The evaluation of in-service materials degradation of low-alloy steels by the electrochemical method
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INTRODUCTION
C H R O M I U M - m o l y b d e n u m CrMo and CrMoV steels possess excellent high-temperature mechanical properties and are widely used in energy conversion industries. However, in-service materials degradation, such as softening, carbide-induced embrittlement, temper embrittlement, etc., can take place during long-term operation. A large percentage of high-temperature plants for electricity, petroleum, and chemical industries have been in operation for periods approaching or exceeding their original design life. Furthermore, it has been predicted that these components, where materials deterioration is in progress, often have to be used under more severe service conditions, for example, daily start-up and shutdown. Therefore, it is extremely important to develop and establish nondestructive evaluation procedures for assessing material conditions for remaining component life. Carbide coarsening is of major significance in the deterioration of CrMo-type steels. It has been suggested that the softening and/or the reduction of creep strength is caused by the decrease both of Mo content in solid solution and of finely dispersed carbide needles due to the carbide coarsening process, t1'2~ Precipitation of carbides during service may have an adverse effect on the toughness. Cheruvu [11 described that Biss and Wada t31 have postulated that the precipitation of M 2 C carbides lowers the impact toughness of the steels. It has also been well known that coarse carbides in steels promote cleavage, t41 and it has been suggested that carbide coarsening increases the ductile-brittle transition temperature of the steels by Qu and Kuo. ~51Generally, carbide coarsening has been thusfar evaluated qualitatively using replication techniques. Recently, a computer-aided X-ray YUTAKA WATANABE, Graduate Student, and TETSUO SHOJI, Professor, are with the Research Institute for Fracture Technology, Faculty of Engineering, Tohoku University, Sendai 980, Japan. Manuscript submitted July 23, 1990. METALLURGICAL TRANSACTIONS A
microanalysis (CMA) method was developed by Masuyama et al.,[6] where the concentration of alloy elements in a matrix during aging is an indicator of the extent of materials deterioration in service, although a sampling of materials is required. CrMo(V)-type steels are extensively used for steam turbine components, petroleum reactor pressure vessels, etc. Temper embrittlement of the upper bainitic microstructure is a critical degradation phenomenon in these steels. It is now well established that segregation of residual elements, such as P, Sn, Sb, and As, at the prior austenite grain boundaries is responsible for the temper embrittlement of low-alloy steels. [7,8,9] For in situ measurement of this phenomenon, chemical etching tl~ and electrochemical polarization techniques tm have been proposed. However, these methods have insufficient sensitivity to the slight or medium levels of embrittlement which are of practical importance in operating plants. In this article, newly developed nondestructive electrochem
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