Interpretation of GB/T 713.1~7-2023 standards

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The new standard GB/T 713.1~7-2023 \"Steel plates and strips for pressure equipment\" will come into effect on March 1st next year, officially replacing existing standards such as GB/T 713-2014 and GB/T 24511-2017; it holds great significance for the pressure vessel industry. To date, GB/T 713 has issued the following parts: Part 1: General requirements; Part 2: Non-alloy and alloy steels with specified temperature properties, replacing GB/T 713-2014 and GB/T 35012-2018; Part 3: Low-alloy steels with specified low-temperature properties, replacing GB/T 3531-2014; Part 4: Nickel alloy steels with specified low-temperature properties, replacing GB/T 24510-2017; Part 5: High-strength steels with specified low-temperature properties; Part 6: Quenched and tempered high-strength steels, replacing GB/T 19189-2011; Part 7: Stainless steels and heat-resistant steels, replacing GB/T 24511-2017. Changes in steel plates and strips for pressure equipment: 1. In Part 1: General requirements, a definition of the heat-treated condition at delivery has been introduced. 2. Part 2: Non-alloy and alloy steels with specified temperature properties – This part specifies steel strips with a thickness of not more than 25.4 mm, as well as shear plates, used for manufacturing pressure equipment operating in the range of -20 to 20°C, as well as single-rolled steel plates with a thickness of 3 to 250 mm. 3. Part 3: Low-alloy steels specifying low-temperature properties – This part specifies low-alloy steel sheets with a thickness of 5–120 mm, used for manufacturing pressure vessels operating at temperatures between -70 and -20°C. 4. Part 4: Nickel alloy steels specifying low-temperature properties – This part specifies nickel alloy steel plates with a thickness not exceeding 150 mm for use in liquefied gas storage and transportation systems. 5. Part 5: High-manganese steel specifying low-temperature properties. This part is newly added: it specifies steel plates with a thickness of 5–60 mm for manufacturing low-temperature pressure vessels at temperatures ranging from -196 to -20°C. 6. Section 6: Quenched and tempered high-strength steel. This section specifies the quenched and tempered high-strength steel plates and strips used for pressure-bearing equipment with a thickness of 10–60 mm. Compared to GB/T19189-2011, the grades have been renamed. Based on welding characteristics, a special code H is introduced to represent steels requiring high wire energy welding, and the codes DEF are introduced. 7. Part 7: Stainless steels and heat-resistant steels. This part applies to hot-rolled stainless steel and heat-resistant steel sheets and strips, as well as cold-rolled stainless steel and heat-resistant steel sheets and strips, for use in pressure vessels with a width of not less than 600 mm. The new standard introduces a new type of surface treatment, BA: cold rolling and bright annealing. After cold rolling, bright annealing is carried out in a controlled atmosphere furnace, usually using a dry hydrogen atmosphere or a mixture of dry hydrogen and dry helium, to prevent oxidation during the annealing process; this is also a common surface treatment method used in subsequent processing steps. Bright annealing of stainless steel sheets and steel pipes: 1. By controlling the temperature in multiple zones within the furnace and employing strong convective cooling in the cooling stage, cooling can be adjusted according to different steel grades and requirements, thereby eliminating work hardening and achieving a satisfactory microstructure. The bright annealing heat treatment process for 300 series austenitic stainless steels is a solution treatment process. During the heating process, the carbides are dissolved into austenite; the heating temperature is around 1000°C to 1150°C. The material is held at this temperature for a suitable short period of time to allow the carbides to dissolve in austenite, after which it is rapidly cooled below 350°C, resulting in a supersaturated solid solution, namely a homogeneous single-phase austenitic structure. The key to this heat treatment process is rapid cooling, requiring a cooling rate of 55°C/s in order to quickly pass through the temperature range for the reprecipitation of carbides after their dissolution (550°C–850°C). The holding time should be kept as short as possible; otherwise, the grain size will increase, affecting the surface finish. 2. It is possible to obtain a surface that is free of oxidation, bright, and has good corrosion resistance. Since bright annealing of stainless steel involves heat-treating strip under a specific protective atmosphere, by strictly controlling the protective atmosphere within the furnace—particularly its purity, residual oxygen content, and dew point—it is possible to restore slightly oxidized materials to their bright state, ultimately yielding a surface that is both oxide-free and shiny. Compared with the surface obtained through ordinary annealing and pickling, the absence of an oxidation process reduces chromium depletion on the strip surface, resulting in better corrosion resistance than that of 2B steel after polishing. 3. Brightly treated stainless steel workpieces can retain the smoothness of their rolled surface, allowing for a bright finish without the need for further processing. Due to the retention of the original metallic luster on the surface after bright annealing, a mirror-like smooth surface is achieved; under normal requirements, this surface can be used directly as the final surface of the workpiece. And no processing is required, **saving polishing costs. 4. Since stainless steel workpieces after bright annealing do not require any further pickling or similar treatments, no media such as acids are used; thus, there is no environmental pollution caused by pickling.