As we all know, corrosion of pipelines by hydrogen sulfide is the most worrying factor in the production and transport of oil and gas. In this work the corrosion behavior of API 5L X70MS and X70MO low carbon steels in hydrogen sulfide environments was investigated. Hydrogen induced cracking and sulfide stress cracking tests were carried out according to NACE TM0177 standard. After testing, blisters and cracks were observed only in X70MO steel, probably due to its lower grain refinement and banded microstructure. Internal cracks seem to be initiated in elongated MnS inclusions. Corrosion process was studied by obtaining potentiodynamic polarization curves, which were registered after open circuit potential measurements, at room temperature. Both steels showed general corrosion in NACE 177A solutions, but the corrosion rate values ​​in H2S-saturated solution were about an order of magnitude higher than those ones in deaerated solution. Hydrogen permeation w::as char::acterized in accordance with ASTM G148 standard. In deaerated H2SO4 solution, permeation measurements were similar for both steels. In H2S-saturated solution, X70 MO exhibited higher hydrogen oxidation current values ​​than X70 MS. H2S seems to promote the reduction of protons and increase the concentration of hydrogen atoms in the solution/steel interface, favoring the diffusion process. As X70MO has a coarse microstructure, it offers more pathways for hydrogen diffusion.

The effect of electromagnetic vibration on the microstructure and impact toughness of API-X70 steel weld metal was investigated. The welding was carried out by gas tungsten arc welding process and using filler metals including ER80S-G and ER309L. Electromagnetic vibration under voltages 0-30 volts concurrent with welding was applied. The microstructure of the base and weld metals using optical microscope and SEM was studied. The impact energy of weld metals at TR, 0° C and-20℃ using charpy impact test was measured. Microstructural investigations revealed that applying electromagnetic vibration have caused the finer and more uniform of Martensitic-Austenitic (MA) islands in the ER80S-G weld metal. The results showed that, due to electromagnetic vibration, the microstructure became finer and the average size of dendrites in the ER309L weld metal decreased from 18. 19 to 8. 01 μ m. . The impact test result, illustrated that the applied vibration cause improvement of impact energy of weld metals at different temperature (TR, 0° C and-20℃ ). It was found that with vibration under 30V, impact energy of ER80S-D weld metal in TR, 0° C and-20℃ was increased equal to 53, 29 and 36 percents, respectively. Besides for ER309L weld metal, the impact energy in TR and-20℃ was increased equal to 18 and 5 percents, respectively. (TR= Room Temperature).

In this research, the hot deformation behavior of API X70 steel was investigated by hot compression tests. A temperature range between 950 and 1150 oC was used for experiments with different strain rates of 0.01, 0.1 and 1 s-1. The work hardening rate versus stress curves were used to reveal if dynamic recrystallization (DRX) occurred. The application of constitutive equations to determine the hot working constants for the tested steel was discussed. Using regression analysis, the stress multiplier (a), the apparent stress exponent (n), and the activation energy (Qd) for DRX were calculated as 0.016 and 4.420, and 382 kJ/mol, respectively. Furthermore, the effect of Zener–Hollomon parameter (Z) on the characteristic points of flow curves was investigated using the obtained relations. The dynamic recrystallization (DRX) kinetics of API X70 steel was also studied and its governing equation was derived.

Welding residual stresses decrease designing stress in natural gas transmission pipes with a large diameter under high internal pressure. The outside diameter and wall thickness of API X70 steel in this research are 142. 3 and 1. 98 centimeters. Hole drilling is the most common technique in order to measure residual stresses. Considering the large diameter of this pipe, its transportation to conduct a hole drilling test is a big problem so cutting a finite sample is desired. In this study, the standard dimension of this sample plate is analyzed, and simulation of the welding process is conducted. The residual stresses in different directions are obtained. Residual stresses for the thickness is presented for the first time. The results show that separating a finite sample with a size of 32×44 centimeters is appropriate to perform the hole drilling test. The location and amount of the maximum compressive and tensile residual stress are obtained, and variation in the hoop and longitudinal residual stresses on both internal and external surfaces of pipe samples are investigated. Also, validation of simulation results is performed by comparing with the experimental results of the residual stress in the same pipe on an industrial scale. The results showed that maximum residual stress in the inner surface of the pipe in the longitudinal direction was 460MPa (96 percent of yield stress) which was reduced to 200MPa (42 percent of yield stress) after the hydrostatic test. Because residual stress after the hydrostatic test is lower than the half of yield stress, the hole drilling technique is validated after the hydrostatic test.

