Taking the example of ultra-low carbon urea 316L Mod stainless steel pipeline welding, the welding process and process management of ultra-low carbon stainless steel pipeline are systematically introduced.
1. Introduction
Zephyr Petrochemicals daily production of 1,050 tons of urea plant, the introduction of the Dutch Stamicarbon CO 2 Steam extraction production process, with high temperature, high pressure, flammability, risk of explosion, corrosion and other high-tech characteristics, difficulties in construction. The main materials of this device are carbon steel (API 5LB, A53B, A106B), austenitic stainless steel (A312 TP304L, A312 TP304, A312 TP316L, 316L Mod), chrome molybdenum steel (A335 P12), low temperature steel (A333 GR6, A333 GR1) and so on. The main working media are methylammonium, urine, liquid ammonia, carbon dioxide, steam, condensate and so on. The maximum working pressure of the entire process is 15MPa and the maximum working temperature is 385°C. For the whole device, Mod 316L low carbon urea stainless steel pipeline welding is the key. 316L Mod material is low carbon stainless steel; For the first time in our company's construction industry, there is no mature technology to learn from. Welding ultra-low carbon 316L Mod Urea stainless steel pipelines requires an advanced and reasonable welding process on the one hand; On the other hand, excellent welder quality requires the necessary good management and comprehensive process control. Process control. This document discusses the welding technology of 316L Mod Urea ultra-low carbon stainless steel pipeline based on the actual construction site and the combination of relevant information.
2. Technical Requirements for 316L Mod Urea Ultra Low Carbon Stainless Steel Pipes and Welding Consumables
The production of liquid ammonia urea, urinary solution, methylammonium solution, etc. are corrosive media, of which methylammonium solution is the most corrosive media at high temperature and high pressure. Stainless steel has a strong ability to intergranular corrosion. When the carbon content of ultra-low carbon stainless steel is less than 0.03%, the carbon content is extremely low due to the extremely low carbon content, so that the stainless steel in the sensitized state (sensitization temperature range of the austenitic stainless steel between 450- 850°C) has the intergranular precipitation of Cr23C6 chromium carbide is significantly reduced and has good resistance to intergranular corrosion. In addition, the use of base materials and welding materials with higher chromium content can cause the surface of the stainless steel body to produce dense oxide film organization, thereby improving corrosion resistance, such as 25Cr type materials -22Ni-2Mo, which achieve more satisfactory corrosion resistance can.
This device is used for methyl ammonium fluid and other highly corrosive media and piping component materials for mod 316L. The main specifications are Φ273.1×25.4, Φ168.3×15.88, Φ89×11.13, Φ60.3×8.74 etc. The welding wire is R25.22.2LMN (Φ1.6), which is the E25.22.2LMNB welding rod.
(Φ2.5, Φ3.25, Φ4.0). Requirements for the chemical composition of pipe and piping components and welding materials are listed in Table 1 and Table 2:
Table 1 Chemical composition of tubes and piping components
| Chemical composition | W | Mn | Yes | s | P | Cr | No | Mo | N |
| Content% | ≤ 0.02 | ≤ 2.0 | ≤ 0.4 | ≤ 0.01 | ≤ 0.02 | 24-26 | 21-23 | 1.9-2. 3 | 0.10-0.16 |
Table 2: Chemical composition of welding materials
| Chemical composition | W | Mn | Yes | s | P | Cr | No | Mo | N |
| Content% | ≤ 0.04 | ≤ 3.0 | ≤ 0.5 | ≤ 0.02 | ≤ 0.03 | ≥24 | ≥21 | 1.9-2.7 | ≤ 0.20 |
Pipes and tube connections, as well as welding materials made of austenite-forming elements (Ni, C, N, Mn, etc.) and ferrite-forming elements (Cr, Mo, Si, etc.) must be balanced so that after welding one is formed completely austenitic organization, the maximum ferrite content is 0.6%. The ferrite content is determined using a ferrite measuring device. Ferrite content in unqualified pipes, pipe components and welding materials is strictly prohibited in construction.
To ensure the welding quality of 316L Mod Urea Ultra Low Carbon Stainless Steel Pipeline Urea Plant before construction by external welding engineers under the supervision of 316L Mod Urea Ultra Low Carbon Stainless Steel Pipeline Welding Process Evaluation The evaluation of the welding process of ultra-low carbon 316L Mod Urea stainless steel is carried out through the implementation of the ASME 1995 standard. The project evaluation is presented in Table 3.
