Analysis of Mechanical Properties of PSL2 X65Q Seamless Steel Linepipe
Conducted are statistics and analysis of experimental research data of PSL2 X65Q seamless steel linepipes as collected over the recent years，involving tensile property，impact property and hardness， etc. The result shows that the PSL2 X65Q seamless steel linepipe features small yield strength fluctuation range(W100 MPd ， low yield ratio(WO.89 ， high plasticity， fine toughness， low hardness and satisfactory corrosion resistance， etc.， being in compliance with the stringent requirements for marine pipeline， and being suitable for the anti-large deformation pipeline， and pipe-laying operation with coiling method.
Key words : X65Q ; seamless steel linepipe ; tensile property ; impact property ; hardness
In recent years, the change of energy structure and the increase of demand for energy have greatly promoted the development of long-distance transmission pipelines. Pipeline transportation is the most economical and safest transportation method for long-distance and large-scale transportation of oil and gas. In order to increase the transmission pressure and improve the transmission efficiency, the pipes used for long-distance oil and gas transmission pipelines are constantly developing towards high steel grades and large wall thicknesses. The application of high-strength pipeline steel can reduce the investment and operating costs of pipeline projects, but at the same time it also puts forward higher requirements for the reliability of pipeline pipes, especially for marine pipelines. Mechanical properties have higher requirements.
1 Requirements for mechanical properties
The API Spec 5L - 2012 standard [ 1 ] requires mechanical properties of seamless steel pipes for PSL2 X65Q pipelines in Table 1 .
2 Tensile properties of seamless steel pipes for PSL2 X65Q pipelines
- Yield Strength and Tensile Strength
According to the requirements of the API Spec 5L - 2012 standard, it is necessary to determine the 0.5% total elongation strength of seamless steel pipes for pipelines. In the tensile test, the maximum nominal tensile stress that the specimen bears before it is broken is called the tensile strength. Yield strength and tensile strength statistics of seamless steel pipes for PSL2 X65Q pipelines.
Table 1 API Spec 5L - 2012 standard to PSL2 X65Q
Requirements for mechanical properties of seamless steel pipes for pipelines
Yield strength /MPa |
Tensile strength /MPa |
Yield ratio |
|
450 ~ 600 |
535 ~ 760 |
W 0.93 |
|
Elongation /% |
0 °C transverse impact energy J |
Hardness HV10 |
|
$ API formula computed value |
Single value $ 20 , average $ 27 |
W 270 |
API Spec 5L — 2012 standard to PSL2 X65Q MPa , the standard deviation is 29.55 MPa , and the standard error of the average value is
0.50 MPa , the yield strength of seamless steel pipes for PSL2 X65Q pipelines is mostly concentrated in 480~530 MPa , which can meet the standard yield strength fluctuation of OED-TP-104 — 2009 Addendum to API 5L For Procurement of C- Mn Seamless Pipe For Reel Lay Installation The requirement of the range W 100MPa ; Fig. 1 ( b ) 3 432 statistical data , the average value is 603.5 MPa , and the standard deviation is
26.94 MPa , the standard error of the average value is 0.46 MPa , and the tensile strength is mostly concentrated in 580 ~ 630 MPa , which well meets the requirements of API Spec 5L - 2012 standard.
- elongation after break
The API Spec 5L - 2012 standard specifies the measurement of the elongation after breaking with a calibration distance of 50 mm . According to this standard, for pipeline pipes with a wall thickness of M 19 mm , a round bar sample with a diameter of 12.7 mm can be used for the longitudinal tensile test; for other wall thicknesses, full-wall strip tensile tests of different widths are taken according to different outer diameters. Sample. There are 3 219 statistical data on the elongation after fracture of the seamless steel pipe used for PSL2 X65Q pipeline , the average value is 38.6% , the standard deviation is 4.65% , the standard error of the average value is 0.098% , and the elongation of the plank after fracture The ratio M is 25% , which meets the standard of API Spec 5L - 2012 . The maximum cross-sectional area is 485 mm 2 .
A 0.2
The formula Af =C is the specified minimum elongation after break, % ;
U 0.9
C is a constant; AXC is the cross-sectional area of the applicable tensile sample, mm 2 ; U is the specified minimum tensile strength, MPa ) The calculated elongation after fracture M is 23% , which can also meet the requirements of 0ED-TP-104 — 2009
standard elongation after breaking $ 25% ; the statistical data is 213 , the average value is 29.5% , the standard deviation is 2.05% , the standard error of the average value is 0.147% , the elongation rate after tensile breaking of the round bar is $ 23% , It meets the requirements of API Spec 5L - 2012 standard that the selected cross-sectional area is 130 mm 2 and the elongation after fracture is $ 18% , which shows that the seamless steel pipe for pipelines of this steel grade has excellent plasticity.
Also need to point out:
① The yield strength and tensile strength of seamless steel pipes of the same specification and steel grade pipelines obtained by lath stretching and round bar stretching are not much different, but the elongation after the lath stretching is much greater than that of the round bar stretching, This is because the cross-sectional area of the strip tensile specimen is much larger than that of the round bar specimen;
② Seamless steel pipes with the same specifications and steel grades shall use API Spec 5L - 2012 standard 50 mm calibration distance and 5.65 ST ( S 0 is the cross-sectional area of the applicable tensile sample, and the elongation after fracture calculated by the mm 2 proportional gauge length also has a huge difference, because the elongation of tensile deformation mainly occurs in the necking part, and the proportional gauge length The specified length is far greater than 50 mm . In the case of little difference in elongation, the larger the gauge length, the smaller the elongation after breaking.
- Yield ratio
It is generally believed that the larger the yield ratio of the metal material, the smaller the plastic range of the material after yielding. This means that the plastic deformability and safety margins of engineering gas pipelines, especially the stress and strain concentration areas at the joints of branch lines, etc. degree of reduction.
- lateral stretch
When the outer diameter of the seamless steel pipe used for the pipeline is ≥ 219.1 mm , some customers also require a transverse tensile test. According to different outer diameters and wall thicknesses, the transverse tensile test can use transversely flattened full-wall thickness plate tensile specimens and Small diameter round bar specimens. On the basis of summarizing a large number of longitudinal and transverse (flattened) tensile tests of seamless steel pipes for PSL2 X65Q pipelines , a typical longitudinal and transverse tensile curve can be obtained. The specification of seamless steel pipes for PSL2 X65Q pipelines used in the test is : 355.6 mm x 15.9 mm , the transverse stretch is a flattened sample.
Compared with the longitudinal tensile curve, the transverse flattened tensile curve of the X65Q pipeline seamless steel pipe is smooth without yield platform, and the obtained yield strength is slightly lower than that of the longitudinal direction, and the tensile strength is higher than that of the longitudinal direction. This is because the transverse sample is deformed during the flattening process. The smaller the outer diameter of the steel pipe, the greater the deformation, which is equivalent to the tensile strain on the inner wall of the steel pipe and the compressive strain on the outer wall. The strain produces a large number of dislocations, which increases the strength.
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