In the present work, API X70 steel, a type of high strength low alloy (HSLA) steel, hot-rolled at different conditions and the effect of various hot rolling parameters on microstructure and mechanical properties investigated through thermomechanical control processing (TMCP). Optical and scanning electron microscopy were used for microstructural investigations and the tensile and hardness test utilized for mechanical characterization. The steel preheated at 1250,for 30 min and hot rolled with the speed of 10 rpm in the temperature range of 1100,to 730,through 3 (sample A), 4 (sample B), and 5 (sample C) passes and cooled in air. A total strain of 0. 62 was imposed on all samples. In the 5 pass condition, a sample rolled with a finishing rolling temperature of 900 (sample D), another with a speed of 20 rpm, and the rest (sample E) rolled and quenched in water (sample F). Results showed that, except for sample E, all samples' microstructure consists of ferrite and pearlite but with different morphology. Sample E showed the least grain size. Due to the formation of the quasi-stable phases, sample F exhibited the highest strength/hardness and the lowest ductility. The best combination of strength and ductility was achieved in sample E.

In this study, first, diffusible hydrogen of cellulosic electrode E8010-P1 and low hydrogen electrode E8018-G was measured by mercury displacement method according to ISO3690. Then, the effect of preheating and post-heating on the sensitivity to hydrogen inducedcold cracking in welding of 18mm API5L X70 steel with these electrodes was investigated according to ISO17642-2. The results of visual inspection, penetrant test, metallographic examination, and hardness test showed that welding with cellulosic electrode leads to cracking unless both preheating and post-heating are applied. While in the case of low hydrogen electrode, cracking occurs only if no preheating or post-heating is applied.

In this study, the simulation of the heat affected zone of welding was carried out based on the practical conditions of four-wire submerged arc welding in two X65 and X70 microalloy steel. Initially, thermal cycles of the heat affected zone were analyzed. By applying heating and cooling thermal cycles to peak temperatures of 950, 1150 and 1350 ° C, in a dilatometric device, transformation behavior and microscopic structure were studied. With increasing peak temperature in transformation, despite the constant cooling rate (23 ° C / s) in the range of 800 to 500 ° C, the start and end temperatures of transformation are delayed and shear structures replace polygonal structures. The reasons are the coarsening of austenite grains, the dissolution of niobium carbide precipitates and the solute-drag effect of solved niobium in austenite. The size of the shear packs and the size of the austenite grains are also noticeably larger. By analyzing the dilatometry results, continuous cooling diagram and the kinetics of decomposition of austenite were investigated. Austenite decomposition modeling was performed using the JMAK classic equation. It was found that the parameter n has a small dependence on temperature; while the parameter k is strongly dependent to temperature, phase fraction transformed and austenite grain size. The results of modeling of phase transformations using the JMAK equation were in good agreement with the dilatometric results.

In this paper, an uncoupled thermo-mechanical nonlinear FE analysis of submerged arc welding process for straight seam large diameter oil and gas pipes to determine residual stresses is presented. The analysis starts with thermal part where, bi-elliptical Goldak equation is used for applying heat input distribution. Afterward, a certain code checks the elements one by one in order to determine the metallurgical phase transformations during cooling process based on cooling time from 800ºC to 500ºC (t8/5). Then the associated mechanical properties of each element is calculated and assigned to the corresponding element to execute the stress analysis phase. A good agreement is observed between simulation results and experimental results obtained from metallography and hole drilling test method. In addition to the results related to the special case studied in this research, undertaking severe nonlinearities due to phase transformation of the API X70-5L material and defining the equivalent material properties during phase transformation are of the new aspects of this research..

In the present work the two microalloyed steel (X65 and X70) used in oil and gas transition pipeline, was obtained as a hot rolled plate with accelerated cooling. First, weld heat affected zone thermal cycles, according to four-wire tandem submerged arc welding process were analyzed. The Baehr 805A/D dilatometer was used for weld heat affected zone thermal cycles’ simulation. The thermal cycles simulated process for heated region involved heating the steel specimens to the peak temperatures of 950, 1150 and 1350 ° C and transformation behaviour and microstructure is investigated. By analyzing the dilatometry results, continuous heating diagram, austenite grain growth and austenite formation kinetics were investigated. Austenite formation modeling was done using Johnson-Mehl-Avrami-Kolmogorov (JMAK) classic equation. The parameter n was found to be relatively independent on temperature (or heating rate); While the parameter k is strongly dependent to temperature, phase fraction transformed and austenite grain growth.