Robert L. Amaro

THE EFFECT OF MICROSTRUCTURE ON THE HYDROGEN-ASSISTED FATIGUE OF PIPELINE STEELS

Tests on the fatigue crack growth rate were conducted on four pipeline steels, two of grade API 5L-X52 and two API 5L-X70. One X52 material was manufactured in the mid-1960s and the other was manufactured in 2011. The two X70 materials had a similar vintage and chemistry, but the microstructure differs. The fatigue tests were performed in 5.5 and 34 MPa pressurized hydrogen gas, at 1 Hz and (load ratio) R = 0.5. At high pressures of hydrogen and high values of the stress intensity factor range (ΔK) there is no difference in the fatigue crack growth rates (da/dN), regardless of strength or microstructure. At low values of ΔK, however, significant differences in the da/dN are observed. The older X52 material has a ferrite-pearlite microstructure; whereas, the modern X52 has a mixture of polygonal and acicular ferrites. The X70 materials are both predominantly polygonal ferrite, but one has small amounts (∼5%) of upper bainite, and the other has small amounts of pearlite (<2%) and acicular ferrite (∼5%). We discuss the fatigue test results with respect to the different microstructures, with particular emphasis on the low ΔK regime.Copyright

Measurements of Fatigue Crack Growth Rates of the Heat-Affected Zones of Welds of Pipeline Steels

Pipelines are widely accepted to be the most economical method for transporting large volumes of hydrogen, needed to fuel hydrogen-powered vehicles. Some work has been previously conducted on the fatigue crack growth rates of base metals of pipeline materials currently in use for hydrogen transport and on pipeline materials that may be used in the future. However, welds and their heat-affected zones are oftentimes the source and pathway for crack initiation and growth. The heat-affected zones of welds can exhibit low resistance to crack propagation relative to the base metal or the weld itself. Microstructural irregularities such as chemical segregation or grain-size coarsening can lead to this low resistance. Therefore, in order to have adequate information for pipeline design, the microstructures of the heat-affected zones must be characterized, and their mechanical properties must be measured in a hydrogen environment. With that in mind, data on the fatigue crack growth rate is a critical need. We present data on the fatigue crack growth rate of the heat-affected zones for two girth welds and one seam weld from two API 5L X52 pipes. The materials were tested in hydrogen gas pressurized to 5.5 MPa and 34 MPa at a cyclic loading rate of 1 Hz, and an R ratio of 0.5.Copyright

Sensitivity Analysis of Fatigue Crack Growth Model for API Steels in Gaseous Hydrogen

A model to predict fatigue crack growth of API pipeline steels in high pressure gaseous hydrogen has been developed and is presented elsewhere. The model currently has several parameters that must be calibrated for each pipeline steel of interest. This work provides a sensitivity analysis of the model parameters in order to provide (a) insight to the underlying mathematical and mechanistic aspects of the model, and (b) guidance for model calibration of other API steels.

Development of an Engineering-Based Hydrogen-Assisted Fatigue Crack Growth Design Methodology for Code Implementation

A primary barrier to the widespread use of gaseous hydrogen as an energy carrier is the creation of a hydrogen-specific transportation network. Research performed at the National Institute of Standards and Technology, in conjunction with the U.S. Department of transportation and ASME committee B31.12 (Hydrogen Piping and Pipelines), has resulted in a phenomenological model to predict fatigue crack growth of API pipeline steels cyclically loaded in high-pressure gaseous hydrogen. The full model predicts hydrogen-assisted (HA) fatigue crack growth (FCG) as a function of applied load and hydrogen pressure. Implementation of the model into an engineering format is crucial for the realization of safe, cost-effective pipelines for the nation’s hydrogen infrastructure. Working closely with ASME B31.12, two simplified iterations of the model have been created for an engineering-based code implementation. The engineering-based iterations are detailed here and the benefits of both are discussed. A case study is then presented detailing the use of both versions. The work is concluded with a discussion of the potential impact that model implementation would have upon future hydrogen pipeline installations.© 2014 ASME

Summary of an ASME/DOT Project on Measurements of Fatigue Crack Growth Rate of Pipeline Steels

The National Institute of Standards and Technology has been testing pipeline steels for about 3 years to determine the fatigue crack growth rate in pressurized hydrogen gas; the project was sponsored by the Department of Transportation, and was conducted in close collaboration with ASME B31.12 Committee on Hydrogen Piping and Pipelines. Four steels were selected, two X52 and two X70 alloys. Other variables included hydrogen gas pressures of 5.5 MPa and 34 MPa, a load ratio, R, of 0.5, and cyclic loading frequencies of 1 Hz, 0.1 Hz, and a few tests at 0.01 Hz. Of particular interest to ASME and DOT was whether the X70 materials would exhibit higher fatigue crack growth rates than the X52 materials. API steels are designated based on yield strength and monotonic tensile tests have historically shown that loss of ductility correlates with increase in yield strength. The X70 materials performed on par with the X52 materials in fatigue. The test matrix, the overall set of data, implications for the future, and lessons learned during the 3-year extensive test program will be discussed.Copyright