Welding technology of 316L Mod urea grade ultra-low carbon stainless steel high pressure pipeline

Taking the welding of 316L Mod urea-grade ultra-low carbon stainless steel pipeline as an example, the welding process and process management of ultra-low carbon stainless steel pipeline are systematically introduced.

1. Introduction

Zephyr Petrochemical’s daily output of 1050 tons of urea plant, the introduction of Dutch Stamicarbon CO₂ vapor extraction production process, with high temperature, high pressure, flammable, explosive, corrosive and other characteristics of high technology, construction difficulties. The main materials of this device are carbon steel (API 5LB, A53B, A106B), austenitic stainless steel (A312 TP304L, A312 TP304, A312 TP316L, 316L Mod), chromium-molybdenum steel (A335 P12), low-temperature steel (A333 GR6, A333 GR1) and so on. The main working media are methyl ammonium, urine, liquid ammonia, carbon dioxide, steam, condensate, and so on. The maximum working pressure of the whole process is 15Mpa, and the maximum working temperature is 385℃. For the whole device, 316L Mod urea-grade ultra-low carbon stainless steel pipeline welding is the key. 316L Mod material is ultra-low carbon stainless steel; for the first time in our company’s construction, there is no mature technology to learn from. 316L Mod urea-grade ultra-low carbon stainless steel pipeline welding, on the one hand, requires an advanced and reasonable welding process; excellent quality of the welder, on the other hand, requires the necessary good management and all-around process control. Process control. This paper discusses the 316L Mod urea-grade ultra-low carbon stainless steel pipeline welding technology according to the actual construction site and the combination of relevant information.

2. 316L Mod urea-grade ultra-low carbon stainless steel pipe and welding consumables technical requirements

Urea production of liquid ammonia, urine solution, methyl ammonium solution, etc., are corrosive media, of which the most corrosive medium is the methyl ammonium solution at high temperature and high pressure; stainless steel has a strong intergranular corrosion ability. When the carbon content of less than 0.03% of ultra-low carbon stainless steel due to the extremely low carbon content, so that the stainless steel in the sensitized state (austenitic stainless steel sensitization temperature range of 450-850 ℃) in the intergranular precipitation of chromium carbide Cr23C6 significantly reduced, and has good resistance to intergranular corrosion. In addition, the use of higher chromium-containing parent material and welding materials can make the stainless steel body surface generate a dense organization of the oxide film, thereby improving corrosion resistance, such as with 25Cr-22Ni-2Mo type materials, which can achieve more satisfactory corrosion resistance.
This device is used for methyl ammonium liquid and other strong corrosive media pipeline and pipeline components material for the 316L Mod; the main specifications are Φ273.1 × 25.4, Φ168.3 × 15.88, Φ89 × 11.13, Φ60.3 × 8.74 and so on. Welding material wire is R25.22.2LMN (Φ1.6), welding rod is E25.22.2LMNB
(Φ2.5, Φ3.25, Φ4.0). The chemical composition requirements of pipes and pipeline components, and welding materials are shown in Table 1 and Table 2:
Table.1 Chemical composition of pipes and piping components

Chemical composition	C	Mn	Si	S	P	Cr	Ni	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	C	Mn	Si	S	P	Cr	Ni	Mo	N
Content%	≤0.04	≤3.0	≤0.5	≤0.02	≤0.03	≥24	≥21	1.9-2.7	≤0.20

Pipelines and piping fittings and welding materials of austenite-forming elements (Ni, C, N, Mn, etc.) and ferrite-forming elements (Cr, Mo, Si, etc.) should be balanced so that after welding to obtain a completely austenitic organization, the maximum ferrite content of 0.6%. The ferrite content is detected with a ferrite meter. Ferrite content of unqualified pipes, pipe components, and welding materials are strictly prohibited in construction.

3. Welding process evaluation

To ensure that the urea plant 316L Mod urea-grade ultra-low carbon stainless steel pipeline welding quality before the construction of the external welding engineers under the supervision of 316L Mod urea-grade ultra-low carbon stainless steel welding process assessment. 316L Mod urea-grade ultra-low carbon stainless steel welding process assessment implementation of the ASME-1995 standard, the assessment of the project is shown in Table 3.
Table.3 Welding process assessment project table

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 rod	Φ2.5	DC reverse connection	60-90	20-22	4–7
	Φ3.25	DC reverse connection	80-120	22-26	5–8
	Φ4	DC reverse connection	120-150	24-28	5–10

316L Mod urea-grade ultra-low carbon stainless steel welding process evaluation welding records all welding parameters (including current, voltage, polarity, welding consumables grade specifications, shielding gas, and the back of the shielding gas type flow, etc.). Evaluation of the tensile test, face bending test, back bending test, Hugh’s test, metallographic test, and ferrite detection. Evaluation of the test sampling location is shown in Figure 1.

