Effect of intercritical heat treatment on microstructure, nano-size precipitation and mechanical properties of Fe-Ni-Cu-Al low carbon steel

In recent years, the co-precipitation of nanoscale Cu and Ni-Al particles in Fe-Ni-Cu-Al low-carbon steel provide an extremely high strengthening increment while minimizing the addition of carbon and alloy elements, how-ever, the toughness and plasticity dramatically decreased. This research focuses on the improvement of tough-ness and plasticity utilizing an intercritical (Quenching-Lamellarization-Aging (QLA)-type) heat treatment for Fe-Ni-Cu-Al steel. The microstructure and nanoparticles precipitation of Fe-Ni-Cu-Al steel were characterized by TEM, HRTEM and phase analysis after a QLA treatment. The results indicate that the multi-phase micro-structure consisting of maraging martensite, intercritical ferrite and austenite reversion were obtained by QLA treatment. With decreasing intercritical quenching temperature between 820 degrees C and 660 degrees C,-80 degrees C V-type impact energy and plasticity of Fe-Ni-Cu-Al steel was obviously improved due to the formation of intercritical ferrite and austenite reversion. Meanwhile, the coarsening of Ni3Al particles were markedly accelerated because of Ni segregation in matrix and abundant Fe atoms for the substitution of Al atoms within Ni3Al particles. The strengthening mechanisms of Ni3Al particles in Fe-Ni-Cu-Al steel was mainly Orowan bypassing strengthening after QLA treatment, and the corresponding strengthening increment was calculated as 89% (214 MPa) at the intercritical quenching temperature of 720 degrees C, which was 17% (259 MPa) lower than the value of order strengthening obtained by 820 degrees C primary quenching. The combination of multi-phase microstructure and co -precipitation strengthening contributed to excellent mechanical properties (yield strength>1050 MPa at 20 degrees C, the elongation>13%, a Charpy impact toughness of >80 J at-80 degrees C) during the intercritical quenching temperature of 720 degrees C-780 degrees C.

Automated summary

This study focuses on the improvement of toughness and plasticity of Fe-Ni-Cu-Al steel by utilizing an intercritical heat treatment. The results show that a multi-phase microstructure consisting of martensite, intercritical ferrite, and austenite reversion can be obtained through this treatment. The reduction in intercritical quenching temperature leads to enhanced impact energy and plasticity due to the formation of intercritical ferrite and austenite reversion. The coarsening of Ni3Al particles is accelerated, resulting in strengthening of the steel through Orowan bypassing mechanism.