15CrMo steel series pearlite structure heat-resistant steel has high thermal strength (δ b ≥ 440MPa) and oxidation resistance at high temperatures, and has a certain ability to resist hydrogen corrosion. Due to the high content of Cr, C, and other alloying elements in steel, the hardening tendency of the steel is more obvious and the weldability is poor.

Welding procedure qualification test

After welding, the test piece shall undergo 100% ultrasonic testing according to JB4730-94 "Non destructive Testing of Pressure Vessels" standard, and the weld seam shall be qualified as Grade I. Conduct welding procedure qualification tests in accordance with the JB4708 "Welding Procedure Qualification for Steel Pressure Vessels" standard. The evaluation results are shown in Table 5.

Table 5 Welding Procedure Qualification Test Results

Test plan: Tensile test, Bending test, Impact toughness test, aky (J/cm2)

Tensile strength δ b/Mpa, fracture location, bending angle surface, bending back, welding seam, fusion line, heat affected zone (HAZ)

Plan I 550/530 base metal 50. Qualified 84.8 162 135.6

Scheme II 525/520 base metal 50. Qualified 79.4 109.2 96.7

From the results of the tensile test, it can be seen that all the tensile specimens of the two schemes broke at the base metal, indicating that the tensile strength of the weld is higher than that of the base metal; All the bending tests are qualified, indicating that the plasticity of the weld seam is good. According to the impact toughness test results in Table 5, it can be seen that the impact toughness of Scheme I is significantly higher than that of Scheme II, which proves that the post weld heat treatment specification of Scheme I is relatively ideal. High temperature tempering not only improves the joint structure and performance, but also makes the toughness and strength match appropriately. According to the mechanical performance results at room temperature, both recommended welding process schemes can be used for on-site construction. Scheme I uses welding rods with similar composition to the base metal, and the weld performance matches the base metal. The weld should have high thermal strength and be not easily damaged after long-term use at high temperatures. The difficulty lies in the strict standards for post weld heat treatment, and improper control of tempering temperature, insulation time, heating and cooling rates can actually lead to a decrease in weld performance. Scheme II uses austenitic stainless steel welding rods for welding, which can eliminate post weld heat treatment. However, due to the different expansion coefficients between the weld and the base material, carbon diffusion and migration can occur during long-term high-temperature operation, which can easily lead to damage of the weld in the fusion zone. Therefore, from the perspective of reliability, it is more reliable to use Scheme I for welding on site

Both welding schemes are feasible for welding thick walled high-pressure pipes made of 15CrMo steel. In order to ensure that the weld performance matches the base material and has high thermal strength, adopting scheme I yields better results. The key is to strictly control the post weld heat treatment process.

Although Scheme II can eliminate post weld heat treatment, the possibility of carbon migration and diffusion in the weld at high temperatures leading to weld damage cannot be ignored. Therefore, it should only be used with caution when post weld heat treatment is not possible.