STUDY OF EMBRITTLEMENT IN AUSTENITIC STAINLESS STEEL AISI 304H WITH 15 YEARS IN SERVICE EXPOSED HIGH TEMPERATURE

Abstract

The present study is aimed at obtaining a model to predict the impact tenacity of austenitic stainless steel AISI 304H exposed to high temperature, in order to define the degree of embrittlement resulting from its metallurgical degradation due to Sigma/Chi phase precipitation. The element studied corresponds to a pipe of 60 inches of diameter associated to the top system of a regenerator that operates continuously at 1300 °F in a Catalytic Cracking plant of an Oil Refinery with 15 years in service. Twelve samples were taken along the studied pipe, taking a sample for each component of the pipeline, a sample for each mitred elbow and a sample for each straight section, of which 3 test-pieces were made per sample, for a universe of 36 test-pieces, complying with the dimensions defined in the ASTM E23-90. The chemical composition of the steel was determined by a portable analyzer based on positive identification of materials, the microstructure of the steel was observed by optical microscopy, the precipitate was sized to determine the shape factor and analyze its grouping, as well as the quantification of the Sigma phase precipitation using a digital image analyzer. The Vickers hardness measurement was performed on site. It was established that knowing the amount and shape of the precipitate, its grouping and hardness, the impact toughness of stainless steel is predicted, which provides an important tool for the study of mechanical integrity of components made of stainless steel AISI 304H exposed to operation continues at elevated temperature. The metallurgical degradation of stainless steel due to Sigma/Chi phase precipitation presents a logarithmic behavior indicating that the impact tenacity presents a sharp decrease in function of the acceleration of the deterioration, that is to say, it loses the ability to plastical deformation and therefore a high probability of occurrence of a fragile failure in operationally critical systems, therefore it should be monitored routinely and rigorously in the industrial field, highlighting its importance of monitoring through an "in situ" methodology based on non-destructive tests. Key Words: stainless steel, 304h grade, Sigma phases, intermetallic phase, embrittlement, impact toughness