Fatigue Crack Growth of a 420 MPa Structural Steel Heat Affected Zone at Low Temperatures

Alvaro, Antonio (SINTEF Materials and Chemistry) | Akselsen, Odd M. (SINTEF Materials and Chemistry) | Ren, Xiaobo (The Norwegian University of Science and Technology (NTNU)) | Nyhus, Bård (SINTEF Materials and Chemistry)

OnePetro 

Abstract

The development of oil and gas fields in the arctic brings to the table several challenges in the use of structural steels, particularly concerning their low-temperature properties. Among others, also fatigue behavior needs to be accounted for when using structural steels for arctic applications. As for static fracture, ferritic steels experience a fatigue ductile to brittle transition (FDBT) when temperature is decreased below a certain temperature. This may result in higher crack growth rate and, consequently, unpredicted fatigue-related failure. In order to shed some more light on this phenomenon, fatigue crack growth tests have been performed on a 420 MPa structural steel weld simulated coarse grained heat affected zone (CGHAZ) at different temperatures: room temperature, -30, -60, -90 and -120 °C, with -60 °C considered as a possible design temperature relevant for the most extreme arctic areas. Post-mortem fracture surface investigations have been also conducted in order to confirm the expected switch in fatigue crack growth mechanisms as temperature is lowered below the FDBT temperature. Finally, two analytical equations, valid for temperature ranges above the FDBT, were established based on the experimental results to relate yield strength and temperature variation of the Paris law constants . These are used to quantify the temperature impact on the designed fatigue life, and the results are compared to the actual design rules (BS 9710).

INTRODUCTION

Exploration of oil and gas in the Arctic regions is increasing due to the large share of the remaining resources (estimates indicate that about 13% of the remaining oil and 1 gas resources is located in the northern regions (Gautier, Bird, Charpentier, Grantz, Houseknecht, Klett, Moore, Pitman, Schenk and Schuenemeyer, 2009) and the possibility for an alternative and direct Asia-Europe connection route keep both oil and gas and maritime industry interest growing. However, the harsh and cold climate characteristic of the arctic regions imposes several challenges when it comes to materials integrity. The combination of long and repeated ice loading together with operating temperatures which are typically lower than the ones at which the offshore industry is used to work with, demands for new research-based development in order to avoid catastrophic leakage and failures. It is well known, in fact, that as ferritic steels is subjected to sub-zero temperature, they undergo a transition from stable, ductile fracture to unstable, brittle fracture. While for pure materials, the transition may occur very suddenly at a particular temperature, for many materials used in practice the transition occurs over a range of temperatures. This causes difficulties when trying to define a single transition temperature and no universally recognized and specific criterion has been established. Similarly, a fatigue ductile to brittle transition (FDBT) can be observed in ferritic steels. Fig. 1 summarizes the qualitative fatigue crack growth behavior variation for ferritic steels as temperature is lowered.

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