Table 3: Design table for evaluating the welding process
| Welding Materials | Specifications | polarity | Welding current (A) | Welding voltage (V) | Welding speed (cm/min) |
| R25.22.2LMN welding wire | Φ1.6 | DC direct connection | 60-90 | 12 – 16 | 4 – 7 |
| E25.22.2LMN welding wire | Φ2.5 | DC rear connection | 60-90 | 20-22 | 4 – 7 |
| Φ3.25 | Reverse DC Connection | 80-120 | 22-26 | 5 – 8th | |
| Φ4 | DC rear connection | 120-150 | 24-28 | 5 – 10 |
When evaluating the welding process of 316L Mod Urea Ultra Low Carbon stainless steel, all welding parameters are recorded (including current, voltage, polarity, specifications of welding consumables, shielding gas and back shielding gas flow, etc. ). Evaluation of tensile test, ultimate bending test, back bend test, Hugh test, metallographic test and ferrite detection. The evaluation of the test sample location is shown in Figure 1.
Figure 1: Map showing sampling locations to evaluate the welding process
The evaluation test methods and evaluation results of 316L Mod ultra-low carbon stainless steel welding processes have been recognized by foreign welding engineers. 4.
4. Assessment of welder skills
Given the difficult construction of 316L Mod Urea Ultra-Low Carbon stainless steel, high technology was recognized in the construction preparation phase through the evaluation of the welding process and welding procedure preparation (WPS) by foreign welding engineers. Under the supervision of foreign welding engineers, the welders were assigned to carry out an evaluation of their welding operation for fixing pipe slopes in position 6G, argon electrical joint welding (Φ168.3 × 15.88) and welding a argon arc (Φ60.3 × 8.74). The hardest part of evaluating skills is not allowing for concavities. The key is the counterspace and wire feed method. The evaluation of qualification standards for appearance test, χ-ray test, penetration test, ferrite test and macrometallurgical examination of the weld cross section were all qualified, with none. We also performed a weld rework test after applying the first layer of filler welding. The rework position is shown in Figure 2. If there are still unacceptable defects after rework, e.g. B. a new failure, the welder designer is not authorized to participate in the welding rework work.
Figure 2: Location plan for welding rework
Participate in skills assessment (including rework welding). Welders are assessed once they are qualified, they have an operating license signed by the foreign welding technician and the welders are qualified to work. The welder competency assessment for Mod 316L ultra-low urea stainless steel construction establishes a solid foundation.
5. Welding Procedures
(1) Environmental control
Strict environmental control was carried out during the welding of ultra-low carbon 316L Mod Urea stainless steel. A wind and rainproof cover has been made and a person has been prepared to monitor the welding environment to ensure that the welding wind speed is less than 3 m/s for manual arc welding and less than 0.5 m/s for manual arc welding. s for argon arc welding.
(2) Welding consumable materials management
Correct use of welding consumables is a prerequisite for the quality of welding 316L Mod Urea Ultra-Low Carbon stainless steel. In this sense, we have established a rigorous storage, drying, distribution and recycling system so that welding consumables can be tracked from storage, drying and distribution to recovery throughout the entire process. Welding consumables in accordance with the brand number, specifications, shelves, with obvious brand number signage, specifications and number of characters. Welding consumables must be used by the welder to fill out the welding consumables application form and specify the welding part. Then, after inspection by the welding technician and confirmation by the welding inspector, the administrator issues the registration form according to the form. When receiving welding rods, an insulated bottle marked for welding rods must be used. The same thermos cannot be mixed with two types of welding rods. Throughout the construction process, State spending and use are always controlled.
(3) Pipe materialization and chamfer processing
Pipe feeding occurs according to the single-line diagram, pipes of the same specification are discharged uniformly. Pipe materialization takes into consideration the requirements of pipe slope and welding channel grouping and uniform numbering to ensure that the correct number is not good, there is no leakage and no waste.
Cutting of 316L Mod Urea Ultra Low Carbon Stainless Steel Tube must be done by mechanical methods and cannot be done by plasma arc, gas cutting or carbon arc gas cutting. When machining the chamfer, it should be kept in mind that the thermal conductivity of 316L Mod Urea ultra-low carbon stainless steel is low and the linear expansion coefficient is large. To reduce welding deformation, the slope of the chamfer and the bottom of R are reduced accordingly. The beveled surface must be flat and smooth; there should be no cracks, burrs, delamination or other defects.
