As a foundry engineer deeply involved in process optimization, addressing persistent quality issues is a core aspect of my work. For several years, the A-series engine block has been a strategic product for our company. While overall quality has stabilized through extensive R&D and process refinement, certain challenges require focused攻关. Among these, water leakage stands out as a critical quality problem demanding a comprehensive solution. This defect is only detected after the block undergoes machining and pressure testing with air and water, incurring significant costs from casting, handling, processing, and assembly—all of which are lost upon scrappage. Quality statistics from a recent annual period revealed that scrap losses due to water leakage accounted for a staggering 45% of the total scrap cost for this engine series. This underscored the urgent need for a systematic resolution to drive cost reduction and efficiency gains for the enterprise.
The engine block in question is a thin-walled, high-strength gray iron casting with approximate dimensions of 940 mm x 392 mm x 427 mm. The production process utilizes a German KW molding line with a stepped gating system designed for smooth filling. The water jacket cores are produced using Mingzhi hot-box core shooters, while other sand cores are made via the cold-box process. Despite this established process, the leakage rate was recorded at 3.3%. A detailed Pareto analysis of the leakage locations was conducted, pinpointing the primary failure points.
The analysis of faulty castings, involving multiple dissections and scanning electron microscope (SEM) examination, identified the root causes for leaks at these critical junctions. Leaks from the ø6 mm and ø20 mm oil holes were attributed to nitrogen porosity defects. In contrast, leaks at the tappet bore locations were traced back to poor fusion between the cast iron and inserted chills or core supports. This paper details the investigative journey and the multifaceted corrective actions implemented to resolve these issues in high-strength gray iron casting.
1. Deep Dive into Failure Analysis and Corrective Actions
1.1 The Challenge of ø6 mm Oil Hole Leakage
The ø6 mm oil hole is machined into a solid cylindrical section approximately 20 mm in diameter. This cylindrical form is created by the water jacket core and the tappet bore core. This region possesses a greater wall thickness compared to the adjacent water jacket walls, making it a localized hot spot that solidifies more slowly. Such areas are inherently prone to shrinkage porosity and gas defects, including nitrogen porosity. Dissection of leaking blocks confirmed the presence of interconnected nitrogen blowholes between the oil hole wall and the water jacket cavity. SEM-EDS analysis conclusively identified these as nitrogen gas pores.
The fundamental cause was linked to the nitrogen content in the base iron. During the solidification of gray iron casting, the solubility of nitrogen decreases as temperature falls. When the nitrogen content exceeds its solubility limit in the iron, it precipitates out, forming gas bubbles that can become trapped, creating defects. The relationship between nitrogen solubility, temperature, and content is crucial. The solubility of nitrogen in molten iron can be described by Sieverts’ law:
$$[N] = K_N \sqrt{P_{N_2}}$$
where $[N]$ is the dissolved nitrogen concentration, $K_N$ is the equilibrium constant (temperature-dependent), and $P_{N_2}$ is the partial pressure of nitrogen at the melt surface. As solidification proceeds and temperature ($T$) drops, $K_N$ decreases, leading to nitrogen supersaturation and pore formation if [$N$] is too high.
Our long-term data analysis confirmed a strong correlation between the base iron’s nitrogen content and the leakage rate. To combat this, we implemented a ferrotitanium (FeTi) alloying practice. When titanium is added and dissolves into the molten iron, it has a high affinity for nitrogen, forming stable titanium nitride (TiN) inclusions:
$$Ti + N \rightarrow TiN_{(s)}$$
$$ΔG^ο = -RT \ln K$$
where $ΔG^ο$ is the standard Gibbs free energy change, which is highly negative for TiN formation, making the reaction favorable. The formation of these TiN particles effectively “ties up” a portion of the nitrogen, removing it from solution and drastically reducing the amount of free nitrogen available to form blowholes during solidification.
