In my experience with manufacturing cast track shoes for heavy-duty tracked equipment, I have encountered a wide range of challenges that stem from both the casting process and subsequent heat treatment. These components are critical for vehicles operating in swampy, marshy, or soft ground conditions because they provide a low ground contact pressure. The complex thin-walled structure with reinforcing ribs, often formed into a “V” cross-section, makes them particularly susceptible to various manufacturing defects. Over the years, my team and I have systematically analysed these problems and developed effective countermeasures. In this article, I will share our findings, focusing on the most common sand casting defects and how we resolved them.
Manufacturing Process and Chemical Composition
Cast track shoes are typically produced using sand casting. The process flow includes melting, sand mixing, moulding, pouring, shake-out, cleaning, normalising, rough machining, quenching, tempering, shot blasting, and painting. The material is a low-alloy cast steel with the chemical composition shown in the table below.
| C | Si | Mn | S | P | Cr |
|---|---|---|---|---|---|
| 0.40–0.47 | 0.3–0.8 | 0.7–1.4 | ≤0.035 | ≤0.035 | ≤0.3 |
The manufacturing process involves two critical special processes: casting and heat treatment. Therefore, the key quality control points are focused on these two stages. In the following sections, I will elaborate on the common problems we have faced, including chemical composition fluctuations, sand casting defects, and unqualified mechanical properties.
Chemical Composition Fluctuations
One of the early challenges we encountered was significant variation in the chemical composition of the molten steel from heat to heat. The raw materials used in the induction furnace are mainly various types of scrap steel from different sources, with inconsistent compositions. During the pouring process, the alloying elements also undergo different degrees of oxidation loss. As a result, the first track shoe poured from a heat may have a different composition from the last one. To control this, we implemented strict procedures:
- Use scrap steel from the same batch as much as possible within one heat.
- Perform chemical analysis before, during, and after melting in the furnace.
- Introduce the hardenability index (DI value) as a reference to ensure consistent heat treatment response. Through process trials, we determined that a DI value in the range of 50–70 provides optimal properties for the wet-area track shoes.
We defined the DI value using a simplified formula:
$$
DI = 0.5 \times \left( 0.7 \times C + 0.3 \times Si + 0.2 \times Mn + 0.1 \times Cr \right) \times 25.4
$$
This formula helped us predict the hardenability and stabilise the final mechanical properties. By controlling the DI value, we significantly reduced the variation in mechanical properties after heat treatment.
Sand Casting Defects: The Main Culprit
Among all quality issues, sand casting defects are the most frequently observed. We performed a statistical analysis of defect occurrences and found that sand inclusion, scabbing, and metal penetration account for the majority of rejections. These defects often require repair welding or even scrap the part, especially when they occur in critical areas such as the bolt holes, where stress concentration can lead to premature failure.
The image above illustrates a typical example of a severe sand inclusion defect that has been ground out from a bolt hole area. Such defects are unacceptable because the bolt hole is the connection point between the track shoe and the chain link. If a defect exists here, it cannot be repaired; the part must be scrapped. Similarly, through-sand holes (porosity caused by loose sand) often appear in thin-walled sections, leading to complete rejection.
We systematically identified the sources of sand that become entrapped in the casting. The following table summarises the main sources and our corrective actions.
| Source of Sand | Defects Caused | Root Cause Analysis | Countermeasure |
|---|---|---|---|
| Ladle lining | Sand inclusion, sand holes | The ladle lining was made from raw moulding sand tamped and hardened, which had low strength and spalled easily when heated. | Replace with high-purity mullite-based neutral refractory with extremely low thermal expansion and conductivity. |
| Furnace lining | Sand inclusion, sand holes | Silica sand rammed lining is prone to spalling at high temperatures, falling into the molten metal. | Use corundum-based neutral lining material with spinel bonding phase instead of acidic silica lining. |
| Sand mould | Severe sand inclusion, sand holes | High moisture content, too fine sand grain size causing low permeability; insufficient CO₂ gassing, incomplete hardening. | Control moisture content of silica sand; ensure sand grain size between 40–70 mesh; standardise CO₂ gassing time. |
| Loose sand | Sand holes, poor surface finish | Loose sand left in the mould cavity after moulding or dropped in during closing. | Blow out loose sand with compressed air at high pressure before closing. |
By implementing these countermeasures, we notably reduced the occurrence of sand casting defects. The change from acidic silica lining to neutral corundum lining required careful optimisation of the sintering schedule. Through process trials, we established the following sintering curve for the neutral lining, which resulted in a dense, smooth, high-strength lining with excellent performance.
$$
\text{Temperature profile: } \begin{cases}
\text{Room temperature} \rightarrow 600^\circ\text{C at }50^\circ\text{C/h}, \text{ hold }2\,\text{h}\\
600^\circ\text{C} \rightarrow 1100^\circ\text{C at }80^\circ\text{C/h}, \text{ hold }4\,\text{h}\\
1100^\circ\text{C} \rightarrow 1300^\circ\text{C at }60^\circ\text{C/h}, \text{ hold }6\,\text{h}\\
\text{Furnace cool to }600^\circ\text{C}, \text{ then air cool}
\end{cases}
$$
This sintering schedule replaced the previous acidic lining practice. After implementation, the lining life improved significantly, and the incidence of lining-related sand casting defects dropped by over 60%.
