Study of microstructures and properties for girth weldingof domestic X70 pipeline steels

With the development of domestic pipeline steels, it is necessary to develop suitable welding technology which can improve the properties of the welded pipeline. In this paper, the microstructures and mechanical properties of domestic XTO pipeline steels and welded joints are discussed. The welding consumables of BOHLER E6010 and HOBART 81N1 are matched for girth welding. The following characteristics in heat-affected zone（ HAZ） are indicated that microstructures of intercritical HAZ（ ICHAZ） is finer and more uniform, the grain sizes of fine-grain HAZ （ FGHAZ） and subcritical HAZ （ SCHAZ） are smaller than that of coarse-grain HAZ（ CGHAZ）. The hardness, tensile strength and toughness of welded joints come up to the standard. The micrographs of impact specimens in welded joints are cleavage, quasi-cleavage and dimple which shows there is typical ductile rupture.

STUDY OF THE X70 PIPELINE STEEL CORRODING IN 3.0wt% NaCl SOLUTION USING ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY TECHNIQUE

The corrosion process of the X70 pipeline steel in 3.0wt% NaCl solution were studied using polarization method, and the chronological characteristics during the entire polarization plot were investigated in detail using EIS technique. In the active region of X70 steel, such as 20mV potential bias applied on open circuit potential (OCP), the impedance spectra was comprised of three parts: a high-frequency capacitive loop, a middle-frequency capacitive loop and a low-frequency inductive component. When positive polarization potential increased, the capacitive loops at high and middle frequency range merged, and the inductive component at low frequency shrunk. At high positive polarization potential bias (500-800mV vs. OCP), the high-frequency capacitive loop and the low-frequency inductive loop exhibited as disheveled points due to the synergism of the inhomogeneity of the corroding material and the localized corrosion. The results were fitted utilizing the equivalent circuits to simulate the impedance spectra and to interpret the electrochemical features shown during the experiments.

DECREASING PITTING SUSCEPTIBILITY OF PASSIVE FILMS ON X70 PIPELINE STEEL IN NaCl SOLUTIONS BY ILLUMINATION

The influence of UV illumination on passivity and pitting susceptibility on X70 pipoeline steel in a borate buffer （pH=8.4 ） solution containing NaCl is described. It is observed that illumination of the sample leads to a decrease in its pitting susceptibility as indicated by pitting potential and incubation time measurements in chloride containing electrolytes. This effect is strongly dependent on the applied potential during passivation. The electronic properties of the passive films on X70 steel were studied by Mott-Schottky analysis and photocurrent transient measurements. The results indicated that illumination during passivation led to modifications in the electronic properties of the passive films, mainly to a decrease of the bulk doping and an increase in the surface state density. The cause for the decrease in the pitting susceptibility is preliminary explained in terms of such modifications of the passive flm.

Study on the microstructure and hardness of in-service welded joint of X70 pipeline steel

Hydrogen induced cracking （HIC） is one of the main problems of in-service welding onto active pipeline. Microstructure and hardness of welded joint have a vital effect on hydrogen induced cracking. The microstructure and hardness of welded joint of XTO pipeline steel were studied using simulation in-service welding device. The results show that the main microstructures of in-service welded seam are grain boundary ferrite , intracrystalline acicular ferrite , as well as small amount of widmanztatten structure. The main microstructures of coarse grain heat-affected zone （CGHAZ） are coarse granular bainite, lath ferrite and martensite. Metastable phases such as martensite and lath ferrite are found in CGHAZ because of the too quick cooling velocity a＇nd the hardness of the CGHAZ is high.

Study on M/A islands in pipeline steel X70

The fine structure of M/A islands in pipeline steel X70 has been studied by transmission electron microscope (TEM). It is shown that the M/A islands are about 1-2 μm in size and distribute at the grain boundary of irregular massive ferrite or acicular ferrite in the microstructure of the steel undergoing TMCP (thermo-mechanical controlled processing). The analysis of diffraction contrast shows that the M/A islands consist of retained austenite and some martensite lamellae different in size and orientation. The microt winning and midrib exist in the lamella of lenticular martensite, which exhibit the typical character of high-carbon martensite. The influences of TMCP parameters on M/A islands have been studied carefully. With the increase of the cooling speed, the amount of M/A islands decrease slightly and the morphology of M/A islands changes to thin dispersive short bars from thick irregular long strips.

