Defect Analysis of the 40CrMnMoA Roll Shaft Forging

The roller shaft is made of 40CrMnMoA steel with a net weight of 36.6 tons, manufactured from a 60.5-ton steel ingot. Its manufacturing process includes: smelting, casting → forging → post-forging heat treatment → rough machining → ultrasonic testing. During ultrasonic testing, continuous defects exceeding the specified limits were detected at the nozzle end, with the maximum equivalent diameter being ∅6 mm, distributed within the central ∅350 mm range. The specific defect distribution is shown in Figure 1; according to the standards, the shaft was deemed non-conforming, and the forging was scrapped. To analyze the cause of the defects, samples were collected from the most severely affected area for defect analysis; the exact sampling locations are indicated by the boxes in Figure 1.

1.1 Low-magnification Test

At the locations shown in Figure 1, fracture specimens and low-magnification test pieces were collected for analysis. The low-magnification test pieces were ground and subjected to a hot acid etching test; after acid washing, they were observed for a specified period. A crack defect was identified at the center of the test piece, with smooth and well-defined crack edges. Non-metallic inclusions were present within the crack, and multiple point-like inclusions were observed near the crack. Preliminary analysis suggests that this defect is a slag-inclusion crack. No other defects such as point-like segregation or porosity were detected. A local magnified view of the defect is shown in Figure 2.

1.2 Macro Fracture Analysis

The fracture specimen was subjected to a slotting and crushing fracture on the back side of the defect location. The morphology of the fractured surface after opening is shown in Figure 3. It can be observed that the crack surface appears gray upon opening under low magnification, with multiple gray defects adjacent to the crack surface; these defects are likely slag inclusion defects. The fracture exhibits a crystalline fracture morphology. The localized magnified morphology of the defect is presented in Figure 4.

1.3 Chemical Analysis

Samples were collected from the low-magnification specimens for chemical composition analysis, with the results presented in Table 1. As shown in Table 1, the chemical composition of the samples complies with the requirements of JB/T 6396-2006. Although the C content is relatively low, it meets the permissible deviation limits for steel composition specified in GB/T 222. The concentrations of all other elements fall within the specified ranges, confirming that the chemical composition is qualified.

1.4 Metallographic Analysis

Samples were collected from the defect location of the fracture specimen for metallographic analysis. After grinding and polishing, microscopic examination revealed numerous non-metallic inclusions in the longitudinal section of the specimen; photographs of these inclusions are shown in Figure 5. Following etching with a 4% nitric acid-alcohol solution, the microstructure exhibited a pearlite-ferrite composition with normal metallographic characteristics. The base metallographic image is presented in Figure 6.

1.5 Electron Microscopy Analysis

Samples were collected at the locations shown in Figure 3 for scanning electron microscopy (SEM) analysis. The morphology of the inclusion under SEM is presented in Figure 7. The fracture morphology of the substrate exhibits characteristics of dislocation fracture, as shown in Figure 8. Energy dispersive spectroscopy (EDS) analysis of the inclusion revealed that its composition primarily consists of elements such as Al, Si, and Ca, as illustrated in Figures 9 and 10.

2 Analysis and Discussion

The roll shaft forging exhibits severe defects at low magnification, with numerous non-metallic inclusions visible under high magnification. Scanning electron microscopy and energy dispersive spectroscopy analysis of the severely affected fracture surfaces revealed that the defects primarily consist of aluminum (Al), silicon (Si), and calcium (Ca). Since Al, Si, and Ca are not the primary alloying elements of the 40CrMnMoA roll shaft forging but rather the main components of the heating and diffusion deoxidizer (Si-Al-Ca-Ba powder) used during smelting, these defects are identified as deoxidation product inclusions containing aluminum, silicon, and calcium. Based on the composition of the inclusions, the deoxidation method employed, and the defect locations, it is concluded that these inclusions formed when deoxidation products failed to fully rise or were entrapped by oxide films during smelting, subsequently entering the ingot with the molten steel during casting. During the subsequent forging process, the浇口 was not completely removed, ultimately resulting in severe defects exceeding acceptable limits and rendering the forgings defective.

3 Conclusion

The excessive defect in this roller shaft forging is slag inclusion, with the primary components being elements such as Al, Si, and Ca.