Welding technology and quality control of urea-grade stainless steel piping

Urea plants with high temperature, high pressure, flammable, explosive, and corrosive characteristics belong to a particularly dangerous production area. The urea plant engineering pipeline is the focus, and applied to the main process media pipeline urea grade stainless steel welding is the top priority. This paper, combined with practice, generalized discussion of the quality control of urea-grade stainless steel piping welding.

Urea plant with high temperature, high pressure, flammable, explosive, and corrosive characteristics is a particularly dangerous production area. The media in urea production are liquid ammonia, ammonia, CO₂, urea solution, and urea methyl ammonium solution. Among them, the most corrosive medium is high temperature and high (medium) pressure under the methyl ammonium solution and urea methyl ammonium solution, which produces a variety of corrosion on the material. Urea plant engineering pipeline is the focus, applied to methyl ammonium liquid and urea methyl ammonium solution and other major process media pipeline urea grade stainless steel pipeline welding is the top priority. In this paper, combined with practice, we go back to the discussion of the quality control of urea-grade stainless steel welding.

1. Urea-grade stainless steel

Used in urea high-pressure ring of austenitic stainless steel formed different from the conventional austenitic stainless steel, urea-grade austenitic stainless steel material chemical composition in line with the German stainless steel material standards in the material number 1.4435 (317L), 1.4429 (00Grl7Nil3M02N) or U.S. standard AISI 316L requirements, but the minimum content of Cr, Ni, Mo additionally Do limit, the purpose is to control the chemical composition of stainless steel in the ultimate heat treatment (including weld deposited metal) to obtain the full austenite organization, and so that the content of ferrite does not exceed 0.6%. In addition, the intergranular corrosion tendency test (Hugh’s test) and selective corrosion requirements. This type of stainless steel material is usually called urea grade (Urea Grade) 316L or modified (Mod) 316L; urea grade or modified austenitic stainless steel is widely used in urea high-pressure equipment and the corresponding pipeline. Some famous foreign stainless steel manufacturers are producing urea-grade 316L stainless steel, the most influential of which is the Netherlands Stamicarbon company’s 316Lmod, the other Swedish Sandvik 3R60UG, Avesta’s 832SKRN, Japan’s Kobe Steel’s KES-UI. Currently, the domestic production of some urea-grade stainless steel cannot fully meet the quality requirements, urea plants with urea-grade stainless steel materials mainly from imports.
Table.1 Several types of urea-grade stainless steel chemical compositions are shown in the table below.

2. Urea grade stainless steel weldability analysis

Generally speaking, urea-grade stainless steel and other austenitic stainless steel with better welding performance, but because the welding process is a complex metallurgical process completed in a short period, the welded joints (including the deposited metal, fusion line, and heat-affected zone) mechanical properties, metallurgical organization, and corrosion resistance are always difficult to achieve the level of the parent material. Weld is a weak link, a lot of equipment corrosion and damage often occurs around the weld, so the correct choice of welding materials and developing a fair welding process in the manufacture of urea equipment is a very important topic.
Urea grade stainless steel for pure austenitic stainless steel, pure austenitic stainless steel in the weld easily produces thermal cracks; this crack belongs to the crystallization of cracks, occurring at high temperatures, room temperature no longer expands. The reason for thermal cracking is due to the austenitic stainless steel thermal conductivity being small, and the coefficient of expansion being large, prolonging the weld metal in the high temperature zone of the residence time, and increasing the weld at high temperatures when the tensile strain. In addition, the pure austenitic weld crystal often exists low melting point inclusions; in the late solidification and crystallization in the form of the liquid film exists between the austenitic grains, in a certain tensile stress cracking and expansion of the formation of intergranular cracks, low melting point inclusions can be formed in the form of elements such as S, P and so on. Prevent pure austenitic stainless steel weld thermal cracking can be used to produce many measures, such as progress in the weld Mn content, ω_Mn = 4% -6% when the prevention of thermal cracking is quite effective, strict limitations on the weld in the S, P, and other impurity elements content. Progress in welding the cooling rate of the weld pool, in the process of taking appropriate measures such as short arc welding, low heat input, and narrow weld channel technology to accelerate the cooling of the molten pool to prevent weld thermal cracking are effective.
Urea-grade stainless steel is a single-phase austenitic organization, but in the steel standard composition within the scope of the provisions, due to the distribution ratio and production process control process often may exist in a small amount of ferrite due to ferrite in the urea medium easily produce phase-selective corrosion, urea-grade stainless steel (including weld deposited metal) requires the ferrite content of no more than 0.6%.
The above analysis can be seen in urea-grade stainless steel welding; there are two key control content: ferrite content and welding hot cracks.