Properties of Surface Film on X70 Pipeline Steel in CO_3^(2-)-/HCO_3^- Environment

The electrochemical behavior of X70 pipeline steel in (0.5mol·L-1 Na2CO3+1 mol·L-1 NaHCO3) solution was studied by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) were used to analyze the composition and microstructure of the surface film. The results showed that there were two anodic peaks at -600 mV and -350 mV. The surface film formed at -600 mV mainly consisted of ferrous carbonates and ferrous hydroxycarbonates. It had a small reaction resistance. It was metastable and possessed poor protective property. Numerous pits and microcracks existed on the film, which could be the active paths for the initiation of stress corrosion cracking. The surface film formed at -350 mV, mainly consisted of ferric oxides. It has high reaction resistance and offered good protection for the substrate.

Influence of cooling conditions on X70 pipeline steel in-service welding HAZ

The chamber device was designed and set up to simulate the in-service welding. The results show ： the t8/5 , t8/3 and inner wall peak temperature Tp decrease with the cooling rate increases. The welding energy is carried off by flowing medium, the cooling rate increases, and many unbalanced microstructures such as granular bainite, martensite and M-A generate ; it worsens the properties of HAZ. Under air-cooling, the cooling rate is slow, the austenite grain grows obviously, the lath ferrite crosses the whole austenite, and it causes the hardness value is also big. The change of HAZ width is not obvious with the increase of cooling rate; and burn-through is not susceptible to the cooling rate. The quench microstructures increase and the hydrogen does not outflow from the HAZ easily when increase the cooling rate, so the susceptibility of hydrogen cracking increases.

Influence of yield-to-tensile strength ratio(Y/T) on failure assessment of defect-free and corroded X70 steel pipeline

The effect of yield-to-tensile strength ratio(Y/T) on failure pressure of X70 pipeline without and with corrosion defects was investigated.The stress-strain response of materials was characterized by a power-law hardening curve.Two formulas to estimate the strain hardening exponent n for a special Y/T were obtained by least squared regression method and the influence of Y/T on n was analyzed.As an application of n-Y/T expression,the analytical solutions of burst pressure for X70 pipeline without and with corrosion defects were also obtained.The results indicate that the burst pressure of defect-free X70 pipe without corrosion defects is a function of the Y/T,pipe geometry t0/D0 and engineering tensile strength,and increases as Y/T or t0/D0 increases; whilst the burst pressure of corroded X70 pipe decreases with the increase of defect depths,d/t.Comparisons indicate that the present analytical solutions closely match available experimental and numerical data.

Oblique Y-groove cracking test of the welding cold cracking susceptibility of domestic X-70 pipeline steel

Gas Transmitting From West to East Project' is significant. It should ensure the welding quality and safety of pipeline. The task is very arduous to guarantee the quality of the project in the condition of long line, complex weather and geology features. In this paper, the welding cold cracking susceptibility of domestic X 70 pipeline steel adopted by the project, which is one of the most interesting questions of welding quality about petrol pipeline, was studied by means of oblique Y groove cracking test. The crack ratio of surface and section was tested under the conditions of different welding materials and preheat temperature .The thickness of plate steel was 14.7 mm and 10.3 mm . The results reveal that X 70 pipeline steel has good crack resistance. The research has important value for the construction of large scale pipeline engineering and the application of domestic X 70 pipeline steel.

Effect of controlled rolling and controlled cooling process on microstructure of Ti-alloyed pipeline for CSP

The pipeline steel is usually produced by adding niobium and vanadium alloying elements.The titanium alloyed pipeline steel was studied in the paper to reduce the production cost and enhance the competitive ability of pipeline steel.The steel containing 0.070%Ti was refined in the laboratory vacuum refine furnace.The dynamic CCT curve of developed steel was conducted on Gleeble-1500 thermal simulator and multi-pass deformation tests of studied pipeline steel were performed to simulate the CSP hot-rolling technology.Corresponding microstructures were observed and the influences of such technology parameters as deformation strain,deformation temperature, cooling rate and finishing temperature etc.on microstructure were analyzed.The dynamic CCT curve was obtained.The test results showed that the volume fraction of acicular ferrite in the microstructure of Ti-microalloyed X70 pipeline steel increased obviously with the increase of cooling rate after thermo-mechanical deformation.However,the volume fraction of acicular ferrite varies scarcely after the cooling rate of 20℃/s.In order to obtain Ti-microalloyed X70 pipeline steel with excellent compound mechanical properties which has acicular ferrite as the main ideal microstructure,the cooling rate should be controlled to be 20℃/s or more.The higher of the finishing temperature,the more of the volume of acicular ferrite and the less of the volume of polygonal ferrite.The quite fine microstructure can be obtained by lowering the finish-cooling temperature.The results have shown that it is possible to produce the Ti-addition X70 pipeline steel from the point of view of microstructure.

