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Title

CROSS-SECTIONAL MICROSTRUCTURAL HOMOGENEITY CHARACTERISTICS ON FATIGUE PERFORMANCE OF STRUCTURAL STEELS IN AIR AND HYDROGEN ENVIRONMENTS

Authorship

Stalheim, Douglas Glenn; Slifka, Andrew; Uranga, Pello; Kang, Dong-Hoon; Lucon, Enrico

Stalheim, Douglas Glenn

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Slifka, Andrew

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Uranga, Pello

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Kang, Dong-Hoon

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Lucon, Enrico

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DOI

10.5151/9785-9785-32231

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Abstract

Structural steel final mechanical properties of strength and ductility are predominately generated by the final ferrite grain size/packet size and homogeneity of that ferrite grain size/packet size through the cross-sectional area. Forty to seventy percent of the strength components comes from how fine of a final cross-sectional ferrite grain size/packet size that can be realized. All the ductility properties of structural steels for a given microstructure are driven by how fine the final cross-sectional ferrite grain size/packet size AND how homogenous of a cross-sectional final ferrite grain size/packet size that can be developed. One key ductility property of structural steels used in construction and energy transmission applications is fatigue. Both low and high cycle fatigue can be realized in these end applications, however low cycle fatigue is typically the predominate mechanism. In construction applications the environment for fatigue is typically air, such as in wind towers or high-rise building construction. In energy applications, there is an interest in gaseous hydrogen as an alternative fuel source to fossil fuels. In these applications, economical transportation of gaseous hydrogen is typically done in pressure vessels or pipelines and requires operating pressures in the 800-3000 psi range. Various structural steel microstructures primarily related to API grade pipeline steels have been fatigue tested and published in both hydrogen and air over the past 10 years by NIST and other US National Laboratories. NIST in particular has developed a unique multi specimen fatigue testing device with capability for both air and high pressure gaseous hydrogen atmosphere hence allowing for capabilities to significantly increase productivity of the fatigue test. One of the microstructures fatigue tested by NIST is a production produced API X60 HIC Sour Service plate grade consisting of a predominately pure microstructure of polygonal ferrite with a low carbon (0.03%), higher Nb content (0.085%) allowing for higher temperature processing (HTP). This microstructure has performed well in both 800 and 3000 psi hydrogen pressure fatigue testing. However, one of the questions that has risen from this testing was while the overall microstructure performed well in fatigue testing what would happen to the fatigue performance if the same alloy design and same predominate pure microstructure of polygonal ferrite had variability in cross-sectional final ferrite grain size homogeneity. To study the fatigue performance of a common microstructure with different cross-sectional final ferrite grain size homogeneity, laboratory produced plate trials were developed duplicating the original API X60 HIC Sour Service alloy design but designing the rolling process to create two different cross-sectional final ferrite grain size/homogeneity situations in the laboratory produced plates. One design utilized a properly optimized rolling strategy to generate an optimum Type I Static Recrystallization behavior and Type II No-recrystallization behavior for the niobium content. The second rolling design strategy was a non-optimized strategy for Type I Static Recrystallization and Type II No-recrystallization behaviors. This “optimized” and “non-optimized” resulted in distinct differences in the cross-sectional final ferrite grain size and homogeneity. Since ductility properties are significantly controlled by high angle grain boundaries (HAGB, >15°), microstructural characterization of the laboratory produced plates of both low angle grain boundaries and high angle grain boundaries was performed. The goal of creating similar microstructures in the two laboratory plates with distinctly different cross-sectional final ferrite grain size/homogeneity was realized. The microstructure in both the optimized and non-optimized plates was predominately polygonal ferrite. The optimized produced plates had an average cross-sectional HAGB ferrite grain size of 3.2 µm with 20% of the cross-sectional HAGB > 8.5 µm. The non-optimized produced plates had an average cross-sectional HAGB ferrite grain size of 4.5 µm with 20% of the cross-sectional HAGB >16.5 µm. Both will be fatigued tested in air and high-pressure gaseous hydrogen to determine the effect the cross-sectional average and homogeneity has on final fatigue performance. This will be compared to the original production produced grade. In addition, austenite grain size evolution modeling with MicroSim® will be done utilizing the processing parameters from the laboratory rolling for comparison to the final polygonal microstructure characterized.

Keywords

Homogeneity, Fatigue, Hydrogen, Air, Microstructure, Microstructural Modeling, Niobium, HTP