How to determine the heat treatment temperature for carbon steel pipes that require post-weld heat treatment
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The standards specify heat treatment temperature and time requirements only for values greater than 19 mm; so how are the heat treatment conditions determined for values less than 19 mm?
Well, you can refer to GB50235, Table 6.0.10-1 in section 6.10.0-1, which outlines the basic requirements for pipeline heat treatment. Material category, nominal thickness t (mm), minimum specified tensile strength of the material (MPa), heat treatment temperature (°C), holding time (minutes per mm), and minimum total holding time (hours): For carbon steel (C) and carbon-manganese steel, when t ≤ 19, no specific requirements apply; for these materials as well, when t > 19, the heat treatment temperature is 600–650 °C with a holding time of 2.4 minutes per mm, and the minimum holding time is 1 hour. For chromium-molybdenum alloy steels (C-Mo, Mn-Mo, Cr-Mo), when Cr ≤ 0.5%, no specific requirements apply; when Cr > 490, the heat treatment temperature is 600–720 °C with a holding time of 2.4 minutes per mm, and the minimum holding time is 1 hour. When 13 ≤ Cr ≤ 490, the heat treatment temperature is 700–750 °C with a holding time of 2.4 minutes per mm, and no specific requirements apply; when Cr > 13, the minimum holding time is 2 hours. For chromium-molybdenum alloy steels of the Cr-Mo type with 0.5% < Cr ≤ 2%, the heat treatment temperature is 700–750 °C with a holding time of 2.4 minutes per mm, and no specific requirements apply when 13 ≤ Cr ≤ 490; however, when 2.25% ≤ Cr ≤ 3%, the minimum holding time is 2 hours. For all types of chromium-molybdenum alloy steels, the heat treatment temperature is 700–760 °C with a holding time of 2.4 minutes per mm. These are the standard requirements
Um, you can refer to GB50235; Section 6.10.0-1 provides detailed information on the allowable thickness range of the materials, whether heat treatment is required, as well as the temperature and duration for such treatment. . . .
In my opinion, the heat treatment thickness specifications for carbon steel and carbon-manganese steel in the standards take welding into account, as thicker wall thicknesses make welding more difficult. For welds that have undergone stress-relief heat treatment, there is no limit on this thickness; in other words, heat treatment is required regardless of the wall thickness. For carbon steel and carbon-manganese steel, those with a thickness of 19 mm or less can be heat-treated using the same process as those with a thickness greater than 19 mm; after heat treatment, it is sufficient to check the hardness of the welds and the heat-affected zone. That’s exactly what we did in our previous projects, and practical testing showed no issues.
We generally don’t make any requests, as most experienced construction teams are familiar with welding evaluations as well as heat treatment; they should be quite experienced in these areas. Therefore, we let the construction party develop a plan for the heat treatment process, and all we require is an inspection of the hardness after heat treatment.