Figure.1 Welding process assessment sampling location map
316L Mod urea grade ultra-low carbon stainless steel welding process assessment test procedures and assessment results have been recognized by the foreign welding engineer. 4.

4. Welder operation skill evaluation

Given that the 316L Mod urea-grade ultra-low carbon stainless steel construction is difficult, high technology in the construction preparation stage by the welding process assessment of the preparation of welding process procedures (WPS), WPS has been recognized by the foreign welding engineer. Under the supervision of the foreign welding engineers, the welders were drawn to carry out the welder operation skill assessment of pipe tilt fixing 6G position, Φ168.3×15.88 argon-electric interconnection welding, and Φ60.3×8.74 argon-arc welding. The most difficult part of the skill evaluation is not to allow concave; the key lies in the counter gap and wire feeding method. Assessment of the qualification standards for the appearance of inspection, χ-ray testing, penetration testing, ferrite testing, and weld cross-section macro-metallurgical examination of all qualified, without one. We also carried out a weld rework welding test after the first layer of filler weld. The rework position is shown in Figure 2. If there are still impermissible defects after rework can be reworked, such as failing again, the welder constructor is not allowed to participate in the weld rework work.

Figure.2 Weld rework location map
Participate in the skills assessment (including rework welding) of the welders are assessed once qualified, signed by the foreign welding engineer operating license, and welders licensed to work. Welder skills assessment for 316L Mod urea grade ultra-low carbon stainless steel construction laid a solid foundation.

