Investigation on Strengthening Mechanism of China Low-Activation Ferrite Steel upon Thermo-Mechanical Treatment

The objective of this study was to investigate the influence of strengthening mechanisms on the high-temperature mechanical properties of China low-activation ferrite (CLF-1) steel, which underwent thermodynamic design and thermo-mechanical treatment (TMT). The microstructure characterization in the normalized and tempered condition and the TMT condition was carried out using optical microscopy, X-ray diffractometer, and scanning electron microscopy with electron backscatter diffraction. High-resolution transmission electron microscopy was employed to determine the crystallographic structures of precipitated phases. The results indicated that the addition of Ti led to an increase in the allocation of C in M C phase and an enhancement in the content of M C phase. Compared to CLF-P steel in the normalized and tempered condition, a 1.5-fold increase in dislocation density and an order of magnitude improvement in MX phase density were achieved after TMT. The formation of high-density nano-scale M C phases during TMT played a significant role in precipitation strengthening due to their favorable coherent relationship with the matrix and low interfacial free energy. The excellent high-temperature mechanical properties observed in CLF-P steel after TMT can be attributed to the combined effects of precipitation strengthening, dislocation strengthening, and lath strengthening.