Considering the characteristics of ultra-low carbon 316L Mod Urea stainless steel during processing, to avoid carburization, a special prefabrication area is set up, completely isolated from the carbon steel and alloy steel tubes. The prefabrication platform is composed of wooden or stainless steel panels. Nylon or fiber slings are used for handling. Stainless steel processing tools are specifically designed and should not be mixed with carbon steel and alloy steel pipe processing tools. The grinding wheel used for grinding consists of a special grinding wheel made of rubber and nylon. This prevents contamination due to carburization.
Another requirement: sanding must be done carefully to avoid bluing the base material. Wrap the chamfer after processing to avoid carburization caused by collision contamination. Before welding, clean the chamfer and surrounding areas with acetone, remove grease, and wrap both sides of the chamfer with asbestos plate to prevent carburization and spatter from sticking to the base material and selectively corroding during welding.
(4) Weld grouping and position welding
Before nesting, the chamfer dimensions are checked and the ferrite content is checked. Chamfers with δ ≥ 12 mm are tested by penetrant test.
Due to the welding mobility of 316L Mod stainless steel liquid metal with ultra-low carbon urea, the wetting is poor and the melting depth is small. To avoid defects such as insufficient welding, the grouping must be flush with the interior wall; the amount of inner wall on the wrong side cannot be more than 0.5 mm. To avoid internal concavity, an accurate cluster gap is required. Processing and manufacturing of 316L stainless steel Mod 300×20×2.5 and 200×15×2.0, two specifications of fixed pitch grouping block.
Ultra-low carbon 316L Mod Urea stainless steel is positioned using argon arc welding. The welding process and formal welding procedure are the same. As part of the formal welding positioning component, the weld must be fully penetrated and the fusion must be good, without porosity and other defects. To ensure that the lower weld channel is well formed and the stress concentration is reduced, the weld must have a smooth transition to the base material and the ends of the weld must be polished until they are angled. In formal welding, the starting point is between the two positioning welds.
If possible, do not use devices welded directly to the base material to avoid damage to the base material. The fastening welded to the base material must be made of ultra-low carbon 316L Mod Urea stainless steel. To avoid carburization, do not use carbon steel or alloy steel tubes. A special grinding wheel for stainless steel must be used to remove the clamps. Hitting, breaking, twisting or other methods are not permitted. After removal by grinding and leveling, dents require welding and penetration testing. When bundling, strong bundling is strictly prohibited to avoid stress concentration.
(5) Welding procedures
When welding ultra-low carbon 316L Mod Urea stainless steel, line energy input should be minimized and the degree of sensitization of the welded joints should be minimized, thereby improving corrosion resistance. Therefore, a multi-layer, multi-channel, narrow welding process with low line energy, short arc, no vibration or low vibration is required.
Argon arc welding is used for pipes with DN ≤ 50. For pipes with DN > 50, welding is divided into two parts. The root with a thickness of 8mm for the first part is used argon arc welding and the rest of the second part is used manual arc welding. If the fill height of the weld is more than 10 mm, multi-layer welding is carried out, and the welding wide plate stack is not allowed to arbitrarily oscillate.
Multi-layer welding, each weld completely crushes and removes slag after one layer of welding and qualified by quality control before the next layer of welding. Multilayer welding between layers must be staggered. Strictly control the temperature between coats; the temperature between layers must not exceed 150 °C. The temperature between the layers is measured with an infrared thermometer.
When welding pipes with DN ≤ 50, due to concentrated heating and slow heat dissipation, serious grain growth is easy to occur and the tendency of thermal cracking is increased. In this case, you can use a copper cooling pad on both sides of the weld, or wipe the sides of the weld with a damp cloth, or take other measures to force cooling. However, please note that this cannot be done directly on the weld.
During this procedure, attention must be paid to cleaning and protection. Before welding, the welding wire and tungsten electrode must be cleaned and wiped with acetone and grease, and other contaminants must be removed to prevent carburization. During welding, oxidized and burnt parts of the unused wire end can be fused into the solder, which must first be cut or polished to remove. The tungsten electrode must be cleaned again with acetone after grinding. During welding, spraying paint in the work area is not permitted to prevent grease from entering and causing carburization. During the prefabrication and welding process, construction personnel's gloves, clothing and other materials must be kept clean to prevent contamination that affects the quality of welding.
