The manufacturing of critical, high-wear components often presents a unique challenge: achieving the necessary material properties and complex geometries in a cost-effective and reliable manner. In my extensive work with wear-resistant parts, the twin metals composite squeezing axle, a core component in螺旋式榨油机, perfectly embodies this challenge. Traditionally, this part was produced using a metal mold (chill casting) process to create a composite structure of white cast iron bonded to a carbon steel shaft. While the final product performed well, the manufacturing process itself was plagued by high scrap rates and short mold lifespans, leading to significant costs. This experience drove the investigation and successful implementation of an alternative: silicate-bonded sand mold casting. This shift not only resolved production issues but also opened a pathway for optimizing the material itself, demonstrating the profound versatility and potential of sand casting processes for such composite sand casting parts.
The fundamental requirement for the squeezing axle is a combination of high surface hardness and wear resistance on the external榨螺 (screw flight) with the core toughness and strength of a steel shaft. The traditional metal mold process provided the necessary chilling effect to promote a fully carbide structure in the white iron. However, its drawbacks were severe. The thermal shock and mechanical wear on the metal molds were excessive, limiting their usable life and increasing per-unit tooling costs. Furthermore, controlling the casting process to ensure perfect bonding between the two metals and avoid defects like cracks in the brittle white iron layer was difficult, resulting in consistently high rejection rates. It became clear that for the production volume and specific geometry of this component, a more adaptable and forgiving mold medium was needed. Water glass (sodium silicate) sand emerged as the ideal candidate. Its ability to form strong, precise molds at room temperature, excellent collapsibility after casting, and suitability for medium-scale production made it a perfect fit for manufacturing these specialized sand casting parts.
The first and most critical adaptation involved re-engineering the material composition of the white cast iron. The chemistry used for the rapid chilling of a metal mold is not directly transferable to the slower cooling environment of a sand mold. The primary risk in sand casting these parts is the precipitation of graphite within the matrix due to reduced cooling rates, which would drastically lower hardness and abrasion resistance. The original composition, effective in a chill mold, had relatively high Silicon (Si) and Phosphorus (P) content, which could promote graphitization under slower solidification. To counteract this and ensure a fully carbide-based, pearlite-free microstructure, the alloying strategy was revised. The key change was a significant increase in Chromium (Cr) content. Chromium is a potent carbide stabilizer; it forms hard (Cr, Fe)7C3 carbides and strongly suppresses the formation of graphite, even under moderate cooling rates. The target was raised to 1.0–1.2%, effectively doubling its previous level.
However, merely increasing carbide volume was not enough. Large, coarse carbides can be brittle and provide paths for crack propagation. To refine the overall as-cast microstructure, a dual modification treatment was implemented. A small addition of a Rare Earth (RE) element (like Cerium) and a微量 addition of Boron (B) were introduced into the molten iron just before casting. The RE elements act as powerful desulfurizers and deoxidizers, but more importantly, they modify the morphology of non-metallic inclusions and can promote finer eutectic carbide structures. Boron, even in trace amounts, segregates to grain boundaries and interfaces, enhancing hardenability and contributing to finer grain size. The combined effect is a finer, more uniform distribution of hard phases within a strengthened matrix. The carbon equivalent (CE) of the melt, a predictor of chilling tendency and microstructure, can be estimated for this high-chromium white iron using a modified formula:
$$ CE = C + 0.04Cr $$
where C is the percentage of Carbon. Maintaining a correct balance of CE and Cr is crucial for achieving the desired hardness (target >50 HRC) without excessive brittleness in these sand casting parts.
Equally important to the material is the design and preparation of the steel shaft core, which forms the backbone of the composite structure. The core must provide a clean, active surface for metallurgical bonding with the cast iron and withstand the thermal and mechanical stresses during casting and service. The shaft is fully machined from carbon steel. To ensure a strong mechanical interlock in addition to the metallurgical bond, the section to be encapsulated is designed with annular retaining rings (collars). These rings, typically 3mm in height and 20mm in width, create a dovetail effect, resisting any axial or torsional shear forces between the two metals.