As a secondary, robust physical barrier, we introduced a cylindrical chill sleeve around the core print that forms the ø6 mm oil hole cavity. This sleeve serves a dual purpose: first, it increases the local cooling rate, modifying the solidification morphology to reduce defect susceptibility; second, even if a nitrogen pore forms, the dense, well-fused chill metal acts as a “armor,” preventing the defect from creating a continuous leak path between the oil and water galleries. This two-pronged approach—chemical (FeTi) and physical (chill)—proved highly effective.
1.2 Addressing Leakage in the ø20 mm Oil Hole
The vertical oil gallery and main oil gallery are cast as solid, heavy sections, with the ø20 mm hole machined afterward. These substantial masses act as major thermal nodes, solidifying very slowly. This leads to coarse grain structure in the center and high susceptibility to shrinkage and gas defects, compromising mechanical properties. Leaks here were also confirmed via SEM to be caused by nitrogen porosity.
The same foundational solution of ferrotitanium addition was applied to mitigate nitrogen pore formation. This area was already equipped with a chill to accelerate cooling. However, analysis of defect locations showed that the existing chill design did not provide complete coverage over the potential defect zone near the water jacket wall. The nitrogen pores could form in areas “unprotected” by the chill. Therefore, we undertook a structural optimization of the chill sleeve for the vertical gallery.
The new design was contoured to follow the shape of the water jacket core more precisely, extending its protective coverage. The goal was to ensure that any potential porosity in the critical region would be encapsulated by the well-fused chill metal, thereby sealing it off. Post-optimization validation included metallographic examination of sectioned castings, which confirmed excellent fusion between the new chill and the gray iron casting matrix, with no gaps or oxide films.
1.3 Solving Tappet Bore Leakage: Fusion Issues
Leaks at the tappet bore manifested in two distinct failure modes related to fusion. The first involved poor metallurgical bonding between the cast iron and the “I-beam” style tappet core support (chill). The second involved poor fusion of the aforementioned ø6 mm oil hole chill sleeve.
1.3.1 Core Support (Chill) Fusion. The core supports, made from low-carbon steel and tin-plated to aid fusion, act as severe chillers. When the pouring temperature was at the lower end of the specification range, the rapid quenching effect could prevent proper melting and alloying of the chill surface, leading to a lack of fusion. Furthermore, research indicates that at the interface of a tin-plated chill and iron, oxygen can segregate, forming micro-oxide films that impede sound bonding.
We conducted designed experiments to solve this:
- Pouring Temperature Optimization: We cast batches of blocks at controlled temperature intervals (4°C steps). Each batch was inspected using a boroscope to assess chill fusion quality. The data revealed a clear threshold: fusion was consistently sound at pouring temperatures above approximately 1410°C. However, at the extreme high end (~1425°C), the chills were found to be completely melted, losing their structural purpose and causing core movement. Therefore, an optimal pouring window of 1410–1423°C was established.
- Chill Design Optimization: To reduce the chilling severity and improve fusion, we redesigned the core support. The overall dimensions were scaled down: the end plate diameter was reduced from 15.88 mm to 12.7 mm, and the connecting web diameter was reduced from 3.05 mm to 2.26 mm. Additionally, the pattern of six vent/fusion holes on the end plates was repositioned closer to the central web. This modification aimed to provide better pathways for gases evolved at the curved interface to escape, further reducing the risk of fusion defects.
1.3.2 ø6 mm Oil Hole Chill Sleeve Fusion. Concurrently, the chill sleeve for the ø6 mm oil hole, originally 1.5 mm thick, was also suspected of contributing to tappet leaks if poorly fused. Drawing from the core support findings, we hypothesized that a thinner sleeve would achieve better fusion by reducing its chilling power and allowing it to reach a semi-molten state more easily. A prototype sleeve with a thickness of 1.2 mm was tested and validated successfully.
