PSI - Issue 51

Alan Vaško et al. / Procedia Structural Integrity 51 (2023) 129–134 A. Vaško et al. / Structural Integrity Procedia 00 (2022) 000–000

133 5

Results of the metallographic analysis (according to the norm and using automatic image analysis) are shown in Table 3. According to STN EN ISO 945, graphite can be found in two different shapes – perfectly spheroidal shape (80%) and imperfectly spheroidal shape (20%), both of which have the same size range (from 30 to 60 μm). According to image analysis, the shape factor of graphite is 0.60, the equivalent diameter of graphite is 26.0 μm, the count of graphitic nodules is 132.9 mm -2 , the graphite content is 7.7%; the content of carbides was not evaluated.

Table 3. Quantitative evaluation of the microstructure of austenitic nodular cast iron EN-GJSA-XNiCr20-2. Microstructure (according to STN EN ISO 945) Shape factor of graphite * Equivalent diameter of graphite ( μ m ) Count of graphitic nodules ( mm -2 )

Content of graphite ( % )

80%VI6 + 20%V6

0.60

26.0

132.9

7.7

* Shape factor S = 4πA/P 2 , where A is an area and P is a perimeter of graphitic particles

Table 4 shows the mechanical properties of EN-GJSA-XNiCr20-2 specified by the norm and the real measured mechanical properties. The yield strength is 251.0 MPa, the tensile strength is 374.7 MPa, the elongation is 12.0%, the absorbed energy is 42.3 J (measured on test specimens without a notch), and the Brinell hardness is 150.3 HBW5/250/10. Measured mechanical properties correspond to the requirements of the norm.

Table 4. Mechanical properties of austenitic nodular cast iron EN-GJSA-XNiCr20-2. Mechanical properties R p0.2 ( MPa ) R m ( MPa ) A ( % ) KV ( J ) K0 ( J )

HBW 5/250/10 140–255

norm

210–250

370–480

7–20 12.0

11–24

–

real

251.0

374.7

–

42.3

150.3

Results of fatigue tests on the specimens of EN-GJSA-XNiCr20-2 are presented in Fig. 3. The measured values of stress amplitude σ a and number of cycles to failure N f were used to construct the Wöhler fatigue curve. Then, the fatigue limit σ c for N = 10 7 cycles was determined from the fatigue curve (Handrik 2017). The fatigue limit σ c of austenitic nodular cast iron EN-GJSA-XNiCr20-2 is 150 MPa, and the ratio of the fatigue limit and tensile strength σ c /R m is 0.40. The fatigue properties of austenitic nodular cast iron were compared with those of another two alloyed nodular cast irons, namely ferrite-pearlitic nodular cast iron EN-GJS-X300SiMo4-1 and pearlite-ferritic nodular cast iron EN-GJS-X300SiCu4-1.5 (Fig. 4) (Vaško 2018). These nodular cast irons have higher fatigue limits σ c than austenitic nodular cast iron, while pearlite-ferritic nodular cast iron has a higher fatigue limit and higher ratio of fatigue limit and tensile strength (σ c = 270 MPa, σ c /R m = 0.41) than ferrite-pearlitic nodular cast iron (σ c = 210 MPa, σ c /R m = 0.37). The results of the fatigue tests show a relationship between the fatigue limit σ c and the tensile strength R m (Lukhi 2018), i.e. the fatigue limit increases as the tensile strength increases.

Fig. 3. Wöhler fatigue curve of austenitic nodular cast iron EN-GJSA-XNiCr20-2, σ c = 150 MPa, σ c /R m = 0.40.