A phase balance control method for 2205 duplex stainless steel pipe GTAW field welding

Abstract

The present application relates to the field of field welding process, in particular to a phase equilibrium control process method for GTAW field welding of 2205 duplex stainless steel pipeline. Through the means such as segmented skip welding process, control of interlayer temperature window, strategy of high-purity inert gas for back and front protection gas window, control of heat input at 0.6-1.8 KJ / mm, replacement threshold value for replacement ratio and pressure stabilization to be satisfied at the same time, interruption reset threshold value for reset upon timeout, etc., the phase content of weld metal and heat affected zone can reach the standard requirement of 35%-65%.

Description

Technical Field

[0001] This invention relates to the field of field welding technology, specifically to a phase balance control process method for field welding of 2205 duplex stainless steel pipes using GTAW. Background Technology

[0002] 2205 duplex stainless steel (containing approximately 22% Cr, 5% Ni, 3% Mo, and 0.15% N) possesses a dual-phase microstructure with approximately 50% ferrite and 50% austenite, exhibiting excellent mechanical properties and resistance to chloride stress corrosion, pitting corrosion, and crevice corrosion. It is widely used in pipeline systems in marine engineering, petrochemicals, papermaking, and flue gas desulfurization. Field welding is a crucial step in the installation of 2205 duplex stainless steel pipelines. The core challenge lies in controlling the phase balance (ferrite / austenite ratio) of the weld metal and heat-affected zone within the range of 30% to 70% to ensure stable corrosion resistance and toughness of the joint. However, the complex field welding environment (intermittent construction, strong winds, humidity fluctuations, and location constraints) can easily lead to phase imbalances if process parameters are not properly controlled, resulting in decreased corrosion resistance or premature joint failure.

[0003] The existing welding process and technology still have the following prominent problems in actual field applications: 1) Parameter control is isolated and lacks system window linkage.

[0004] 2) The protective gas coverage on the back of the roots is discontinuous, which easily leads to root oxidation.

[0005] 3) Continuous long weld sections lead to temperature accumulation, resulting in a high risk of σ phase precipitation.

[0006] 3) The lack of a reset mechanism after construction is interrupted leads to prominent problems of arc crater contamination and re-oxidation.

[0007] 4) The rework process is highly arbitrary, and the proportion of components after re-welding is even more difficult to control.

[0008] 5) The composition of the shielding gas did not take nitrogen compensation into account, and the risk of post-weld heating was not clearly eliminated. Summary of the Invention

[0009] The purpose of this invention is to provide a phase balance control method for GTAW field welding of 2205 duplex stainless steel pipes. By employing a segmented skip welding process, controlling the interpass temperature window, and using high-purity inert gas in the back and front protective gas windows, the heat input is controlled between 0.6 and 1.8 KJ / mm. Furthermore, the replacement compliance threshold is achieved by simultaneously satisfying the replacement ratio and maintaining stable pressure. The interruption reset threshold is reset upon timeout. These measures aim to achieve the standard requirement that the phase content of the weld metal and heat-affected zone reaches 35%-65%.

[0010] To achieve the above objectives, the present invention employs the following technical solution: A phase balance control method for GTAW field welding of 2205 duplex stainless steel pipes: First, confirm the object and set the process window: confirm that the base material is 2205 duplex stainless steel pipe, and set three types of process windows for the butt welding joint: ① Interlayer temperature window: The upper limit of the interlayer temperature shall not exceed 150℃.

[0011] ② Heat input window: The heat input is controlled within the range of 0.6-1.8 KJ / mm.

[0012] ③ Back-side shielding gas and front-side shielding gas windows: The back-side shielding gas at the root is a high-purity inert gas, and it is kept stably covered at least until the root pass is completed; welding is carried out using duplex filler material, and the front-side shielding gas is preferably a mixture of inert gas and nitrogen, wherein the nitrogen gas fraction is 1%-3%, or an equivalent nitrogen-containing mixture is used.

[0013] Dehumidification and Prohibited Items: When there is a risk of condensation or dampness on site, dehumidification heating is only allowed on the joint area before welding. The dehumidification temperature shall not exceed 100℃. It is only used to remove moisture and condensation and is not used as routine preheating. Post-weld heating is not recommended.

