1. Structure of nodular cast iron
The material grade of the ductile iron parts for the main shaft of the high-power offshore fan is EN-GJS-400-18U-LT (- 20 ℃), and the overall dimension is 5280 mm × 5 280 mm × 2 460 mm, unit weight 32 672 kg, the structure of nodular cast iron is as shown in the figure. The maximum wall thickness is at the connecting flange of the rotor. The wall thickness is 200 mm. The minimum wall thickness is 50 mm for the main cylinder wall. It belongs to low-temperature ductile iron castings with large wall thickness difference.
2. Technical requirements
2.1 Mechanical properties
The mechanical properties shall meet the requirements of DIN EN 1563-2005 Ductile Iron. The mechanical properties of U70 attached cast test block and ductile iron body shall be tested, and the V-notch specimen shall be subjected to – 20 ℃ low temperature impact test. The specific requirements are shown in Table 1.
| Category | Tensile strength/MPa | Yield strength/MPa | Elongation/% | Hardness/HB | Impact value/J (- 20 ℃) average | Impact value/J (- 20 ℃) single value |
| Attached cast test block | ≥360 | ≥220 | ≥12 | 130~180 | ≥10 | ≥7 |
| Casting body | ≥310 | ≥200 | ≥8 | 130~180 | — | — |
2.2 NDT requirements
Carry out 100% magnetic particle inspection (MT) and 100% ultrasonic inspection (UT) for ductile iron castings. UT flaw detection requirements: in accordance with EN 12680-3-2003 Ultrasonic Testing: Ductile Iron, the key area is of grade 2, and the non-key area is of grade 3.
MT inspection requirements: use fluorescent medium to carry out magnetic particle inspection in accordance with EN 1369-2012 Magnetic Particle Testing. The key area is level 2 and the non-key area is level 3.
2.3 Metallographic structure
Metallographic inspection was carried out on the attached cast test block and the body of nodular cast iron. The requirements for the microstructure of the test block and the body were consistent. The specific requirements are shown in Table 2.
| Category | Matrix structure | Graphite morphology | Graphite size/grade |
| Attached cast test block | Ferrite matrix, pearlite volume fraction ≤ 10% | V+VI≥90% | 4~6 |
| Casting body | Ferrite matrix, pearlite volume fraction ≤ 15% | V+VI≥75% | — |
2.4 Dimension and quality tolerance
The dimensional tolerance of the spheroidal graphite cast iron blank shall be in accordance with the CT11 level of ISO 8062-3-2007 Dimensional Tolerances for Castings. The wall thickness tolerance shall reach the CT12 level. The actual quality of the spheroidal graphite cast iron after processing shall not exceed ± 3% of the theoretical quality.
