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Welding Technology of 316L Mod Urea Grade Ultra Low Carbon Stainless Steel

The 316L Mod urea grade ultra-low carbon stainless steel material and its welding technical requirements used in urea plants working in high temperature, high pressure, and high corrosive medium are introduced. The key to welding technology is that the welded joint has excellent welding hot crack resistance and intergranular corrosion resistance. Based on welding procedure qualification, a reasonable on-site welding process for products was developed, including the selection of welding materials, groove form and size design, welding specification parameters, problems, and solutions in the welding process. The practice has proved that strict control of the whole process of product welding can obtain excellent welding quality.

1. Introduction

316L Mod urea-grade ultra-low carbon stainless steel is 316L improved material for the U.S. AISI standard grade, is austenitic-ferritic duplex stainless steel because of its excellent resistance to weld hot cracking performance and resistance to intergranular corrosion, commonly used as a petrochemical urea plant high-pressure pipeline material. Urea device with high temperature, high pressure, flammable, explosive, corrosive, and other characteristics, a maximum working pressure of 15MPa, a maximum working temperature of 385 ℃, the main working medium for methylamine, urine, liquid ammonia, and other highly corrosive substances. The stainless steel material has a strong intergranular corrosion ability. 316L Mod ultra-low carbon stainless steel welding is the key to the whole device, technical difficulties, good technical management, reasonable welding process, and a full range of process control to ensure the welding quality.

2. Material technical requirements

2.1 Base material technical requirements

Urea device corrosion resistance of stainless steel materials through Hugh’s test for evaluation must have excellent resistance to high corrosive medium methylamine liquid, urine produced by intergranular corrosion performance. When the carbon content of less than 0.03%, stainless steel in the sensitized state, in the 450-850 ℃ sensitization temperature range, the precipitation of Cr23C6 significantly reduced, with excellent resistance to intergranular corrosion. The material in the austenite-forming elements (Ni, C, N, Mn) and ferrite-forming elements (Cr, Mo, Si) should be balanced so that the weld metal in the weld all the final austenitic organization, the maximum ferrite content of 0.6%, according to the Schaeffler diagram for the calculations, limited to the range of Table 1. The use of higher chromium content of the base material can make the surface of stainless steel generate dense oxide film and improve corrosion resistance, such as 00Cr17Ni14Mo2N, its main size specifications are Φ168.3 × 16, Φ60.3 × 8.8, Φ273.1 × 25.5, etc., the chemical composition of table 2.
Table.1 Chemical composition of urea device stainless steel range of values (mass fraction)

Element C Cr Ni Mo N
Content ≤0.030 ≥17.0 ≥13.0 ≥2.2 ≤0.20

Table.2 Chemical composition of 316L Mod (00Cr17Ni14Mo2N) (mass fraction) %

Element C Cr Ni Mo Si Mn S P N
Content 0.03 17.3 13.6 2.53 0.45 1.78 0.001 0.022 0.19

2.2 Technical requirements and selection of welding consumables

Welding consumables should meet all the technical requirements of the base metal of the molten metal. Considering the loss of alloying elements and the weld cooling method’s complexity, controlling the ferrite content of the weld deposit metal must shift the austenite isothermal transformation C-curve to the right. The higher the alloying element content, especially the increase in the content of Ni, can make the austenite isothermal transformation C-curve shifted to the right so that the austenite organization is more stable in the cooling process of the weld, the austenite organization into ferrite organization of the chance to reduce. The welding consumables with higher alloy content than the base metal were selected, and the welding wire R25.22.2LMN, Φ1.6, and the welding rod BM310MoL or E25.22.2LMNB, Φ2.5, Φ3.2, were used, and the chemical compositions are shown in Table 3 and Table 4.
Table.3 Chemical composition of welding wire R25.22.2LMN

Element C Cr Ni Mo Si Mn S P N
Content 0.014 24.99 21.69 2.64 0.17 4.43 0.001 0.012 0.13

Table.4 Chemical composition % of electrode BM310MoL (mass fraction)

Element C Cr Ni Mo Si Mn S P N
Content 0.03 24.54 21.02 2.32 0.45 4.43 0.004 0.02 0.12

Before welding and the welding process, they should be tested for ferrite content, ferrite content of all unqualified base metals, welding consumables, and their contact with the metal material. It is strictly prohibited to put into construction.

