Mechanical properties of a low alloy steel in a molten nitrate salt environment
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I.
INTRODUCTION
A D V A N C E D solar central receivers (SCR) designs call for the use of a condensed phase fluid as the coolant (heat transport fluid) and/or as the sensible heat storage medium. The choice of a particular fluid is largely one of economics based upon a combination of factors including cost and optimal thermophysical characteristics. For current receiver designs, this choice has been narrowed to the nitrate based salts. Within an SCR, a containment alloy could be used in the construction of a variety of components which would be required to withstand continuous exposure to the salt at temperatures on the order of 400 to 500 ~ (i.e., lowtemperature thermal storage or evaporator assemblies). While nitrate salts have been used for some time at these temperatures in a variety of applications including heat transfer and metal heat treating applications, they are generally contained in austenitic stainless steel vessels. ~ There are, therefore, a number of questions regarding the compatibility of a lower alloy material, such as 2.25Cr-lMo, in these molten nitrate salt environments. This report details the results of an experimental program in which the effect of long-term salt exposure on the mechanical properties of 2.25Cr-lMo has been evaluated through a series of slow strain rate tests (SSRT). These tests consist of the application of a constant extension rate imposed on tensile specimens having uniform cross-sections which are exposed to the environment of interest. The behavior of the specimens in salt is compared to the behavior of specimens exposed to an inert or reference environment (in the present case, air) under identical conditions of strain rate and temperature. Fracture strain, reduction in area (RA), and ultimate tensile strengths (UTS) are measured as parameters which determine the susceptibility of an alloy to environmental degradation. The test is a versatile method for both the detection of the presence of an environmental embrittlement or stress corrosion cracking phenomenon as
well as for analysis of the critical variables which may contribute to the observed material degradation. The variables which may be screened include temperature, strain rate, the particular nitrate salt chemistry or impurity content, and the metallurgical condition of the alloy under investigation. Previous studies have shown that both austenitic and ferritic stainless steels suffer uniform surface attack when immersed in nitrate salts for extended periods of time. 2'3'4 Therefore, in order to examine the effect of continuous deformation on the corrosion characteristics of materials in these salts, it was necessary to perform the mechanical testing at rates slow enough to allow the corrosive environment to act upon the test alloy before purely mechanical failure occurs. Figure 1 schematically illustrates the expected strain rate dependence of the ductility for a material which suffers from an environmentally induced loss of ductility. In Region I the strain rate is rapid and the resulting test time is short
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