In the manufacturing sector, steel casting plays a pivotal role, particularly in the production of railway components where durability and precision are paramount. The casting industry has been evolving towards greener practices to ensure sustainability, and one significant advancement is the adoption of ester-hardened sodium silicate sand technology. This method represents a major improvement over traditional ordinary water glass sand processes, which were prevalent in the late 20th century. Originally, steel casting for railway locomotives relied on a dual-sand system using ordinary water glass sand as facing sand and wet mold sand as backing sand, but this approach often led to issues like poor collapsibility, difficulty in cleaning castings, and defects such as porosity and cold cracks due to gas generation from high-temperature metal contact. To address these challenges, we embarked on a process optimization initiative, developing an ester-hardened sodium silicate sand molding system combined with modified water glass sand for core making. This shift not only enhances the quality of steel casting products but also aligns with mechanized production demands, reduces labor intensity, and improves the working environment. In this article, I will delve into the details of this optimization, including the advantages of the new process, equipment specifications, experimental setups, and results, all while emphasizing the critical role of steel casting in railway applications.
The traditional process for steel casting involved using ordinary water glass sand as the facing sand, with a thickness of 30–50 mm around the pattern, and wet mold sand as the backing sand. This was typically carried out on Z2520 molding machines, where the backing sand was compacted using air rammers at a pressure of at least 0.5 MPa. After molding, the backing sand surface was leveled, and vent holes were punched to facilitate gas escape. For core making, ordinary water glass sand was employed, with manual techniques to place core reinforcements, sand blocks, ropes, and lifting lugs during sand filling. The sand mixtures were prepared using specific ratios: for the facing sand, a S112 mixer was used to blend refined quartz sand in batches of 250 kg, dry-mixed for 2–3 minutes, followed by the addition of 45–50 kg of water glass (up to 6% by weight), and mixed for 8–10 minutes before being transported via conveyor belts. The backing sand, processed in a S115 mixer, involved adding 250 kg of used sand per batch, with 7–14 kg of bentonite and 30–40 kg of water, mixed for 1–3 minutes. However, this method resulted in high water glass consumption, poor sand collapsibility, and increased defects in steel casting, such as gas-related issues, due to the inherent properties of wet mold sand when exposed to molten metal. Consequently, we decided to overhaul this system by introducing ester-hardened sodium silicate sand for molding and modified water glass sand for core making, aiming to overcome these limitations and boost the efficiency of steel casting production.
Ester-hardened sodium silicate sand offers several advantages over conventional methods, making it ideal for steel casting applications. The hardening process occurs in three distinct stages: First, the organic ester hydrolyzes in an alkaline aqueous solution, producing organic acids or alcohols, which determines the usable time of the sand mixture. This can be represented by the hydrolysis reaction: $$ \text{RCOOR}’ + \text{H}_2\text{O} \rightarrow \text{RCOOH} + \text{R}’\text{OH} $$ where R and R’ represent organic groups. Second, the ester reacts with sodium silicate, increasing its modulus and leading to a dehydration reaction that solidifies the sand. This stage involves the formation of a gel-like structure: $$ \text{Na}_2\text{O} \cdot n\text{SiO}_2 + \text{RCOOH} \rightarrow \text{Silicate Gel} + \text{By-products} $$ Finally, further dehydration strengthens the固化, enhancing the overall durability. Compared to ordinary water glass sand, this ester-hardened system provides superior flowability, enabling easier mechanization in steel casting processes. The sand molds harden before pattern removal, ensuring dimensional stability and improved casting accuracy. Moreover, the lower moisture content in the sand reduces the risk of gas defects in steel castings. Another significant benefit is the enhanced collapsibility, which simplifies shakeout and cleaning, while facilitating sand reclamation and reuse of materials like chills and core reinforcements. This not only improves product quality but also cuts costs in raw materials and labor. Furthermore, the integration of ester-hardened sand with automated equipment, such as continuous mixers and robotic systems, lowers manual effort and boosts safety, creating a better work environment for steel casting operations.
To implement this optimized process, we utilized a range of equipment tailored for ester-hardened sodium silicate sand in steel casting. The sand molds were designed for various sizes, including 1600 mm × 1000 mm × 450 mm, 1200 mm × 1200 mm × 350 mm, 1000 mm × 1000 mm × 300 mm, and 1600 mm × 1000 mm × 320 mm, accommodating different steel casting components. A dual-arm continuous mixer was employed for sand preparation, with a capacity of 25 t/h, consisting of a base, large arm (conveying arm), small arm (mixing arm), pneumatic system, binder dosing system, sand flow control valve, and electrical control system. This mixer ensures uniform blending of sand, ester-hardened sodium silicate, and organic ester, critical for consistent steel casting quality. A vibration table with a load capacity of 6 t and a platform size of 2000 mm × 1300 mm was used for compaction, employing low-amplitude, high-frequency vibrations to achieve dense sand molds. A flip-and-draw machine facilitated pattern removal, featuring components like a base, O-type rotary frame, fixed and mobile roller conveyors, clamping cylinders, drawing devices, and hydraulic drives. For coating application, a move-out flow coater was integrated, equipped with support beam guides, sandbox clamping and flipping manipulators, a hydraulic station, and a coating circulation system, with a clamp opening range of 1500–2500 mm. Additionally, a hot air surface drying oven was used for post-molding drying, operating at temperatures of 120–200°C with ±5°C uniformity, a maximum sandbox size of 2000 mm × 1500 mm × 800 mm, and a drying cycle of 5–7.5 minutes per station, ensuring residual moisture below 0.2% for optimal steel casting results.
