Effects of tungsten content on the creep-rupture properties of low-carbon cobalt-base heat-resistant alloys
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I.
INTRODUCTION
IT has recently been reported that the precipitates of tungsten (W) solid solution occurring during hightemperature creep improve the rupture strength of nickelbase 20 wt pct Cr-20 wt pct W alloys, [~J although W is a typical solid-solution strengthening element in heatresistant alloys, t2,3] The present authors [4,5] have found that the precipitates of W solid solution and M 6 C carbide are principal strengthening phases in commercial HAYNES* 25 (L-605) alloys containing about 14 wt *HAYNES is a trademark of Stoody Delore Stellite, Inc., Industry, CA.
pct W during high-temperature creep. HAYNES 25 alloys are often used for combustion chamber liner of jet engines and high-temperature springs. Since W is a unique alloying element which raises the melting temperature of cobalt-base heat-resistant alloys, t6] the increase in W content may lead to both the improvement of creep-rupture strength and an increase in the service temperature of cobalt-base heat-resistant alloys. But the increase in W content also increases the electron hole number of the solid-solution matrix of the alloys, |7,8] which increases the tendency to form topologically close-packed (tcp) phases such as Laves phase (Co2W), which is harmful to ductility in HAYNES 25-type cobalt-base alloys, tgt Thus, the effects of W content on the creep-rupture properties have not been studied sufficiently for the cobaltbase heat-resistant alloys. In this study, the effects of W content on the creeprupture properties are investigated using low-carbon HAYNES 25-type cobalt-base alloys at 1089 K (816 ~
MANABU TANAKA, Associate Professor, and HIROSHI IIZUKA, Research Associate, are with the Department of Mechanical Engineering, Mining College, Akita University, Akita 010, Japan. Manuscript submitted February 20, 1991.
METALLURGICAL TRANSACTIONS A
and 1311 K (1038 ~ The precipitated phases in the ruptured specimens are identified with X-ray diffraction. The relationships between W content, microstructures, and the creep-rupture properties are experimentally discussed.
II.
EXPERIMENTAL PROCEDURE
The materials used in this study were low-carbon HAYNES 25-type cobalt-base alloys containing various amounts of tungsten. The alloys were supplied by Mitsubishi Material Corporation (Okegawa, Japan) in the form of hot-forged round bars of 20-mm diameter. Table I shows the chemical composition of these alloys. The alloy containing about 14 wt pct W is the commercial HAYNES 25 alloy of the standard chemical composition. For convenience, the alloys are described as 14W, 17W, or 20W according to W content in this article. The alloy bars were solution-treated to give almost the same grain diameter of about 260 /xm. Table II shows the solution treatment and the resulting grain diameter of the alloys. The heat-treated specimens were machined to test pieces of 6-mm diameter and 30-mm gage length for creep-rupture experiments. Creep-rupture experiments were carried out at 1089 K (816 ~ and 1311 K (1038 ~ in air. The specimens were aged for 10.8 ks (3 hours) at e
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