3. Urea-grade stainless steel welding technology and quality control measures

(1) Material acceptance, storage
Materials must have material test reports and certificates of compliance.
Pipes, and pipe components, piece by piece determination of ferrite content. Inspection equipment using a ferrite measuring instrument, ferrite content test before, should be tested parts and testing probe with acetone or alcohol clean, and then test; testing process, often clean the testing probe to ensure that it is not contaminated by the ferrous body, to ensure the correctness of the test results. Divergent ferrite content of the pipeline and pipeline components and welding materials are strictly prohibited in construction.
After the test, qualified raw materials and welding materials storage requirements to set up a special library for the same governance do not promise to contact with other materials. Qualified stainless steel test materials should be made to confirm the mark. The construction process should be transplanted to the obvious parts of the mark. The use of non-metallic pigments, sulfide or chloride content ≤ 25mg/L of waterproof ink or paint, etc., in the stainless steel writing mark.
Welding rods, wires, and other materials according to the furnace (batch) number test ferrite content. When in doubt, chemical composition analysis, ferrite content determination, crystal corrosion tendency test, selective corrosion inspection, and metallographic examination.
(2) Welding method
Wall thickness ≤ 8mm, using full argon arc welding; wall thickness > 8mm, using argon arc welding and electrode arc welding combined welding, argon arc welding bottom welding two layers, the rest of the layers of the electrode arc welding, can only be used to narrow overlay weld. Argon arc welding tube filled with argon protection.
Argon arc welding joints with corrosion resistance than electrode arc welding to be better due to argon arc welding weld pool protection than electrode arc welding weld quality is high, while argon arc welding heat input low sensitization of the weld and heat affected zone is small, reducing the possibility of cracking.
(3) Welding materials
The correct use of welding consumables is to ensure the welding quality of urea-grade ultra-low carbon stainless steel conditions. Urea-grade stainless steel for the single-phase austenitic organization, so you must use pure austenitic welding consumables. Generally, using 25-22-2 type welding consumables can achieve more satisfactory results.
Establish a strict storage, baking, distribution, and recycling system so that welding consumables from the warehouse to the recovery can be tracked throughout the process. Welding consumables by the welding welder to fill out the welding consumables application form by the welding technician audit custodian according to the single release. When receiving welding rods, one must use the insulation Jane, the same insulation tube is not promised to be installed at the same time with two or more welding rods to prevent misuse in the whole construction process of welding materials issued and used to be in a controlled state.
(4) Welding process evaluation
Strictly carry out welding process evaluation before welding. During the welding process, record all welding data (e.g., amperage, volts, AC, DC, polarity), as well as the type and diameter of the welding material, the flow of inert gas (shielding gas and back shielding gas), the type of welding equipment, tungsten electrode diameter, nozzle diameter, and other relevant content.
Evaluation test for tensile test, face bending test, root bending test, metallographic examination of the weld cross-section, Hugh’s test, and so on.
(5) Welding staff requirements
Welding urea-grade stainless steel welders must have the appropriate qualifications and be experienced, well-trained, and familiar with stainless steel welding.
(6) Stainless steel welding bevel preparation
Urea-grade stainless steel pipe is mainly used in mechanical processing methods to prepare welding bevels. The thick-walled pipe is mainly used in the form of a bevel, shown in the figure below, to reduce heat loss and welding deformation, which is conducive to the corrosion resistance of the weld.
The surface of the machined bevel shall be subject to ferrite content determination and penetration testing.
Bevel must be flat and clean; any impurities (such as water, rust, oil, paint, etc.) will hurt the quality of the weld. Organic solvents such as acetone are used for cleaning if necessary. The welding process should also keep the welding channel and the entire welding area clean and clean; this article is very important to welding urea-grade stainless steel.
Figure.1 Pipe wall thickness δ>15mm, U-shaped bevels
(7) Welding technology measures
Welding should minimize heat transfer and reduce the sensitization of welded joints, thereby improving corrosion resistance. Therefore, it is required to use a multi-layer multi-channel narrow welding process, using a small heat input, short arc, no swing, or small swing welding method.
Multi-layer welding, each weld, after a layer of weld, must be thoroughly polished slag; grinding the arc is very important. Multi-layer welding layer joints should be staggered after the quality check is qualified before the next welding layer.
Strictly control the temperature between the layers; the layer temperature does not exceed 100 ℃. The infrared measuring instrument can detect layer temperature.
(8) Welding inspection
Pipe diameter ≤ 40mm, the ultimate post-welding inspection: appearance check, ferrite content check, penetration check (for butt welds using ray detection instead of penetration test).
Pipe diameter >40mm, wall thickness ≤5mm weld, the ultimate welding after the following checks should be performed: appearance check, ferrite content check (if possible, to check the weld inside the pipe), penetration testing, and ray detection 100%.
Steel pipe diameter > 40mm, wall thickness > 5mm weld inspection:

