PSI - Issue 14

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

731

3

slabs/plates have been furnace cooled from 950°C.

Table 1. Chemical composition

Chemistry

C

Mn

Si

Mo

Cr

Co

Al

Nb

P

S

Fe

B1

0.85 (0.04) 0.85 (0.03)

2.01 (0.05)

1.64 (0.05)

0.21 (0.02)

0.99 (0.02)

1.52 (0.04)

0.84 (0.02)

Nil

0.006 (0.0002) 0.006 (0.0002)

0.006 (0.002) 0.006 (0.002)

Bal

B1Nb

2.10 (0.005)

1.50 (0.004)

0.25 (0.008)

1.04 (0.003)

1.60 (0.005)

0.97 (0.003)

0.037 (0.0001)

Bal

15 X 15 X 200 mm strips have been cut from the rolled plates for heat treatments and have been used to prepare tensile and CVN impact samples. In addition to the above samples 15x15x15 mm cubes were used to estimate the progress of the transformation in both the steels. Both the steels were austenitized at 900°C for one hour followed by austempering at 300°C. All the heat treatments were carried out using electrically heated atmospheric furnaces. Tensile and CVN impact tests were done at room temperature. The tensile tests were done at a crosshead speed of 1 mm/min. The microstructural characterization has been carried out using Optical Microscopy, Scanning Electron Microscopy (SEM), Electron Back Scattered Diffraction (EBSD) and Transmission Electron Microscopy (TEM). Tensile and impact fractography has also been studied using SEM. In order to reveal PAGS, both the steel samples were tempered at 650°C for one hour after austempering at 300°C and etched using saturated aqueous solution of picric acid with few drops of HCl. 3. Results and discussion Optical micrographs showing the PAGS of B1 and B1Nb steel samples are given in figure 1. Addition of Nb has resulted in substantial refinement of PAGS. The optical micrographs showing the progress of the transformation at different time intervals are shown in figure 2. The fine PAGS has caused increased nucleation sites resulting in acceleration of the transformation and finer sheaf size of bainite in B1Nb. The time to observe visible bainite in optical micrograph has decreased from 2 hours to 1 hour because of the Nb addition. Also, the size and fraction of the bulk retained austenite reduced in B1Nb compared to B1 as shown in the SEM micrograph in figure 3. However undissolved primary niobium carbo-nitride has been found to be present in the microstructure and has been confirmed by EDAX as shown in figure 4. Sizes of these primary Nb carbo-nitrides were less than 3µm.

(a)

(b)

Fig.1. Optical micrographs showing prior austenitic grain size (PAGS) of the steel samples (a) B1 and (b) B1Nb