5. Welding process

(1) Environmental control
316L Mod urea-grade ultra-low carbon stainless steel welding carried out strict environmental control. Produced a windproof and rainproof canopy and set up a person to monitor the welding environment to ensure the welding wind speed manual arc welding control at 3m/s below, argon arc welding control at 0.5m/s below.
(2) Welding consumables management
The correct use of welding consumables is to ensure the welding quality of 316L Mod urea-grade ultra-low carbon stainless steel prerequisite. In this regard, we have established a strict storage, drying, distribution, and recycling system so that welding consumables from storage, drying, and distribution to the recovery of the whole process can be tracked. Welding consumables according to the brand number, specifications, shelves, with signage on the brand number, specifications, and the number of obvious signs. Welding consumables must be used by the welder to fill in the welding consumables application form, indicating the welding part. Then the custodian will issue the application form according to the form after the welding technician’s examination and the welding inspector’s confirmation. Receiving welding rods must be used with a welding rod labeling insulation cylinder; the same insulation cylinder cannot be mixed with two kinds of welding rods; in the entire construction process, the issuance and use of the state are always under control.
(3) Pipeline materialization and bevel processing
Pipeline feeding according to the single-line diagram, the same specification pipe uniform discharge. Pipeline materializing considers pipe slope and welding channel grouping requirements and unified numbering to ensure that the right number is not good, no leakage, no waste.
The cutting of 316L Mod urea-grade ultra-low carbon stainless steel piping must be done by mechanical methods and is not allowed to be processed by plasma arc, gas cutting, or carbon arc gas planning. When processing the bevel, considering the characteristics of 316L Mod urea grade ultra-low carbon stainless steel, thermal conductivity is small, and the linear expansion coefficient is large. To reduce the welding deformation, the bevel inclination and the bottom of the R are reduced accordingly. The bevel surface should be flat and smooth; there shall be no cracks, burrs, delamination, or other defects.
Considering the characteristics of 316L Mod urea-grade ultra-low carbon stainless steel during processing, to prevent carburization, a special prefabrication area is set up, which is fully isolated from carbon steel and alloy steel pipes. The prefabrication platform is built with wooden or stainless steel plates. Nylon or fiber slings are used for handling. Stainless steel processing tools are dedicated, prohibiting mixing with carbon steel and alloy steel pipe processing tools. The grinding wheel sheet used for grinding adopts a rubber nylon special grinding wheel sheet. This avoids contamination carburization.
Another requirement, sanding should be gentle to avoid bluing of the parent material. Wrap after bevel processing to avoid collision pollution carburization. Before welding with acetone to clean the bevel and its adjacent areas, remove grease, and wrap both sides of the bevel with an asbestos sheet to prevent the welding of carburization and spatter fusion adhesion to the base material to selective corrosion.
(4) Welding grouping and positioning welding
Before grouping, the bevel dimensions are verified, and the ferrite content is checked. δ ≥ 12mm bevels are checked by penetration.
Due to 316L Mod urea-grade ultra-low carbon stainless steel welding liquid metal mobility, wetting is poor, and the depth of fusion is small. To prevent defects such as under-welding, grouping should be flush with the inner wall; the amount of the inner wall on the wrong side is not more than 0.5mm. In order to prevent the inner concave, accurate grouping gap, processing and production of 316L Mod stainless steel 300 × 20 × 2.5 and 200 × 15 × 2.0, two specifications of the Grouping fixed-pitch block.
316L Mod urea-grade ultra-low carbon stainless steel position welding using argon arc welding, welding process, and formal welding process is the same. As part of the formal weld component of the positioning, the weld must be completely welded through, and fusion is good, with no porosity and other defects. In order to ensure that the bottom weld channel is well formed and reduces stress concentration, the weld should be a smooth transition to the base material, and the ends of the weld will be polished into a slope. When formally welding, the starting point is between the two locating welds.
As far as possible, do not use fixtures welded directly to the base material to avoid damage to the base material. Welded to the base material fixture must be 316L Mod urea grade ultra-low carbon stainless steel material, do not allow carbon steel or alloy steel tubes to prevent carburization. Removing the clamps needs to use stainless steel special grinding wheel grinding, shall not use knocking, breaking and twisting, or other methods. After the removal of grinding and leveling, if there is a dent must be replenished welding and penetration checks must be done. The grouping process is strictly prohibited strong grouping so as not to cause stress concentration.
(5) Welding method
316L Mod urea grade ultra-low carbon stainless steel welding should minimize the line energy input and reduce the degree of sensitization of the welded joints, thereby improving corrosion resistance. Therefore, it is required to use a multi-layer multi-channel narrow welding process, using small line energy, short arc, no swing, or small swing welding method.
For DN ≤ 50 pipelines using full argon arc welding. For DN>50 pipelines, the welding is divided into two parts. The root of the thickness of 8mm for the first part, using argon arc welding, and the remaining part of the second part, using manual arc welding. When the filler height of the weld is more than 10mm, multi-layer welding is carried out, and arbitrary swinging of the pile of wide weld passes is not allowed.
Multi-layer welding, each weld after a layer of weld, thoroughly grinding and slag removal, and qualified by the quality inspection before the next layer of welding. Multi-layer welding between layers should be staggered. Strictly control the temperature between layers; the temperature between layers shall not exceed 150℃. The temperature between layers is measured by an infrared thermometer.
DN ≤ 50 pipe welding, due to the concentration of heating, slow heat dissipation, easy cause grain growth is serious, thermal cracking tendency to increase, can be used on both sides of the weld with a cooling copper block or wipe the sides of the weld with a wet cloth and other measures to force the cooling, but note that it cannot be carried out directly on the weld.
The process should pay attention to cleaning and protection. Before welding, use acetone to clean and wipe the welding wire and tungsten electrode and remove grease and other impurities to prevent carburization. The welding process of an unused wire end oxidized burned parts of the prohibited fusion into the weld, which must first be sheared or polished to remove, can be used for welding. The tungsten electrode should be re-cleaned with acetone after grinding. Paint spraying work is not allowed in the work area during welding to prevent grease from entering and causing carburization. During the prefabrication and welding process, the construction personnel’s gloves, clothes, and other supplies should be kept clean to avoid contamination, affecting the welding quality.
(6) Argon arc welding argon protection
Argon arc welding argon protection is crucial. Argon arc welding should be sent ahead of time when welding, stop welding, lag stop gas. Welding protective gas flow is generally 10-15L/min. To prevent oxidation of the surface of the welding channel at the back of the argon arc welding and reduce the quality of the weld, the back of the welding channel must be taken to fill the argon protection measures. Argon-filled protection can be used as a whole or part of the argon two methods. Pipe filled with argon should be left in the head at the blowout hole (Φ5-10mm). Argon filling start flow can be appropriately increased to ensure that the air inside the tube is completely excluded. Welding argon flow should be appropriately reduced to maintain a smooth so as not to cause the back of the weld due to argon blowing support in the molding of the depression. Back protective gas flow is generally 20-25L/min. Argon, as far as possible, is partially filled with argon so as not to cause waste.
Argon protection effect can be judged according to the color change of the weld. During the welding process, the welder can adjust the protective gas according to the color so that the weld can achieve the best protective effect. Weld seam color and protection effect comparison is shown in Table 4.
Table.4 Comparison of welding seam color and protection effect

