Structural steel mechanical properties of strength and ductility for a given microstructure are predominately driven by the average ferrite grain/packet size and by the cross-sectional homogeneity of the ferrite grain/packet size in the final product. Fatigue performance, a ductility property, in air for applications of wind towers, bridges or high-rise buildings along within environments of high-pressure gaseous hydrogen for various pipeline systems is critical to the end-use design. Fracture and fatigue testing of a commercially produced low carbon 20 mm API X60 Sour Service had been completed which showed very good stable performance when compared to other commercially produced pipeline and structural steel microstructures. This commercially produced API steel was reported as “Alloy D” in prior published work. The microstructure was predominately polygonal ferrite with industrial quality of steel cleanliness, microstructural banding and cross-sectional grain size/homogeneity required for a successful API X60 Sour Service specification/application.  Based on the initial fatigue performance reported for the “Alloy D” cross-sectional microstructure a more comprehensive study on the effect of the cross-sectional grain size/homogeneity on fatigue was initiated. To isolate and study the effect of the cross-sectional ferrite grain size/homogeneity only on fatigue performance, laboratory developed samples of a low carbon API X60 Sour Service with the same alloy design as “Alloy D”, predominately single-phase polygonal ferrite microstructure with excellent cleanliness and no microstructural banding with having the only difference of variations in average cross-sectional and homogeneity of the final ferrite grain size were produced. To create the desired cross-sectional grain size differences, per pass reduction strategies and austenite recrystallization behaviors were modified. The processing conditions were considered as “optimized” and “non-optimized” for the alloy design and processing capabilities of the laboratory pilot rolling mill. The cross-sectional microstructure (optically, LAGB, HAGB, averages, distributions, kernel mapping, etc.) of each processing conditions, “optimized” and “non-optimized” were fully characterized prior to fatigue testing in air and high-pressure gaseous hydrogen.  The characterization analyses show that there is a strong correlation between the “optimized” and “non-optimized” processing conditions and the homogeneity in the final microstructure. The mean grain size value is reduced in the “optimized” condition to 3.2 µm from 4.7 µm.  Also, the degree of homogeneity, evaluated as the Dc20% value, which gives an idea of the extension of the distribution tail, is reduced from 16.5 µm in the “non-optimized” to 8.5 µm in the “optimized” condition. Therefore, a convenient process design provides a finer and more homogeneous final microstructure. In addition, MicroSim® austenite evolution modeling of the pilot mill was conducted which also confirmed the differences noted in the cross-sectional microstructural characterization average and heterogeneity differences. Traditional mechanical property testing of the “optimized” and “non-optimized” showed expected slight strength differences from the cross-sectional ferrite grain size/homogeneity. Ductility properties of elongation and transverse charpy toughness transition temperature also showed expected differences with elongation decreased by 5.4% and charpy transition temperature increased by 25 °C for the “non-optimized” steel. For fatigue comparison purposes to the original “Alloy D” along with other pipeline/structural steel microstructures completed by NIST and others, hydrogen pressures of 800 and 3000 psi will be used at NIST. NIST Initial fatigue testing in air has been completed while fatigue testing in hydrogen is currently on-going. Pre-cracking the samples for fatigue testing showed the “optimized” condition took 2-7 times the number of cycles to pre-crack (1 -3.5 million vs. 0.5 million) which illustrates the strong effect on the cross-sectional microstructure homogeneity on the ductility properties of toughness. Fatigue testing in air showed that “optimized” had improved fatigue performance than that of the “non-optimized” and when compared to some previously tested API X70. This paper will detail the trial design, cross-sectional microstructure created, mechanical properties and fatigue test results generated to date of the effect of cross-sectional ferrite grain size/homogeneity.


Cross-Sectional Grain Size Homogeneity Effect on Structural Steel Fatigue Performance in Air and Hydrogen Environments

Division / Sponsors: 
Pipeline Systems Division

Paper Proceedings: https://asmedigitalcollection.asme.org/IPC/IPC2020/volume/84447

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