn | RE | Mg |
|---|---|---|---|---|---|
| 1.8 | 0.3 – 0.5 | 0 – 0.8 | 0.03 – 0.08 | 0.01 – 0.025 | 0.08 – 0.020 |
Table 3 Wire-feeding Parameters Control for Vermicularizing and Inoculating Treatments
| Mg Wire/m/min | Inoculating Wire/m/min | Liquidus Temperature/°C | Mg Index | Inoculating Index |
|---|---|---|---|---|
| 45 – 55 | 21 – 25 | 1140 – 1150 | 12 – 20 | 2 – 8 |
Table 4 Inspection Results of Metallographic Structure and Mechanical Properties
| Item | Vermicularity/% | Volume Fraction of Pearlite/% | Tensile Strength/MPa | Elongation/% | Hardness/HB |
|---|---|---|---|---|---|
| Value | 80 – 85 | 85 – 95 | 480 – 510 | 1.5 – 3 | 200 – 215 |
To further expand the content, we can provide more detailed explanations and examples for each section. For example, in the section on the improvement of vermicularizing treatment equipment, we can describe in more detail how the change in the transportation method of the treatment ladle affects the production process and the advantages of the new wire-feeding treatment station. We can also mention any challenges or considerations that were taken into account during the modification.
In the section on the development of the composition of cored-wire for vermicularizing treatment, we can explain the reasons why the original cored-wire with a lower Mg content is not suitable for heavy-section castings and how the new cored-wire with a higher Mg content addresses these issues. We can also provide more information about the exploration and testing process for determining the optimal composition of the cored-wire.
When discussing the development of the vermicularizing treatment process, we can elaborate on the factors that need to be considered when determining the wire-feeding speed and how the orthogonal contrast test scheme was designed to optimize this speed. For the amount of wire added, we can provide more data or examples to illustrate the relationship between the wire-feeding amount and the vermicularity, as well as the factors that affect the absorption rates of Mg and RE. In the alloying process section, we can explain in more detail how the addition of Cu and Sn alloys improves the performance of heavy-section vermicular iron castings and the specific effects of these alloys on the microstructure and properties of the castings.
In the section on the pouring method, we can provide more details about the design of the quantitative ladle and the quantitative pouring basin, including how the ladle diameter/height ratio was determined and how the quantitative pouring basin helps to reduce defects and improve the surface quality of the castings. We can also mention any additional measures or techniques that are used in conjunction with the pouring method to ensure the quality of the castings.