Effects of alloying additions and austenitizing treatments on secondary hardening and fracture behavior for martensitic steels containing both Mo and W |
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Authors: | K B Lee H Kwon H Kwon H R Yang |
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Affiliation: | (1) the School of Metallurgical and Materials Engineering, Kookmin University, 136-702 Seoul, Korea;(2) the Center for Advanced Aerospace Materials, POSTECH, 790-784 Pohang, Korea;(3) the Department of Mechanical Engineering, Inchun Junior College, 402-750 Inchun, Korea |
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Abstract: | The effects of alloying additions and austenitizing treatments on secondary hardening and fracture behavior of martensitic
steels containing both Mo and W were investigated. The secondary hardening response and properties of these steels are dependent
on the composition and distribution of the carbides formed during aging (tempering) of the martensite, as modified by alloying
additions and austenitizing treatments. The precipitates responsible for secondary hardening are M2C carbides formed during the dissolution of the cementite (M3C). The Mo-W steel showed moderately strong secondary hardening and delayed overaging due to the combined effects of Mo and
W. The addition of Cr removed secondary hardening by the stabilization of cementite, which inhibited the formation of M2C carbides. The elements Co and Ni, particularly in combination, strongly increased secondary hardening. Additions of Ni promoted
the dissolution of cementite and provided carbon for the formation of M2C carbide, while Co increased the nucleation rate of M2C carbide. Fracture behavior is interpreted in terms of the presence of impurities and coarse cementite at the grain boundaries
and the variation in matrix strength associated with the formation of M2C carbides. For the Mo-W-Cr-Co-Ni steel, the double-austenitizing at the relatively low temperatures of 899 to 816 °C accelerated
the aging kinetics because the ratio of Cr/(Mo + W) increased in the matrix due to the presence of undissolved carbides containing
considerably larger concentrations of (Mo + W). The undissolved carbides reduced the impact toughness for aging temperatures
up to 510 °C, prior to the large decrease in hardness that occurred on aging at higher temperatures. |
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