Welding process for 15CrMo containers

Refer to the relevant technical requirements in the SH-T3520-2004 Welding Code for Chromium-Molybdenum Heat-Resistant Steels in the petrochemical industry, carry out proper welding procedure qualification; the welding procedure qualification that passes the evaluation can be used

Preheat to 150–300 degrees, and carry out stress-relief treatment after welding; it’s a very mature process.

How is preheating carried out and the interlayer temperature maintained during welding? What types of fixtures are commonly used?

1) Pressure vessels made of 15CrMo are generally quite thick, so preheating is necessary. The preheating parameters should comply with the requirements of JB4709-2000; the temperature is usually between 150 and 200 degrees. Preheating is required both for the inner and outer surfaces. If manual welding is used, workers can take turns carrying out the welding work. A single-sided groove can also be used, with TIG welding used for the root pass; the workers carry out welding on the outside of the cylinder. However, the quality of welding on the inside may not be as good as that achieved through double-sided manual welding, so it is sometimes necessary to go inside to check and then apply TIG welding again. 2) Chromium-molybdenum steel has a strong tendency to harden upon welding, which makes it prone to the formation of cold cracks, especially delayed cracks; therefore, after the groove is prepared, MT testing is usually carried out to ensure that no defects are present before welding proceeds. If this is the first time you are manufacturing such equipment, conduct a dry magnetic particle inspection after the base construction is completed to ensure that there are no crack-like defects at the root. RT is performed 24 hours after welding, and MT is also added to the inner and outer surfaces. 3) CO2 gas shielded welding is not recommended for pressure vessels, as the mechanical properties of the welds produced by this method are somewhat inferior; its use is also not advised in China.

