The microstructures and mechanical properties of isothermally transformed 3$\frac{1}{2}$ Cr-Mo alloy steel were studied.
Isothermal transformation characteristics were determined using dilatometric and metallographic techniques. Also microstructural features were observed using optical, scanning and transmission electron microscopes for specimens isothermally transformed in the 650 to 740℃ range. Associated mechanical properties were measured by tensile and Charpy impact tests.
3$\frac{1}{2}$ Cr-Mo alloy steel exhibited two distinct temperature regions in which "C-curve" austenite decomposition occurred. Direct decomposition of austenite resulted in the complex microstructure in which various forms of alloy carbides were precipitated, and fibrous $Cr_7C_3$ predominated in the temperature range 650 to 740℃. Interphase precipitation was competitive with fibrous precipitation and the tendency of interphase precipitation was increased as transformation temperature was decreased.
As the temperature was lowered, yield strength, tensile strength and reduction of area were increased with the finer interlamellar spacing and interfiber spacing. The investigation of nonpropagating microcracks near the fracture surface in the tensile specimens showed that microcracks in lamellar type carbides were formed by shear cracking process. At the lower temperature, transformation colony size and cleavage plane size were decreased, which resulted in higher impact energy.