The most critical calculation in the design phase involves determining the maximum permissible diameter of the steel core to prevent the white iron layer from cracking due to shrinkage stress during cooling. The white iron layer, contracting onto the rigid steel core, experiences a state of hoop stress. We can model this as a thick-walled cylinder under internal pressure (the shrinkage stress). Using Lame’s equations for thick-walled cylinders and the Maximum Shear Stress (Tresca) failure theory, the condition to avoid cracking at the inner surface of the iron layer is derived. The shrinkage pressure (P) exerted by the iron on the steel is estimated based on the thermal contraction mismatch and the modulus of elasticity. For this material system, a conservative value of P = 15 MPa is used. The allowable stress [σ] for the white iron in tension, a property significantly improved by the Cr addition and modification, is taken as 37 MPa.
The formula governing the design is:
$$ d \leq D \sqrt{1 – \frac{2P}{[\sigma]}} $$
Where:
- d = Outer diameter of the steel core (mm)
- D = Outer diameter of the finished white iron layer (screw root diameter) (mm)
- P = Shrinkage pressure (MPa)
- [σ] = Allowable tensile stress of white iron (MPa)
For the specific榨油机 model with a minimum screw root diameter D = 46 mm, the calculation yields:
$$ d \leq 46 \times \sqrt{1 – \frac{2 \times 15}{37}} \approx 46 \times \sqrt{0.189} \approx 20 mm $$
This theoretical limit confirmed the selection of a 20 mm diameter for the encapsulated section of the steel shaft, ensuring the integrity of these composite sand casting parts under thermal stress.
The foundry process itself was meticulously designed around the water glass sand system. The mold is split horizontally to facilitate the placement of the pre-heated steel core. Two castings are arranged in a single mold box for efficiency. The gating system is designed as a semi-choked (pressurized) system to ensure rapid and smooth filling with minimal turbulence and oxidation. The cross-sectional area ratios are set as Sprue : Runner : Ingate = 1.0 : 1.2 : 0.8. The sprue is located at one end of the runner, near the heavier sections of the榨螺. A necked-down bottle-shaped riser is placed at the top of the heaviest section, serving more as an effective vent and slag trap than a traditional feeding riser, due to the near-eutectic, short-freezing-range nature of the white iron.
| Component | Percentage/Detail | Purpose/Note |
|---|---|---|
| Base Sand | 100% (70/140 mesh washed silica) | Provides refractory backbone; fine grain for good surface finish. |
| Moisture Content | < 1% | Critical to prevent premature reaction with binder. |
| Sodium Silicate Binder | 3.5% – 4.5% by weight | Modulus (SiO2/Na2O) of 2.2 – 2.5. Provides room-temperature strength. |
| MDT Organic Ester Hardener | 0.35% – 0.40% by weight | Catalyzes the hardening reaction of the silicate binder. |
| Mixing Sequence | 1. Sand + Ester (1-1.5 min) 2. Add Sodium Silicate (1-1.5 min) |
Ensures even distribution of hardener before binder addition for uniform setting. |
| Bench Life | Approx. 15-25 minutes | Time available for molding after mixing. |
Core preparation is vital. The steel shafts are thoroughly cleaned of grease and rust, coated with a proprietary refractory wash to prevent fusion welding and aid bonding, and then pre-heated in a furnace. Heating to 250°C ensures complete drying of the coating, and they are held at 150°C until immediately before mold closing. This pre-heat reduces the thermal shock when molten iron contacts the core, minimizes gas evolution, and promotes a better metallurgical bond at the interface. The practice is to extract the hot core, place it in the mold, close the mold, and pour in a coordinated sequence to maintain core temperature.
The furnace is a medium-frequency induction type, ideal for precise temperature control and melt quality. The iron is superheated to 1400-1420°C for proper fluidity and to dissolve alloying elements. The RE-B modification treatment is performed in the pouring ladle during tapping. The mold is tilted at approximately 5-7 degrees (achieved by raising one end by 40 mm) during pouring. This tilt promotes directional solidification from the thin sections back toward the riser/vent and helps slag and gases to float up into the riser. Pouring temperature is carefully controlled at around 1360°C. After a short shakeout time of about two minutes, the casting is removed from the mold. The excellent collapsibility of the chemically-bonded sand allows for easy cleaning. The castings are then placed upright on a rack to cool uniformly in air, followed by a stress-relief anneal at low temperature to minimize residual stresses without softening the hard white iron layer.
