Microstructure Of X80 Pipeline Steel Research Articles

Study on Microstructure and Properties of X80 Pipeline Steel

Experimental methods, such as OM, SEM and X-EDS, were used to study the microstructure of X80 pipeline steel. It mainly consists of fine acicular ferrite (AF). X80 pipeline steel possesses high strength and impact energy at-30°C approaches to 400J. Grain refinement and precipitation hardening are the main reasons for high strength, and toughness improvement can be attributed to grain refinement and particular microstructural characteristics of AF.

Effects of Cooling Processes on Microstructure Evolution of X80 Pipeline Steel

With the development of pipeline industry, the pipeline steels with higher strength and plasticity, better low-temperature toughness and weldability are the main development trend. For bainitic pipeline steels, M/A constituent is the main hard phase. Although the M/A constituent can enhance the strength, the larger block-form M/A constituent can deteriorate low-temperature toughness. Therefore, it is essential to further investigate how to refine the M/A constituent. In the present paper, X80 pipeline steel was cooled to room temperature with various cooling paths after hot compression deformation at the temperature of 800oC. The evolution of microstructure of X80 pipeline steel has been analyzed by optical microscope (OM) and scanning electron microscope (SEM). The experimental results show that increasing the cooling rate can significantly refine M/A constituent and promote the formation of granular bainite, and the bainitic ferrite can be also greatly refined. In addition, the effects of five final temperatures of fast cooling were also investigated.

Effect of Nano CaO and MgO Addition on the Inclusions Composition in the Cast Microstructure of X80 Pipeline Steel

Nanometer calcium and magnesium oxides were added into molten steel by the carrier method in the experiment. The experiment takes the X80 pipeline steel as the research object and analyses the effect of nanometer calcium and magnesium oxides addition on the inclusions in the cast microstructure of the X80 pipeline steel. The scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) were used to analyze the composition of the inclusions in the cast microstructure of the X80 pipeline steel. The result reveals that when no addition of nanooxides, the inclusions in the cast microstructure are the C-Fe-Si-Al-Mn-based inclusions. When adding nanoMgO, the inclusions are mainly the composite oxides of Fe-Mg-Si-O, Fe-Mg-Si-Mn-O and Mg-Si-Mn-Al-O. When adding nanoCaO, the inclusions are mainly the composite oxides of Fe-Ca-Al-O, Fe-Ca-Si-O, Fe-Ca-Si-Mn-O and the composite oxides or sulfides of Fe-Ca-Si-Mn-O-S. There is more Fe in some inclusions and Fe is not uniformly dispersed in the molten steel. As the result, some certain segregation phenomenon takes place.

Effect of Nanometer Calcium and Magnesium Oxides Addition on the Inclusions in the Cast Microstructure of X80 Pipeline Steel

Nanometer calcium and magnesium oxides were added into molten steel by the carrier method in the experiment. The experiment takes the X80 pipeline steel as the research object and analyses the effect of nanometer calcium and magnesium oxides addition on the inclusions in the cast microstructure of the X80 pipeline steel. The scanning electron microscope (SEM) was used to analyse the morphology and distribution of the inclusions in the cast microstructure of the X80 pipeline steel. The results reveal that there are a small amount of inclusions in the sample of no nanometer oxides addition. With the amount of the nanoMgO oxide addition rising, the number of the inclusions in the sample firstly increases and then decreases gradually. When adding 0.05 wt% nanoMgO or CaO oxides, the size of the inclusions is big and the aggregation phenomenon appears. When adding 0.02 wt% nanoMgO or CaO oxides, the number of the inclusions in the cast microstructure is the most, whose size is smaller, about 2~4 μm and the inclusions are the most uniformly distributed.

Effect of Nanometer Magnesium Oxide Addition on the Cast Microstructure of X80 Pipeline Steel

The pipeline steel as an application in pipeline construction must have good comprehensive mechanical properties due to the harsh environment of the pipeline engineering. So this experiment takes the X80 pipeline steel as the research object, the thermal stability second phase particles which would not be dissolved or aggregated at high temperature will be expected by means of adding nanomagnesium oxide into the steel with the method of carrier dispersion addition. The effect of nanometer magnesium oxide addition on the cast microstructure of X80 pipeline steel was analysed. The results show that the cast microstructure is consist of the ferrite and a small amount bainite. And the bainite is distributed at the boundary of the ferrite grains. When adding 0.02 wt% nanometer magnesium oxides, the number of bainite increases significantly in the cast microstructure, which is mostly distributed at the boundary of the ferrite grains.

Study on Mechanical Properties Anisotropy of X80 Pipeline Steel

The microstructure of X80 pipeline steel in different directions were observed by SEM technique and its effective grain size and misorientation were statistically analyzed by EBSD system. Based on these results, the mechanical properties at 0°, 45° and 90° to the rolling direction of X80 pipeline steel were studied. The results show that, owing to finer grain size and less low-angle grain boundaries, strengths and impact toughness of X80 pipeline steel at 90° direction are optimal. While the pipeline steel possesses finer grain size, more high-angle grain boundaries and less low-angle grain boundaries, the crack propagation is effectively suppressed, then its impact toughness is improved.

Microstructure and Texture Comparison of an Acicular Ferrite Pipeline Steel with Different Hot Rolling Process

Microstructure of X80 pipeline steel with different hot rolling process was compared using Optical microscopy (OM), Bulk X-ray texture and micro orientation analysis was carried out by Orientation distribution function (ODF) and Electron back-scattered diffraction (EBSD), to analyze the various texture components of the pipeline steels under two different rolling processes. The results show that the final microstructures under the two schedules both present typical acicular ferrite characteristic. On the other side, the corresponding textures were found mainly comprised of two fibers in the rolling and normal direction in hot rolled X80 steel plate, there were obvious {112} , {110} , and {111} fiber, which seemed to be related with the mechanical properties anisotropy. Therefore, the influences of the microstructure and texture on the anisotropy were also discussed in this paper.

Characterization of inclusions of X80 pipeline steel and its correlation with hydrogen-induced cracking

In this work, the microstructures of an X80 pipeline steel were characterized, and their susceptibilities to hydrogen-induced cracking (HIC) were investigated by hydrogen-charging, electrochemical hydrogen permeation and surface characterization. It is found that the microstructure of X80 pipeline steel consists of a polygonal ferrite and bainitic ferrite matrix, with martensite/austenite (M/A) constituents distributing along grain boundaries. The inclusions existing in the steel include those enriched with Si, Al oxide, Si–ferric carbide and Al–Mg–Ca–O mixture, respectively. The majority of inclusions are Si-enriched. Upon hydrogen-charging, cracks could be initiated in the steel in the absence of external stress. The cracks are primarily associated with the Si- and Al oxide-enriched inclusions. The diffusivity of hydrogen in X80 steel at room temperature is 2.0 × 10 −11 m 2/s, and the estimated hydrogen trapping density in the steel is as high as 3.33 × 10 27 m −3.