PSI - Issue 7

Šulko Miroslav et al. / Procedia Structural Integrity 7 (2017) 262–267 Šulko Miroslav et Al./ Structural Integrity Procedia 00 (2017) 000–000

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The value of micro hardness in 24CrMoV55 bainitic steel significantly decreases at cyclical loading. At high stress amplitudes, the micro hardness rate of decrease is high (for σ a =696MPa more than 40%) – (Fig.2). At lower cyclic stress levels, the micro hardness decrease rate is lower (for σ a =610MPa more than 12%) – (Fig.3). The same tendency has Brinell hardness on the surface of the material, the rate of change is lower (for σ a =610MPa more than 5%) – (Fig.3). For steel S355J0 were obtained dependency between development of hardness and the level of fatigue damage for amplitudes of alternating stress cycle of 325MPa (N f =11800 cycles) and 280MPa (N f =105000 cycles) - see (Fig. 4).

Fig.4 Development of Vickers micro hardness HV0,02 and Brinell´s hardness HB1/30/10 depending on the degree of fatigue damage – steel S355J0

The value of micro hardness of low carbon steel S355J0 varies significantly with cyclic loading. During the first 40% of the fatigue lifetime it increases (about 40% in ferrite and about 20% in perlite) and decreases during further loading. The same trend has also Brinell's hardness, only the change rate is lower (about 12-15%) - (Fig.4). 4. Conclusions The hypothesis about the change of surface properties of steels (probably of all metals) during cyclic loading was supported. The Vickers micro hardness and Brinell´s hardness values obtained vary significantly over different cyclic loading amplitudes. Based on the measured micro hardness and hardness of the two different steels, several preliminary conclusions can be made: - the rate of change of Vickers micro hardness is in all cases higher than the change in Brinell´s hardness - the rate of change in both micro hardness and hardness is higher at higher loading amplitudes with decreasing amplitude of cyclic loading also declining - the changes in micro hardness and hardness copy the cyclical behavior of the respective steel that means cyclic softening i.e. cyclic hardening