Microstructure Optimization to Prevent Hydrogen-Induced Cracking of API X70 Pipeline Steels in Sour Environments
These days, by increasing the demand for oil and gas resources, the failure of pipeline steels has become one of the crucial topics in the oil and gas transportation industry. The failure of pipeline steels is divided into two major categories: stress corrosion cracking (SCC) and hydrogen assistant cracking (HAC). Hydrogen-induced cracking (HIC) is the main cause of pipeline steels failure in sour environments, which is categorized as a type of HAC. In this type of failure, hydrogen atoms accumulate in the steel and cause cracking by different mechanisms. The main focus of this thesis is to find the effects of texture and microstructural parameters on HIC nucleation and propagation in steels exposed to sour environments and also the effect of such environment on the mechanical properties of steels with different microstructure and texture. In this thesis, the electrochemical hydrogen charging experiment was used to introduce HIC in API 5L X70 pipeline steels with four different thermo-mechanical control processing (TMCP) parameters. Using the scanning electron microscope (SEM), HIC cracks at the cross-section of samples were observed. The SEM observations indicate that the sample which had more finish rolling steps and smaller grain size was more susceptible to HIC. However, the specimen with larger grains was more susceptible to hardness increment than the steels with smaller grains. The energy dispersive Spectrometer (EDS) results showed that two types of inclusions, namely calcium sulfide (CaS) and aluminum oxide (Al2O3), were responsible for the nucleation of HIC in all examined API 5L X70 samples in sour environments. Comparing the length and quantity of the cracks in the middle and top layers of all samples, it could be concluded that the middle layer was more susceptible to HIC. The martensite phase accumulated around the HIC crack was recognized as the weakest phase that could resist HIC propagation and the specimen which had more deformed grains was more susceptible to HIC. The hydrogen permeation (HP) test was used to indicate the type of traps in each sample. My experimental results indicated that there are both reversible and irreversible traps in all samples, but the quantity of reversible traps was higher in the samples with smaller grains. These results showed that the increasing the finish rolling steps and cooling rate made the steel more susceptible to the HIC. However, lower cooling rate, more roughing and less finishing reduction steps led to large grains, which were more resistance to the HIC. The effect of different testing environments, including sour and acidic environments, on the tensile strength and type of failure of X70 pipeline steels were investigated. The results indicate that the acidic environment does not have any significant effect on the failure type and mechanical properties of the samples while the sour environment causes hydrogen embrittlement, and as a result, a brittle fracture happens. The other result achieved by tensile test is that the middle layer of the pipeline slab is generally weaker than the top layer, and the reason is the difference in the microstructure and texture of these layers.
Pipeline steel, Hydrogen induced cracking, Corrosion, API X70, Sour environment
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