Development of a copper containing 690 MPa yield strength quenched and tempered steel

Demands placed on modern steels mean that higher strengths are being sought while alloy contents, particularly carbon, are decreased. These two conflicting requirements have forced steel designers to consider micro-alloying, heat treatment and precipitation strengthening to meet market demands. This study concentrated on the use of precipitation hardening by copper to maintain the strength of reduced carbon, quenched and tempered steels. Many studies have concentrated on copper proportions above 1% in as rolled steels. It was the intent of this study to examine the effect of copper levels between 0 and 1% in the development of a low carbon, 690 MPa yield strength quenched and tempered structural steel. The changes in hardness upon tempering, tensile testing, Charpy v-notch impact testing, Jominy hardenability, bead-on-plate weld testing and optical microscopy were used to evaluate the steels. Also covered was the development of empirical descriptions of the response of quenched and tempered steels to heat treatment. Both copper free and copper containing steels were used in the analysis. In studying the copper containing steels, precipitation hardening was observed in a steel containing 0.4% copper (nominal composition). It was shown that the strength and fracture toughness targets of military and commercial specifications can be met by the steels developed. As a result of copper based precipitation hardening, Jominy hardenability tests showed a hardenability much higher than would be predicted on the basis of carbon content and the volume fraction of martensite produced as a function of cooling rate. It was demonstrated that the weldability of the low carbon copper steels, as assessed by the bead on plate test, is superior to that of currently used alloys. Therefore, the aim of maintaining strength and increasing weldability by reducing carbon content was clearly achieved.