Chapter 2 - Metallurgical Failure Analysis

Abstract

The methodology for undertaking a metallurgical failure analysis is described. Emphasis is placed on practical hints and checklists for carrying out the analysis. A failure analysis of threaded fasteners or bolted joints is discussed, including consideration of why the joints fail and a summary of the failure analysis steps that should be undertaken. The causes of shaft failures are discussed. With the exception of wear as consequential damage of bearing failure, metal fatigue is the most common cause of shaft failure. Stress systems and stress raisers in shafts are considered. The impact of changes in shaft diameter, press and shrink fitting, longitudinal grooves, the manufacturing process, metallurgical factors, and surface discontinuities on shaft failure are discussed. A case study of a failed turbine rotor shaft is presented. Failures through material surface changes are usually caused by wear, fatigue and corrosion and each of these failure forms is discussed and a methodology for wear-failure analysis is described. In a typical process unit there will be a large number of failures during the construction and start-up phases, fewer failures in the middle years, and an increasing number of failures as the plant becomes older. Case histories of different types of failure at different stages of a plant’s life (research and development, engineering phase, construction, start-up, and aging plant) are examined and preventative action plans that could prevent such failures identified.