A new equation for the Cr equivalent in 9 to 12 pct Cr steels
- PDF / 833,587 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 96 Downloads / 171 Views
I.
INTRODUCTION
In order to improve the thermal efficiency of fossil power plants, there is a drive to develop high Cr (usually 9 to 12 pct Cr) ferritic steels with excellent creep properties as well as superior oxidation and corrosion resistance properties to those of conventional 1CrMoV, 2.25Cr1Mo, or existing 9 to 12 pct Cr steels. In alloy designing so-called advanced 9 to 12 pct Cr steels, it is very important to estimate the amount of d-ferrite imbedded in tempered martensite microstructure, because d-ferrite is known to reduce the notch toughness catastrophically and should be avoided in turbine steels. Effects of alloying elements on the formation of the d-ferrite are basically related to the role of these elements in expanding or constricting the austenite field. Strong carbide forming elements, such as Ti, V, Mo, W, and Nb, and noncarbide forming elements, such as Si, which are known to constrict the g field, are expected to increase the d-ferrite formation, while austenite stabilizing elements, such as Ni, Mn, Co, Cu, C, and N, are expected to play the opposite role. In an attempt to predict the formation of d-ferrite in high Cr alloy steels, the concept of Cr equivalent (in wt pct) was introduced, which strictly corresponds to the equivalent amount of Cr in the Fe-Cr binary system that would form the same amount of d-ferrite at the same temperature. In that context, various equations for the Cr equivalent (Creq) are proposed by Thielemann,[1] Newhouse et al.,[2] Schneider,[5] Pickering,[6] and other researchers as well,[7–14] which are summarized in Table I. All these equations express the Cr equivalent as linear equations of various alloying elements through the regression analysis. Here, coefficients ahead of each alloying element are potency factors normalized to the same amount of Cr in their ability to expand or constrict the g field, which is related to the d-ferrite formation. Here, the net Cr equivalent equation by Schneider is deduced by the Schaeffler diagram modified by Schneider showing the effects of Ni and Cr equivalents on the constitutional diagram of stainless steel. In the diagram, S.H. RYU, Senior Researcher, is with the Life Assessment Research Team, Research & Development Center, Korea Heavy Industries & Construction Co., Ltd., Kyungnam 641–792, Korea. JIN YU, Professor, is with the Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Taejon 305-701, Korea. Manuscript submitted June 27, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
it was shown that the d-ferrite exists when Creq 2 0.691 Nieq . 10 pct, where formulas of the Cr and Ni equivalents are given in References 5 and 6. Existing equations differ substantially among themselves and do not predict the formation of d-ferrite accurately in many cases, which can be ascribed to a different number of alloying elements being included and to the exclusion of higher order and interaction terms among alloying elements. In the present analysis, several new 9 to 12 pct Cr steels were p
Data Loading...