3. 316L Mod ultra-low carbon stainless steel welding technology requirements

3.1 Prevent carburization and reduce corrosion resistance

Room temperature carbon in the austenite solubility of about 0.02% -0.03%, when the austenitic carbon content exceeds its solubility at room temperature, carbon is constantly to the austenitic grain boundaries diffusion, and chromium and chromium combination, precipitation of chromium carbide Cr23C6, resulting in the austenite boundary chromium-poor, in the role of corrosive media, resulting in intergranular corrosion, which reduces the corrosion resistance.
Due to the role of high temperature, weldments, and weld metal surface oil, the welding process may produce a large amount of carbon and infiltration into the weld. Therefore, strict control of the source of carbon is necessary.

3.2 Prevention of weld cracking

As a result of the alloying elements Si, B, Ni, and impurity elements can generate a low melting point liquid interlayer S, P, the formation of low melting point eutectic in the molten pool, Si and Ni to form Ni3S2, the melting point of 645 ℃, Ni-Ni3S2 eutectic melting point is only 625 ℃; and the austenitic stainless steel thermal conductivity is small, the coefficient of linear expansion is large, the welding of the internal stress is large, hot cracking is easy to produce a defect of austenitic stainless steel welding, especially the welding of the austenitic stainless steel. Produce a defect, especially single-phase austenitic stainless steel, is more likely to produce.
To prevent weld cracking, strictly control the weldments and welding consumables in the content of S, P, etc.; reduce the heat input; improve the cooling rate and the structure of the welded joints to reduce the degree of constraint.

3.3 Control ferrite content

Due to methylamine, urine corrosion characteristics are mainly overall corrosion and selective erosion, and ferrite organization is conducive to developing selective erosion. Hence, the pure austenitic organization of methylamine urine corrosion performance is the best, so it should be strictly controlled ferrite content.
To this end, should be strictly prohibited stainless steel base material, welding consumables, and carbon steel material contact; grinding joints should use a special stainless steel corundum grinding wheel; welding process using a smaller welding line energy, control the interlayer temperature does not exceed 60 ℃, reduce the welding process to stay at high temperatures, to avoid austenitic continuous cooling C-curve and austenitic isothermal transformation C-curve intersection, to ensure that the weld metal is ultimate all the uniform Austenitic organization.
Figure 1, austenite continuous cooling C-curve 1 for the smaller welding line energy obtained by the curve, and austenite continuous cooling C-curve 2 for too large welding line energy obtained by the curve. When the welding line energy increases, the weld cooling time increases, and the austenite continuous cooling C-curve shifts to the right, in the austenite continuous cooling C-curve is not intersected with the austenite isothermal transformation C-curve before the weld molten metal finally all of the austenitic organization. Continue to increase the welding line energy, the austenite continuous transformation C-curve continues to shift to the right, the austenite continuous transformation C-curve meets the austenite isothermal transformation C-curve, the weld metal final organization of the molten metal has a ferrite organization.
20230815114128 50512 - Welding Technology of 316L Mod Urea Grade Ultra Low Carbon Stainless Steel
Figure.1 Austenite transformation C-curve diagram

4. Welding process evaluation

Welding process evaluation according to JB 4708-92 “steel pressure vessel welding process evaluation” of the relevant provisions of the 316L Mod ultra-low carbon stainless steel welding process evaluation record welding current, welding voltage, power supply polarity, welding consumables, specifications, argon gas flow on both sides, etc., for the tensile, bending test, Hewitt’s test, metallurgical test, and ferrite test. And ferrite test.

4.1 Welding method selection

316L Mod ultra-low carbon stainless steel alloy element content is high, and the media contact side of the inert gas shielded TIG welding method. For thick-walled materials, considering the production and economic efficiency, the use of argon arc welding bottom and arc welding cover argon-electric welding methods. Welding process parameters are shown in Table 5.
Table.5 Welding process parameters

Welding level Welding method Welding material grade and specification Power polarity Welding current/A Welding voltage/V Welding speed/mm.s-1 Argon flow rate/L.Min-1
1-2 GTAW 25.22.2LMN Φ1.6 DC SP. 90-110 12-14 1.0-1.2
Positive: 8-12
Back: 10-15
3 SMAW BM310MoL Φ2.5 DC RP. 65-75 21-24 1.0-1.2
4-5 SMAW BM310MoL Φ4.0 DC RP. 110-130 23-26 1.7-1.9
6 SMAW BM310MoL Φ2.5 DC RP. 65-75 21-24 1.0-1.2

Note: welding process control line energy is generally less than 25kJ/cm.

