Method for improving corrosion resistance of austenitic stainless steel weld joint

Abstract

The invention discloses a method for improving corrosion resistance of an austenitic stainless steel weld joint. A base material is 304L austenitic stainless steel; laser welding conditions; the welding seam is cleaned; 3-6 times of single cycle treatment is carried out; performing cold deformation; bright annealing; cooling is performed; circulating according to the step 4) until a set single cycle number is reached; and performing vacuum dehydrogenation heat treatment. Through the combined effect of cold deformation and heat treatment, the austenitic stainless steel welding seam structure performance is improved, stress and crystal defect distribution of a welding seam area are changed, the welding seam structure is subjected to static recrystallization, original thick columnar crystals are converted into fine isometric crystals, and therefore the corrosion resistance of the welding seam area is remarkably improved; and in addition, the process is simple, and implementation can be effectively combined with an existing welding production line.

Description

Technical Field

[0001] This invention relates to the field of welding of metallic materials, and more specifically to a method for improving the microstructure and corrosion resistance of austenitic stainless steel welds through cold deformation and heat treatment. Background Technology

[0002] Austenitic stainless steel, a typical face-centered cubic alloy, is widely used in harsh operating environments such as chemical equipment (e.g., reaction vessels, pipelines) and nuclear power plant reactor internals (steam generator heat transfer tubes, containment structures) due to its excellent corrosion resistance at room temperature, good low-temperature toughness, and high-temperature strength. However, during the welding process, the weld metal undergoes a rapid non-equilibrium solidification process, easily leading to the formation of coarse columnar crystal structures and δ-ferrite in the weld area. This heterogeneous interface between the columnar crystal weld and the equiaxed base metal can induce significant galvanic corrosion. The directional growth of the columnar crystals results in highly consistent grain boundary orientations, creating a crystallographic mismatch with the random orientation of the equiaxed crystals in the base metal. This generates a potential difference in the electrolyte, with the columnar crystals acting as the anode and the equiaxed crystals as the cathode. The columnar grain boundaries exhibit parallel alignment, forming a connecting channel with the network grain boundaries of the equiaxed crystals in the base metal. Corrosion current preferentially conducts along the weld-base metal interface, leading to accelerated dissolution of the anodic region (columnar crystals) at the interface. Once galvanic corrosion is initiated, the corrosion extends along the columnar grain boundaries, leading to rapid failure in the depth direction.

[0003] Chinese Patent Publication No. CN119820091 A discloses a welding process method for improving the corrosion resistance of 304 stainless steel welds. The core of this method is to attempt to directly improve weld quality by adjusting the laser welding process itself, thereby enhancing its corrosion resistance. Experimental results show that, compared to traditional laser welding, this method can improve the corrosion resistance of the weld joint in 3.5% NaCl solution to some extent. However, this method essentially still seeks optimization during the welding process and does not change the fundamental problem of the inherent coarse columnar crystal structure of the weld after solidification. Its optimization effect is limited and mainly targets specific plate thicknesses (2mm) and specific welding parameters. It is not significantly effective in addressing the galvanic corrosion mechanism between columnar crystals and equiaxed crystals in the base material, nor can it fundamentally eliminate the drawback of columnar grain boundaries acting as preferential corrosion pathways.

[0004] Chinese Patent Publication No. CN112355516 A discloses an "Active Flux for Improving the Corrosion Resistance of Stainless Steel Welds." The core of this document lies in developing a novel flux formulation aimed at improving the corrosion resistance of welds by enhancing molten pool behavior, increasing deoxidizing capacity, reducing surface tension, improving wettability during welding, and forming a dense protective film after welding. Experimental results show that, using this active flux, the corrosion current density of austenitic 316L stainless steel welds in 15% sulfuric acid solution is approximately 1.3 × 10⁻⁻⁻⁶.6 A / cm². Although this method can improve the corrosion resistance of the weld to some extent by forming a protective layer on the weld surface through optimized flux composition, its mechanism of action is mainly limited to surface protection. It also cannot change the inherent coarse columnar crystal structure within the weld metal, and therefore cannot eliminate the driving force (potential difference) for galvanic corrosion caused by crystallographic mismatch and grain boundary morphology differences between the columnar crystals and the equiaxed crystals of the base metal, nor the deep corrosion channels formed by the parallel arrangement of columnar crystals. This surface protection effect may be insufficient in terms of durability and fundamental effectiveness in harsh corrosive environments or long-term service.

