Metallurgical Study of a AISI 316L Stainless Steel Used in a Gas Turbine Exhaust System

Abstract

Several high temperature fatigue and possibly creep-fatigue cracks have nucleated and propagated through the 3.7 mm wall thickness of a gas turbine exhaust system of a navy combat ship made of a grade type AISI 316L annealed stainless steel. The main cracks propagated near some welded joints, where the measured working temperature was approximately equal to 350°C (Fig.1). The paper presents tensile, fatigue and creep data obtained from experimental tests that were performed in several test specimens obtained from steel plates used in-service. Results of optical microscopy for the microstructure of the material and analysis of the fracture surfaces carried out with the SEM have identified the failure mechanisms at test temperatures. The paper also presents microhardness and grain size measurements carried out together with microstructural observations in the SEM. A research work to investigate carbide precipitation in virgin thin sheet specimens, as used in these exhaust tubes, was also performed and it is presented. The influence of stages time (100, 200, 100+100 and 4x50 hours) and of thermal exposure temperatures (500 and 550°C) was assessed to compare the metallurgical properties of the material. Finally, the paper shortly analyses other materials that could replace the used one.