4.2 Sample preparation

  • (1) Tensile specimen, side bending specimen preparation according to JB 4708-92 “steel pressure vessel welding process assessment” provisions.
  • (2) Metallographic specimen (ferrite determination specimen) preparation: metallographic specimen should include the weld, heat-affected zone, and the base material of three areas. Specimen size is shown in Figure 2.
  • (3) Hugh’s specimen preparation: due to the use of two welding methods, namely, argon arc welding bottoming, arc welding filler, and cover, the preparation of the specimen, the specimen should include two welding methods of molten metal. Specimens should be taken from the side in contact with the medium; the tube should be taken from the inner wall to test its corrosion resistance more accurately. Pre-processing of the specimen should be removed from the material by sawing and shearing and then mechanically planed or milled, not allowed to use the grinding method. Pre-processing deformation as small as possible, control the amount of cutting less than 2mm and 0.1-0.3mm. Specimen finishing cutting tool material should be high-speed steel, the last 1mm of the specimen planing depth of 0.6mm, 0.3mm, and 0.1mm, respectively, 0.05-0.1mm, cutting rate of 4-10m/min. Hugh’s surface area of the specimen for 20-30cm2, specimen surface area for 20-30cm2, and specimen surface area 20-10mm. The surface area of Hugh’s specimen is 20-30cm2; the preparation of the specimen is shown in Figure 3.

20230815112208 18984 - Welding Technology of 316L Mod Urea Grade Ultra Low Carbon Stainless Steel
Figure.2 Metallographic specimen diagram
20230815112353 90251 - Welding Technology of 316L Mod Urea Grade Ultra Low Carbon Stainless Steel
Fig.3 Hugh’s specimen

4.3 Assessment results

(1) Welded joints are qualified by appearance and 100% RT II standard test.
(2) The test results of the tensile and side bending tests are shown in Table 6.
Table.6 Test results of mechanical properties of welded joints

Project σb/MPa Fracture location Result
Tensile specimen 650 Broken weld seam Qualified
Tensile specimen 670 Broken weld seam Qualified
Side bending specimen Qualified

(3) Metallurgical organization test results are as follows: the parent material organization for the single-phase austenitic organization, along the rolling direction, is banded distribution. Grain uniformity, fine, local twin state, grain size 7-8. Weld organization for the austenitic matrix + a small amount of dendritic ferrite, measured, ferrite content is less than 0.6%. Heat-affected zone organization is the same as the base material, slightly grown. There is a good organization; the assessment results are qualified.
(4) Ferrite determination results are shown in Table 7.
Table.7 Ferrite measurement results

Test site Standard Actual Measurement Results
Base material ≤0.6% Qualified
Heat affected zone ≤0.6% Qualified
weld seam ≤0.6% Qualified

(5) Hugh’s test results: after 5 cycles, each cycle for 48h boiling nitric acid corrosion test (Hugh’s test), test results, a variety of properties are qualified; see Table 8, Table 9.
Table.8 Boiling nitric acid corrosion test (Hugh’s test) results in 1

Weekly corrosion rate (48 hours) μm
Cycle 1 2 3 4 5 Average
Standard<3 <3 <3 <3 <3 <3
Actual measurement 2.28 1.83 1.44 1.24 1.14 1.59

Note: 1,3,5,7 for perpendicular to the parent material rolling direction of the part; 4,8 for the parent material along the rolling direction of the part; 2,4 for the weld.
Table.9 Boiling nitric acid corrosion test (Hugh’s test) results in 2

Selective corrosion test results (maximum corrosion depth on each side) μm
Position 1 2 3 4 5 6 7 8
Standard <70 <200 <70 <200 <70 <200 <70 <200
Actual Measurement <40 <40 <40 <40 <40 <40 <40 <40

Note: 1,3,5,7 for perpendicular to the parent material rolling direction of the part; 4,8 for the parent material along the rolling direction of the part; 2,4 for the weld.

5. Welding process

5.1 Welding process requirements

5.1.1 Environmental control

316L Mod urea-grade ultra-low carbon stainless steel welding site must be strict environmental control, should be set up special prefabrication area to take measures against wind and rain, special processing tools and handling tools. Wind speed greater than 0.5m/ s shall not be argon arc welding; wind speed greater than 3.0m/s shall not be manual arc welding.

5.1.2 Welding material storage and distribution

Construction site by ISO 9001: 2000 international standards to establish a strict welding material storage, drying, distribution, and recycling management program, the welding material storage, distribution, use, and recycling for the whole process of control. Welding consumables are stored according to the brand number and specifications, arranged in racks, and marked clearly. Welding consumables by the welding welder fill out the welding consumables receipt, the welding technology responsible person audits, welding quality assurance engineers confirm the signature and the custodian receives the single release. Welding rods must be dried according to the specified drying temperature, receive welding rods must be used to insulate the cylinder, and the same insulated cylinder shall not be mixed with various welding rods.

