Precipitate Phase Evolution in a 11Cr Ferritic/Martensitic Steel During Tempering
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I.
INTRODUCTION
IN recent years, 9-12Cr ferritic/martensitic (F/M) steels with lower carbon (0.1 pct max) contents and additions of some alloy elements, which possess superior resistances to irradiation embrittlement and radiation-induced swelling, higher thermal conductivities, lower thermal expansions, and good oxidation and corrosion resistances at elevated temperatures, have been developed and considered for applications in Generation IV nuclear reactors.[1–3] For instance, 9-12Cr F/M steels are considered for use not only as a fuel cladding material for supercritical water reactors (SCWRs) but also as core and reactor vessel materials.[4] Given the temperature requirements in the design of Generation IV reactors, the creep resistance of a F/M steel at high temperatures is extremely important. Precipitation hardening is one of the most effective ways to improve the creep resistance of 9-12Cr F/M steels. When this method is used, 9-12Cr F/M steels usually contain several types of carbonitrides and intermetallic compounds in the matrix and at grain boundaries that result in sufficient strengthening.[5] In other words, precipitate phases may strikingly affect the creep resistance of 9-12Cr F/M steels. Consequently, investigation of precipitate phases in 9-12Cr F/M steels is very important to understanding the effect of precipitates on the creep rupture strength of these steels.[6] ZHIQIANG XU and YINZHONG SHEN are with the School of Mechanical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Minhang District, Shanghai 200240, P. R. China Contact e-mail: [email protected] Manuscript submitted December 29, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
Before being put into service, 9-12Cr F/M steels are commonly subjected to a final heat treatment consisting of normalizing and tempering. Normalizing is conducted at a temperature of approximately 1323 K (1050 C), and tempering is performed at temperatures ranging from 923 K to 1053 K (650 C to 780 C).[7] Generally, some different precipitate types of carbides/carbonitrides, including M23C6, MX, M7C, M2X, and M6C, where M denotes a metallic element and X represents carbon and/or nitrogen atoms, were detected in 9-12Cr F/M steels after normalizing and tempering treatments.[2,5,6,8–18] In our previous work,[19] the precipitate phases in a 11Cr F/M steel fabricated with the normalized-and-tempered condition have been investigated using transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDX). The formation mechanism of the carbide phase observed in the steel is not completely understood and thus requires further investigation. Moreover, there is a lack of information about the evolution of the precipitate phase in 9-12Cr F/M steels at lower tempering temperatures.[2,8–11] The aims of the present investigation are to further understand the influence of the tempering temperatures on the types of precipitate phases in 9-12Cr F/M steels and to determine the changes in the precipitate phases of 11Cr F/M steel during tempering proce
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