Selection of tensile specimen size for hot bend

The size of tensile sample has a great influence on the tensile test results. In order to improve the accuracy of the tensile test results of hot bend and reduce the test cost, combined with the sampling location of common hot bend and the requirements of GB/T 228.1 and ASTM A370 for the size of tube tensile sample, the tensile test results of different sample sizes under the two standards were compared and analyzed. The results show that it is more practical to use the specimen with 50 mm gauge distance as required by ASTM A370; In the tensile test, the samples with different sizes should be selected according to the different diameters of the hot bend and the corresponding standards, so as to improve the accuracy of the test results.

The hot bending pipe is an indispensable pipe fitting for each major pipeline project, and its own strength plays a decisive role in the pressure of the pipeline. The strength of the hot bend is verified by tensile test. The size of tensile specimen has a great influence on the tensile test results. In the relevant standards of hot bending, the room temperature tensile test only refers to the test according to GB/T228.1 or ASTM A370 or ISO 6892 method, and no suggestions or recommendations are made on the size of tensile specimens. The author combined with the requirements of relevant standards, summarized the size of tensile specimens of hot bend, in order to provide reference for relevant inspectors.

Requirements of different standards for tensile test of hot bend

SY/T 5257—2012

Test sample

The tensile test sample of hot bending pipe shall be plate type. When the wall thickness of the bend is ≥ 20 mm, the rod shaped sample processed by the unevening specimen can be adopted by consensus, but the weld shall be ground flat, and the partial defects and rolling chips shall be removed.

Test method

At normal temperature, tensile test shall be conducted in accordance with GB/T 228.1 and GB/T 17600.1. The elongation after time fracture is determined according to the gauge length 5.65. However, other gauge lengths are also allowed. At this time, the corresponding elongation shall be determined according to GB/T 17600.1 with the reference of gauge length.

GB/T29168.1—2012(ISO15590-1: 2009MOD)

Test sample

If the round bar sample is adopted through negotiation, the round bar sample is taken from the flat test block. The residual height of joint weld should be removed by grinding, and the partial defects and surface metal oxide layer should be removed.

Test method

At room temperature, tensile test shall be conducted according to GB/T 228.1 or ASTM A370.

CDP-S-OGP-PL-016—2014-3

The tensile specimens of the pipe body and weld can be processed into rectangular or cylindrical specimens. Tensile test shall be conducted according to GB/T 228, ISO 6892 and ASTM A370.

ASME B16.49—2012

The tensile properties of hot bend shall be evaluated according to ASTM A370.

Sampling position of hot bend

The common sampling position of hot bending pipe is shown in Figure 1. It can be seen from Figure 1 that the standard requires that the sampling positions of tensile samples for hot bend are more, and there are longitudinal and transverse parts; The smaller the size of tensile sample is, the more favorable it is to the preparation of the sample. But the smaller the sample, the greater the error of the test results, and the higher the measurement accuracy of the test equipment, which will not be conducive to the operation of the test. Therefore, the appropriate size of the specimen should be selected for tensile test.

20210719080507 25772 - Selection of tensile specimen size for hot bend
Figure 1 Schematic diagram of common sampling position of hot bending pipe
Generally, the diameter of hot bend pipe is DN200( Φ All tensile specimens below 219.1 mm) are taken longitudinal samples with diameter of DN200( Φ 219.1 mm) or more specimens shall be taken in transverse and longitudinal directions.

Requirements for the size of tensile specimens of hot bend in different standards

GB/T 228.1—2010

The type of specimen used in tensile test of hot bend is specified in Appendix E of GB/T 228.1-2010 standard. The shape of longitudinal arc sample is shown in Figure 2, and the processing dimensions of the sample are shown in Table 1. The transverse test specimen of tensile test is recommended to be carried out according to Appendix D of GB/T 228.1-2010. See Table 2 and table 3 for the processing dimensions of the sample, and the shape of transverse sample is shown in Figure 3.

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Figure.2 Schematic diagram of longitudinal arc tensile sample
Table.1 dimensions of longitudinal arc tensile specimens

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Note: if there is no specific provision in relevant product standards, the proportion sample with proportion coefficient k = 5.65 is preferred.
Table.2 dimensions of tensile proportional specimens of rectangular cross section

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Note: if there is no specific provision in relevant product standards, the proportion sample with proportion coefficient k = 5.65 is preferred.
Table.3 dimensions of non proportional specimens for rectangular cross section tensile

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Figure.3 Schematic diagram of transverse tensile sample

ASTM A370—2016

The type of test sample used for the pipe is specified in Appendix A2 of ASTM a370-2016 standard. The longitudinal plate tensile test sample is shown in Figure 4. See Table 4 for the dimension and tolerance of the longitudinal tensile specimen of tubular products.