5.1.3 Material discharging and bevel processing

Field pipe material must be considered for welding joint assembly requirements. 316L Mod urea-grade ultra-low carbon stainless steel pipe beveling must be a boring machine or beveling machine, and other mechanical methods, do not allow plasma cutting, gas cutting, or carbon arc planing processing. 316L Mod stainless steel thermal conductivity is small, the coefficient of linear expansion is large, and the bevel angle and the grouping of the smaller size reduce the welding deformation. For bevel form, see Figure 4.
20230815112446 93715 - Welding Technology of 316L Mod Urea Grade Ultra Low Carbon Stainless Steel
Figure.4 Bevel production diagram

5.2 Welding preparation

  • (1) Grouping before should be checked for ferrite content, welding wire, bevel, and inside and outside the 30mm range of oil, grease, water, and other dirt, cleaned with acetone.
  • (2) Clean up the bevel, grinding welds, and other applications of stainless steel wire brush, corundum grinding wheel, and other special tools.
  • (3) Welders should wear clean labor insurance clothing, welding gloves, etc., to prevent contamination.

5.3 Welding process measures

  • (1) In using larger line energy welding, grain size is easy to coarse, and residual stress is larger. At the same time, the austenite continuous cooling C-curve right shift will lead to an increase in ferrite content and ultimately reduce the corrosion resistance of welded joints, directly affecting the product resistance to methylamine liquid, urine corrosion performance, so it should be as much as possible to use multi-layer multi-channel narrow weld channel welding process, the use of a small line energy input, a short arc, not swinging or a small pendulum, Fast welding method.
  • (2) Welding, to ensure the quality of welding, to ensure that the reverse side of the molten metal is not oxidized, the reverse side of the argon gas is always protected, bottoming layer and the second layer of welding are used argon arc welding method.
  • (3) For wall thickness less than 8mm 316L Mod urea grade ultra-low carbon stainless steel, using full argon arc welding. For wall thickness greater than 8mm, 8mm from the root of the argon arc welding, above the part of the manual arc welding. Weld filler height of over 10mm using a multi-layer multi-channel fast welding method. The welding process should ensure the interlayer temperature does not exceed 60 ℃.
  • (4) The welding process should consider cleaning and protection to prevent carburization. Multi-layer welding, every welding layer of weld, must be thoroughly ground slag, and after passing the inspection before the next layer of welding. Multi-layer welding should be staggered. Welding process, an unused wire, the end of the oxidation of the burned part, must be removed before being reapplied to the welding.

5.4 Welding inspection

Pipe diameter ≤ 40mm pipe, the final weld for the appearance of the inspection, ferrite content check. Pipe diameter > 40mm, wall thickness ≤ 5mm of the pipeline, the final weld for the appearance of the inspection, ferrite content check penetration test, RT test. Pipe diameter > 40mm, wall thickness > 5mm of the pipeline, argon arc welding for priming and the first layer of welding, appearance inspection, ferrite content check, penetration testing, such as no defects, with acetone cleaning bevel surface, cleaning after the second layer of welding. After the second layer of welding penetration test, if there are no defects, RT test. After the final welding, appearance check, penetration test, and RT test are carried out.
The ferrite content is detected by probe-type ferrite measuring instrument, and the maximum content should be less than 0.6%. To ensure the accuracy of the test, before the test, with acetone cleaning calibration specimens, test probes, and welds, the test process often cleans the probe and removes oil and rust.

5.5 Welding repair

After testing, the unqualified welds must be reworked. Defective parts with a mechanical grinding method to remove prohibit the use of carbon arc gas planning to prevent carburization. Before repair, penetration testing should be carried out to confirm that the defects have been removed. Rework using argon arc welding. The same part of the rework should be done at most two times. Otherwise, it should analyze the reasons, develop measures, and be approved by the person in charge of technology.

6. Conclusion

  • (1) Welding process assessment results show that: the choice of 25.22.2LMN wire, BM310MoL electrode, the use of argon arc welding bottom, arc welding cover welding method, welding 316L Mod urea-grade ultra-low-carbon stainless steel in the process is feasible.
  • (2) Through the adoption of strict control of the base material and welding consumables ferrite content, multi-layer multi-channel short arc, fast non-swing welding, reduce the line energy input, control the interlayer temperature and other welding process measures, welding the whole process of control, petrochemical engineering urea plant pipeline system urea-grade stainless steel site welding construction quality is good. All welds by 100% RT and ferrite content determination are qualified.

Author: Yang Guohui



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