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.

The critical temperatures of a thermomechanical process have a great importance for the final microstructure and mechanical properties of high strength low alloy steels. In this study, the average scheduling was used to determine the critical temperatures of API X70 steel. This steel is extensively used in Iran for large diameter, high-pressure gas transportation pipelines as well as the oil transmission networks. The critical temperatures in various conditions including different strains, strain rates (from 0.1 to 1 s-1) and rolling interpass times were determined and the effects of these parameters on no-recrystallization critical temperature (Tnr) was investigated. The results showed that Tnr decreased with an increase in the strain and strain rate. In addition, it was observed that Tnr increased with a decrease in the interpass time. A good agreement was found between the results of this research with those of existing empirical relations and those of similar steel. With regard to the lack of experimental data, the results obtained in the present study can be used for production of API X70 steel in Iran.

The production of microalloy steel plates is associated with the combination of heat treatments and hot working known as thermo-mechanical process. Superior mechanical properties of the milled products could be obtained by controlling the microstructural evolution during hot rolling process and accurate identification of phase transformation temperatures. In this paper, the influence of austenitizing time and temperature, cooling rates, banded structure of ferrite and perlite and microalloying element on the phase transformation of austenite in API-X70 steel were investigated. The dilatometry test and microstructure analysis were carried out on the samples with different direction (RD, TD and ND) to the steel plate. The results indicate that increasing of austenitizing time and temperature decrease of the start and the finish temperature of austenite decomposition; this was associated with changing of microstructure from ferrite-perlite to ferrite-bainite. The impact of small changes of cooling rate was found to be insignificant on the phase transformation temperatures. However, by increasing the cooling rate, the microstructure becomes bainitic and the critical temperatures decrease rAPIdly. Both the dilatometry data and the microstructure micrographs prove that the sample direction has negligible effect on critical temperatures.

The welded zone of API X70 steel pipe, due to the inherent defects of welding, is a potential area for initiation and propagation of cracks which can eventually lead to damage of the structure. In this research, mechanical behavior of spiral seam weld was evaluated in API X70 pipe steel in three zones (base metal, HAZ and weld metal) using uniaxial tensile and three point bend (3PB) experiments. Three tensile specimens were used in each zone for measurement of mechanical properties. For studying the mechanical behaviour of the pipe, one specimen with 3PB geometry was tested for each zone. Specific values including yield, peak and final load for 3PB specimen were determined from load-displacement plots. The associated energy for each load plus initiation and propagation energies were calculated and the results were analyzed in relation to microstructure in each zone. With an analytical equation based on slip-line field analysis, yield strength was determined with 3PB specimen in each zone and compared with the result of uniaxial tensile experiment. From uniaxial tensile experiment, yield strength levels of 560, 514 and 507 MPa were found for base metal, HAZ and weld metal respectively and from 3PB experiment 604, 582, 575 MPa respectively.

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 the present study, a combination of X-ray diffraction spectroscopy, field emission scanning electron microscopy (FESEM) and optical microscopy methods were used to investigate the microstructure and crystallographic texture changes of API 5L X70 pipeline steel following an extreme cold working process that resulted in reduce the thickness of 80% in the sample. All investigations were done on two different sections of the sample, including the surface and the inclined section rotated 45 degrees relative to the surface. In fact, these two cross-sections are chosen to show the angle between the two important crystallographic planes of {100} and {110}. FESEM and optical microscope analyzes showed that the microstructure of the primary steel includes granular bainite, conventional upper bainite, and martensite-austenite islands. The results showed that the morphology of the initial microstructure consisted of mostly equiaxed grains, which after applying cold work due to being subjected to severe pressure, turns into a microstructure with the morphology of elongated grains, especially in the rotated section. Analysis of crystal orientation changes by X-ray diffraction spectroscopy showed that the density of {100} crystallographic planes decreased due to applied cold work on the surface and at the same time the density of {110} crystallographic planes on the surface increased. Also, the results proved that {100} crystallographic planes were replaced by {110} crystallographic planes on the sample rotated by 45 degrees.