Abstract: Impact toughness reflects the ability of a material to absorb plastic deformation work before fracture under impact loading, and it reveals the tendency of materials toward brittle fracture. Higher absorbed impact energy indicates a lower possibility of brittle failure. Ductile iron castings are prone to brittle fracture when subjected to impact loading at low temperatures. In order to broaden the application range of ductile iron casting, it is essential to develop ductile iron with sufficient impact toughness at -50 °C. In this study, without adding any additional alloying elements, the chemical composition of the cast iron was optimized and the castings were subsequently annealed to obtain a nearly 100% ferritic matrix. The resultant ductile iron exhibited a tensile strength of ≥380 MPa and a yield strength of ≥230 MPa, satisfying the mechanical property requirements of both the national standard QT400-18L and the European standard EN-GJS-400-18LT. Moreover, the average absorbed impact energy at -50 °C reached ≥12 J, with individual values ≥9 J, which reached the expected target. This research provides a practical reference for the production of ductile iron casting used in extremely cold environments.
Keywords: ductile iron casting; low temperature (-50 °C); impact toughness; ferrite matrix; annealing treatment; mechanical properties; casting process
1. Introduction
Ductile iron is an important engineering material produced through nodularization and inoculation treatments, giving rise to spheroidal graphite in the microstructure. This unique graphite morphology effectively reduces stress concentration and enhances the elongation and impact toughness of the iron, making it comparable to carbon steel in many applications. Ductile iron casting has been widely used in the railway industry, heavy machinery, wind power equipment, and other fields where high strength and moderate toughness are required. However, with the expansion of transportation networks into colder regions, components made from ordinary ductile iron are increasingly exposed to extremely low ambient temperatures, sometimes dropping below -50 °C. Under such conditions, ductile iron casting tends to exhibit a ductile-to-brittle transition, resulting in a sharp decrease in impact toughness and a higher risk of catastrophic brittle fracture.
Several previous studies have explored methods to improve the low-temperature toughness of ductile iron. Some researchers investigated the effect of Si and Ni on impact toughness and achieved acceptable results after normalizing. Others adjusted the chemical composition and heat treatment to obtain ductile iron with −60 °C impact energy above 12 J. The influence of Cu content on low-temperature impact toughness was also examined, showing that 0.3% Cu could provide a tensile strength of 510 MPa and good toughness. Furthermore, trace additions of Ti have been proposed to refine the microstructure. Nickel addition and annealing treatments have also been studied to improve the performance of QT400-18L. However, many of these approaches rely on the addition of expensive alloying elements such as Ni, Cu, or Mo, which increases production costs. From an economic perspective, it is desirable to achieve the required low-temperature toughness in ductile iron casting without resorting to such alloying additions.
The product investigated in this work is a motor shell for railway locomotives, as shown below. This ductile iron casting has a contour size of 640 mm × 601 mm × 625 mm and a mass of 301 kg. The main wall thickness is generally within 30 to 60 mm, with some local sections exceeding 60 mm. According to the national standard GB/T 1348—2019, a C-type 40 mm cast-on test block was used to evaluate the mechanical properties. The image below illustrates a typical ductile iron casting component similar to the product.
Because the service environment of this component can reach -50 °C, the ductile iron casting must possess reliable impact toughness at that temperature. The specific requirements for the cast-on test block are summarized in Table 1. The tensile strength must be at least 380 MPa, the yield strength at least 230 MPa, and the elongation at least 15%. In addition, the average impact absorbed energy at -50 °C must be greater than or equal to 12 J, with no individual value below 9 J. These strict requirements demand careful control of both composition and heat treatment.
| Standard | Grade | Wall thickness (mm) | Rm (N/mm²) | Rp0.2 (N/mm²) | Elongation A (%) |
|---|---|---|---|---|---|
| GB/T 1348—2019 | QT400-18L | t ≤ 30 | 400 | 240 | 18 |
| GB/T 1348—2019 | QT400-18L | 30 < t ≤ 60 | 380 | 230 | 15 |
| DIN EN 1563:2019 | EN-GJS-400-18LT | t ≤ 30 | 400 | 240 | 18 |
| DIN EN 1563:2019 | EN-GJS-400-18LT | 30 < t ≤ 60 | 380 | 230 | 15 |
2. Materials and Experimental Procedure
2.1 Raw materials
To obtain ductile iron casting with high low-temperature impact toughness, the raw materials must be clean and free from harmful impurities. In this investigation, we selected Q10 high-purity pig iron, quality steel scrap, and ductile iron return scrap as the metal charge. The charge ratio was set at 7:2:1 (pig iron:steel scrap:return scrap). In addition, carburizer containing 97.50%–99.50% carbon, ferrosilicon with 72.0%–80.0% silicon, a 6RE nodulizer containing 5.00%–5.50% magnesium, and a high-calcium barium inoculant with 68.00%–75.00% silicon were used. The use of low-manganese and low-phosphorus raw materials helps to minimize the negative effects of these elements on impact toughness.
