Microstructural assessment of nitrogen-strengthened austenitic stainless-steel welds using thermoelectric power
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I. INTRODUCTION
THE electronic properties of an alloy correlate directly to its microstructure. This investigation studies the behavior of the correlation between the thermoelectric power coefficient and the interstitial nitrogen content associated with the weld microstructure. The thermoelectric power coefficient is sensitive to microstructural changes and will be demonstrated on nitrogen-strengthened austenitic stainlesssteel welds. Austenitic stainless steels are being strengthened with high levels of nitrogen for the enhancement of mechanical properties and corrosion resistance.[1,2] Austenitic stainless steel has a high solubility for nitrogen; however, in particular alloys, the solubility of the nitrogen decreases with increasing temperature. During the welding of nitrogen-strengthened stainless steel, the solubility of nitrogen can become exceeded, resulting in the partitioning of nitrogen on cooling into solid-solution nitrogen and nitrides. If the microstructure and the properties are to be properly correlated, it is essential that both the solid-solution nitrogen and the nitridenitrogen contents be easily and rapidly determined. In this investigation, electronic properties are used to develop insight into the nature and role of nitrogen in these nitrogen-strengthened stainless steels. Thermoelectric Power To gauge the magnitude of the influence of the soluble nitrogen relative to the nitride nitrogen, the thermoelectric
A.N. LASSEIGNE, Graduate Research Scientist, and D.L. OLSON and H.-J. KLEEBE, Professors, are with the Department of Metallurgical and Material Engineering, Colorado School of Mines, Golden, CO 80401. Contact e-mail: [email protected] T. BOELLINGHAUS, Vice President, is with the Federal Institute of Materials Research and Testing, Berlin, Germany 12205. Manuscript submitted February 4, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
power coefficient, also referred to as the Seebeck coefficient, is introduced. The thermoelectric power coefficient, Z, is a temperature-dependent electronic property of the material that can be described as the entropy of the free electrons in the alloy. Given the voltage difference, V, between two points held at a constant temperature difference, T, on the specimen surface, the thermoelectric power coefficient can be determined using the mathematical expression: Z
V Zref T
[1]
where Zref is the thermoelectric power coefficient of the reference material, which in this investigation is a tungsten tip in contact with the specimen surface. A schematic diagram of the thermoelectric power measurement setup is shown in Figure 1. From solid-state electronic models, the thermoelectric power coefficient is a function of the electron concentration, the effective mass of the electron, and the electronic scattering behavior in an alloy, which are all influenced by the solute content, lattice strain, microstructural changes, material processing, and time-dependent phase changes. As the nitrogen content changes in the austenitic stainless steel, there is a re
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