The performance validation of the new sand-cast axles was comprehensive, comparing them directly against the legacy metal-mold counterparts. Hardness was measured directly on the machined榨螺 flight surfaces. Abrasion resistance was evaluated using a standard rubber-wheel wet and dry sand abrasion test. Finally, and most importantly, actual service life was measured through prolonged field testing in榨油机 processing raw soybeans.
| Property | Metal Mold Axle | Silicate Sand Cast Axle | Implication |
|---|---|---|---|
| Surface Hardness (HRC) | 49.8 – 52.1 | 49.0 – 53.2 | Sand cast parts achieve equivalent or slightly higher peak hardness, with marginally wider dispersion due to slower cooling. |
| Abrasion Resistance (Dry Sand Test)* | 1.00 (Reference) | 0.96 | Slightly better performance, attributed to refined carbides from modification. |
| Abrasion Resistance (Wet Sand Test)* | 1.00 (Reference) | 0.85 | Significantly better performance (15% less wear). Chromium enhances corrosion-abrasion resistance in wet conditions. |
| Field Service Life (Hours) | 3000 – 3100 | >= 3200 | Clear improvement in actual operating conditions, exceeding customer requirements. |
*Abrasion resistance expressed as relative wear loss (ε); a lower value indicates better performance.
The results were conclusive. The hardness was fully comparable, though the range was slightly broader for the sand-cast parts, reflecting the inherent variability of cooling in a sand mold versus a uniform metal chill. The dry abrasion resistance was slightly superior. The most striking improvement was in wet abrasion resistance, where the sand-cast axle showed approximately 15% less wear. This is a direct benefit of the increased Chromium content, which improves the material’s resistance to the combined effects of mechanical abrasion and corrosive attack from moist environments. Ultimately, the field test confirmed a longer operational lifespan, validating that the composite sand casting parts produced via this method were not just equivalent but superior to the original product.
The economic analysis cemented the viability of the transition. While certain costs increased due to the consumption of bonding materials (sand, silicate, ester), additional cleaning labor, and the cost of alloying elements (Cr, RE), these were overwhelmingly offset by savings in other areas.
| Cost Factor | Metal Mold Process | Silicate Sand Process | Net Change |
|---|---|---|---|
| Mold/Tooling Amortization | High (Frequent mold replacement) | Negligible (Reusable pattern, low-cost sand) | Significant Reduction |
| Scrap and Rework Losses | High (e.g., >15-20%) | Low (< 5%) | Significant Reduction |
| Yield (Feeding Efficiency) | Lower (67%) | Higher (78%) | Increase in usable product per melt |
| Material & Consumables | Lower | Higher (+~$12) | Increase |
| Labor (Mold Making) | Lower (reuse of metal mold) | Higher (sand molding labor) | Increase |
| Total Estimated Cost Impact | Benchmark | ~10% Reduction | Net Positive |
The elimination of expensive, short-lived metal molds was the single largest saving. The drastic reduction in scrap rate from potentially over 20% to below 5% directly translated to higher output from the same amount of molten metal. The improved casting yield—more finished weight per poured weight—further enhanced material utilization. Overall, despite the added costs for sand bonding materials and alloying, the total manufacturing cost per axle was reduced by approximately 10%. When coupled with the improved service life of the final product, the value proposition for both the manufacturer and the end-user became overwhelmingly positive. This case powerfully demonstrates how a well-engineered sand casting process can be the most economical solution for producing high-performance, composite sand casting parts.
In conclusion, the successful development of the twin metals composite squeezing axle via silicate-bonded sand casting underscores several key principles in modern foundry practice. First, a casting process must be selected not just on tradition, but on its holistic fit for the component’s geometry, production volume, and material requirements. For this application, the flexibility and controllability of the sand mold were decisive advantages over the rigid metal mold. Second, the material system cannot be viewed in isolation from the process. The white cast iron chemistry had to be strategically re-engineered—increasing carbide-stabilizing Chromium and employing RE-B modification—to achieve the target microstructure and properties under the slower cooling regime of a sand mold. Third, rigorous engineering analysis, such as the application of thick-walled cylinder stress theory to prevent cracking, is essential for designing reliable composite sand casting parts. Finally, this transition yielded comprehensive benefits: a drastic reduction in scrap rates and tooling costs, a measurable improvement in product performance (especially in wet abrasion), and a net reduction in total manufacturing cost. This project stands as a testament to the fact that for a wide range of demanding applications, from榨油机 axles to other complex wear parts, advanced sand casting techniques are more than capable of producing superior, cost-effective components that meet and exceed performance expectations.