2.2 Chemical composition design
For ductile iron casting, the carbon equivalent (CE) is a critical parameter affecting fluidity and solidification behavior. Based on industrial practice for castings with a wall thickness of 30–60 mm, the carbon equivalent should be controlled in the range of 4.3% to 4.5%. The empirical formula for carbon equivalent is often expressed as:
$$ \text{CE} = \text{C} + \frac{\text{Si}}{4} + \frac{\text{P}}{2} $$
However, in this work the values were adjusted according to production experience. Silicon increases the tensile strength and yield strength, but excessive silicon significantly deteriorates impact toughness. Manganese and other alloying elements can negatively affect the impact energy and raise the ductile-to-brittle transition temperature. Phosphorus tends to segregate at grain boundaries, forming brittle carbides, so its content must be kept extremely low. Sulfur consumes nodulizer and causes irregular graphite, so it must also be minimized. Magnesium content is essential for nodularity but should be controlled within a narrow range.
The designed chemical composition of the ductile iron casting is given in Table 2. No intentional addition of copper, nickel, molybdenum, or other alloying elements was made. Only trace amounts from the raw materials were expected.
| C | Si | Mn | P | S | Mg |
|---|---|---|---|---|---|
| 3.65–3.75 | 1.95–2.05 | < 0.20 | ≤ 0.04 | ≤ 0.02 | 0.035–0.045 |
The charge materials were weighed according to the designed ratio and melted in a medium-frequency induction furnace. After melting, a sample was taken to verify the composition, and adjustments were made by adding carburizer or ferrosilicon as needed. When the desired composition was reached, 0.1%–0.3% of silicon-barium was added for deoxidation pre-treatment. The melt was then tapped into a ladle for nodularization treatment using the sandwich (pouring-over) method. The nodulizer (6RE) was placed in the bottom of the ladle together with an inoculant and steel chips, all preheated at 100–200 °C for 1 hour. The nodulizer addition amount was 1.2%–1.3% of the melt mass, and the inoculant addition was 0.7%–0.9% of the melt mass. After the reaction was complete, the slag was removed, the melt temperature was measured, and the iron was poured. During pouring, a stream inoculation with 0.1%–0.2% inoculant was applied. After solidification, the castings were allowed to cool slowly in the mold to room temperature. The castings with attached test blocks were then removed from the molds, shot blasted, and separated. Some test blocks were tested in the as-cast condition; the remaining blocks were heat-treated together with the castings.
3. Annealing Heat Treatment
As-cast ductile iron often contains a small amount of pearlite, which is harmful to low-temperature impact toughness. For the casting with a wall thickness of 30–60 mm, a ferritic annealing treatment was adopted to transform any pearlite into ferrite. The heat treatment cycle is shown in Figure 4 in the original paper. In brief, the castings and attached test blocks were loaded into an annealing furnace and heated to 850 °C, holding for 0.5 h. The temperature was then raised to 920 °C and held for 3.5 h. After that, the furnace was cooled to 550 °C and then opened for air cooling. The heating rate was strictly controlled at 70 °C/h and the cooling rate at 35 °C/h. This slow heating and cooling prevents thermal stress and ensures complete decomposition of pearlite.
After the annealing treatment, test specimens were machined from the cast-on test blocks. Tensile tests were performed according to ISO 6892-1, and Charpy V-notch impact tests were conducted at -50 °C using a pendulum impact tester. Metallographic samples were prepared and examined under an optical microscope to evaluate the graphite nodularity, nodule count, and the fraction of ferrite. Five independent batches (labeled 1 to 5) were evaluated to confirm repeatability.
4. Results and Discussion
4.1 Chemical composition of the tested batches
Table 3 presents the measured composition of five batches after nodularization and inoculation. All conventional elements are within the designed ranges. The contents of Cu, Ni, Mo, Sb, and Sn are very low, introduced only as trace impurities from the charge. Since these elements are present in such small amounts, their alloying effect is negligible. Thus the dominant factors affecting mechanical properties are C, Si, Mn, P, and S.