Welding procedure qualification for thick-walled high-pressure pipes made of 15CrMo steel. Abstract: 15CrMo steel is a heat-resistant steel with a pearlitic microstructure. In accordance with the special requirements for on-site welding and welding quality, the welding process for 15CrMo thick-walled high-pressure pipes was studied. Welding procedure qualification tests were conducted by using different welding materials and processes, and Scheme I was determined to be the optimal welding process. Keywords: pearlitic heat-resistant steel, thick-walled high-pressure pipes, welding process. During the technical renovation of our plant’s nitrogen synthesis tower, there are multiple high-pressure welds on the pipe running from 105D to 123C; the material used for these pipes is 15CrMo steel, with dimensions of φ14″×3/4″. The medium flowing inside these pipes is a mixture of hydrogen and ammonia nitrogen, at a working pressure of up to 150 Kg and a working temperature of up to 500°C. To ensure welding quality, different welding materials and processes were selected for welding procedure qualification tests, providing reliable welding procedure data for on-site welding. 1 Weldability of 15CrMo steel: 15CrMo steel is a heat-resistant steel with a pearlitic structure; it exhibits high thermal strength at high temperatures (δb≥440 MPa) and good oxidation resistance, as well as certain resistance to hydrogen corrosion. Due to the high contents of Cr, C, and other alloying elements in steel, it has a pronounced tendency to harden, resulting in poor weldability. The chemical composition of 15CrMo steel is as follows: C: 0.16%, Si: 0.27%, Mn: 0.55%, Cr: 0.95%, Mo: 0.50%, S≤0.40%, P≤0.035%. Its carbon equivalent, calculated using the formula recommended by the International Welding Institute 11W: Ceq = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15, is 0.545%. Based on experience, when Ceq exceeds 0.4%, there is a high tendency for the welded joint to harden, which can lead to cold cracks. Since the Ceq value for 15CrMo steel is 0.545%, this steel has a high tendency to harden and poor weldability. Therefore, when welding 15CrMo steel, it is crucial to select appropriate welding materials and implement strict welding procedures in order to prevent cracks and ensure satisfactory performance. 2 Welding Process 2.1 Welding Materials Considering the weldability of 15CrMo steel and the operating conditions of high-pressure pipelines in field applications, and based on past experience as well as welding procedure cards provided by foreign sources, we selected two approaches for welding tests. Plan I: Welding preheating is employed, using ER80S-B2L wire for root welding with T1G process, and E8018-B2 electrode for surfacing welding by shielded metal arc welding; subsequent local heat treatment is carried out after welding. Option II: Use ER80S-B2L welding wire for root welding with T1G process, and E309Mo-16 electrode for fill welding with arc welding; no heat treatment is performed after welding. The chemical composition and mechanical properties of welding wires and electrodes are shown in Table 1. Table 1 Chemical composition and mechanical properties of welding materials
Model C Mn Si Cr Ni Mo S P δb/Mpa δ,%
ER80S-B2L: ≤0.05, 0.7, 0.4, 1.2, <0.2, 0.5, ≤0.025, ≤0.025, ≤500, 25
E8018-B2: 0.07, 0.7, 0.3, 1.1, 0.5, ≤0.04, ≤0.03, 550, 19
E309Mo-16: ≤0.12, 0.5–2.5, 0.9, 22.0–25.0, 12.0–14.0, 2.0–3.0, ≤0.025, ≤0.035, 550, 25
2.2 Pre-welding preparations
The test pieces were made from 15CrMo steel pipes with a diameter of φ325×25; the groove type and dimensions are shown in Figure 1. Before welding, use an angle grinder to grind the inside and outside of the groove as well as the area within 50 mm of the groove edges until a metallic shine is visible, then clean it thoroughly with acetone. The test piece is fixed horizontally, with a gap of 4 mm between the mating surfaces. Six spot welds are made evenly around the perimeter using manual TIG welding, with each spot weld having a length of not less than 20 mm. The welding electrodes are baked in accordance with the specifications in Table 2. Table 2 Welding rod baking specifications. Welding rod type, Baking temperature, Holding time: E8018-B2 – 300 ℃ for 2 hours; E309Mo-16 – 150 ℃ for 1.5 hours. 2.3 Welding process parameters: Preheating is required before welding according to Plan I. The formula for calculating the preheating temperature proposed by Tto-Bessyo and others is: To = 350√-0.25 (℃), where To represents the preheating temperature in ℃. =x p p=0.005Sx x=C (Mn Cr)/9 Ni/18 7Mo/90 Where, x—is the carbon equivalent of the alloy composition ； p——dimensional carbon equivalent ； S – thickness of the test piece (in this text, S = 25 mm); x = C(Mn Cr)/9 + 7/90Mo = 0.361, p = 0.045; therefore To = 138°C. Hence, the preheating temperature is set at 150°C. The specimen is heated using an oxygen-acetylene flame. First, a temperature probe is used to roughly determine the surface temperature of the specimen (by estimating it based on the speed of change in the color of the probe’s indication), and then a semiconductor thermocouple is used for precise measurement. At least three measurement points should be selected to ensure that the entire specimen reaches the required preheating temperature. During welding, the first layer is laid using manual TIG welding as a base layer. To prevent depressions on the back side of the weld in areas where welding is performed in an upward position, the wire feeding method of internal feeding is used, that is, the welding wire is fed into the tube through the alignment gap. The remaining layers are welded using shielded metal arc welding; a total of 6 layers are welded, with one weld