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Pipelines must have sufficient impact toughness at low temperature to prevent brittle fracture. Charpy V-notch impact tests are normally required to achieve the specified absorbed energy value at the specified temperature. For example, GB/T 9711 requires the average value of transverse impact energy ≥ 27J (0 ℃ test temperature), and X80 steel grade has higher requirements. Impact test shall be carried out for low temperature materials used at - 20 ℃ and below, welds and heat affected zones.

In accordance with API 5L and GB/T 9711, pipeline pipes shall be subject to hydrostatic test one by one, and the standard does not specify that non-destructive testing is allowed to replace hydrostatic test, which is significantly different from some Chinese steel pipe standards (such as SY/T 5037). The test pressure and pressure holding time shall be calculated and determined according to the standard formula.

① PSL1 is not required to be subject to impact performance test, and PSL2 must be subject to and specifies the minimum absorbed energy (e.g. transverse ≥ 27J); ② PSL1 does not require NDT, and PSL2 requires NDT one by one; ③ PSL1 only specifies the minimum value of yield strength, PSL2 specifies the minimum value and the maximum value at the same time, and control the yield ratio ≤ 0.93; ④ PSL2 has more strict restrictions on the content of sulfur, phosphorus and other impurities (S ≤ 0.015%, P ≤ 0.025%); ⑤ PSL2 requires an upper limit of carbon equivalent to ensure weldability.

General principle: LSAW or SSAW shall be selected for large-diameter trunk line (above Φ 508mm); ERW is selected for small and medium-sized branch pipelines; LSAW is preferred for high steel grade, large wall thickness and severe working conditions; LSAW or seamless pipes are preferred for submarine pipelines and crossing sections; SSAW can be selected for large-diameter water transmission pipeline with low pressure rating.

The seamless steel pipe has no longitudinal weld and uniform overall performance. It is mainly used in the crossing section, complex terrain area and high pressure, high temperature and other harsh working conditions. However, the large-diameter seamless pipe is difficult to manufacture and the cost is high, so it is less used in the large-diameter trunk line.

ERW is applicable to medium and small-sized pipelines (Φ 168~660mm, wall thickness of 8~25mm), with steel grade of X56~X80. It is commonly used for branch pipelines and urban natural gas pipelines, submarine transmission pipelines and pipelines in high and cold areas. Its welding speed is high and it can be continuously produced, but it cannot be used as the main natural gas pipeline.

Advantages: narrow-band steel can be used to produce large-diameter steel pipes, with a maximum diameter of more than 2500mm; Less equipment investment. Limitation: complex residual stress distribution; Weak resistance to large deformation; The wall thickness is generally ≤ 25mm; the development of high steel grade (above X90) is limited.

Advantages: Mature forming process, easy to ensure welding quality; The residual stress is small after the pipe is expanded; Good geometric accuracy and mechanical property; It can reach high steel grade (≥ X120) and large wall thickness (≥ 40mm). Limitation: the diameter is limited by the plate width (generally ≤ 1422mm); Expensive investment in UOE molding; Less economical with diameter

Line pipe can be manufactured in two main ways: seamless steel pipe (SMLS) and welded steel pipe (Welded). Welded steel pipes are divided into resistance welding (ERW/HFW) and submerged arc welding (SAW), and the latter is further divided into longitudinal submerged arc welding (LSAW) and spiral submerged arc welding (SSAW).

The yield strength of X80 pipeline steel is equivalent to X70, but the tensile strength is higher than X70. X80 offers higher strength levels, reduced wall thickness, reduced engineering costs, but higher weld process and toughness control requirements. At present, X80 has been widely used in major projects such as West-East Natural Gas Transmission Project in China.

PSL2 has strict limits on chemical composition. With X46 PSL2 as an example, carbon equivalent (CE) is subject to mandatory upper limit requirements. Impurities such as phosphorus and sulfur are strictly controlled to improve toughness and HIC/SSC resistance. Microalloy elements such as niobium, vanadium and titanium are added to refine grains. Different steel grades and wall thicknesses have different carbon equivalent limits, and the larger the wall thickness, the more strict the requirement.

In the process of using pipeline steel, in addition to high compressive strength, it is also required to have high low temperature toughness and excellent welding performance. Specific indicators include yield strength, tensile strength, yield strength ratio (PSL2 ≤ 0.93), Charpy impact energy, carbon equivalent (CE), hardness, etc

The "L" series of GB/T 9711 corresponds to the "X" series of API 5L one by one, but the unit is different - the L series is in MPa (MPa) and the X series is in ksi (ksi). For example, L360 corresponds to X52 (360MPa ≈ 52ksi), L415 corresponds to X60, L485 corresponds to X70 and L555 corresponds to X80. The number of the steel grade number represents the minimum yield strength: X52 minimum yield strength is 52ksi, L360 minimum yield strength is 360MPa.

The steel grades specified in GB/T 9711 and API 5L cover a wide range, from low steel grade to ultra-high steel grade: L175/A25、L210/A、L245/B、L290/X42、L320/X46、L360/X52、L390/X56、L415/X60、L450/X65、L485/X70、L555/X80、L625/X90、L690/X100、L830/X120 Etc. At present, the commonly used steel grades in the project are L245/X42 to L555/X80.