PSI - Issue 28

Avanish Kumar et al. / Procedia Structural Integrity 28 (2020) 93–100 Avanish et al. / Structural Integrity Procedia 00 (2019) 000–000

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growth. The enhanced sub-critical fatigue life may be by virtue of the fact that the steel transformed at higher temperature contains more amount of RA in between BF laths. When the fatigue crack tip reaches a ductile austenite lath, a great amount of the input mechanical energy, which could have contributed to the crack propagation, is absorbed in phase-transformation of RA to martensite (Huo and Gao, 2005). In the present study, nano-indentation experiments were performed to verify the TRIP effects of RA on fatigue crack growth behavior suggested in our prior work (Kumar and Singh, 2019). It is well known that with an increase in Δ K , plastic zone size increases with an increase in the crack length, and since NB350 steel has thicker RA with low carbon concentration, it should show the maximum TRIP effect amongst the three steels. However plastic zone size will be smallest for NB250 steel which has the highest yield strength. Therefore, an array of indents were made below the crack surface on the transverse section cut at mid-thickness of the tested specimen. This region corresponded to stage-II in d a /d N – Δ K plot for NB250 and NB350 steel. Fig. 4 shows the variation in hardness as a function of depth below the crack surface at a crack length of ~17 mm where Δ K was equal to 20.7 and 25 MPam -1/2 for NB250 and NB350 steel respectively. In case of NB250 steel, hardness profile does not show any variation with respect to the mean bulk hardness of the steel. The possible reason can be the finer microstructure and large density of dislocation in this steel such that increase in hardness due to phase-transformation is very small, that too in a small cyclic plastic zone size of ~14 μm. However in case of NB350 steel, the hardness near the crack surface is high and it gradually decreases to bulk hardness of the steel at a distance of ~45 μm. This suggests that there is significant amount of transformation of RA to harder phase martensite which led to increase in hardness of this small region beneath the crack surface. Knowing the yield strength of NB350 steel and stress intensity factor range at that crack length, cyclic plastic zone size (Cheng et al. , 2008) was calculated as 47 μm, which is very close to the observed size of the zone where the transformation has happened i.e. 45 μm. This result supports the fact that there is phase-transformation of RA to martensite which can retard the crack growth, because a part of input mechanical energy is absorbed in phase transformation rather than driving the crack growth. The other factor for phase-transformation-induced crack closure can be the compressive stress (about 4% volume expansion when austenite transforms to martensite) and it was promoted by increased meta-stability of the austenite (Pineau and Pelloux, 1974; Hornbogen, 1978; Suresh, 1998). Therefore it can be stated that the steel with the higher content of RA will offer higher resistance to fatigue crack growth. The continuous RA film between the adjacent BF also acts as a barrier for slip band transmission and hence reduces the mean free path for dislocation glide (Yoder and Analysis, 1983; Kaneshita, Miyamoto and Furuhara, 2017).

Fig. 4. Hardness as a function of distance below the crack surface in stage-II of fatigue crack growth