Qiang Chi

Influence of Dual-Phase Microstructures on the Properties of High Strength Grade Line Pipes

Abstract The influence of dual-phase microstructures on mechanical properties of X70, X80, and X90 line pipes is investigated. It is found that the line pipes with dual-phase microstructures possess both larger uniform elongation and higher hardening exponent, especially for high grade steel X90. The tensile deformation of dual-phase line pipe does not follow the trend predicted by the Hollomon formula, and a stable strain-hardening exponent is not found. This stress-strain behavior is different from the normal line pipe. In the initial stage of plastic deformation, the strain-hardening capacity of dual-phase line pipe increases rapidly. However, it reaches a stable stage after 2.0% total strain. The dual-phase pipeline steel is composed of soft phase (polygonal ferrite) and hard phase (bainite), and thus the relatively soft ferrite is good for its deformability. Besides, the fraction of large angle grain boundaries in the dual-phase microstructures is greater than that of the normal line pipe, which is proven to be critical for improving the resistance to plastic deformation and crack propagation.

Microstructure and Mechanical Properties of Twinning M/A Islands in a X100 High Strength Pipeline Steel

Fine microstructure of twinning Martensite/austenite (M/A) islands in a X100 high strength pipeline steel were analyzed by the scanning electron microscope (SEM) and high-resolution transmission electron microscope (HRTEM), and a uniaxial compressive experiment of micro-pillar for a twinning M/A island was conducted in present paper. The experimental results showed that M/A islands in X100 pipeline steels were consisted of retained austenite and nanoscale twins with sizes of less than ten nanometers. There were a few small blocks of nanoscale twins in an M/A island. Volume fraction of twinning M/A islands had an important effect on mechanical properties of X100 pipeline steels, with the increase of twinning M/A islands fraction, yield strength of X100 pipeline steel increased, and impact toughness of X100 pipeline steel decreased. The micro-pillar compression showed that the nanoscale twinning M/A island exhibited the higher deformation hardening during the compressive test, and its uniaxial compressive strength could up to 1.35GPa ultrahigh stress level.

Evaluation Method of Large Inclusions in High-Grade Pipeline Steel

Good internal quality of steel pipes is a basic guarantee for the safe and efficient operation of pipeline system. While presence of large inclusions has adverse effect on the mechanical properties, weldability and corrosion resistance of the steel pipes, and then it would bring huge risk to the safe operation of oil and gas transmission pipeline. In order to reduce the risk to a minimum extent, it is quite necessary to analysis and study the characteristics of large inclusions in high grade pipeline steel, and then it would provide evidence for the formulation of level-determination standard of large inclusions during steel pipe acceptance. Optimized production technology such as electromagnetic stirring is put forward accordingly to prevent the occurrence of large inclusions in pipeline steel.Copyright

Effect of Cold Bending Process and Strain-Age on Properties of X80 Linepipe

Through the field cold bending and mechanical properties tests of Φ1219 mm × 22 mm X80 longitudinal submerged-arc welding line pipe, the effects of different bend radii consisting of 20D, 28D and 47D (D is the diameter of steel pipe) on properties of X80 linepipe were studied. The tests results indicate that the change of the transverse tensile property, roughness, hardness and bend performance of cold bends manufactured by various radius of curvature compare with the mother pipe is relatively small. When the radius of curvature is decreased, the transverse yield strength and yield ratio in intrados of bend increase slightly, but those in extrados of bend decrease in a little range. The changes of longitudinal tensile properties are bigger. When the radius of curvature is decreased, the yield strength and yield ratio in intrados of bend decrease obviously, but those in extrados of bend increase. The longitudinal tensile curves in different position of bend exhibit a rather large difference. After the strain-age for mother pipe and bends, the effects of material deformation in the pipe production process appear which result in that the transverse tensile strength and yield ratio increase obviously. The longitudinal tensile strength and yield ratio in different position of bend also present a increase tendency.

The Effect of Temper Temperature on Property and Microstructure of X80 Induction Bends

Tempering test of X80 induction bends with diameter of 1219mm, thickness of 22mm, are performed at different temperature. and then the tensile tests. CVN tests, chemical analysis and microstructure examination are tested on base metal and welding of X80 induction bends, which is tempered at different temperature. The results showed that the strength and toughness of X80 induction bends has been improved by. temper. And the best match of strength and toughness are obtained when temper temperature is 550° C.

Characters of Lorenz Force Generated by a Single Linear Electromagnetic Stirrer

The internal flow and level fluctuation of liquid metal exposed to linear electromagnetic stirring are mainly induced by the electromagnetic force. As a result, it is necessary to estimate the electromagnetic force amplitude generated by electromagnetic stirrers. The Maxwells equations were simplified on some reasonable assumption so as to estimate the electromagnetic force amplitude in this study. During the electromagnetic force analysis, the characters of the electromagnetic force in liquid metal exposed to a linear electromagnetic stirrer were determined at the same time.