Spheroidize annealing and mechanical property evaluation of AISI 1040 steel

The importance of medium carbon steels as engineering materials is reflected by the fact that out of the vast majority of engineering grade ferrous alloys available and used in the market today, a large proportion of them are from the family of medium carbon steels. Typically medium carbon steels have a carbon range of 0.25 to 0.65% by weight, and a manganese content ranging from 0.060 to 1.65% by weight. Medium carbon steels are more resistive to cutting, welding and forming as compared to low carbon steels. From the last two decades a number of research scholars reported the use of veritiy of heat treatments to tailor the properties of medium carbon steels. Spheroidising is the novel industrial heat treatment employed to improve formability and machinability of medium/high carbon low alloy steels. This exclusive study covers procedure, the effects and possible outcomes of various heat treatments on medium carbon steels. The austenite phase present in steel above the critical temperature has the tendency to form variety of non equilibrium phases depending upon the degree of supercooling or cooling rates. The near spherical or curved shaped carbides records least resistance for machining because the blunt cornered shapes are having least free energy with minimum internal stresses. There is a need to formulate the heat treatment process to tailor the characteristics in line with the application. The age old normalizing treatment provides more nucleation sites required for so that finer spheroids are dispersed in the matrix to improve toughness with balanced bulk hardness related properties to improve machinability. In line with requirements, the heat treatment cycle to balance mechanical and microstructural properties of AISI 1040 structural steel is designed. It is observed that lower spheroidizing temperature gives finer spheroids, more in number with better improvement in toughness whereas higher Spheroidization temperature reduces hardness values with lesser spheroid density, accordingly reduces strength and impact resistance. The balanced improvement in properties may be incorporated for metal removal operations to improve productivity and tool life.