Spring steels are widely used in advanced applications such as industrial engines, aircraft equipment, car suspension systems, springs and industrial machine parts. This category of steel has very high strength, flexibility and hardness. In this research, various heat treatment cycles of 55Cr3 spring steel, which are used in the production of spring hangers, have been investigated. In this research, the samples were first austenitized at a temperature of 850 Cᵒ for 30 minutes. The steels then were cooled in three different media i. e. water, oil and compressed air. After that, the quenched steels were tempered at 350, 450, and 550 Cᵒ. The microstructure of the samples was investigated using optical microscope and field emission scanning electron microscope. Also, the mechanical properties of the samples were eva‎luated using tensile test, hardness test and charpy impact test. In addition, the fracture surfaces of the fractured samples were examined by a scanning electron microscope. It was found that with the increase of the tempering temperature, the hardness value of the oil quenched samples have been decreased from about 500 Hb down to about 350 Hb and the tensile strengths have been decreased from about 1700 MPa to about 850 MPa. Also, the toughness of these samples has been increased, as well as the elongation from about 4% to about 10%. In addition, the impact energy increases from about 10 J to about 45 J. The fracture surfaces changed from brittle to semi-brittle and ductile with increase in tempering temperature. It was also concluded that the oil media is the best environment for quenching this kind of spring steel and the best tempering temperature range is between 450-550 Cᵒ. Water media is not suitable for quenching 55Cr3 steel due to the relative susceptibility of steel to cracking and cooling in compressed air due to poor results of mechanical properties.

علي شفيعي , محمود منيرواقفي

حسين مستعان

احمد رضائيان , محمود منيرواقفي