Mechanical Properties Failures
Another major category of problems we faced was unqualified mechanical properties, particularly elongation and impact toughness. Track shoes must bear the full weight of the vehicle and endure severe impact loads during operation. Failures in impact toughness, especially after tempering, were alarmingly common. The required mechanical properties are listed below.
| Condition | Hardness | Yield Strength (MPa) | Tensile Strength (MPa) | Elongation (%) | Impact Toughness (J/cm²) |
|---|---|---|---|---|---|
| Normalised | ≥183 HBW | 343 | 588 | ≥15 | 29.4 |
| Quenched | ≥50 HRC | — | — | — | — |
| Tempered (final) | 269–321 HBW | 539 | 784 | ≥10 | ≥39.2 |
Through thorough analysis of both the casting and heat treatment processes, and by comparing test results with the requirements, we identified the following root causes for poor impact toughness, along with our solutions.
3.1 Casting Defects in Test Specimens
The test specimens machined from the castings sometimes contained internal defects such as porosity, shrinkage, and inclusions. These sand casting defects directly reduced the measured mechanical properties. To eliminate this variable, we standardised the specimen location to a critical stress area of the track shoe, and we introduced ultrasonic inspection to screen out specimens with internal defects. This ensured that the test results reflected the true material quality rather than casting imperfections.
3.2 Inadequate Normalising and Tempering
Normalising is a critical step for homogenising the as-cast structure. In our initial practice, we used a car-bottom furnace at $$850 \pm 10^\circ\text{C}$$ with a holding time of 2.5 hours after the furnace reached temperature. However, the furnace temperature uniformity was poor. Parts near the hot zone reached temperature quickly, while those near the door or corners did not. As a result, the core of thicker sections retained as-cast structures, leading to hardness variations outside specification. We measured the temperature uniformity of the furnace and upgraded it to meet Class A standard (see table below). We also performed core temperature profiling by drilling a hole to half-thickness at three locations (top of door, middle of furnace, bottom near back wall) to determine the actual time for the core to reach the normalising temperature. Based on this data, we adjusted the holding time to ensure full transformation throughout the cross-section.
| Grade | Quenching, Carburising, Nitriding Furnace | Normalising, Annealing Furnace (≥400°C) | High-Temperature Tempering Furnace (>400°C) | Low-Temperature Tempering Furnace (<400°C) |
|---|---|---|---|---|
| A | ±7.5°C | ±10°C | ±7.5°C | ±5°C |
| A1 | ±10°C | ±15°C | ±10°C | ±7.5°C |
| B | ±15°C | ±25°C | ±15°C | ±10°C |
After achieving grade A uniformity, we set the normalising parameters to $$850 \pm 10^\circ\text{C}$$ with a holding time of 3 hours (based on core temperature data) followed by air cooling. This eliminated the core structure problem.
For tempering, we used pit furnaces. The main issue was that the tempering temperature was too low or the holding time insufficient, resulting in excessive hardness (above 321 HBW) which caused poor elongation and impact toughness. To optimise the tempering process, we designed a series of orthogonal experiments investigating tempering temperature (ranging from 520°C to 580°C), holding time (2–4 hours), and cooling rate. The response variables were hardness, yield strength, tensile strength, elongation, and impact toughness. Our experiments showed that a tempering temperature of $$560 \pm 10^\circ\text{C}$$ with a holding time of 3 hours followed by air cooling gave the best combination of strength and toughness, consistently meeting all requirements. We also found that the cooling rate must be moderate (air cooling) to avoid re-hardening or excessive residual stresses.
Conclusion and Continuous Improvement
Through systematic analysis and targeted countermeasures, we have significantly reduced the rejection rate of cast track shoes. The key lessons learned are:
- Careful control of raw materials and DI value minimises composition fluctuations.
- Eliminating sources of sand contamination—from ladle lining, furnace lining, mould sand, and loose sand—directly reduces sand casting defects. The adoption of neutral furnace lining and optimised sintering greatly improved lining life and reduced defects.
- Improving furnace temperature uniformity and establishing proper normalising and tempering parameters based on core temperature measurements and orthogonal experiments resolved the mechanical property failures.
These improvements have brought consistent quality to our production line. While we have addressed the most common issues, we continue to monitor process variables and conduct regular audits to prevent sand casting defects from reappearing. The journey of quality improvement never ends, and I believe that sharing our experience can help other manufacturers facing similar challenges in the production of complex cast steel components.