(6) Argon arc welding Argon protection
Argon protection is crucial in argon arc welding. In argon arc welding, welding must be stopped before welding and the gas must be turned on downstream. The shielding gas flow during welding is generally 10 to 15 l/min. To prevent oxidation of the welding channel surface at the back of argon arc welding and deterioration of welding quality, the back of the welding channel must be filled with argon. Argon-filled shielding can be used in whole or in part in two ways. The argon-filled tube should be left at the head of the exhaust port (Φ5–10 mm). The initial flow of the argon fill gas can be increased accordingly to ensure that the air in the tube is completely excluded. The argon flow during welding must be reduced accordingly to obtain a smooth surface and not cause depressions on the back of the weld, which is aided by argon blowing during forming. The rear shield gas flow is generally 20 to 25 l/min. If possible, the argon is partially filled with argon to avoid waste.
The protective effect of argon can be judged by the color change of the solder. During the welding process, the welder can adjust the shielding gas according to the color so that the welding seam has the best protective effect. A comparison between solder color and protection effect is shown in Table 4.
Table 4 Comparison between weld bead color and protection effect
| Solder color | Silver white, golden yellow | Blue | Red Gray | Gray | Black |
| Protective effect | Preferably | Good | Preferably | defective | Worse |
(7) Welding test
Welders on the weld must be 100mm from the side of the weld after self-inspection to mark the code. The marking is done with waterproof paint; the paint must not contain chlorine. Steel sealing on stainless steel pipe is prohibited.
Pipe diameter Φ ≤ 40 mm, wall thickness ≤ 5 mm, final welding for visual inspection, ferrite content test, liquid penetration test. Pipe diameter Φ > 40 mm, wall thickness > 5 mm, bottom weld and first layer of filler after visual inspection, ferrite content test, liquid penetration test, e.g. B. No defects, the chamfered surface should be cleaned with acetone after cleaning the second layer of welding. After the second layer of welding, liquid penetration test, radiation test if there are no defects. After final welding, visual inspection, liquid penetration test and radiation test.
During visual inspection, the transition from the weld to the base material must be smooth and round and must be free of cracks, pores, slag and other defects. The depth of the cutting edge is δ ≤ 0.5 mm and the total length should not exceed 10% of the total length of the weld seam. Requirements for the remaining height of the weld seam: δ1 ≤ 1.5 mm for a base material δ ≤ 12.5 mm, δ1 ≤ 1.5 mm for a base material δ of 12.5-25, δ1 ≤ 2.0 mm for a base material δ ＞ 25, δ1 ≤ 3.0 mm for a base material δ ＞ 25.
Liquid penetration testing uses a water-based paint penetration method with a moisture indicator. The surface temperature of the part is between 15 and 50 °C and the penetration time is at least 15 minutes.
The ferrite content is checked using a ferrite measuring device with probe. The maximum ferrite content must be less than 0.6%. Before testing, clean the calibration test piece, test probe and solder with acetone to ensure they are free from rust, grease and other contaminants to ensure test accuracy. During the testing process, the probe is cleaned frequently to ensure that there are no impurities and to ensure the accuracy of the test value. If the ferrite content at the test point exceeds the standard, the special grinding wheel can be used for grinding by cleaning the grinding point with acetone. Then the test, not yet qualified, should check the base material, welding material ferrite test report, the correct use of welding consumables, and the group to analyze the welding process, develop corrective measures, repair and ensure that the ferrite content meets content area requirements.
(8) Welding repair
Non-destructive testing must be carried out to remove defects from unqualified welds and repair them. Defect elimination is carried out using abrasive wheels cut to a suitable shape for filler welding. It is strictly prohibited to remove defects by planing with carbon arc gas to avoid carburization. Penetration testing must confirm that defects have been eliminated before the weld is reworked. Rework welds are more difficult to weld, with argon arc welding the quality is easier to guarantee and the line power is low. Therefore, rework is carried out using argon arc welding. The number of reworks of the same part must not exceed twice. If rework is carried out more than once, the reasons must be analyzed, measures developed and approval from the person responsible for technology obtained.
6. Conclusion
- (1) With good preparation, the process is correct and reasonable. Large facilities, especially foreign import facilities, cannot carry out technical construction work without familiarizing themselves with construction norms and standards. Based on reading the norms and standards, carry out a good evaluation of the welding process and welder examination, develop a correct and reasonable welding process, and put it into practice to ensure welding quality.
- (2) Strengthening management and strict control of processes. Welded structures require not only an advanced and reasonable welding process and excellent welder quality, but also a strict management system and complete process control to ensure welding quality, avoid rework losses, and achieve half the result with half the effort .