[0005] Existing technologies mainly focus on optimizing welding process parameters, selecting suitable welding materials, and performing subsequent heat treatments. For example, optimizing welding parameters can control the temperature field distribution during welding by adjusting heat input, welding speed, and welding sequence, thereby affecting the microstructure of the weld. Selecting welding materials with good corrosion resistance can improve the corrosion resistance of the weld to some extent. Subsequent heat treatments, such as solution treatment and aging treatment, can improve the microstructure distribution of the weld joint. However, these methods often have drawbacks such as limited improvement effects, complex processes, and high costs, and they are difficult to completely solve the problem of decreased corrosion resistance caused by the columnar crystal structure in the weld.

[0006] In order to meet the ever-evolving high standards required in the fields of chemical equipment and other technologies, it is imperative to provide a simple and efficient process to improve the microstructure of the weld, especially the effect of columnar crystal structure, thereby enhancing the corrosion resistance of austenitic stainless steel welds and meeting the increasingly demanding working environment requirements. Summary of the Invention

[0007] This invention overcomes the shortcomings of existing technologies by proposing a method to induce static recrystallization in the weld region through a combination of cold deformation and heat treatment. This transforms the average columnar grain size from coarse grains of 144–172 μm to fine equiaxed grains of 10–15 μm. Simultaneously, in a 3.5 wt% NaCl solution at room temperature, the corrosion current density in the weld region decreases from 6.5 × 10⁻⁶ before treatment. -7 A / cm²~1.0×10 -6 A / cm² decreased to 1.5×10 after treatment. -8 ~1.6×10 -7 A / cm²; Methods to improve the corrosion resistance of austenitic stainless steel welds by increasing the open-circuit corrosion potential from -334 to -281 mV to -215 mV to -164 mV.

[0008] Technical measures to achieve the above objectives A method for improving the corrosion resistance of austenitic stainless steel welds, comprising the following steps: 1) The base material for welding is 304L austenitic stainless steel, and the welding is done by butt joint, with a thickness of 1-3mm; 2) Laser welding conditions: welding speed 4.2~42mm / s, seam width less than 1mm, welding power 5~20kw; 3) Cleaning treatment of the weld: After grinding and polishing the weld, immerse it in a mixed solution containing 10% nitric acid and 3% hydrofluoric acid for no less than 4 minutes; rinse with clean water and let it air dry until there are no water stains on the weld surface. 4) Perform 3 to 6 single-loop processing cycles, wherein the single-loop processing includes the following steps: A. Perform cold deformation, rolling 4 to 8 times, controlling the cumulative reduction rate at 25% to 35%; B. Bright annealing is carried out under the protection of a mixture of hydrogen and nitrogen gas, and the annealing temperature is controlled at 1020-1060℃, and the temperature is held for no less than 4.5 min. C. Cool the temperature to below 400°C at a cooling rate of not less than 10°C / s. 5) Repeat step 4) above until the set number of single cycles is reached; control the annealing temperature of the last single cycle at 980-1020℃, and hold at this temperature for 4-6 minutes; 6) Perform vacuum dehydrogenation heat treatment at a temperature of 400–500℃ and hold at this temperature for 1–2 hours; control the vacuum level at 10. -3 ~ 10 -4 Pa, and control the hydrogen content at the end to be ≤1.5PPm.

[0009] The key difference is that the cooling method described in step 4) is either water cooling or forced air cooling.

[0010] The key feature is that in the hydrogen and nitrogen mixture, hydrogen accounts for 5-15% of the volume, and the remainder is nitrogen.

[0011] The austenitic stainless steel has the following elemental chemical composition and weight percentage content: 0.02-0.03% C, 0.3-0.8% Si, 1.0-1.2% Mn, 0.02-0.03% P, 0.008-0.01% S, 18-20% Cr, 8-10% Ni, 0.3-0.5% Mo, 0.3-1.0% Cu, with the remainder being Fe and unavoidable impurities.