Weld color	Silver white, gold yellow	Blue	Red gray	Grey	Black
Protective effect	Best	Good	Preferably	Defective	Worst

(7) Welding inspection
Welders in the weld, after self-inspection, should be within 100mm of the side of the weld for code marking. Marking with waterproof ink writing, ink should not contain chlorine. Prohibit the steel seal on the stainless steel pipe.
Pipe diameter Φ ≤ 40mm wall thickness ≤ 5mm pipe, final welding for visual inspection, ferrite content inspection, liquid penetration inspection. Pipe diameter Φ> 40mm, wall thickness > 5mm pipe, bottom welding and the first layer of filler weld after visual inspection, ferrite content check, liquid penetration inspection, such as no defects, acetone should be cleaned bevel surface, cleaning after the second layer of welding. After the second layer of welding liquid penetration inspection, ray detection, if there are no defects. After the final weld visual inspection, liquid penetration inspection, and ray inspection.
Visual inspection requires that the weld transition to the base material is smooth and round, not allowing cracks, pores, slag, and other defects. The depth of the biting edge δ≤0.5mm, and the cumulative length does not exceed 10% of the total length of the weld. Requirements for the residual height of the weld: δ1≤1.5mm when the base material δ≤12.5mm, δ1≤1.5mm when the base material δ is 12.5-25, δ1≤2.0mm when the base material δ＞25, δ1≤3.0mmmm when the base material δ＞25.
Liquid penetration inspection adopts a water-washing coloring penetration method with a wet display agent. The surface temperature of the workpiece is between 15-50℃, and the penetration time is at least 15 minutes.
The ferrite content is checked using a probe-type ferrite meter, and the maximum ferrite content should be less than 0.6%. Before the inspection, use acetone to clean the calibration test piece, inspection probe, and weld so that it is free of rust floats, grease, and other impurities, to ensure the inspection accuracy. The inspection process often cleans the probe to ensure there is no contamination to ensure the accuracy of the test value. When the test point ferrite content exceeds the standard, the special grinding wheel can be used for grinding treatment, with acetone to clean the grinding place. Then the test, still unqualified, should check the parent material, welding material ferrite test report, the correct use of welding consumables, and the group to analyze the process of welding, the development of corrective measures, repair and ensure that the ferrite content in the requirements of the scope of the content.
(8) Weld repair
By non-destructive testing, unqualified welds must be eliminated defects and repair treatment. Defect elimination using the grinding wheel grinding method trimmed to a shape suitable for filler welding. It is strictly prohibited to use carbon arc gas planing to remove defects to prevent carburization. Penetration testing should be used to confirm that the defects are removed before refinishing the weld. Rework welds are more difficult to weld, argon arc welding is easier to ensure quality, and the line energy is small. Therefore, the rework is carried out using argon arc welding. The number of reworks of the same part should be at most two times; if more than once rework, should analyze the reasons, develop measures, and approval by the person in charge of technology.

6. Conclusion

• (1) Well-prepared, the process is correct and reasonable. Large-scale installations, especially foreign imported installations, cannot carry out engineering construction without familiarizing themselves with the construction norms and standards. Based on eating through the norms and standards, make good welding process evaluation and welder’s examination, formulate correct and reasonable welding process, and put it into practice to ensure the quality of welding.
• (2) Strengthen management and strict process control. Welding construction not only requires an advanced, reasonable welding process and excellent welder quality but also must have a strict management system and grasp the full range of process control to ensure the quality of welding, avoid rework losses, and achieve half the result with half the effort.

This project 316L Mod urea-level ultra-low carbon stainless steel pipeline welding weld a total of 350 welds, ray flaw detection a pass rate of 100%, weld ferrite testing a pass rate of 99.5%. The practice has proved that the 316L Mod urea-grade ultra-low carbon stainless steel pipeline welding technology discussed in this paper is effective and feasible, and its welding process and process control for the future of the same steel and other austenitic ultra-low carbon stainless steel welding provides a reference.
Author: Li Xiaosong,Wang Fuyin,Bu Shanjuan