Effects of cooling rate on microstructure, mechanical properties, and residual stress of Fe-2.1B (wt%) alloy

The continuous cooling transformation (CCT) curve of an Fe-2.1B (wt%) alloy is obtained using a Gleeble 1500D thermomechanical simulator. The microstructure, mechanical properties, and residual stress of alloy specimens with various cooling rates are examined. The results reveal that the cooling rate has a great influence on the matrix microstructure of the Fe-2.1B (wt%) alloy. Pearlite is formed at the cooling rate of 0.1 K/s, pearlite and martensite are formed in the cooling rate range of 0.2-0.5 K/s, and only martensite remains in the matrix when the cooling rate exceeds 0.5 K/s. In addition, as the cooling rate increases, the dislocation density in the matrix increases, and this, in turn, leads to an increase in the volume fraction of the M-23 (B,C)(6) phase. The precipitation of M-23(B, C)(6) causes the decrease in the (B + C) contents of the matrix, which, in turn, reduces the microhardness of the matrix to some extent. Meanwhile, the large residual compressive stress of the alloy increases with increasing cooling rate. The maximum residual compressive stresses induced by the cooling rates of 0.5 and 30 K/s are approximately 18% and 36%, respectively, higher than that induced by the cooling rate of 0.1 K/s. Moreover, when the cooling rate increases from 0.1 K/s to 0.5 K/s, the macrohardness and bending stress increase significantly owing to an increase in V-m/V-pr (where V-m represents volume fraction of martensite, V-pr is the volume fractions of pearlite and austenite). When the cooling rate exceeds 0.5 K/s, the macrohardness and bending stress decrease gradually because of the decrease in the (B + C) contents of the matrix and an increase in the residual stress. The critical cooling rate (0.5 K/s) may be the optimal cooling rate of Fe-B alloys.