Operating temperature of Q235B steel plate pipes

In Teacher Ying’s responses to questions regarding GB20801-2020, the following view was expressed: Regarding GB/T 20801.2-2020, if carbon steel of grade Q235B is used, is there a restriction that the design pressure cannot exceed 1.6 MPa? In the original GB 20801.2-2006, Clause 6.2(c) clearly states that Q235B killed steel can be used under conditions where the design pressure does not exceed 3.0 MPa and the design temperature does not exceed 350°C. Why was this clause removed in the 2020 version? Answer: Q235B can only be used as a billet in the field of pressure pipelines. With the formulation of GB/T13401, billets are no longer used as material standards in any standards for pipeline components, hence it is not included. Moreover, in the past there were standards for welded pipe fittings such as GB/T 12459 and 13401. In GB/T 3091, welded pipe billets were classified as Q235B; however, the technical requirements for these products remained at Grade A, so the welded pipes could still only be considered as Q235A. Although welding pipe standards such as GB/T 3091 and SY/T 5037 include room-temperature impact testing as a specified requirement, GB/T 20801, as the basic standard for industrial pipes, can determine the scope of use for such products only based on the mandatory requirements of product standards. Question ①: Why isn’t Q235B mentioned in GB/T 20801.2-2020 materials? And isn’t there a table of allowable stress values either? Why is this? ②In design, Q235A is commonly used for non-toxic and non-flammable liquid media such as water. However, the coke oven gas in coking plants has a diameter of two to three meters and is not under much pressure, so Q235B is used instead. GB/T 20801-2006 specified usage restrictions for grade Q235B, as well as allowable stress values; these are not specified this time. What is the reason for that? Is it believed that for plate-welded pipes, Q235A should be used when they are non-flammable, and L245 should be used for pipes handling other types of media? Is that correct? ③For coal gas pipelines that are not under pressure, is it possible to avoid using GB/T 20801 and instead use Q235B? Answer: Coal gas pipelines, regardless of the level of pressure they are under, still fall under the category of pressure pipelines for combustible gases (as defined by GB/T 20801, rather than the definition applied to pressure pipelines for special equipment). Therefore, it is recommended to adhere to the standards applicable to the construction of industrial pipelines. For non-standard pipe fittings, such as shrimp bends, if Q235B is still to be used, its application scope can be referred to the provisions of GB/T 20801-2006, and its allowable stress value can be selected based on that of Q235A. When using B-grade killed steels such as Q215B and Q235B, the design pressure should not exceed 3.0 MPa, and the design temperature should be no higher than 350°C and no lower than the temperature indicated by Curve A in Figure 1 (or Table 1) of GB/T 20801.2-2020. The thickness should not exceed 20 mm. For 21/4Cr-1Mo steel and other 2Cr-grade alloy steels, when the operating temperature exceeds 454°C, the carbon content in the weld metal must be greater than 0.05% ； The aforementioned opinion is because GB/T 20801.2-2020 lists Q235B as a steel grade for steel plates or strips under GB/T 700, solely on the grounds that Q235B is used only as a billet; it is not included in the product standards for pipe components. Therefore, GB/T 20801.2-2020 does not list this material grade. For non-standard pipe components such as shrimp-shaped bends, GB/T 700 Q235B steel plate can still be used as the material grade, just like in the case of pressure vessels. It should be noted that since Q235B, which has requirements for impact resistance at room temperature, has been selected, both the welding procedure qualification for these pipeline components and the impact resistance requirements at room temperature for such components must comply with the corresponding requirements of GB/T 20801.2-2020. Based on the positioning of Q235B in the aforementioned pipeline specifications, the following question may arise: Why are non-standard pipeline components made from Q235B subject to restrictions regarding pressure, temperature, and wall thickness, even though their welds and the base material itself pass the tests for impact resistance at room temperature? Moreover, their minimum operating temperature is determined according to Curve A in Table 1 of GB/T 20801.2-2020. For welded pipe fitting manufacturers that use Q235B steel plates as raw material to produce large-diameter welded elbows of type CF 415K in accordance with GB/T 13401, there are no restrictions on their scope of application, and the minimum allowable operating temperature is determined according to curve B. The reason for this is that the difference between the two lies in the fact that: 1. The welds in the latter case must pass radiographic inspection at 100, and the products must undergo overall heat treatment. 2. For the former, the inspection rate for steel plates is based on the same batch number, with batches being no larger than 60 tons; whereas in pipe manufacturing plants, the batch sizes are much smaller than those in steel mills, which accordingly improves reliability.

According to the provisions of GB/T 3274-2007, the operating temperature range for Q235B steel plates is 20°C to 300°C, whereas the operating temperature range for Q235C steel plates is 0°C to 300°C. Given that the operating temperature of the water pipes in your project is 7 degrees, it can be considered that this temperature falls within the range suitable for Q235B steel plates. When selecting materials for water pipes, factors such as the operating pressure and temperature of the pipes, the properties of the fluid flowing through them, and the diameter of the pipes need to be taken into account. In the situation you mentioned, where the water pipes have a diameter of less than DN300, 20# seamless steel pipes in accordance with GB8163 are used, which is appropriate for normal operating conditions; indeed, 20# seamless steel pipes possess good mechanical properties and sealing capabilities, making them suitable for transporting various fluids. For water pipes larger than DN350, you mentioned using Q235B welded steel pipes. Under normal circumstances, steel pipes made of Q235B material possess good plasticity, toughness, and weldability, making them suitable for the construction of general engineering structures and the manufacture of ordinary mechanical parts. In the specific circumstances of your project, a temperature of 7 degrees falls within the allowable range, so from a temperature perspective, it is feasible to use Q235B welded steel pipes. However, it should be noted that when selecting pipe materials, various factors such as the design pressure of the pipe, the properties of the fluid, and the stresses acting on the pipe must also be taken into consideration. In practical applications, strength calculations and corrosion resistance analyses may also be required to ensure that the pipes meet the safety and reliability requirements of the project. Therefore, it is recommended to consult relevant pipeline design and engineering experts before making a final decision on the pipeline material, in order to carefully assess the specific engineering conditions and standards. .

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