pass per layer. The welding process parameters for Plan I and Plan II are shown in Tables 3 and 4. Welding according to Plan I Table 3 Welding process parameters for Plan I Weld bead name Welding method Welding material Weld rod specification/mm Welding current/A Arc voltage/V Preheating and interpass temperature Heat treatment specifications Root pass Tungsten inert gas welding ER80S-B2L φ2.4 110 12 Fill pass Shielded metal arc welding E8018-B2 φ3.2 5 85–90 23–25, 150°C 715. ×75 min Cover layer Shielded metal arc welding E8018-B2 φ3.2 5 85–90 23–25 Table 4 Welding process parameters for Scheme II Weld bead name Welding method Welding material Weld rod specification/mm Welding current/A Arc voltage/V Preheating and interpass temperature Heat treatment specifications Root pass Tungsten inert gas arc welding ER80S-B2L φ2.4 110 12 Fill pass Shielded metal arc welding E309Mo-16 φ3.2 90–95 22–24 / / Cover layer Shielded metal arc welding E309Mo-16 φ3.2 90–95 22–24 During welding, the interpass temperature should be no lower than 150°C; to prevent the specimen from cooling down due to interruptions in welding, two welders should take turns performing the welding, and insulation measures should be applied immediately after welding to allow for gradual cooling. 2.4 Post-weld heat treatment: For specimens welded using Method I, local high-temperature tempering treatment should be carried out after welding. The heat treatment process is as follows: the temperature is raised at a rate of 200°C/h, reaching 715°C where it is held for 1 hour and 15 minutes; then the temperature is lowered at a rate of 100°C/h, and after reaching 300°C, the material is cooled in air. Specifically, a JL-4 type crawler-type electric heater (1146×310) is used to wrap around the weld, with an aluminum silicate cotton layer providing insulation; the thickness of this insulation layer is 50 mm. Temperature control is achieved using a DJK-A type electric heater temperature controller. 3 Welding procedure qualification tests: After welding, the test pieces are subjected to 100% ultrasonic testing in accordance with the JB4730-94 standard \"Non-destructive testing of pressure vessels\", and the welds meet Grade I requirements. Welding procedure qualification tests shall be conducted in accordance with the JB4708 standard \"Welding Procedure Qualification for Steel Pressure Vessels\". The evaluation results are shown in Table 5. Table 5 Results of welding procedure qualification tests Test procedures Tensile test Bend test Impact toughness test aky (J/cm2) Tensile strength δb/Mpa Fracture location Bend angle Face bend Back bend Weld Fusion line Heat-affected zone (HAZ) Procedure I 550/530 Base metal 50° Passable Passable 84.8 162 135.6 Plan II 525/520 Base material 50. Pass Pass 79.4 109.2 96.7 The results of the tensile tests show that in both cases the test specimens broke at the base metal, indicating that the tensile strength of the weld is higher than that of the base metal ； All bending tests passed, indicating good plasticity of the weld. According to the impact toughness test results in Table 5, the impact toughness of Scheme I is significantly higher than that of Scheme II, indicating that the post-weld heat treatment parameters for Scheme I are relatively ideal. The high-temperature tempering not only achieved the goal of improving the microstructure and properties of the joint but also ensured an appropriate balance between toughness and strength. The results of mechanical properties at room temperature show that both of the recommended welding process options can be used for on-site construction. Plan I uses welding rods with compositions similar to those of the base material, ensuring that the properties of the weld match those of the base material. The weld should possess high heat resistance, so as not to be damaged under prolonged use at high temperatures. The difficulty lies in the fact that the post-weld heat treatment specifications are quite strict; improper control of the tempering temperature, holding time, as well as the heating and cooling rates can lead to a decline in the properties of the weld. Plan II uses austenitic stainless steel electrodes for welding; although this eliminates the need for post-weld heat treatment, the different expansion coefficients of the weld metal and the base material can lead to carbon diffusion during long-term operation at high temperatures, which may result in damage to the weld in the fusion zone. Therefore, from the perspective of operational reliability, it is more prudent to use Solution I for welding on-site. 4 Conclusions Both welding methods are feasible for welding thick-walled high-pressure pipes made of 15CrMo steel. To ensure that the weld properties match those of the base material and to achieve high heat resistance, Scheme I is more effective; the key is to strictly control the post-weld heat treatment process. Although Option II eliminates the need for post-weld heat treatment, the possibility of weld failure due to carbon migration and diffusion at high temperatures cannot be ignored; therefore, it should only be adopted cautiously when heat treatment is not possible after welding. Reference 1 Edited by the Harbin Welding Research Institute and the Welding Society. Welding (J). 1990 2 Compiled by the Beijing Technology Association Committee. Practical Welding Manual (M). 1983 3 Mechanical and Electronic Industry, compiled by Chengdu Welding Machine Research Institute. Welding machine (J). 1989, 4, edited by Zhou Zhenfeng from Jilin University of Technology. Principles and Processes of Metal Fusion Welding (M). 1985 5 **Compiled by the Machinery Industry Bureau. Welding procedure qualification for steel pressure vessels