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In recent years, failures of carbon and carbon 0.5-molybdenum steel refinery equipment in high temperature hydrogen service have justifiably raised questions about the advisability of operating such equipment, even at conditions below their respective Nelson curves, as published until recently in API 941. However, in recent editions of API 941, the curves for C- molybdenum steels were removed from Figure 1, the operating limits diagram, and relegated to a separate Annex, Annex A. The purpose of Annex A as stated in that document "is to provide a brief summary of the information and experience regarding the use of 0.5Mo (C-0.5Mo, Mn-0.5Mo) steels in elevated temperature and pressure hydrogen service". The API document further states that "C-0.5Mo steels vary in their resistance to high temperature hydrogen attack, HTHA. Many heats seem to have resistance at conditions indicated by the 0.5Mo curve on Figure A.1. However, some heats seem to have HTHA resistance only marginally better than carbon steel. The document then attributes the difference in the incidence of high temperature hydrogen attack to the thermal history.

The technical papers in this WRC Bulletin provide insight into metallurgical reasons for the variability in performance of the 0.5Mo alloy, especially in regard to the effects of composition and heat treatment on toughness. In addition, results of a study of the effects of HTHA on mechanical properties are provided. In that study, the degree of attack was characterized and quantified by measurement of methane content at through wall positions of material taken from a refinery heat exchanger that suffered HTHA. The degree of attack was then correlated to the reduction in Charpy V-notch toughness at the same position. This method, developed by Professor Carl Lundin at the University of Tennessee in Knoxville, can be an important and effective tool for assessing the safety of operating petroleum refinery equipment. The final paper is based on a study of the compositions and microstructures of a broad range of commercial heats of C-0.5 Mo steel as reported in this Bulletin. Utilizing the comprehensive characterizing data for the large number of heats studied "Toughness Prediction Factors" are proposed. These factors should prove very useful in assessing safety and rationalizing run-retire-replace decisions.

WRC is pleased to have supported the practical experiences of the student co-authors of these papers who conducted the research under the direction of Professor Lundin. The practical experience they received performing the research reported in this Bulletin will prove to be invaluable as they pursue their respective careers.

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Published: 2014 Number of Pages: 109 File Size: 1 file , 21 MB