[0012] The role and mechanism of the main processes in this invention The reason why this invention employs multiple cold deformations, controlling the total reduction rate at 25-35% in each cold deformation and rolling 4-8 times, is to eliminate residual stress and avoid early cracking caused by stress superposition during subsequent service. At the same time, recrystallization transforms columnar crystals into equiaxed crystals and eliminates compositional segregation to improve corrosion resistance.

[0013] The reason this invention combines cold deformation with bright annealing under a hydrogen and nitrogen mixed gas protection is to control the grain size by controlling the final annealing temperature at 980–1020°C and holding it at this temperature for 4–6 minutes, thereby suppressing grain growth. The remaining passes are annealed at 1020–1060°C and held at this temperature for at least 4.5 minutes. This is because recrystallization occurs at this temperature after cold deformation, releasing the energy stored during cold deformation at high temperatures. New grains nucleate at defects, transforming coarse columnar crystals into fine equiaxed crystals, fundamentally improving the corrosion resistance of the weld. Hydrogen, as a protective gas, ensures a complete and dense passivation film on the stainless steel surface. Simultaneously, hydrogen's high thermal conductivity effectively shortens the heating time and improves the uniformity of the holding temperature difference. Hydrogen-protected annealing is the preferred process for achieving high surface quality and excellent corrosion resistance in stainless steel, especially suitable for high-end fields such as medical, food, and nuclear energy. Despite the higher cost of gas, its advantages of eliminating the pickling process and reducing the scrap rate make the overall cost lower than that of nitrogen annealing.

[0014] The present invention employs vacuum dehydrogenation heat treatment at a temperature of 400–500°C, held at this temperature for 1–2 hours; the vacuum level is controlled at 10. -3 ~ 10 -4 The requirement to maintain a hydrogen content of ≤1.5 ppm at the end of heat treatment is due to the introduction of hydrogen atoms during the process, which leads to hydrogen embrittlement. Vacuum low-temperature dehydrogenation can effectively reduce the partial pressure of hydrogen, promote hydrogen desorption, and restore the material's toughness after the loss of hydrogen atoms.

[0015] Compared with existing technologies, this invention improves the microstructure and properties of austenitic stainless steel welds through a combination of cold deformation and heat treatment. It also alters the stress and crystal defect distribution in the weld region, causing static recrystallization and transforming the original coarse columnar crystals into fine equiaxed crystals. This significantly improves the corrosion resistance of the weld region. Furthermore, the process is simple and can be effectively integrated into existing welding production lines, exhibiting strong industrial applicability. While improving the mechanical properties of the weld, it also significantly extends its service life. Detailed Implementation

[0016] The following is a further detailed description. It should be noted that the chemical composition of the base material in the following embodiments is within the specified range, namely 0.02-0.03% C, 0.3-0.8% Si, 1.0-1.2% Mn, 0.02-0.03% P, 0.008-0.01% S, 18-20% Cr, 8-10% Ni, 0.3-0.5% Mo, 0.3-1.0% Cu, with the remainder being Fe and unavoidable impurities.

[0017] Example 1 A method for improving the corrosion resistance of austenitic stainless steel welds, comprising the following steps: 1) The base material for welding is 304L austenitic stainless steel, and the welding is done by butt joint, with a thickness of 2.5mm; 2) Laser welding conditions: welding speed 4.5 mm / s, weld width 0.85 mm, welding power 5.5 kW; 3) Cleaning treatment of the weld: After grinding and polishing the weld, immerse it in a mixed solution of 10% nitric acid and 3% hydrofluoric acid for 4.3 minutes; rinse with clean water and let it air dry until there are no water stains on the weld surface. 4) Perform three single-loop processing cycles, wherein the single-loop processing includes the following steps: A. Perform cold deformation, control the total reduction rate at 27%, and roll 6 times; B. Bright annealing is carried out under the protection of a hydrogen and nitrogen mixture, the annealing temperature is controlled at 1033℃, and the temperature is held for 4.8 min. The hydrogen and nitrogen mixture contains 8% hydrogen by volume and the remainder is nitrogen. C. Cool the water to 396°C at a water cooling rate of 10°C / s; 5) Repeat step 4) above until the set number of single cycles is reached. The annealing temperature for the third single cycle is 983℃, and the temperature is held for 5 minutes. 6) Perform vacuum dehydrogenation heat treatment at 405℃ and hold at this temperature for 1.5 hours; control the vacuum level at 10. -3 Pa, with the hydrogen content at the end being 1.4 ppm.

