Abstract:
This study delves into the application of digital wire feeding spheroidization stations in the production of QT800-5 bracket castings. By integrating digital technology, thermal analyzers, and iron transfer systems, we have established a digital operation platform. By selecting high-quality, high-purity raw materials, controlling elements that affect graphitization and anti-spheroidization, ensuring high spheroidization rates and reasonable alloy element contents, and controlling the proportion of ferrite and pearlite, we have effectively and stably produced QT800-5 bracket castings that meet technical requirements in terms of chemical composition, mechanical properties, and metallographic structure.
1. Introduction
Wire feeding technology is widely used in the casting industry, with operations and management of wire feeders evolving from semi-automation to full automation. The adoption of new intelligent, interconnected digital wire feeding spheroidization stations, combined with automated iron melt transfer and quantitative pouring devices, has shortened the time required for tapping, testing, and transferring iron melts, increased safety, improved the current situation of crane and manual trolley transportation of iron melts, enhanced the efficiency of melting equipment, and realized green and automated melting and pouring processes.
2. Technical Requirements for Bracket Castings
Bracket castings are subjected to longitudinal and lateral forces as well as moments from the guide arm. The strength and lifespan of this component are crucial to the safety of the entire vehicle during driving. With the rapid development of automotive energy conservation and emission reduction, lightweight and thin-walled castings, as well as increasingly stringent requirements for the quality of key components such as chassis suspension systems in heavy-duty trucks, the adoption of high-strength, high-ductility QT800-5 material has replaced lower-grade materials like QT450-10 or QT500-7. This switch not only meets product requirements but also enhances economic benefits and corporate reputation.
3. Production Process and Process Control for Bracket Castings
3.1 Equipment Conditions
We use a 2-ton medium-frequency induction furnace for melting and specialized wire-fed iron ladles.潮模砂 and KW static pressure molding lines are employed, along with a new digital wire feeding spheroidization platform that integrates digital means, intelligent digital control systems, environmental protection systems, and iron melt transfer systems to achieve fully automated operation and management of wire feeders. Equipment such as a German OBLF direct-reading spectrometer, an iron melt thermal analyzer for pre- and post-furnace analysis, a microcomputer-controlled electronic universal testing machine, and a metallographic microscope are used for testing.
3.2 Melting Process
3.2.1 Composition Requirements for Bracket Castings
The carbon content affects the mechanical properties of ductile iron by influencing the graphite structure and metal matrix, and should be between 3.6% and 3.8%. Silicon strongly promotes graphitization, reduces the tendency for white cast iron, and increases the ferrite content in the matrix. Considering the wall thickness of the casting, silicon is controlled between 2.4% and 2.6%. Manganese promotes and stabilizes pearlite, increases tensile strength, and reduces elongation, and is controlled between 0.3% and 0.5%. Alloy elements copper and tin are basic control elements used to regulate the pearlite and ferrite content within the matrix, but attention should be paid to tin’s solid solution effect, which can degrade graphite morphology and mechanical properties. Copper is controlled between 0.4% and 0.6%, and tin between 0.02% and 0.04%. Antimony is a strong pearlite and carbide-forming element that can increase the number of graphite nodules and refine the graphite, so antimony is controlled between 0.015% and 0.03%. Residual magnesium is set at 0.03% to 0.05%. The final chemical composition of the bracket casting, determined based on years of production experience, is shown in Table.
Table: Chemical Composition of Main Elements in Bracket Castings (Mass Fraction, %)
| Item | C | Si | Mn | Cu | Sn | Sb | P | S | Mg | RE |
|---|---|---|---|---|---|---|---|---|---|---|
| Raw Iron | 3.7~3.9 | 1.3~1.5 | <0.3 | <0.05 | <0.02 | – | – | – | – | – |
| Final Iron | 3.6~3.8 | 2.3~2.5 | 0.3~0.5 | 0.4~0.6 | 0.02~0.04 | 0.015~0.03 | <0.05 | <0.01 | 0.03~0.05 | 0.005~0.015 |
3.2.2 Charge Ratio for Bracket Castings
To obtain high strength and high elongation, which are required for QT800-5 castings, it is first necessary to ensure good metallurgical quality and control the influence of harmful elements on spheroidization. Therefore, high-purity pig iron, high-quality scrap steel, and a small amount of recycled material (free of sand and rust after shot blasting) are used. After comprehensive consideration, the charge ratio is determined to be: 40% high-purity pig iron, 40% scrap steel, 20% recycled material, plus carburizer and other alloys.
3.2.3 Wire Feeding Spheroidization and Inoculation for Bracket Castings
(a) Digital Wire Feeding Station
The digital wire feeding spheroidization station mainly consists of cored wire, a spheroidization ladle, a wire feeding platform, a wire feeder, an inlet cage and outlet conduit, a safety door and its lifter, a ladle cover and its lifting mechanism, dust removal piping, a control system, and a spheroidization roller path, as shown in Figure 2. Figure 3 depicts the construction and operational site of the wire feeding station.
(b) Selection of Cored Wire and Its Addition Amount
Yttrium-based heavy rare earth spheroidizing cored wire has characteristics such as iron melt purification, spheroidization, resistance to degradation, strong resistance to graphite distortion, low tendency for white cast iron, refinement of the matrix structure, and a wide range of applicability. To ensure consistency in spheroidization rates between the first and last pours, yttrium-based heavy rare earth spheroidizing cored wire is selected. The quality of the cored wire powder affects the spheroidization quality, so it is important to select material with small Mg and RE fluctuation ranges, uniform particle size distribution, and control the MgO/Mg ratio in the spheroidizing cored wire to ≤0.4%. Based on the structural characteristics, production conditions, and melting temperature requirements of the bracket casting, Longyi BXX-Q1E spheroidizing cored wire is selected, with a wire diameter specification of φ13 mm+0.5 mm and a chemical composition of ω(Mg) 24%~26%, ω(RE) 2%~3%, ω(Si) 40%~50%, ω(Ca) 3.5%~4.5%, ω(Ba) 2%~3%, ω(Sb) 0.5%. The preliminary calculated addition amount of spheroidizing cored wire is 24 m/t.
(c) Selection of Spheroidizing Ladle and Wire Feeding Speed
During wire feeding spheroidization treatment, iron melt splashing is severe. To prevent iron melt from splashing out of the ladle during wire feeding spheroidization, a certain safety height of 400~500 mm is required, and the aspect ratio of the spheroidizing ladle should reach 1.5~2.0, with an iron melt weight of 600 kg and a treatment temperature of 1,520~1,550 °C.
The wire feeding speed in the wire feeding spheroidization process determines the absorption rate of magnesium from the cored wire. An appropriate wire feeding speed should ensure that the spheroidizing wire detonates when it reaches 100~200 mm from the bottom of the liquid surface, to ensure sufficient desulfurization of the iron melt within the ladle. The main factors affecting the wire feeding speed are the size of the treatment ladle (aspect ratio) and the weight of the iron melt to be treated (more precisely, the depth of the iron melt), as well as the iron melt treatment temperature. Therefore, iron melt temperature and liquid surface height have a significant impact on wire feeding speed. Based on the melting mechanism of cored wire and experience, a temporary wire feeding speed of 20 m/min is set.