Post-Weld Heat Treatment Requirements for Carbon Steel Pipes

Abstract: Carbon steel is a type of steel that exhibits good weldability. If appropriate welding materials and processes are selected, the welded joints are unlikely to develop hardened structures or cold cracks. This article analyzes the welding and preheating requirements for carbon steel pipelines.
 

Pipelines made from carbon steel (CS) are widely used in the construction of petrochemical plants. The workload associated with welding and heat treatment of carbon steel during pipeline prefabrication and installation is substantial. The necessity for post-weld heat treatment (PWHT) for carbon steel pipes varies by standard, with differences also existing between domestic and international standards.

 
Stress corrosion cracking can occur in carbon steel pipes under specific media and working conditions. For example, consider the circulating hydrogen pipeline of a hydrocracking unit at a particular refinery. The material used is ASTM A106 Grade B, with a wall thickness of 7.82 mm. Post-weld heat treatment was not performed according to design requirements during construction. During startup, wet hydrogen sulfide stress corrosion cracking occurred at a DN20 socket joint, causing a gas leak containing hydrogen sulfide and hydrogen, which led to the urgent shutdown of the unit and significant economic losses. Implementing post-weld heat treatment for parts that can be exempted or applying excessive heat treatment will increase construction costs and energy consumption and may also degrade material performance. Therefore, it is essential to study the conditions for post-weld heat treatment of carbon steel pipelines and propose specific requirements for those that do require such treatment.
 

Common steel grades or types of carbon steel pipes used in petrochemical plants include 10 steel, 20 steel, ASTM A53 Grade A, ASTM A53 Grade B, ASTM A106 Grade A, and ASTM A106 Grade B. The properties of carbon steel primarily depend on its carbon content. The mechanical properties of 20 steel are equivalent to ASTM A53 Grade B and ASTM A106 Grade B, while the mechanical properties of 10 steel are equivalent to ASTM A53 Grade A and ASTM A106 Grade A. Carbon steel is categorized into ordinary carbon steel, high-quality carbon steel, and high-grade steel based on the phosphorus and sulfur content. The phosphorus (P) and sulfur (S) content in 10 steel and 20 steel, as specified in GB/T 8163-2018 "Seamless Steel Pipe for Fluid Transportation," is slightly higher than in GB9948-2013 "Petroleum Cracking Pipe." The ASTM standards stipulate that the P and S content in the A53 series is higher than in the A106 series, and that the upper limit of P and S content for carbon steel in ASTM standards is higher than the values specified in national standards. Thus, according to domestic classification standards for carbon steel, ASTM A53 Grade A and ASTM A53 Grade B are categorized as ordinary carbon steels, ASTM A106 Grade A and ASTM A106 Grade B as high-quality carbon steels, and 10 steel and 20 steel as high-grade steels.
 

Due to its low carbon content and minimal alloy elements, low carbon steel is highly weldable. Using standard welding methods, it is unlikely to produce hardened structures or cold cracks in the joint. Carbon steel with a carbon content (w(C)) greater than 0.15% is sensitive to hydrogen-induced cracking. During welding, it is important to ensure a low-hydrogen environment. Prior to welding, the weld joint should be free of oil and rust, and low-hydrogen welding materials must be selected. Cold cracks may occur when welding rigid carbon steel structures at low temperatures, so preheating is necessary, and low-hydrogen electrodes should be used for arc welding. The main purpose of preheating is to prevent hydrogen-induced delayed cracking, remove moisture from the groove and adjacent surfaces, reduce the cooling rate, prevent hardened martensitic structures in the welding seam metal and heat-affected zone, and help maintain interlayer temperature. In multi-layer and continuous welding, the rear weld bead helps remove hydrogen and improves the heat-affected zone structure of the previous layer, while the residual heat of the front weld bead preheats the rear weld bead. Multi-layer welding is proven to be an effective method for reducing peak hardness in the heat-affected zone near the fusion line.
 

ASME B31.3-2018 stipulates that for carbon steel pipes with a wall thickness greater than 25 mm, when the carbon content (w(C)) exceeds 0.30%, the minimum preheating temperature is 95°C; when w(C) is less than or equal to 0.30%, the minimum preheating temperature is 10°C. According to GB/T 20801.4-2020, when the minimum tensile strength (Rm) of carbon steel pipe materials exceeds 490MPa, the minimum preheating temperature is 95°C; when Rm is less than or equal to 490MPa, the minimum preheating temperature should follow the provisions in Table 2. GB 50236-2011 specifies that when the minimum tensile strength (Rm) exceeds 490MPa, the minimum preheating temperature is 80°C; when Rm is less than or equal to 490MPa and the wall thickness (δ) is greater than 25 mm, the minimum preheating temperature is 80°C, with no provisions for wall thickness less than or equal to 25 mm. SH/T 3501-2021 states that when the wall thickness (δ) of a carbon steel pipeline exceeds 25 mm, the minimum preheating temperature is 95°C. For commonly used carbon steel pipes in petrochemical plants, such as 20 steel, ASTM A106 Grade B, and ASTM A53 Grade B, with a minimum tensile strength (Rm) less than or equal to 490MPa and a carbon content (w(C)) less than or equal to 0.30%, the minimum preheating temperature during welding can be determined according to the different standards in Table 2. Table 2 shows that for pipelines with a wall thickness greater than 25 mm, the minimum preheating temperature specified in ASME B31.3 is significantly lower than that in other standards.

Table 2 Minimum preheating temperature of carbon steel pipelines commonly used in petrochemical plants