[0014] The welding method specifically includes the following steps: 1) Replacement compliance gate control: Replace the inner cavity protective gas at the root of the opening with replacement volume ratio and pressure stabilization: the replacement volume ratio shall not be less than 6-12 times the effective volume of the inner cavity; after replacement, the back protective gas flow rate shall be controlled at 5-15L / min to maintain a slightly positive pressure state.

[0015] The completion of the above replacement is taken as the replacement standard threshold. If the standard is not met, the arc shall not be started.

[0016] 2) The locating weld length is 10-15mm, and the welding current is 95A. The formal root pass welding is done in segments with symmetrical skip welding, each segment being 50-80mm long. After four segments, once the weld temperature is below 150℃, the remaining positions are completed using symmetrical welding with a root pass welding current of 95A. After no more than two consecutive weld segments in the same direction, it is necessary to switch to a distant weld segment at least one segment long before backfilling the middle segment to create a cooling cycle.

[0017] 3) After the root pass welding is completed, the weld temperature should be reduced to 300℃-400℃. Use clean water to cool the weld and heat-affected zone. Once the temperature is ≤50℃, proceed with the filler weld. The filler weld current should be 120A. The reason is to allow the σ phase to precipitate quickly through the dangerous embrittlement zone of 300-600℃, maintaining a good ferrite + austenite dual-phase ratio in the weld microstructure. Complete the filler weld in the same welding sequence as the root pass welding in step 2), controlling the interpass temperature to ≤150℃.

[0018] 4) Narrow weld bead rules for root pass and fill pass: Root pass and fill pass should be deposited with narrow weld beads. The width of a single weld bead should not exceed 0.8 times the effective width of the groove. Large swings and large molten pool dwells are prohibited. After the root pass is completed, the back shielding gas should be maintained until the fill and cap passes are completed to avoid fluctuations in the root region. Use duplex filler material. The front shielding gas should be a mixture of inert gas and nitrogen, with nitrogen comprising 1%-3%.

[0019] 5) When the temperature of the weld bead and heat-affected zone is ≤50℃, perform cover welding with a welding current of 145A; the welding sequence is the same as the root pass welding sequence in step 2).

[0020] Interpass temperature gating: Before each arc strike, measure the interpass temperature at a distance of 20-30mm from the weld toe. Arc strike can only be performed if the measured temperature does not exceed the threshold (cover weld starting temperature ≤50℃, fill weld starting temperature ≤50℃, other interpass temperatures ≤150℃); if the temperature exceeds the threshold, it must be cooled down.

[0021] Furthermore, starting from the completion of the previous arc termination, if the welding shielding gas interruption time exceeds 8-20 minutes (i.e., the interruption replacement threshold exceeds 8-20 minutes), then a three-step reset must be performed before the next arc ignition: ① Mechanical cleaning of the arc initiation zone and the area to be welded, removing discoloration and contaminants; ② The back protective gas is re-replaced and meets the replacement threshold of step 1); ③ After starting the arc, first sweep back to cover the previous arc-closing crater, and then proceed with normal movement.

[0022] Repair cleaning and re-welding window reuse: When incomplete fusion, root oxidation, porosity or slag inclusions are found, the defective area must be cleaned until the metal is bright and expanded outwards by 5-20mm on both sides before re-replacing and segmenting the weld according to steps 1) to 5).

[0023] Phase balance acceptance and remediation (magnetic testing method, standard ISO8249, GBT1954): After welding, representative points are selected in the weld metal and heat-affected zone for phase content acceptance. The acceptance target is that the ferrite content in the two phases is within the range of 35%-65%. If the limit is exceeded, rework and re-welding are required, and steps 1) to 5) gate control must be strictly followed during re-welding.

[0024] In controlling the duration of the back protective gas, nitrogen-containing multi-component mixed protective gas can be used during the filling and covering stages to balance anti-oxidation and nitrogen compensation.

[0025] Any timeout, rework, or restart must be performed by cleaning, re-replacing, and sweeping back to cover the arc-closing crater to prevent problems from being carried over to the next stage.

[0026] In segmented cycle and skip welding control, narrow weld beads and moderate layer thickness are used for filler and cover layers to avoid excessive thickness of each weld bead, thus reducing defect sensitivity.

[0027] In interpass temperature control, the upper limit of the interpass temperature is set at 150℃, and gated temperature measurement is performed before each welding pass. The heat input is controlled within the range of 0.6-1.8 KJ / mm.

