Failure Causes and Solutions for Ductile Iron Pipes

1. Cause Analysis

1.1 Ingredient Analysis

Under specific cooling rates and inoculation conditions, increasing the carbon equivalent can enhance the number of graphite nodules in ductile iron, improving the morphology of the graphite. This, in turn, reduces the amount of carbon existing in carbide form while boosting the proportion in graphite form, thereby diminishing the tendency for supercooling in the molten iron after cavitation treatment. On the binary Fe-C phase diagram, the eutectic carbon content is 4.26%. Typically, for main pipes ranging from DN800 to DN1000 mm, the carbon equivalent is controlled within the range of 4.2% to 4.3%, which helps enhance the fluidity of the molten iron and promotes densification of the microstructure in ductile iron pipes. However, if the carbon equivalent becomes too high, it increases the tendency for graphite floatation, leading to graphite removal. Moreover, excessive carbon results in a higher effective volume fraction occupied by graphite pores, ultimately compromising the overall performance of ductile iron and resulting in inferior mechanical properties of the ductile iron pipes.

1.2 Metallographic Analysis

1.2.1 Graphite Spheroidization Analysis

After magnifying the sample 100 times and observing sequentially from the outer wall to the inner wall, the distribution of graphite is shown in Figure 2. In Figure 2a, the graphite on the inner wall is predominantly flaky, with significant creep and cavitation levels exceeding Grade 6; the graphite particles are classified as Grade 8 in size. In Figure 2b, most of the graphite in the central region appears spherical, while a small fraction remains blocky or exists in trace amounts—resulting in a sphericity grade of 2. The graphite particles here are also rated as Grade 8 in size. Finally, in Figure 2c, the black research area at the outer wall exhibits a smaller volume, with a sphericity grade of 3. This means that the majority of the graphite particles are blocky or spherical, with only a minor portion remaining blocky. Notably, the graphite particles in this region measure over 1.5 millimeters in diameter.

1.2.2 Metallographic Analysis

The ferrite content is higher on the inner wall of the specimen, while the lower-left corner shows an accumulation of graphite that has yet to fully globalize. In Figure 3b, compared to the inner wall, the ferrite content at the center of the end face (1) has decreased, while the presence of LEY—a mixture of pearlite and cementite—has increased. At the center of Figure 3c, there is a noticeable increase in cementite migrating from the interior toward the exterior. Finally, Figure 3d reveals that the outer wall of the end face contains 18% pearlite, with reduced ferrite levels and a notable rise in proeutectoid cementite, pearlite, and ledeburite.

The specimen achieved a ductility rating of Level 3 according to the national standard, while the ductility near the inner wall was rated at Level 6. Residual debris adhered to the inner wall of the pipe, leading to the accumulation of numerous impurity elements. Notably, the spheroidalization effect on the outer wall was superior to that observed in the center and inner wall regions. In the microstructure of the outer wall, Ray's segregation was particularly pronounced.

The mechanical properties of the pipe wall vary significantly. The decomposition and transformation of eutectic cementite and pearlite in the inner wall lead to the enlargement of black studies, causing the microstructure to expand in volume and increasing the radius of the main pipeline. Moreover, the higher the degree of graphitization, the easier it becomes for black studies to grow larger and swell further. Notably, the decomposition of cementite and pearlite requires specific temperature and time conditions. Analysis reveals that, under high external pressure, the outer wall—composed of hard, brittle ledeburite with low strength and toughness—tends to fracture first. If the internal pressure on the entire main pipe's inner wall is particularly intense, localized pipe wall failures may occur. To prevent such issues, appropriate preventive measures should be implemented.

2.1 Adjust the appropriate carbon equivalent and increase the amount of scrap steel added, ensuring the scrap steel ratio reaches 7% to 10%. Meanwhile, silicon levels must be precisely controlled, while maintaining the carbon equivalent within the range of 4.2% to 4.3%.

2.2 Strengthen control over the annealing process.

The size of the pipeline mixing and cooling process is controlled by the parameters of the larger pipe, and when entering the cooling pipe, the two chain claws are re-separated relative to the hot pipe.

2.3 Enhance sampling frequency, elevate monitoring standards, and improve annealing quality by enabling timely, accurate, and rapid responses to annealing outcomes. Additionally, install a rapid metallographic sampling device downstream of the annealing furnace, increasing the sampling frequency from 1 in 100 to 25 strategically placed points. The resulting metallographic inspection data will be directly available on-site at the annealing furnace, facilitating precise control and adjustment of annealing parameters.