The formulation of sand mixtures is crucial for achieving high-quality steel casting. For ester-hardened sodium silicate sand, we used bagged refined quartz sand with specific grades: ZGS 98-30/50 (35C) for molding sand and ZGS 98-40/70 (50D) for core sand. Through extensive testing, we determined the optimal ratios: ester-hardened sodium silicate was added at 2.8–4.0% of the sand weight, significantly lower than the 6% in traditional methods, while organic ester was incorporated at 15–18% of the sodium silicate weight. This combination ensures adequate hardening and flowability for steel casting molds. For core making with modified water glass sand, the mixture involved adding refined quartz sand (100 kg for cores like the 102-type coupler, processed in an 80 kg shooting machine) and modified water glass at 4.5–6.0% of the sand weight, mixed for 30–60 seconds. Other steel casting cores were prepared manually using a mixer, with modified water glass at around 4.0%. The quality of these sands depends on factors like raw material purity, mixing efficiency, and temperature, all of which influence the performance in steel casting. To validate the process, we conducted small-batch trials on representative steel casting components, such as the 102-type coupler and new eight-axle locomotive axle boxes, producing two molds each (with two couplers per mold and one axle box per mold) and corresponding cores using the modified water glass sand. The organic ester and modified water glass were automatically dispensed in the continuous mixer, eliminating the need for conveyor belt transport and streamlining the steel casting production.
The properties of raw materials are fundamental to the success of steel casting. For silica sand, key indicators include SiO2 content, mud content, and grain size, which directly affect sand strength and defect rates. The table below summarizes the performance requirements for silica sand used in steel casting:
| No. | Test Item | Molding Sand: ZGS 98-30/50 (35C) | Core Sand: ZGS 98-40/70 (50D) | Inspection Frequency |
|---|---|---|---|---|
| 1 | SiO2 Content (%) | ≥ 98 | ≥ 98 | Per batch |
| 2 | Mud Content (%) | ≤ 0.3 | ≤ 0.3 | Per batch |
| 3 | Ignition Loss (%) | < 0.5 | < 0.5 | Annually |
| 4 | Angularity Factor | ≤ 1.45 | ≤ 1.45 | Annually |
| 5 | Fine Powder Content (%) | ≤ 1.0 | ≤ 1.0 | |
| 6 | Dry Sand Moisture (%) | < 0.2 | < 0.2 | Annually |
| 7 | Refractory Temperature (°C) | > 1700 | > 1700 | Annually |
| 8 | Average Fineness | 31–39 | 45–55 | Per batch |
| 9 | Three-Sieve Content (%) | ≥ 75 | ≥ 75 | Per batch |
| 10 | Four-Sieve Content (%) | ≥ 85 (30/40/50/70) | ≥ 85 (30/40/50/70) | Per batch |
For the binder systems, we used ester-hardened sodium silicate and organic ester from a supplier, adhering to standards like GB/T 4209-2022 and JB/T 8835-2013. The technical specifications and actual test results are shown in the following table, which highlights their suitability for steel casting:
| No. | Product Name | Performance Indicator | Actual Test Value | Result |
|---|---|---|---|---|
| 1 | Ester-Hardened Sodium Silicate | Density (g/cm³): 1.45–1.54 | 1.49 | Qualified |
| 2 | Organic Ester | Density (g/cm³): 1.05–1.25 | 1.1–1.2 | Qualified |
Additionally, modified water glass for core making in steel casting was evaluated, with density ranging from 1.47–1.52 g/cm³ and a modulus of 2.0–2.35, as per test results. The performance requirements for the sand mixtures are critical; for instance, modified water glass sand must have a permeability of at least 180 cm³·cm/(g·min), a dry tensile strength of over 1.3 MPa, and a moisture content of 2.0–4.0%, while ester-hardened sodium silicate sand requires a permeability of ≥ 100 cm³·cm/(g·min) and a dry tensile strength of ≥ 0.1 MPa. These parameters ensure that the sands can withstand the rigors of steel casting, such as high temperatures and mechanical stresses, without compromising integrity.
The production trials for steel casting involved manufacturing four 102-type couplers and two axle boxes using the optimized process. For molding, ester-hardened sodium silicate sand was prepared with sodium silicate at approximately 3% of the sand weight and organic ester at 15–18% of the sodium silicate weight. The cores were made with modified water glass sand, containing about 4% modified water glass, and achieved a hardening time of around 30 minutes. After molding, the mold cavities were clean, and the core surfaces were free of loose sand, meeting dimensional specifications upon inspection. Pouring was conducted at 1575°C, with a pouring time of 20–30 seconds for the coupler molds. Shakeout revealed excellent collapsibility, making cleaning straightforward; the internal cavities of the couplers were residue-free, and critical areas like the ear holes and spring seat mounting holes were smooth, suitable for assembly. Non-destructive testing on the coupler traction surfaces showed no cracks, and density checks on a sample coupler confirmed that defect levels were within acceptable limits for steel casting. These results demonstrate that the new process effectively reduces defects, enhances product quality, and supports mechanized production for steel casting components.
In conclusion, the optimization of the molding and core-making processes for steel casting through ester-hardened sodium silicate sand and modified water glass sand has proven highly beneficial. By reducing the reliance on ordinary water glass sand, we have minimized issues like cracking and gas defects, leading to superior steel casting outcomes. The mechanized approach not only lowers labor intensity and production costs but also fosters a safer and more efficient work environment. This advancement underscores the importance of continuous innovation in steel casting technologies, particularly for railway applications where reliability is critical. Future work could focus on further refining the sand mixtures and expanding the automation capabilities to enhance steel casting production on a larger scale.