[0028] In the interrupt reset threshold control, the replacement threshold is used to stabilize the welding conditions.

[0029] Compared with existing technologies, the beneficial effects of this invention are: 1) This invention achieves the standard requirement of 35%-65% phase content in weld metal and heat-affected zone by using segmented skip welding process, controlling the interpass temperature window, and using high-purity inert gas for back and front protective gas windows to control heat input between 0.6 and 1.8 KJ / mm; and by simultaneously satisfying the replacement ratio and voltage stabilization threshold and the interruption reset threshold of resetting upon timeout.

[0030] 2) This invention uses multi-dimensional process windows, gate thresholds, forced reset, and narrow-channel skip welding techniques to adjust the heat input to the middle of the international standard value window for heat input of duplex stainless steel. This significantly improves the phase balance control accuracy, rework controllability, and long-term service corrosion resistance of GTAW welding of 2205 duplex stainless steel pipes under harsh field conditions, and overcomes the problems of easy oxidation, phase ratio imbalance, and low rework success rate of traditional processes. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of GTAW welding of 2205 duplex stainless steel pipe according to the present invention.

[0032] In the diagram: ① - steel pipe. Detailed Implementation

[0033] The present invention will be described in detail below with reference to the embodiments, but the scope of the present invention is not limited to the following embodiments.

[0034] Example: In this example, a 2205 duplex stainless steel pipe connection is used in an engineering project. The pipe has a nominal diameter of DN200, a wall thickness of 12mm, and uses GTAW welding method with 3-layer all-position welding. Figure 1 As shown in the figure, 1-8 are the welding positions.

[0035] Follow these steps for details: 1) The bevel is a V-shaped bevel; the root gap is 3.0mm; the blunt edge is 0.8mm; and the misalignment is 1.0mm.

[0036] 2) Window settings: Interpass temperature not exceeding 150℃. Heat input 0.6 to 1.8 KJ / mm. Backside shielding gas meets normal welding conditions: For butt joints at the root of the opening, the internal shielding gas is replaced, with a replacement volume ratio of not less than 10 times the effective volume of the internal cavity; after replacement, the backside shielding gas flow rate is controlled at 10L / min to maintain a slightly positive pressure state.

[0037] 3) Narrow weld beads are used for root and filler welds, with a single weld bead width of 7.1mm. No wobbling or slight wobbling is required, and the arc crater is filled when the arc is closed. The shielding gas on the back side is not stopped after the root weld is completed.

[0038] 4) Welding: such as Figure 1 As shown, 1, 3, 5, and 7 are tack welds with a tack weld length of 15mm and a welding current of 95A. The formal root pass welding sequence is 5, 1, 3, and 7, with a weld length of 80mm and a welding current of 95A. After the weld temperature drops below 120℃, weld 4, 8, 6, and 2 to complete the root pass welding.

[0039] After the root pass welding is completed, the weld temperature is lowered to 350℃. The weld bead and heat-affected zone are cooled with clean water until the temperature reaches 45℃. Then, the filler weld is performed with a filler weld current of 120A, following the same welding sequence as the root pass welding, while controlling the interpass temperature at 120℃. When the weld bead and heat-affected zone temperature reach 45℃, the cover pass welding is performed with a welding current of 145A; the welding sequence is the same as the root pass welding sequence.

[0040] Both the filler and the cover are welded in segments with a length of 80mm. After welding no more than two segments in the same direction, the welding will jump to the opposite far end segment to form a cooling cycle.

[0041] Before each arc strike, the interpass temperature is measured 25mm from the weld toe. The weld is only allowed to proceed if the measured temperature before each strike does not exceed the temperature threshold.

[0042] The back shielding gas is maintained until the fill and cover welding are completed; the front shielding gas is a mixture of inert gas and nitrogen, with nitrogen comprising 2%.

[0043] 5) Interrupt Reset: If the interruption time exceeds 8 minutes and the detected humidity value is 70%-85%, dehumidify using a heating temperature of 85℃, and then perform a three-step reset: ① Mechanical cleaning of the arc initiation zone and the area to be welded, removing discoloration and contaminants; ②The protective gas on the back is replaced; ③ After starting the arc, first sweep back to cover the previous arc-closing crater, and then proceed with normal movement.

