Improvement of a Thin-Walled Ductile Iron Valve Casting Process

Abstract

This study focuses on a thin-walled, compressor valve-type ductile iron casting with a complex internal cavity, weighing 3.4 kg and featuring a main wall thickness of 4 mm. The casting is prone to defects such as cold shuts, core breakage, and gas porosity, all while having strict requirements for surface quality and airtightness. To address these challenges, silicon carbide foam filters were integrated into the gating system. Due to the casting's intricate geometry and multiple hot spots, risers were strategically positioned to counteract shrinkage, thereby ensuring both internal quality and airtightness. During the initial process design and production trials, the casting’s upper surface was found to be highly susceptible to cold shuts, gas porosity, and core breakage, leading to an elevated scrap rate. Consequently, process improvements were implemented, effectively eliminating these defects and ensuring stable batch production.

Introduction

Thin-walled ductile iron valve castings are essential components in compressor valves. The material used is high-performance A-395 (QT400-18L), with a unit weight of 3.4 kg. The external appearance and internal cavity of the casting are illustrated in Figure 1. One of the key characteristics of this component is its extremely thin walls in relation to its overall size. The casting has uneven wall thickness, with the minimum thickness being just 4 mm, which makes it prone to core breakage. The external dimensions are 172 mm × 164 mm × 116 mm, and the mechanical properties must meet a minimum tensile strength of 400 MPa, with a Brinell hardness range of 143–187 BHN 3000. Users specify high dimensional accuracy and surface quality, with a CT7 tolerance grade and strict consistency across production batches. Surface defects, such as blowholes and gas porosity exceeding φ3 mm, are unacceptable. Furthermore, before shipment, the castings must be uniformly coated with a rust-preventive fluid. Figure 1 illustrates the casting’s shape and key dimensions.

Figure 1 Appearance and Internal Cavity of Valve Body Casting

1. Original Casting Process, Trial Production, and Defect Analysis

Due to the relatively narrow diameter of the three valve holes in the casting’s inner cavity (φ15, φ15, and φ12), breakage is a particular risk during the pouring process. To prevent defects such as misalignment and poor sealing of the valve holes, the sand core features a frame-style core head that connects the three valve hole cores, forming a ring-like structure around the φ60 main valve hole. This design not only enhances the overall strength and bending resistance of the sand core but also optimizes sand flow during the core-making process.

Additionally, multiple markings on the casting surface, such as the casting drawing number, customer trademark, and supplier code (all in font size 5), complicated demolding and made the markings prone to deformation during cleaning and transportation. As a result, some of the cast markings were designed as recessed characters. The original sand core casting process and upper and lower mold plate configurations are shown in Figures 2 and 3.

Figure 2: Original Sand Core Casting Process Diagram

The sand core process shown in Figure 3 was initially designed with the sand injection directed at one side of the φ60 main hole, featuring four sand injection ports. The sand core is a solid structure, with each core weighing 1.2 kg. To ensure efficient removal of residual sand from the casting’s inner cavity, the surface of the sand core is evenly coated with a 0.15 mm layer of refractory paint. The core should be used promptly after drying. Additionally, to strengthen the sand core’s solidification layer, the sand core head frame has a width of 20 mm, ensuring sufficient overall strength.

The mold is split at the midsection, allowing molten iron to flow into the cavity via the core head of the sand core frame. A φ65 hot riser is added for shrinkage compensation, with one riser serving two castings. The gating system follows a closed-open design, with a cross-sectional area ratio of A_vertical : A_horizontal : A_inner = 1.2 : 1.0 : 1.4. A foam ceramic filter block is positioned in the horizontal runner to prevent slag from entering the cavity, maintaining smooth molten iron flow. Molten iron is directed to the hot riser though the horizontal runner. This gating system offers several benefits: molten metal enters the cavity from the upper mold flange, promoting smooth filling and facilitating sequential solidification from bottom to top. Additionally, four exhaust pins are installed on the core head and upper surface of the casting to ensure proper venting, with four castings produced per mold.

