PSI - Issue 14

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

733

5

(a)

(b)

(c)

Fig.4. (a) and (b) BSE micrograph showing the presence of undissolved primary Nb (C N), (c) EDS spectrum corresponding to (b).

Table 2. Mechanical properties

Material

%Bainite

%Ret.Austenite

YS (MPa)

UTS (MPa)

%El

%RA

CVN (J)

B1 B1Nb

75 85

23 14

1262 ± 40 1187 ± 1

1633 ± 8 1621 ± 2

16 ± 4 28 ± 2

39±7 28±4

23 ± 2 25 ± 3

Phase maps of both the steels obtained through EBSD are given in figure 6. The size of bulk retained austenite has been refined significantly. The volume fraction of total retained austenite has decreased from 23% to 14% due to Nb addition whereas the fraction of bainite has increased from 75% to 85%. The fraction of Nb(C,N) in B1Nb has been 0.134%. It is known that the ductility of nano bainitic steel is governed by the fineness and mechanical stability of the retained austenite (Avishan et al (2015)). The instability and necking in tensile test occur upon reaching a threshold fraction (10%) of retained austenite called as percolation threshold defined as the minimum amount of austenite required for maintaining a continuous network of retained austenite in nano bainitic steel (Bhadeshia (2008)).