Age hardening and mechanical properties of a 2400 MPa grade cobalt-free maraging steel
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I. INTRODUCTION
II. EXPERIMENTAL PROCEDURE
MARAGING steels have high strength, high toughness, and good machining properties and are used in many military and commercial industries. Over the past half-century, two major types of maraging steels have been developed:[1–9] the 18Ni maraging steels and the cobalt-free maraging steels. Among the two types, the 18Ni maraging steels are in a more advanced and mature stage of development and applications, with maximum strength levels reaching 2400 MPa, accompanied by good toughness and ductility. However, these steels contain an expensive alloying element, cobalt, to levels as high as 8 to 13 pct. This keeps the steels rather expensive, preventing wider selection and application. Therefore, developing cobalt-free maraging steel with reduced quantities of expensive alloying elements to lower the production cost has been an important direction of maraging steels research. Over the past two decades, enormous advances have been achieved in the development of cobalt-free maraging steels, to a strength level of 2000 MPa.[5–9] However, cobalt-free maraging steels at even higher strength levels have not been reported. Earlier work by the authors[10,11] has studied the microstructure of a 2000 MPa grade cobalt-free maraging steel. The conclusion is that Ni3Ti is the major precipitation hardening phase in cobalt-free maraging steels. In this paper, based on the earlier work, the titanium content is further increased to boost the strength. At the same time, the amounts of other alloying and impurity elements are controlled to avoid detrimental effects on toughness. The effect of heat treatment on mechanical properties has been investigated, and the effects of cobalt, molybdenum, and titanium on steel toughness have been discussed. Some of the results were presented at the 2002 Aviation Exhibition in Zhuhai, Guangdong, People’s Republic of China.[12]
Commercially pure iron, electrolytically pure nickel, molybdenum, and titanium were used to produce laboratory melts, using vacuum induction and vacuum arc melting. Two steels were made, each of the two ingots weighing 20 kg. The chemical compositions of the steels are given in Table I. The ingots were homogenized at 1473 K for 24 hours, cooled, and reheated to this temperature for 1 hour before being forged and hot rolled to plates of 14 80 mm cross-section. The forging start temperature was 1423 K and the finishing temperature was higher than 1123 K. The specimens were cut along longitudinal directions from the plates and solutionized at 1083 K for 1 hour, followed by ageing treatment at different temperatures before hardness and mechanical properties testing. Air cooling was used after all heat treatments. Three individual hardness measurements were performed to determine the presented mean values. Standard tensile specimens were prepared, with the gauge diameter 3 mm and gauge length 15 mm. KIC tests used standard three-point bending specimens, 5 10 55 mm. All mechanical tests were carried out at room temperature. Two samples
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