| No. | C | Si | Mn | P | S | Mg | Cu | Ni | Mo | Sb | Sn |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 1 | 3.76 | 1.99 | 0.19 | 0.030 | 0.0091 | 0.041 | 0.027 | 0.014 | 0.0032 | 0.0010 | 0.0021 |
| 2 | 3.70 | 2.05 | 0.20 | 0.030 | 0.0098 | 0.038 | 0.041 | 0.012 | 0.0028 | 0.0010 | 0.0023 |
| 3 | 3.67 | 2.07 | 0.18 | 0.028 | 0.012 | 0.036 | 0.04 | 0.011 | 0.0038 | 0.001 | 0.0024 |
| 4 | 3.68 | 1.96 | 0.17 | 0.029 | 0.010 | 0.039 | 0.028 | 0.011 | 0.0026 | 0.0010 | 0.0019 |
| 5 | 3.65 | 2.04 | 0.18 | 0.032 | 0.011 | 0.043 | 0.048 | 0.013 | 0.0030 | 0.0009 | 0.0021 |
4.2 As-cast microstructure and mechanical properties
The as-cast test blocks were first examined. Table 4 summarizes the tensile properties, hardness, graphite characteristics, pearlite content, and -50 °C impact absorbed energy for the five batches. The tensile strength values are all above 380 MPa, and the yield strength is above 236 MPa, meeting the standard. However, the impact absorbed energy at -50 °C is insufficient. The average values range from 6.95 J to 10.33 J, and some individual values are much lower than 9 J. This indicates that the as-cast ductile iron casting cannot reliably meet the low-temperature impact requirement. The metallographic observation reveals a nodularity of 2–3 grade and a pearlite content below 5%. Figure 3 in the original paper shows the as-cast microstructure before and after etching. The small amount of pearlite present in the matrix is responsible for the reduced impact toughness.
| No. | Rm (N/mm²) | Rp0.2 (N/mm²) | A (%) | Hardness (HB) | Nodularity grade | Nodule count (/mm²) | Pearlite (%) | KV2 at -50 °C (J) – value 1 | KV2 – value 2 | KV2 – value 3 | Average KV2 (J) |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 1 | 395 | 248 | 24.5 | 144 | 2–3 | 155 | <5 | 10.12 | 10.35 | 10.51 | 10.33 |
| 2 | 395 | 251 | 20.5 | 145 | 2–3 | 170 | <5 | 8.94 | 9.08 | 8.63 | 8.88 |
| 3 | 400 | 252 | 26.0 | 148 | 2–3 | 125 | 5 | 7.09 | 7.11 | 6.65 | 6.95 |
| 4 | 386 | 236 | 25.5 | 143 | 2–3 | 125 | <5 | 9.11 | 9.83 | 10.07 | 9.67 |
| 5 | 392 | 245 | 25.5 | 144 | 2–3 | 130 | <5 | 10.26 | 9.53 | 10.74 | 10.17 |
The correlation between pearlite content and impact energy is evident. Even a small fraction of pearlite can significantly lower the impact toughness of ductile iron casting at low temperatures. Pearlite acts as a brittle phase, providing an easy path for crack propagation and reducing the energy absorbed during fracture. Therefore, it is necessary to eliminate pearlite through heat treatment.
4.3 Effects of annealing on microstructure
After the annealing treatment, the pearlite is decomposed into ferrite and graphite. The microstructure of the annealed samples is shown in Figure 5 in the original paper, which indicates a nearly fully ferritic matrix. The measured pearlite content after annealing was less than 2% for all batches. The nodularity remained at grade 2–3, and the nodule count only slightly decreased due to the high temperature holding, but still remained in the range of 135–150 per square millimeter. A fully ferritic matrix is essential for low-temperature toughness because ferrite has a body-centered cubic crystal structure that can still provide some ductility at low temperatures, whereas pearlite is much more brittle.
4.4 Mechanical properties after annealing
Table 5 lists the tensile properties, hardness, graphite parameters, and impact absorbed energy at -50 °C for the annealed ductile iron casting. All five batches now satisfy the standard requirements. The average impact energy ranges from 14.06 J to 14.52 J, which is much higher than the required 12 J. The minimum single impact value is 13.63 J, well above the 9 J limit. The tensile strength values are between 384 and 390 MPa, which is slightly lower than the as-cast values because of the elimination of pearlite, but still comfortably above the 380 MPa minimum. The yield strengths are all above 239 MPa. The elongation values are in the range of 23.5%–26.5%, significantly exceeding the required 15%. Hardness ranges from 138 to 144 HB, which is typical for ferritic ductile iron.
