PSI - Issue 14

P Ponguru Senthil et al. / Procedia Structural Integrity 14 (2019) 729–737 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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(2015)). Even though the fraction of retained austenite has been reduced in B1Nb, a remarkable increase in total elongation has been observed. It is expected that the material will reach percolation threshold early because of the reduced retained austenite fraction. However, despite the decrease in the fraction of total retained austenite the total elongation has increased substantially.

(a)

(c)

(b)

(d)

Fig.7. Tensile fractographs of B1 steel (a & b) and B1Nb steel (c & d) at low and high magnifications.

The above results can be reasoned with the stability of the retained austenite. Coarse blocky retained austenite is less stable due to non-uniform distribution of carbon. Because of reduced fraction and fineness of retained austenite the carbon content in the retained austenite increases and will be distributed uniformly. Increased carbon content increases mechanical stability of retained austenite and hence has maintained a continuous network of austenite to a large strain which has led to delay in reaching percolation threshold. This is also evident from the drop in reduction in area (%RA). Increased stability of retained austenite has led to increased strain hardening and hence increased uniform elongation in B1Nb whereas the necking occurred early in B1 leading to a localized deformation and failure as evident from the low magnification SEM images of the fractured tensile specimens in figure 7(a) and (c). Also, strained induced transformation of coarse blocky austenite to martensite easily leads to localized loss of continuity