Hyperfine Interactions 139/140: 495–499, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands. 495 Long-Term Changes in a Heat Exchanger Steel ETTORE CARPENE 1 , PETER SCHAAF 1 , SHMUEL ARIELY 2 and RONY SCHNEK 3 1 Universität Göttingen, II. Physikalisches Institut, Bunsenstrasse 7, 37073 Göttingen, Germany; e-mail: pschaaf@uni-goettingen.de 2 Israel Electricity Company, Haifa 31000, Israel 3 Ben-Gurion University of the Negev, Department of Materials Engineering, Beer-Sheva, Israel Abstract. The steel ASTM A213 P22 is used for superheater outlet header in power plants. During duty cycles lasting over tens of years the temperature is of the order of 545 ◦ C and the pressure 125 atm. The microscopic changes in these steels are hard to analyze under working conditions, but they are believed to be responsible for the appearance of creeps in such devices. Investigation of the microscopic modifications will help to predict future failures due to creeps, increasing the reliability and saving a lot of money. We report the investigation of such changes in ASTM A213 P22 steel in use for 30 years as a Superheater header at Haifa power station. The analysis has been carried out using Mössbauer Spectroscopy, X-ray diffraction and optical microscopy. As a result it seems that the morphology and phase change of the carbide phases M 3 C/M 7 C 3 and M 23 C 6 are the most prominent changes leading to the material failures. Key words: ASTM A213 P22, creep. 1. Introduction In the electricity production process with fossil fuel power plants, highly pres- surized water is superheated to steam through convoluted steel pipes gathered to a large diameter tube called the “superheater header”. The steam temperature is about 545 ◦ C and its pressure is 125 atm. The outer environment is combustion gas con- taining vapors of sulfuric acid. During service, these parts are subjected to different kinds of damages like creep, thermal fatigue, oxidation and de-carbonization. Mi- crostructure changes in the morphology of these low alloy steels show the progres- sive transformation of normal perlite into spheroidization of iron carbides. Further changes show that the fine dispersed iron carbides coalesce to larger particles and eventually to graphite [1]. In this work, a superheater outlet header has been subjected to various investi- gation in order to monitor the changes in the iron carbides after 30 years service (about 200,000 hours). The samples have been analyzed by means of Conversion Electron and Transmission Mössbauer Spectroscopy (CEMS and TMS), Grazing incidence X-ray Diffraction (GXRD) and optical microscopy.