Precipitation reactions and strengthening behavior in 18 Wt Pct nickel maraging steels
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I.
INTRODUCTION
MARAGING steels
are low-carbon martensitic steels developed in the 1960s for applications requiring ultrahigh strength combined with good fracture toughness, tl] These steels usually contain about 18 wt pct Ni, together with substitutional elements such as Co, Mo, and Ti. The initial steels developed, referred to as "C-grade," contained about 8 to 9 wt pct Co, 4 to 5 wt pct Mo, and small amounts of Ti. More recently, "T-grade" steels free of Co but containing higher amounts of Ti have been developed. Typically, the steels are solution treated in the fully austenite region (>800 ~ quenched to produce a completely martensitic matrix, and subsequently aged at intermediate temperatures (400 ~ to 500 ~ to cause precipitation hardening. Substantial strengthening, often in excess of 300 ksi, can be achieved in this way. The high strength, combined with good fracture toughness, is obtained by the precipitation of intermetallic phases in the soft martensite matrix. Numerous studies have been carried out to determine the nature of the maraging precipitates. However, there exists a considerable diversity of opinion on the nature of the strengthening precipitates, particularly in the C-grade steels. For instance, in C-grade steels of similar composition, several different kinds of precipitates, such as Ni3Mo, t2-51 Ni3Ti, t4'5'6] F e 2 M o , [6'7'8] FeTi, [2] and Fe2Ti, [3] have been reported. Often more than one kind of precipitate has been reported. I2-7] In the T-grade steels, NiaTi has been VIJAY K. VASUDEVAN, formerly with the University of Illinois, is Assistant Professor, Department of Materials Science and Engineering, University of Cincinnati, Cincinnati, OH 45221-0012. SUNG J. KIM, formerly with the University of Illinois, is Senior Engineer with the Korea Institute of Machinery and Metals, Kyungnam, Republic of Korea. C. MARVIN WAYMAN, Professor, is with the Department of Materials Science and Engineering, University of l]linois, Urbana, IL 61801. Manuscript submitted July 3, 1989. METALLURGICALTRANSACTIONSA
reported to be the dominant precipitate, t9'~~ This study was undertaken to determine the crystallography, structure, and composition of the strengthening precipitates in two commercial grades of maraging steels, designated C-250 and T-250. The results of electrical resistivity, microhardness, analytical electron microscopy, and computer-simulated electron diffraction patterns are presented and discussed. II.
EXPERIMENTAL METHODS
Maraging steel grades C-250 and T-250 in ingot form were obtained from commercial sources. The alloy compositions are shown in Table I. The principal difference in the composition of the two steels is in the amounts of Co, Ti, and Mo. The C-grade contains about 8 wt pct Co and 0.46 wt pct Ti, whereas the T-grade is free of Co but contains about 1.4 wt pct Ti. The Mo content is about 5 wt pct in the former and 3 wt pet in the latter. The alloy ingots were sectioned and cold swagged, with intermediate anneals at 900 ~ to rods about 3 mm in diameter. Cut rods were sea
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