• 1) The following inspections shall be carried out after the bottoming weld layer and the first filler weld: appearance inspection, ferrite content inspection (if possible, inspection of welds inside the pipe), and penetration testing.
• 2) The following inspections shall be made after the second filler weld layer: appearance inspection, penetration testing, and 100% radiographic testing.
• 3) The following inspections shall be made after the ultimate weld: appearance inspection, ferrite content inspection, penetration testing, and radiographic testing 100%.

(9) Protection during construction
316L urea-grade stainless steel high-pressure pipeline prefabrication is best carried out in a special car asked; the workshop is not carried out at the same time as other steel construction operations. The prefabrication platform is made of carbon steel plate but must be spread on the platform board, asbestos sheet, or rubber skin; its prefabrication platform requires cleaning. Whether it is stored in the prefabricated car asked or temporarily stacked at the construction site the prefabricated pipe sections should be spread on the bottom of the asbestos sheet or rubber skin, and prefabricated pipe sections should be cleaned and closed promptly.
During the construction process, should prevent the tire, fixture, arc, and splash damage to the surface of the material, the damage affecting the corrosion resistance must be repaired and ground, and the depth of repair should be by the relevant provisions.
The construction process should prevent water, rust, oil, paint and chlorine-containing materials and other pollution. The welding area promises to paint at different times.
The construction process is not promised in the stainless steel parts on the welding of non-urea stainless steel parts. If you need to weld process assembly with temporary parts, fixtures, etc., use the same stainless steel material. Remove the temporary weldments and temporary support, only using special stainless steel grinding wheels Ammon ‘grinding eradication. Clean up the weld does not promise to use an ordinary wire brush, you need to use stainless steel brush cleanup.

4. Conclusion

Urea grade stainless steel welding process requires high, welding process evaluation, welder training test, advanced fair welding process and strict process quality control, guaranteeing excellent welding quality.
Practice has confirmed that: in the urea-grade stainless steel pipe joint welding, only in the above links to good control, in order to ensure the welding quality, to meet the product requirements.
Author: Li Xiaosong