Effect of Continuous Cooling on the Microstructure and Mechanical Properties of Sintered Low Alloy Steels

Abstract

In order to improve the mechanical properties of sintered low alloy steels, Fe-1.5%Si-2.0%Ni-(0.5%Mo)-0.1-0.5%C compacts sintered at 1250°C for 3.6 ks were cooled in various conditions after the solution treatment at 960°C. The structures of specimens were controlled by changing the cooling rate. The relation between the structures which transformed during continuous cooling and the mechanical properties was studied. The results are summarized as follows:
On the continuous cooling transformation diagram, the addition of molybdenum shifted the ferrite and pearlite noses to longer time and enlarged the sections of bainitic and martensitic transformations.
It was necessary, under the optimum carbon content and cooling rate, to get the fine mixed structure of polygonal ferrite, bainitic ferrite and upper bainite which resulted in good combination of strength, ductility and toughness. But lower bainite and martensite appeared constantly in the structure of the specimens cooled in uniform cooling rate and decreased the ductility and toughness.
The STEP cooling, which was the treatment of changing the cooling rate at 600°C, lead the fine mixed structure mentioned above containing 1025 vol% retained austenite. The STEP cooled specimens showed good mechanical properties; for example in 0.3-0.4%C specimen, 750 MPa of tensile strength, 8% of elongation and 120 kJ/m² of Charpy impact value.