[0018] Testing revealed that in this embodiment, the average grain size changed from columnar crystals of 144 μm to equiaxed crystals of 11 μm; and in a 3.5 wt% NaCl solution at room temperature, the corrosion conductivity of the weld area decreased from 7.8 × 10⁻⁶. -7 A·cm² decreased to 9.3×10 -8 A·cm², the corrosion potential increased from -281mV to -164mV (vs. SCE).

[0019] Example 2 A method for improving the corrosion resistance of austenitic stainless steel welds, comprising the following steps: 1) The base material for welding is 304L austenitic stainless steel, and the welding is done by butt joint with a thickness of 3mm; 2) Laser welding conditions: welding speed 10mm / s, weld width 0.95mm, welding power 8kw; 3) Cleaning treatment of the weld: After grinding and polishing the weld, immerse it in a mixed solution of 10% nitric acid and 3% hydrofluoric acid for 5 minutes; rinse with clean water and let it air dry until there are no water stains on the weld surface. 4) Perform 5 single-loop processing cycles, wherein the single-loop processing includes the following steps: A. Perform cold deformation, controlling the reduction rate of each pass at 25%, and roll for 7 passes; B. Bright annealing is carried out under the protection of a hydrogen and nitrogen mixture, the annealing temperature is controlled at 1060℃, and the temperature is held for 6 minutes. The hydrogen and nitrogen mixture contains 10% hydrogen by volume and the remainder is nitrogen. C. Perform water cooling, cooling to 390℃ at a water cooling rate of 11℃ / s; 5) Repeat step 4) above until the set number of single cycles is reached. The annealing temperature for the 5th single cycle is 980℃, and the temperature is held for 4 minutes. 6) Perform vacuum dehydrogenation heat treatment at 500℃ and hold at this temperature for 1 hour; control the vacuum level at 10. -3 Pa, with the hydrogen content at the end being 1.39 ppm.

[0020] Testing revealed that in this embodiment, the average grain size changed from columnar crystals of 165 μm to equiaxed crystals of 14 μm; in a 3.5 wt% NaCl solution at room temperature, the corrosion conductivity of the weld area decreased from 9.2 × 10⁻⁶. -7 A·cm² decreased to 1.5×10 -8 A·cm², the corrosion potential increased from -298mV to -167mV (vs. SCE).

[0021] Example 3 A method for improving the corrosion resistance of austenitic stainless steel welds, comprising the following steps: 1) The base material for welding is 304L austenitic stainless steel, and the welding is done by butt joint, with a thickness of 1.5mm; 2) Laser welding conditions: welding speed 20mm / s, weld width 0.65mm, welding power 12kw; 3) Cleaning treatment of the weld: After grinding and polishing the weld, immerse it in a mixed solution of 10% nitric acid and 3% hydrofluoric acid for 4.2 minutes; rinse with clean water and let it air dry until there are no water stains on the weld surface. 4) Perform three single-loop processing cycles, wherein the single-loop processing includes the following steps: A. Perform cold deformation, controlling the reduction rate of each pass at 28%, and roll for 8 passes; B. Bright annealing is carried out under the protection of a hydrogen and nitrogen mixture at a temperature of 1050°C and held at this temperature for 8 minutes. The hydrogen and nitrogen mixture contains 15% hydrogen by volume and the remainder is nitrogen. C. Cool the air to 366°C using forced air cooling at a rate of 12.5°C / s. 5) Repeat step 4) above until the set number of single cycles is reached; the annealing temperature for the third single cycle is 990℃, and the temperature is held for 6 minutes. 6) Perform vacuum dehydrogenation heat treatment at 410℃ and hold at this temperature for 2.2 hours; control the vacuum level at 10. -3 Below Pa, the hydrogen content at the end is 1.29 ppm; Testing revealed that in this embodiment, the average grain size changed from columnar crystals of 136 μm to equiaxed crystals of 10 μm; and in a 3.5 wt% NaCl solution at room temperature, the corrosion conductivity of the weld area decreased from 6.5 × 10⁻⁶. -7 A·cm² decreased to 8.2×10 -8 A·cm², the corrosion potential increased from -312mV to -172mV (vs. SCE).