| No. | Rm (N/mm²) | Rp0.2 (N/mm²) | A (%) | Hardness (HB) | Nodularity grade | Nodule count (/mm²) | Pearlite (%) | KV2 at -50 °C (J) – value 1 | KV2 – value 2 | KV2 – value 3 | Average KV2 (J) |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 1 | 384 | 242 | 23.5 | 138 | 2–3 | 145 | <2 | 14.42 | 13.81 | 14.42 | 14.22 |
| 2 | 389 | 245 | 24.5 | 142 | 2–3 | 150 | <2 | 13.87 | 14.25 | 14.07 | 14.06 |
| 3 | 390 | 243 | 25.0 | 144 | 2–3 | 145 | <2 | 13.63 | 14.42 | 14.35 | 14.13 |
| 4 | 386 | 239 | 25.5 | 140 | 2–3 | 140 | <2 | 14.42 | 14.63 | 14.52 | 14.52 |
| 5 | 385 | 240 | 26.5 | 139 | 2–3 | 135 | <2 | 14.51 | 13.89 | 14.34 | 14.25 |
The average -50 °C impact energy of all batches is 14.37 J, which is approximately 45% higher than the as-cast average (9.20 J). This improvement confirms that annealing efficiently transforms the residual pearlite into ferrite, thereby enhancing the toughness of ductile iron casting in low-temperature service.
4.5 Discussion of key factors
To ensure the success of this development, several interacting factors must be carefully controlled.
Effect of silicon: Silicon is a strong graphitizing element and promotes the formation of ferrite. However, silicon also increases the ductile-brittle transition temperature. In this design, the silicon content was kept at about 2.0%, which provides sufficient strength while maintaining the impact toughness at -50 °C. Higher silicon content would raise the yield strength but at the cost of excessive toughness degradation. Therefore, the range of 1.95%–2.05% is considered optimal for this wall thickness and heat treatment.
Effect of manganese: Manganese is a strong pearlite stabilizer and also segregates at cell boundaries. For low-temperature applications, manganese should be kept as low as possible. Our measured values were below 0.20%. The complete decomposition of pearlite during annealing becomes easier when the manganese content is low. If manganese were higher, a longer holding time or a higher temperature would be necessary, which might coarsen the graphite and reduce nodule count.
Effect of phosphorus: Phosphorus is a harmful element in ductile iron casting. It forms steadite (iron phosphide eutectic) at grain boundaries, which severely lowers impact toughness and increases brittleness. The raw materials chosen had phosphorus content between 0.028% and 0.032%, ensuring that the detrimental effect is minimized.
Heat treatment parameters: The annealing cycle was chosen to ensure complete decomposition of pearlite without causing excessive ferrite grain growth. The two-stage holding at 850 °C and 920 °C allows for homogenization and decomposition of any carbides. Cooling at 35 °C/h to 550 °C followed by air cooling prevents the formation of secondary pearlite and avoids high residual stresses. The final pearlite content of less than 2% guarantees that the impact requirement is reliably met.
Nodule count and nodularity: The nodule count of 135–150 per square millimeter ensures a short diffusion distance for carbon during annealing and produces small graphite nodules that are less detrimental to toughness. Nodularity of 2–3 grade is acceptable, but higher nodularity would improve consistency. The graphite shape affects the stress concentration at the graphite-matrix interface; spherical particles are ideal for ductile iron casting to achieve high toughness.
It should be noted that without the addition of elements like nickel or copper, the cost of production is significantly reduced. The use of post-inoculation and controlled pouring also contributed to the uniform microstructure and consistent properties. Based on these results, the developed ductile iron casting is suitable for railway components operating in extremely cold regions.
5. Conclusion
In this work, a ductile iron casting capable of withstanding -50 °C impact loading was successfully developed without adding costly alloying elements. The following conclusions can be drawn:
- By selecting clean raw materials and carefully controlling the chemical composition, particularly limiting Mn, P, and S, a ductile iron casting with a carbon equivalent of about 4.3%–4.5% can be produced with a predominantly ferritic matrix after annealing.
- The as-cast material contained less than 5% pearlite, which was insufficient to meet the -50 °C impact energy requirement. The average impact energy was around 9.2 J, below the 12 J target.
- After a two-stage annealing treatment (850 °C for 0.5 h, 920 °C for 3.5 h, furnace cooling to 550 °C, then air cooling), the pearlite was almost completely decomposed. The pearlite content dropped below 2%, and a nearly 100% ferrite matrix was achieved.
- The annealed ductile iron casting exhibited a tensile strength of 384–390 MPa, yield strength of 239–245 MPa, elongation of 23.5%–26.5%, and an average -50 °C impact energy of 14.37 J with the lowest individual value of 13.63 J. All these values meet the requirements of QT400-18L and EN-GJS-400-18LT.
- This economical approach provides a practical solution for producing low-temperature impact-resistant ductile iron casting without the need for expensive alloy additions, offering an important reference for the casting industry.
Acknowledgments: The authors would like to thank the laboratory staff for their assistance in mechanical testing and metallographic analysis.