Effects of Co and Ni on secondary hardening and fracture behavior of martensitic steels bearing W and Cr
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Table I.
Chemical Compositions of Alloys, Weight Percent
Alloy Designation
C
W
Cr
Co
Ni
W W-Cr W-Cr-Co W-Cr-Co-Ni
0.25 0.22 0.31 0.24
5.47 2.72 3.33 3.13
— 2.44 2.41 3.07
— — 15.11 14.18
— — — 9.83
H. KWON, C.M. KIM, K.B. LEE, H.R. YANG, and J.H. LEE AF1410 (14Co-10Ni-2Cr-1Mo-0.16C)[1] and Aermet100 (13.4Co-11.1Ni-3.1Cr-1.2Mo-0.23C),[2] possess superior toughness even at ultrahigh strength levels. These steels are based on the design of HY180 (8Co-10Ni-2Cr-1Mo0.1C).[3,4] The precipitates providing the secondary hardening are fine M2C-type carbides that are formed by the dissolution of M3C-type cementite during aging at temperatures near 500 7C for the high-Co-Ni secondary hardening alloy steels. The alloying elements Mo and W form the carbides of M2C type. In contrast, because Cr alone does not form M2C carbides but M7C3 or M23C6 carbides, hardening with precipitates of the latter carbides could only occur by Cr additions greater than about 9 wt pct.[5,6] However, even the lower Cr contents combined with Mo and/or W additions can contribute to the formation of M2C carbides in which Cr is also dissolved. Recently, the effect of alloying additions on the secondary hardening behavior has been systematically analyzed for alloy systems from the basic system of the Fe-C-Mo ternary alloy, which has a strong M2C carbide former Mo, to the multicomponent high Co-Ni alloy system by means of the stepwise alloying additions of Cr, Co, and Ni.[7] However, another M2C carbide former W has been neglected because W has a weak effect on secondary hardening. Because the 14Co-10Ni addition to the 2Mo-3Cr steel, which exhibited little hardening, results in a strong secondary hardening by the fine dispersion of M2C carbides, there may be a similar effect in W-containing steels. Although the secondary hardening behavior in some Wcontaining steels, such as 6W,[8,9] 6W-3Ni,[10,11] 6W6Ni,[11,12] and 3W-3Cr,[13] was studied in our laboratory, no systematic approach to the high Co-Ni steels containing W has been conducted. In contrast, many studies on the fracture behavior and toughness of commercial high Co-Ni systems containing Mo have been made by Garrison and co-workers.[14,15,16] Kwon and co-workers reported the fracture behavior of both the basic secondary hardening steels containing the M2C carbide forming elements Mo and/or W[8–13,17] and the MoCr, MoCrCo, and MoCrCoNi steels.[18] The purpose of this study was to analyze the effect of
H. KWON and J.H. LEE, Professors, and K.B. LEE, Postdoctor, are with the Department of Metallurgy and Materials Engineering, Kookmin University, Seoul 136-702, Korea. H. KWON is also jointly appointed to the Center for Advanced Aerospace Materials. C.M. KIM, Research Engineer, is with the R & D Training Center, Korea Gas Corporation, Ansan, Kyunggi-Do 425-150, Korea. H.R. YANG, Professor, is with the Department of Mechanical Engineering, Inchun Junior College, Inchun 402-750, Korea. Manuscript submitted March 18, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
Fig. 1—Hardness as a functi
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