The influence of alloying elements to achieving the mechanical properties of high tensile bolts and nuts

In this paper, the influence of alloying elements on mechanical properties and microstructure of steel grade, according to (ASTM) has been studied for manufacturing of high tensile bolts and nuts. Three materials, one plain carbon steel and other two low alloys steel were selected for the purpose of heat treatment. The three materials were austenised and tempered at different temperatures. As the microstructure and mechanical properties of tempered steel depend on the temperature and duration of tempering and the carbide particles become large and fewer in number as the temperature and time increase, this change in microstructure lower the hardness and strength but higher ductility and toughness. Tempered at very low tempering temperature may not show change in hardness but yield strength may increase. In case of alloy steel which contain one or more of strongly carbide forming elements (Cr, Mo, V and W) are capable of secondary hardening and these steel harden on tempering. The temperability of plain carbon steel decreases with an increase in carbon content, in case of alloy steel, the alloying elements increase hardenability and retard the softening rate. Increased strength based on martensite transformation alone generally means a drastic reduction in ductility and high sensitivity to notches. In the development of high strength steels attempt has been made to overcome these produced during rapid cooling. Small amount of alloying elements are used to increase hardenability and matrix strength and some ductility. Plain carbon steel can be heat treated to bring about high tensile properties but this material has been found unsuitable for high temperature applications such as headers for refineries, while tow alloy steel can be heat treated to bring about high tensile properties