Key Performance Requirements for Steel in Oil and Gas Pipelines
(1) Strength General oil and gas pipelines are designed according to the yield strength of steel. The use of higher yield strength steel pipe, you can improve the working pressure of the pipeline, to obtain better economic benefits: therefore. Pipeline steel yield strength has been gradually developed from the initial carbon steel, the 1940s) X42-X52 steel grade. 60s to reach X60-X70 steel grade. Has been officially produced and officially used yield strength has reached a higher X80 ~ X100 steel level.
(2) toughness 1950s and 1960s, many parts of the world have occurred in the oil and gas pipeline rupture accident, through the analysis of these accidents, greatly promote the understanding of pipeline toughness indicators. API 5L regulations, in addition to conventional mechanical property testing, the production plant should also be SR5 and SR6 supplementary requirements for V-notch Charpy impact test and drop hammer tearing test (i.e., the V-notch impact test). (DWTT), strict nondestructive testing should be carried out before the pipes leave the factory. Nevertheless, for long-distance pipelines, it is difficult to avoid ruptures altogether, and it is necessary to focus on stopping the propagation of crack instability. Research shows that the control of DWTT can be worthwhile ways to achieve anti-cracking. To this end, many countries around the world on the pipeline steel provisions of the DWTT test port shear area percentage of the minimum value.
(3) weldability As a result of the field laying pipeline site conditions are harsh, steel pipe butt welding requirements have good weldability. Poor weldability of steel pipe in the welding is easy to produce cracks at the weld, and cause the weld and heat-affected zone hardness increases, toughness decreases, increasing the possibility of pipeline rupture. Steel weldability design principle is the martensite transition point and the control of hardenability. According to the influence of alloying elements on the martensitic transition point and practical experience to determine the carbon equivalent formula, which can be used to assess the weldability of steel. The commonly used formula for carbon equivalence is.
Ceq=w(C)+w(Mn)/6+[w(Cr)+w(Mo)+w(V)]/5 +(W(Ni)+w(Cu))/15
Ceq should generally be controlled at less than 0.40%. In practice, most steel mills control below 0.35 per cent.
(4) Ductility If the ductility is insufficient, it will lead to cold roll-forming process steel splintering or laminar tearing in the welding process. Therefore, the API standard requires the guide bending test in addition to the flattening test for pipe welded pipes. The key to improve ductility is to reduce non-metallic inclusions in the steel and control the shape and distribution of inclusions.
(5) corrosion resistance in the transport of sulfur-containing oil and gas, the pipeline wall contact with hydrogen sulfide and carbon dioxide, resulting in hydrogen embrittlement and stress corrosion rupture. Generally used to reduce the sulfur content of steel, control the sulfide morphology, improve the toughness along the wall thickness of the direction of such measures. The main feature is that through micro-alloying and controlled rolling, high strength, high plasticity, high toughness and good weldability can be obtained in the hot-rolled state. In order to fully meet the performance requirements of oil and gas pipeline to steel, in addition to the tight alloy design, the control of sulfur, phosphorus and other harmful elements is also very strict. General sulfur control at 0.010% or less, to improve the plasticity of steel, toughness, especially transverse toughness.