2. Systematic Summary of Improvements and Results
The implemented improvements were multifaceted, targeting specific root causes. The table below summarizes the key actions and their intended effects:
| Leakage Location | Root Cause Identified | Primary Corrective Action | Supporting/Secondary Action | Principle of Action |
|---|---|---|---|---|
| ø6 mm & ø20 mm Oil Holes | Nitrogen Porosity | Ferrotitanium (FeTi) Addition | Optimized Chill Sleeve Design & Placement | Chemical getering of nitrogen (TiN formation). Physical barrier and enhanced cooling. |
| Tappet Bore (Mode 1) | Poor Core Support Fusion | Optimized Pouring Temperature Range (1410-1423°C) | Redesigned, smaller core support with modified vent hole pattern | Ensures sufficient superheat for fusion. Reduces chilling severity and improves gas venting. |
| Tappet Bore (Mode 2) | Poor ø6 mm Chill Sleeve Fusion | Reduced Chill Sleeve Thickness (1.5mm → 1.2mm) | Maintained within optimized pouring temperature window | Reduces chilling power, promoting better surface melting and bonding. |
The effectiveness of these measures was quantitatively significant. The leakage rate, which initially stood at 3.3%, was reduced to 1.95%, representing a total reduction of 40.9%. The individual contributions were substantial:
- ø6 mm oil hole leakage rate decreased by 43%.
- ø20 mm oil hole leakage rate decreased by 74%.
- Tappet bore leakage rate (from both modes) decreased by 44%.
The integrated process controls can be visualized in the following flowchart, which outlines the critical control points (CCPs) established in the gray iron casting process for this block:
| Process Stage | Control Parameter | Target Specification | Monitoring Method |
|---|---|---|---|
| Melting & Treatment | Base Iron Nitrogen Content | < [Threshold Value] ppm | Spectroscopic Analysis (Periodic) |
| Melting & Treatment | FeTi Addition | X kg/ton of iron ± Y% | Weight-based process control |
| Molding & Pouring | Pouring Temperature | 1410 – 1423 °C | Immersion thermocouple (per pour) |
| Core Making | Chill/Support Dimensions & Placement | Per updated design drawings | Visual inspection and fixture checks |
3. Conclusions and Technical Insights
This comprehensive quality improvement project for a high-strength gray iron casting yielded several critical conclusions and technical insights that are transferable to similar casting applications:
- Nitrogen Control is Paramount: The implementation of a ferrotitanium alloying practice proved to be a foundational strategy for managing dissolved nitrogen levels. The chemical reaction $$Ti + N \rightarrow TiN_{(s)}$$ is highly effective in reducing the free nitrogen available to form blowholes during solidification, thereby directly addressing a major root cause of leakage in thick sections and thermal nodes.
- The Critical Role of Pouring Temperature: The fusion quality of inserted chills and core supports is exquisitely sensitive to pouring temperature. An optimal window must be determined empirically, balancing sufficient superheat for fusion against the risk of complete melt-back and core instability. For this specific gray iron casting and chill design, the window was 1410–1423°C.
- Chill Design is a Precision Engineering Task: Chills are not merely thermal sinks but are integral functional components. Their geometry, thickness, and placement must be engineered to achieve multiple goals: promote directional solidification, act as a physical seal against potential subsurface defects, and ensure reliable metallurgical bonding with the parent metal. Optimizations based on defect mapping (like the contoured ø20 mm hole chill) and fusion studies (like the thinner ø6 mm hole sleeve) are essential.
- A Systemic Approach is Necessary: No single solution was sufficient. The final outcome resulted from a synergistic combination of metallurgical control (FeTi), thermal process control (pouring temperature), and geometric design optimization (chills and supports). This highlights the interconnected nature of variables in gray iron casting production.
The successful reduction of the leakage rate by 40.9% underscores the effectiveness of a data-driven, root-cause-based approach. The knowledge gained—particularly regarding the interaction between nitrogen, titanium, chill design, and thermal parameters—has not only stabilized the quality of the A-series engine block but has also enriched our foundational expertise in producing complex, high-integrity gray iron castings. The principles established here provide a robust framework for tackling similar leakage and porosity challenges in other critical casting components.