Effect of Zr on the microstructure and mechanical properties of 12Cr ferritic/martensitic steels

The effect of Zr in the range of 0--1.36 wt.% on the microstructure and mechanical properties of 12Cr ferritic/ martensitic steels was investigated. The steels were prepared by melting and hot rolling, then normalized at 1050 C for 30 min and tempered at 760 degrees C for 90 min. The steels with 0--0.11 wt.% Zr had a mixture of martensite and ferrite, while the steels with-1.36 wt.% Zr showed fully ferrite. The area fraction of the ferrite phase in steels increases as an increase in the Zr content, which indicates that Zr is a ferrite forming alloy element in steels. The precipitates in steels with 0--0.11 wt.% Zr were mainly distributed along martensitic lath boundary and ferritic/martensitic interface. The coarser precipitates distributed at the ferrite/martensite inter-face were M23C6 carbides and obey Kurdjumov-Sachs orientation relationship, namely (110)(alpha-Fe)//(111)(M23C6) and [-111](alpha-Fe)//[011](M23C6). There were two kinds of precipitates in the steels containing-1.36 wt.% Zr. The coarser precipitates, which distributed randomly, were Zr-rich (Fe, Zr) compounds. The Laves phases found in smaller precipitates were distributed at the grain boundary. Tensile tests showed that the strength decreased and the ductility increased as an increase in the Zr content. The steels with 0--0.11 wt.% Zr demonstrated a superior strength balance, of which the tensile strength and elongation at room temperature were 750-830 MPa and 19%- 22%, respectively. At the elevated temperature (600 C), the tensile strength of the steels decreased to 390-450 MPa and the elongation of steels increased to 27%-34%.