[0022] Example 4 A method for improving the corrosion resistance of austenitic stainless steel welds, comprising the following steps: 1) The base material for welding is 304L austenitic stainless steel, and the welding is done by butt joint with a thickness of 2mm; 2) Laser welding conditions: welding speed 6mm / s, seam width 0.75mm, welding power 6kw; 3) Cleaning treatment of the weld: After grinding and polishing the weld, immerse it in a mixed solution of 10% nitric acid and 3% hydrofluoric acid for 4.5 minutes; rinse with clean water and let it air dry until there are no water stains on the weld surface. 4) Perform three single-loop processing cycles, wherein the single-loop processing includes the following steps: A. Perform cold deformation, controlling the reduction rate of each pass to 30%, and roll for 6 passes; B. Bright annealing is carried out under the protection of a hydrogen and nitrogen mixture at a temperature of 1045°C and held at this temperature for 6 minutes. The hydrogen and nitrogen mixture contains 15% hydrogen by volume and the remainder is nitrogen. C. Cool the water to 355°C at a water cooling rate of 12.8°C / s; 5) Repeat step 4) above until the set number of single cycles is reached; the annealing temperature for the third single cycle is 992℃, and the temperature is held for 4.5 minutes. 6) Perform vacuum dehydrogenation heat treatment at 435℃ and hold at this temperature for 1.5 hours; control the vacuum level at 10. -3 The concentration should be below Pa, and the hydrogen content should be controlled to be ≤1.4 ppm at the end. Testing revealed that in this embodiment, the average grain size changed from columnar crystals of 156 μm to equiaxed crystals of 13 μm; in a 3.5 wt% NaCl solution at room temperature, the corrosion conductivity of the weld area decreased from 8.6 × 10⁻⁶. -7 A·cm² decreased to 1.2×10 -7 A·cm², the corrosion potential increased from -317mV to -186mV (vs. SCE).

[0023] Example 5 A method for improving the corrosion resistance of austenitic stainless steel welds, comprising the following steps: 1) The base material for welding is 304L austenitic stainless steel, and the welding is done by butt joint, with a thickness of 2.8mm; 2) Laser welding conditions: welding speed 30mm / s, weld width 0.90mm, welding power 15kw; 3) Cleaning treatment of the weld: After grinding and polishing the weld, immerse it in a mixed solution containing 10% nitric acid and 3% hydrofluoric acid for 6 minutes; rinse with clean water and let it air dry until there are no water stains on the weld surface. 4) Perform four single-loop processing cycles, wherein the single-loop processing includes the following steps: A. Perform cold deformation, controlling the reduction rate of each pass at 34.5%, and roll for 10 passes; B. Bright annealing is carried out under the protection of a hydrogen and nitrogen mixture, the annealing temperature is controlled at 1050℃, and the temperature is held for 5 minutes. The hydrogen and nitrogen mixture contains 15% hydrogen by volume and the remainder is nitrogen. C. Cool the water to 330°C at a water cooling rate of 12°C / s; 5) Repeat step 4) above until the set number of single cycles is reached; the annealing temperature for the fourth single cycle is 970℃, and the temperature is held for 4 minutes. 6) Perform vacuum dehydrogenation heat treatment at 500℃ and hold at this temperature for 2 hours; control the vacuum level at 10. -3 Pa, with a hydrogen content of 1.30 ppm at the end; Testing revealed that in this embodiment, the average grain size changed from columnar crystals of 161 μm to equiaxed crystals of 12 μm; in a 3.5 wt% NaCl solution at room temperature, the corrosion conductivity of the weld area decreased from 1.0 × 10⁻⁶. -6 A·cm² decreased to 1.6×10 -7 A·cm², the corrosion potential increased from -334mV to -187mV (vs. SCE).