Effect of Environmental Factors on Electrochemical Behavior of X70 Pipeline Steel in Simulated Soil Solution

Potentiodynamic polarization measurement was used to investigate the effects of temperature, dissolved ox- ygen concentration and pH on the electrochemical behavior of X70 pipeline steel in simulated solution according to the orthogonal testing method. The results showed that temperature, dissolved oxygen concentration and pH had great influence on corrosion current density （icorr） of X70 steel. Corrosion current density of X70 steel was most influenced by dissolved oxygen concentration in simulated solution. The corrosion degree of X70 steel was the least under the environment of low temperature, deficient oxygen and weak acid.

Effect of pH Value on Stress Corrosion Cracking of X70 Pipeline Steel in Acidic Soil Environment

The effect of pH value on the stress corrosion cracking （SCC） of API XT0 pipeline steel in simulated acidic soil solutions was investigated by using slow strain rate test, electrochemical polarization curves, electrochemical impedance spectroscopy, and scanning electron microscopy, pH plays an important role in the susceptibility and electrochemical mechanism of SCC. The pH higher than 5 has no significant effect on electrochemical processes. By contrast, the pH lower than 5 intensifies cathodic hydrogen evolution reactions, thus increasing the cathodic current and corrosion potential. Under different pH values, the SCC mechanism of XT0 pipeline steel varies among anodic dissolution （AD）, hydrogen embrittlement （HE）, and the combination of AD and HE （AD ＋ HE） with variations of applied potential. At -850 mVscE, the SCC mechanism is HE if pH is less than 4 or AD 4- HE if pH value is more positive.

Microstructure transformation of X70 pipeline steel welding heat-affected zone

The continuous cooling transformation curve of heat-affected zone （HAZ） of X70 pipeline steel was mea- sured by Gleeble-3500 thermal mechanical simulator, optical microscope （OM） and hardness analysis. The microstructure transformation rule at different cooling rates and solution behaviors of microalloy carbonitride during heating process of simulated specimens were investigated. When the cooling rate changes from 10 to 20 ℃.s-l, microstructures at HAZ are identified as granular bainite, lathy bainite, and quasi-polygonal ferrite. This micro- structure is featured with fine ferrite grains, martensite/ austenite islands dispersed, high-density dislocations, and fine carbonitride particles, resulting in improving the strength and toughness of HAZ. With the cooling rate increasing to above 40 ℃.s-1, the microstructure is pre- dominantly coarse lathy bainite with clear primary aus- tenite grain boundary. While the cooling rate decreases to below 1 ℃.s-1, a fairly small amount of pearlite can be observed at the boundaries. The strength and toughness of HAZ are deteriorated because of coarse grains among these microstructures. Most of microalloy carbonitrides in HAZ could be dissolved in the matrix during heating process. A few of TiN particles existing as residues in the matrix can prevent austenite grain from growing, and then improve the strength and toughness of HAZ.

Source and Control of Nitrogen for X70 Pipeline Steel

The effects of some key factors on nitrogen absorption during the smelting process of X70 pipeline steel were studied,and the source of nitrogen pick-up was analyzed to find the bottleneck for nitrogen control.A series of measures were put forward to decrease the nitrogen pick-up.The results indicated that an exponential relationship existed between the nitrogen absorption index and the free oxygen in molten steel.Nitrogen absorption index could decrease below 0.3when free oxygen in molten steel was above 100×10^（-6） after tapping.For low sulfur killed steel,the nitrogen absorption ratio and sulfur content satisfied a linear relationship with a slope of-0.007.Low free-oxygen and sulfur were beneficial to the deep desulfurization during vacuum treatment.The contradiction of high desulfurization ratio and low nitrogen pick-up during LF process could be resolved by skimming oxidizing slag after tapping and making new high basicity top slag.After optimization,the average content of nitrogen in final product decreased from 46×10（-6）to 35×10（-6）.