PSI - Issue 58

Alan Vaško et al. / Procedia Structural Integrity 58 (2024) 48–53 A. Vaško et al. / Structural Integrity Procedia 00 (2019) 000–000

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2. Experimental material and methods For mechanical and corrosion tests, two types of austenitic nodular cast iron with different chemical compositions (Table 1) were chosen, namely:  nickel-manganese nodular cast iron designated as EN-GJSA-XNiMn13-7 by symbol and 5.3506 by number;  nickel-chromium nodular cast iron designated as EN-GJSA-XNiCr20-2 by symbol and 5.3500 by number. The mechanical and corrosion properties of these austenitic nodular cast irons were compared with the properties of two types of alloyed non-austenitic nodular cast irons, namely:

 silicon-molybdenum ferritic-pearlitic nodular cast iron designated as EN-GJS-X300SiMo4-1;  silicon-copper pearlitic-ferritic nodular cast iron designated as EN-GJS-X300SiCu4-1.5.

Table 1. Chemical composition of selected austenitic nodular cast irons. NCI ( weight % ) C Si Mn Ni Cr

Cu

P

S –

Mg

C E

NiMn

required

max 3.00

2.00– 3.00 2.177 1.50– 3.00 2.305

6.00– 7.00 6.365 0.50– 1.50 1.079

12.0– 14.0 13.45 18.0– 22.0 19.77

max 0.20

max 0.50

max 0.08

–

–

real

2.634

0.061 1.00– 3.50 1.859

0.038

0.035

<0.015

0.094

3.82

NiCr

required

max 3.00

max 0.50

max 0.08

–

–

–

real

2.284

0.033

0.025

<0.015

0.078

3.72

The mechanical properties of all four mentioned types of nodular cast irons were determined by the following mechanical tests: tensile test (according to STN EN ISO 6892-1), impact bending test (according to STN EN ISO 148-1), Brinell hardness test (according to STN EN ISO 6506-1) and low-frequency fatigue test (according to STN 42 0362) with alternating symmetrical cyclic loading (stress ratio R = –1) (Bokůvka 2014, Kopas 2016). The corrosion properties were determined by two corrosion tests: the exposure immersion test and the electro chemical potentiodynamic polarisation test (Baboian 2005, Yau 2003, Mansfeld 2003, Roberge 2008). Both corrosion tests were performed at ambient temperature (T = 23  5 °C) in a 3.5% NaCl solution (to simulate seawater). For the exposure immersion test, 12 samples from each type of nodular cast iron were used. The samples were immersed in the salt solution and left in it for the required time. Three samples were taken after 1 week, another three samples after 2 weeks, another three samples after 4 weeks and the last three samples after 8 weeks. From the weight of the samples before and after the test, the weight loss and the average corrosion rate were calculated. For the electrochemical potentiodynamic polarisation test, 3 samples from each type of nodular cast iron were used, on which potentiodynamic polarisation curves were determined. From them, the corrosion potential, corrosion current density and average corrosion rate were determined using Tafel analysis. 3. Experimental results and discussion The microstructure of chosen austenitic nodular cast irons is shown in Fig. 1. The austenitic matrix was obtained by the addition of alloying elements, namely nickel, manganese or chromium. Perfectly and imperfectly nodular graphite can be found in the austenitic matrix. The structural parameters, expressing the shape, size and amount of graphite, are listed in Table 2. The microstructure of nickel-manganese nodular cast iron (Fig. 1a) is without carbides, but the microstructure of nickel-chromium nodular cast iron (Fig. 1b) contains regularly distributed chromium carbides (Hasse 2008). The microstructure of chosen non-austenitic nodular cast irons is described in detail in the article (Vaško 2018). Silicon-molybdenum nodular cast iron has a ferritic-pearlitic matrix and silicon-copper nodular cast iron has a pearlitic-ferritic matrix.