[0044] 6) Rework: If severe blackening or oxide strips are found at the root, the defective section should be removed until the metal is bright and expanded outward by 10mm before re-welding; if there is no fusion or slag inclusion, the exposed part should be expanded outward by 15mm before removal and re-welding, and the heat input should be adjusted to the middle of the window.

[0045] 7) After welding, representative points were selected in the weld metal and heat-affected zone for phase content acceptance. The acceptance result was that ferrite in the two phases was 45%, which meets the target requirements for two phases.

[0046] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A phase balance control method for GTAW field welding of 2205 duplex stainless steel pipes, characterized in that, Specifically, the methods and steps are as follows: 1) Replace the inner cavity of the butt joint at the root of the opening with protective gas, and the replacement volume ratio shall not be less than 6-12 times the effective volume of the inner cavity; after the replacement is completed, the flow rate of the protective gas on the back side shall be controlled at 5-15L / min. 2) The length of the locating weld is 10-15mm, and the welding current is 95A; the formal root pass welding is segmented symmetrical skip welding with a length of 50-80mm. After 4 segments, when the weld temperature is below 150℃, the root pass welding of other positions is completed by symmetrical welding with a root pass welding current of 95A; after no more than 2 consecutive weld segments in the same direction, it is necessary to switch to the far end weld segment that is at least 1 segment apart, and then backfill the middle segment to form a cooling cycle; 3) After the root pass is completed, the weld temperature drops to 300℃-400℃. Use clean water to cool the weld and heat-affected zone. Cool to ≤50℃ before filling the weld. The filling weld current is 120A. Complete the filling weld in the same welding sequence as the root pass in step 2) and control the interpass temperature to ≤150℃. 4) The root pass and fill pass welds should be deposited with narrow weld beads, and the width of a single weld bead should not exceed 0.8 times the effective width of the groove. After the root pass weld is completed, the shielding gas on the back side should be maintained until the fill and cover passes are completed. The shielding gas on the front side should be a mixture of inert gas and nitrogen, with nitrogen comprising 1%-3%. 5) When the temperature of the weld bead and heat-affected zone is ≤50℃, perform cover welding with a welding current of 145A; the welding sequence is the same as the root pass welding sequence in step 2).

2. The phase balance control method for GTAW field welding of 2205 duplex stainless steel pipes according to claim 1, characterized in that, The welding heat input is controlled within the range of 0.6-1.8 KJ / mm.

3. The phase balance control method for GTAW field welding of 2205 duplex stainless steel pipes according to claim 1, characterized in that, When there is a risk of condensation or dampness on site, dehumidification heating is only permitted on the joint area before welding, and the dehumidification heating temperature shall not exceed 100℃.

4. The phase balance control method for GTAW field welding of 2205 duplex stainless steel pipes according to claim 1, characterized in that, Before each arc strike, measure the interpass temperature at a distance of 20-30 mm from the weld toe.

5. The phase balance control method for GTAW field welding of 2205 duplex stainless steel pipes according to claim 1, characterized in that, If the welding shielding gas interruption time exceeds 8 minutes, starting from the completion of the previous arc re-ignition, a three-step reset must be performed before the next arc re-ignition: ① Mechanical cleaning of the arc initiation zone and the area to be welded, removing discoloration and contaminants; ②The protective gas on the back is replaced; ③ After starting the arc, first sweep back to cover the previous arc-closing crater, and then proceed with normal movement.

6. The phase balance control method for GTAW field welding of 2205 duplex stainless steel pipes according to claim 1, characterized in that, When incomplete fusion, root oxidation, porosity, or exposed slag inclusions are found, the defective area should be cleaned until the metal is bright and expanded outward by 5-20mm to both sides before re-replacing and segmenting the weld according to steps 1) to 5).

7. The phase balance control method for GTAW field welding of 2205 duplex stainless steel pipes according to claim 1, characterized in that, After welding, representative points were selected in the weld metal and heat-affected zone for phase content acceptance. The acceptance target was that the ferrite content in the two phases was in the range of 35%-65%.

8. The phase balance control method for GTAW field welding of 2205 duplex stainless steel pipes according to claim 1, characterized in that, In step 4), during the control of the duration of the back protective gas, a nitrogen-containing multi-component mixed protective gas is used in the filling and covering stages to balance anti-oxidation and nitrogen compensation.

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