The tensile test shall be carried out in accordance with gb/t 228-2002 tensile test method for metallic materials at room temperature, and the sample shall be a standard short cylindrical sample; The impact test shall be conducted according to GB 229-2007 metallic materials Charpy pendulum impact test method, and the sample shall be 10 mm × The test equipment used for the 10 mm notchless impact specimen has been qualified by the relevant measurement department and meets the test requirements.
Ten standard samples of nodular cast iron were divided into a group, heated to 890 ℃ with the same batch of nodular cast iron crankshafts, and then isothermal cooled at 300 ℃, 280 ℃, 260 ℃ and 240 ℃ respectively. After being processed into standard samples, tensile tests and impact tests were carried out. At the same time, hardness measurement and metallographic analysis were carried out on the samples after tensile test and impact test. Among them, the arithmetic mean value (10 samples / group) was taken for the data of tensile strength, elongation after fracture and impact toughness. The results are shown in the table.
| Group number | Isothermal temperature / ℃ | Impact toughness /j · cm-2 | Tensile strength /MPa | Elongation after fracture /% | Surface hardness /HRC |
| 1 | 300 | 75.1 | 1 469 | 3.7 | 43-45 |
| 2 | 280 | 67.9 | 1 470 | 3.2 | 44-46 |
| 3 | 260 | 60.1 | 1 513 | 2.8 | 46-48 |
| 4 | 240 | 43.7 | 1 536 | 1.2 | 50-51 |
It can be seen from the table that at the constant temperature of 240~300 ℃, with the decrease of temperature, the positional strength of nodular cast iron sample increases slightly, and the elongation and toughness after fracture decrease significantly. Among them, the decrease of toughness affects the fatigue strength of crankshaft. At 300 ℃, the microstructure is mainly composed of upper bainite and a small amount of retained austenite; When isothermal at 240 ℃, the microstructure is mainly bainite below. According to JB 3021-1981 metallographic standard for isothermal quenching of rare earth magnesium nodular cast iron, the number of white areas is grade 2-3, and the content is about 10%. After observation, it was found that there were more quenched martensite in the white zone besides the residual austenite.
Theoretically, the strength and toughness of upper bainite are lower than that of lower bainite, but the toughness of nodular cast iron sample after isothermal treatment at 240 ℃ is significantly lower than that of 300 ℃, which is obviously related to the existence of quenched martensite in the structure. The brittleness of quenched martensite reduces the toughness of nodular cast iron sample.