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Figure 4 Schematic diagram of longitudinal plate tensile sample
Table.4 dimensions and tolerances of longitudinal tensile specimens of tubular products

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Note: 1 in = 25.4 mm.
ASTM a370-2016 specifies that 4 # specimens shall be used except for the capacity of the sample device or the size and nature of the tubular products to be tested, except for the 1 # 2# or 3 # test pieces in Table 4.
The dimension of the transverse tensile specimen machined by cutting the ring from the bend product is shown in Figure 5.

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Figure 5 dimension of transverse tensile specimen cut from bend products

Comparison of tensile sample sizes in different standards

with Φ 219 mm × The 8 mm hot bend is compared and analyzed as an example. When plate tensile test sample is used, if GB/T 228.1 is used, longitudinal tensile sample shall be in accordance with S6 in Table 1, and transverse sample shall be short proportion sample with standard distance as per P11 in Table 2. If r = 20 mm is taken and length of clamping end is 75 mm, the total length of longitudinal test sample is obtained, lt ≈ 310 mm, and the total length of transverse sample LT ≈ 295 mm; If the non proportional test sample is used for transverse tensile test, the total length of the sample, lt ≈ 256 mm, shall be in accordance with p15 in Table 3.
same Φ 219 mm × When the 8 mm hot bending pipe is used, if the plate tensile test sample is used, if ASTM A370 is followed, the transverse and longitudinal tensile specimens are processed according to 4 # test pieces and figure 5 in Table 4, and the length of clamping end is 75 mm and the total length of the sample LT ≈ 250 mm.
From the above comparison, if the sample processed according to GB/T 228.1 is processed according to the proportion sample, the sample size is too large and the longitudinal and transverse dimensions are different. It is not conducive to processing when preparing the sample, and the efficiency will be reduced. The requirements for the degree of the sample tube will also increase during sampling, which increases the manufacturing cost. With the increase of wall thickness, the tensile samples processed according to GB/T 228.1 will be processed, Whether it is a proportional sample or a non proportional sample, the larger the wall thickness of the specimen, the longer the length of the sample, and more stringent the sampling and preparation conditions. If the processing is conducted according to ASTM A370, the requirements of longitudinal and transverse specimens are basically the same, the length of the sample is not affected by the wall thickness, and the length of the sample is shorter than that of GB/T 228.1, which is more favorable for sample preparation.
Compared with the tensile test results of two standard processed samples, there is no obvious difference between yield strength and tensile strength, and the true strength of bend can be measured accurately, but the elongation is different.
According to the requirements of the standard for elongation of hot bend, the elongation under the standard distance of 50 mm is often selected.

Conclusion

  • (1) When the size of the drawing sample is selected, the standard distance of 50 mm required by ASTM A370 is more practical, which can not only save cost, but also improve the processing efficiency, and also facilitate the unified sample size of each bend manufacturer, and facilitate the comparison of tensile test results.
  • (2) According to the practical experience and the requirements in Table 1, the pipe diameter of the hot bending pipe is Φ the size of tensile test sample with 168.3 mm or more shall be 4 × 4 in Table 4, with diameter of Φ 88.9~ Φ 168.3 mm, 3 # test pieces in Table 4 are used, with pipe diameter of Φ 42.4~ Φ 88.9 mm, 2 # test pieces in Table 4 are used, with pipe diameter of Φ 32~ Φ 38 mm, use 1 # sample in Table 4, with diameter of Φ full size longitudinal specimens below 32 mm are more suitable.

Authors: Wang Junhua, Zhang Fufeng, Hu Weijie, Liu Heng, Tang Li

Source: Network Arrangement – China Pipe Bend Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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Reference:

  • [1]ASTM A370—2016,Standard Test Methods and Definitions for Mechanical Testing of Steel Products[S].
  • [2] GB/T 228.1-2010, tensile test of metallic materials Part I: room temperature test method [s]
  • [3] ISO 6892-2016, metallic materials – Tensile testing – Part 1: room temperature test method [s]
  • [4] SY/T 5257-2012, steel induction heating bend for oil and gas transportation [s]
  • [5] GB/T 29168.1-2012, induction heating elbows, pipe fittings and flanges for pipeline transportation system of petroleum and natural gas industry, part I: induction heating bend [s]
  • [6] Cdp-s-ogp-pl-016-2014-3, technical specification for induction heating bend for oil and gas pipeline engineering [s]
  • [7]ASME B16.49—2012,Factory-Made,Wrought Steel,Buttwelding Induction Bends for Transportation and Distribution Systems[S].
  • [8] GB/T 9711-2011, steel pipe for pipeline transportation system of petroleum and natural gas industry [s]
  • [9] API Spec 5l-2012, specification for pipeline steel pipe [s]
  • [10] GB/T 2975-1998, sampling position and sample preparation for mechanical properties test of steel and steel products [s]

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