During the initial trial production of the sample, several surface defects were identified in the castings. The defects appeared in the following areas: cold shuts in the upper mold far from the ingate, gas porosity in the independent blind hole cavity, insufficient wall thickness (4 mm), and core breakage at the φ15 valve hole. The rejection rate for the castings was 30%. To address these issues, a numerical simulation was conducted. Based on the simulation results, an analysis was conducted to identify the potential causes:

• Pouring Temperature Control: A low pouring temperature (1380–1420°C) can result in cold shuts. However, pouring temperature requires careful control—excessively high temperatures can cause sand adhesion and core breakage. Following evaluation, the pouring temperature was increased by 20°C.
• Exhaust System: Inadequate venting during pouring can lead to gas porosity. Three-dimensional molten iron flow simulations indicated the need to adjust exhaust pin positions for improved cavity ventilation.
• Gating System: The ingate position was inadequate. To improve molten iron filling speed, three additional ingates were added to the two originally positioned on the flange.

Figure 4: Cold shut and gas porosity in castings

Figure 7: Modified Sand Core Diagram

2. Process Design Optimization

(1) Enhancement of the Sand Core Injection Process

The direction of the sand core injection is modified by rotating the core 180° along the vertical axis. An additional sand injection port is added, and the φ60 main valve hole is evacuated. The wall thickness is 12 mm, and the core box is fitted with an evacuated base and heating pipe. These modifications reduce the sand core weight from 1.5 kg to 0.8 kg, improving gas release and optimizing the partial filling area.

(2) Increase the Number of Inner Gates

The number of inner gates per casting has been increased from 2 to 5. These gates have a thin, sheet-like design with a thickness of 2.5 mm and are strategically positioned to ensure even and rapid pouring, stable filling, and effective gas escape, thereby reducing cold shut defects.

(3) Increase the Pouring Temperature

The pouring temperature is increased by 20°C, from 1380–1420°C, to improve gas removal from the thin-walled casting cavity and enhance the molten alloy’s filling capacity. The final pouring temperature is controlled between 1400–1450°C, while the molten iron temperature after treatment ranges from 1460–1480°C. This adjustment successfully reduces gas porosity on the upper surface of the casting.

(4) Modify the Pouring and Riser System Structure

The horizontal runner of the lower box is repositioned to the upper box, and the height of the upper riser is increased by 30 mm. An exhaust needle is added to the outer mold with the following dimensions: lower diameter φ10 mm, upper diameter φ6 mm, and height 60 mm. The exhaust needle is positioned at the highest point of the sand core head in the outer mold and connected to the casting via an exhaust sheet.

(5) Adjust Alloy Composition and Riser Dimensions

The copper content in the molten iron is reduced from 0.6% to 0.5%, the flow inoculation is set to 1.5%, and the riser height is increased to address partial shrinkage in the thin-walled ductile iron valve body casting. Thin-walled castings are designed with simultaneous solidification in mind. The molten iron is greatly influenced by the cooling of the mold during pouring, resulting in surface solidification shrinkage and internal liquid shrinkage that occur earlier. This shrinkage is supplemented by the poured molten iron, which is delivered through multiple, dispersed entrapments, ensuring a ductile iron casting free from shrinkage defects.

Figure 8: Modified New Upper and Lower Mold Plate Diagram

2. Production Verification

Following the improvement plan, the tooling mold was modified, and casting trials and small-batch production were conducted. Thirty trial samples were produced, followed by over 200 castings in small batches. The results showed that the castings exhibited well-defined contours and a high-quality finish. The scrap rate remained below 2%, primarily due to defects such as cold shuts and surface porosity. No issues arose during subsequent processing. The finished castings after polishing are shown in Figure 9. Following successful small-batch verification, over 2,000 castings were mass-produced. The processing of these castings, shipped to customers, was closely monitored, and it was observed that the previous defects were resolved, leading to stable mass production.

Figure 9: Appearance of the Casting After Cleaning

4. Conclusion

By implementing the process improvements outlined above, the casting process for ductile iron compressor valve castings has been optimized, resulting in significant improvements in product quality. The casting yield rate has increased from 52% to 58%, and defects such as gas porosity, broken cores, and cold shuts have been significantly reduced. The overall scrap rate has been reduced to below 3%, while production efficiency has improved.

• A bottom-pouring, closed-open system has been adopted, with an increase in pouring temperature. This ensures smooth mold filling and unobstructed exhaust, effectively preventing surface porosity and producing denser castings.
• The frame-type core head process design creates a sand core with high strength and excellent bending resistance.
• The choice of shot blasting is critical to casting quality. Since drum shot blasting damages the casting surface, an inner cavity shot blasting machine must be used for both surface and inner cavity cleaning after rough casting.
• The optimal smelting process and material ratio for high-performance A-395 (EN-GJL-400) material have been established. Adjusting the proportion of molten iron alloy, carbon equivalent, inoculation amount, and riser height has resolved the issue of partial shrinkage in the castings.