The Effects of Large Scale Forgings and Heat Treatment on the Mechanical Performance of Mooring Connectors

摘要

A shackle failure in the Far East in 2007, called into question the integrity of some processes used in the manufacture of long-term mooring systems. This paper reports the results, so far, from a joint First Subsea Ltd and University of Sheffield's Institute for Microstructural and Mechanical Process Engineering (IMMPETUS) research into larger scale metal forging processes and the resulting mechanical properties.The objective of the Large Scale Forging research project is to more fully understand what happens during the forging process and, in so doing, better characterise the micro structure of forged metals, its relation to Charpy toughness values (CVN), fracture toughness values and the design and integrity of mooring connectors.Large scale metal forging is a complex, and far less understood process, compared with many other metals treatment techniques. Mooring components need to exhibit high levels of strength and toughness, as well as outstanding fatigue behaviour over an extended period. While the latter relies heavily on established computer modelling techniques, the factors governing strength and toughness are less well understood. The paper reports on the different aspects of the forging process including: chemical composition of the steel material, heat treatment processes, quenching and cooling rates and test procedures. In particular it will focus on the role and importance of heat treatment, cooling rate and geometry of the material on the mechanical performance of the finished forged connector.Forging trials have been extensively carried out, comparing parameters like soak and quench times as well as cooling rates and durations. The results of the forging trials, so far, has been a better understanding of the optimum conditions for metals forging with respect to chemical composition and the relevance of microstructure due to internal thermo-couples and thermal imaging cameras. In addition, improved testing methodologies have been reviewed together with the development of a heat transfer model used to predict toughness properties along the ingot.