[0024] Example 6 A method for improving the corrosion resistance of austenitic stainless steel welds, comprising the following steps: 1) The base material for welding is 304L austenitic stainless steel, and the welding is done by butt joint with a thickness of 3mm; 2) Laser welding conditions: welding speed 42mm / s, weld width 0.55mm, welding power 20kw; 3) Cleaning treatment of the weld: After grinding and polishing the weld, immerse it in a mixed solution containing 10% nitric acid and 3% hydrofluoric acid for 4 minutes; rinse with clean water and let it air dry until there are no water stains on the weld surface. 4) Perform 5 single-loop processing cycles, wherein the single-loop processing includes the following steps: A. Perform cold deformation, with a reduction rate of 35% per pass, and roll for 9 passes; B. Bright annealing is carried out under the protection of a hydrogen and nitrogen mixture, the annealing temperature is controlled at 1025℃, and the temperature is held for 5 minutes. The hydrogen and nitrogen mixture contains 15% hydrogen by volume and the remainder is nitrogen. C. Cool the air to 386°C using forced air cooling at a rate of 12°C / s. 5) Repeat step 4) above until the set number of single cycles is reached. The annealing temperature for the 5th single cycle is 992℃, and the temperature is held for 4 minutes. 6) Perform vacuum dehydrogenation heat treatment at 450℃ and hold at this temperature for 1.5 hours; control the vacuum level at 10. -3 Pa; the hydrogen content at the end was 1.23 ppm; Testing revealed that in this embodiment, the average grain size changed from columnar crystals of 172 μm to equiaxed crystals of 15 μm; and in a 3.5 wt% NaCl solution at room temperature, the corrosion conductivity of the weld area decreased from 7.2 × 10⁻⁶. -7 A·cm² decreased to 9.3×10 -8A·cm², the corrosion potential increased from -334mV to -215mV (vs. SCE).

[0025] This specific embodiment is merely a best example and is not intended to limit the implementation of the technical solution of the present invention.

Claims

1. A method for improving the corrosion resistance of austenitic stainless steel welds, comprising the following steps: 1) The base material for welding is 304L austenitic stainless steel, and the welding is done by butt joint, with a thickness of 1-3mm; 2) Laser welding conditions: welding speed 4.2~42mm / s, seam width less than 1mm, welding power 5~20kw; 3) Cleaning treatment of the weld: After grinding and polishing the weld, immerse it in a mixed solution containing 10% nitric acid and 3% hydrofluoric acid for no less than 4 minutes; rinse with clean water and let it air dry until there are no water stains on the weld surface. 4) Perform 3 to 6 single-loop processing cycles, wherein the single-loop processing includes the following steps: A. Perform cold deformation, rolling 4 to 8 times, controlling the cumulative reduction rate at 25% to 35%; B. Bright annealing is carried out under the protection of a mixture of hydrogen and nitrogen gas, and the annealing temperature is controlled at 1020-1060℃, and the temperature is held for no less than 4.5 min. C. Cool the temperature to below 400°C at a cooling rate of not less than 10°C / s. 5) Repeat step 4) above until the set number of single cycles is reached; control the annealing temperature of the last single cycle at 980-1020℃, and hold at this temperature for 4-6 minutes; 6) Perform vacuum dehydrogenation heat treatment at a temperature of 400–500℃ and hold at this temperature for 1–2 hours; control the vacuum level at 10. -3 ~ 10 -4 Pa, and control the hydrogen content at the end to be ≤1.5PPm.

2. The method for improving the corrosion resistance of laser welds in austenitic stainless steel as described in claim 1, characterized in that: The cooling method described in step 4) is water cooling or forced air cooling.

3. The method for improving the corrosion resistance of austenitic stainless steel welds as described in claim 1, characterized in that: The hydrogen and nitrogen mixture contains 5-15% hydrogen by volume, with the remainder being nitrogen.

4. The method for improving the corrosion resistance of austenitic stainless steel welds as described in claim 1, characterized in that: The austenitic stainless steel has the following elemental chemical composition and weight percentage content: 0.02-0.03% C, 0.3-0.8% Si, 1.0-1.2% Mn, 0.02-0.03% P, 0.008-0.01% S, 18-20% Cr, 8-10% Ni, 0.3-0.5% Mo, 0.3-1.0% Cu, with the remainder being Fe and unavoidable impurities.