3.2 Hardness and strength of SZ
The hardness distribution of the original joint and rolled joints is shown in Fig. 3 (a). It was observed that the hardness of the as-welded SZ was approximately 68.4 HV and was lower than that of BM (approximately 108 HV). After rolling process, the hardness of the SZ was gradually increased to 151.6 HV (2 mm) as shown in Fig. 3 (b).