A production method for improving the residual stress and toughness of girth welds of pipeline steel

Abstract

This invention discloses a production method for improving the residual stress and toughness of pipeline steel ring welds, comprising: controlling the heat input heat source at 0.8–1.5 kJ / mm using a low-heat-input dual-heat-source composite welding method; stabilizing the interpass temperature at 100–180℃ through infrared thermography and air cooling; implementing controlled atomized cooling or water mist cooling after a 30–60 s delay post-weld to maintain the average cooling rate of the weld at 4–12℃ / s in the 500–200℃ range; subsequently subjecting the weld to online short-time tempering at 580–650℃ for 1.5–4 min to achieve substructure recovery and martensite reduction. Through the synergistic strengthening effect of microalloying Nb–V–Ti–Mo–Cu, the weld microstructure is refined and the Martensite-Alder (M-A) component is significantly reduced. Ultimately, the residual tensile stress of the weld is reduced to 50–120 MPa, and the impact toughness at −20℃ and −40℃ is increased by 30–70 J, respectively.

Description

Technical Field

[0001] This invention relates to the field of pipeline steel welding manufacturing and heat treatment processes, and in particular to a comprehensive production method for residual stress control and low-temperature toughness improvement of circumferential welds in long-distance oil and gas pipelines, belonging to the technical field of high-strength low-alloy pipeline steel welding quality control. Background Technology

[0002] As my country's oil and gas pipelines are constructed with higher steel grades, larger diameters, and longer distances, the circumferential weld area has become a weak point throughout the pipeline's service life. Numerous studies and engineering failure cases have shown that a high-amplitude tensile residual stress field (typically reaching 300–500 MPa) is generated in the circumferential weld area during welding, which continues to affect stress corrosion cracking (SCC) initiation, low-temperature brittle fracture, and fatigue crack propagation during subsequent service. Particularly in medium- and high-strength pipeline steels such as X60–X80, phenomena such as coarsening of the weld heat-affected zone (HAZ), embrittlement of the martensite / bainite mixed structure, and an increase in MA components at grain boundaries result in significantly inferior impact toughness of the circumferential weld compared to the base metal. Existing processes typically employ traditional welding process control, post-weld overall heat treatment, or external force correction to eliminate residual stress. However, these methods suffer from high costs, low efficiency, inapplicability to large-diameter pipelines, and an inability to simultaneously improve toughness. Furthermore, residual stress redistributes after pressure testing, leading to insufficient process stability.

[0003] Therefore, it is necessary to develop a new composite production method that is suitable for pipeline steel ring welds, and simultaneously features significantly reduced residual tensile stress, greatly improved weld and HAZ toughness, controllable process costs, and applicability to engineering sites. Summary of the Invention

[0004] This invention addresses the problems of existing pipeline steel ring welds, such as the difficulty in completely eliminating high tensile residual stress, poor low-temperature toughness in the HAZ coarse-grained region, uneven high-temperature cooling of the weld leading to MA component enrichment, and the difficulty in implementing post-weld heat treatment on-site. By combining process innovation and process control, this invention proposes a comprehensive method for improving the microstructure and properties of ring welds with significant creativity. This method reduces the residual tensile stress of the ring weld to the range of 50–120 MPa, while increasing the impact absorption energy at −20℃ and −40℃ by 30–70 J, achieving a "dual improvement" in both residual stress and toughness.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] One aspect of the present invention provides a production method for improving the residual stress and toughness of pipeline steel ring welds, the technical solution of which includes the following steps:

[0007] (1) Adopting dual heat source composite welding technology with controlled heat input: When the circumferential weld is welded, the "low heat input main welding heat source + low power auxiliary stable heat source" mode is used to keep the heat input of the weld at 0.8–1.5 kJ / mm, so that the weld metal can be rapidly solidified and have a fine equiaxed grain structure. Its heat cycle peak is controlled at 1150–1250℃, which effectively inhibits the growth of coarse grains and provides basic structural conditions for subsequent toughness improvement;

[0008] (2) Adopting intelligent interpass temperature control technology: During the welding process, infrared temperature measurement and air cooling are used to control the interpass temperature at 100–180℃ to avoid excessive interpass temperature from inducing HAZ tempering embrittlement. At the same time, it ensures that the next weld has enough heat to replasticize the top grains of the previous weld, so that the microstructure of the circumferential weld transition zone is homogenized.

[0009] (3) Introduce a controlled forced cooling strategy to regulate the residual stress field: After welding, without overall heat treatment, implement a three-stage forced cooling of “30–60 s delay + controlled water mist cooling + air cooling stabilization” to control the rate at which the circumferential weld temperature drops from high temperature to 200–250℃ at 4–12℃ / s, so that the weld can achieve plastic coordinated deformation in the shrinkage stage, thereby transforming the residual tensile stress in the weld and HAZ area into low stress or even some compressive stress.

[0010] (4) Improve weld toughness by using local online tempering: After forced cooling, the weld zone is subjected to short-term online tempering at 580–650℃ for 1.5–4 min by a mobile annular tempering device, so that the martensite / bainite substructure is slightly restored, eliminating the brittleness of the supercooled structure, improving the impact absorption energy, and avoiding the problems of high cost and inapplicability to large diameters caused by traditional PWHT.

[0011] (5) Implement microalloying composition stabilization measures: By reasonably controlling the solid solution and precipitation behavior of Nb, V and Ti in the weld metal, high-density nanoscale precipitates are formed during the cooling process of the weld, which stabilizes the grain boundaries and hinders dislocation slip, so that the weld exhibits higher plasticity and toughness at low temperature; the addition of trace amounts of Mo and Cu further delays the tempering embrittlement process.

[0012] (6) Through the final optimization of process-organization-stress integration, residual stress and toughness are improved: Through the synergistic effect of the above processes, the residual stress cloud map in the circumferential weld and HAZ is transformed from the "high tensile peak" of traditional welding to the "uniformly distributed low value field". At the same time, the microstructure is refined, the average impact absorption energy is increased by 30-70 J, the elongation after fracture is increased by 2-4%, and the impact brittle temperature is reduced by 10-25℃.

[0013] In some embodiments, the dual-heat source composite welding method in step (1) includes: the main arc is used for welding at 260–350A and 24–30V, and the auxiliary heat source is used at 50–80A and 16–20V to stabilize the droplet transfer in order to suppress the formation of coarse grain zone in the weld metal.

[0014] In some implementations, the interpass temperature in step (2) is controlled by “welding intermittent heat management + air cooling regulation”, with an upper limit of no more than 180°C, in order to avoid excessive transformation of bainite to tempered sorbite and improve the low-temperature toughness of HAZ.

[0015] In some embodiments, the controlled cooling in step (3) employs air cooling, atomized cooling, water mist cooling, or a combination thereof, so that the average cooling rate during the initial cooling stage (500–300°C) is maintained at 6–12°C / s, in order to plastically coordinate shrinkage deformation and reduce residual stress peaks; optionally, the pressure of the atomized cooling is 0.3–0.6 MPa, so that the time for the weld to cool from 500°C to 230°C is controlled within 15–35s, in order to ensure a uniform temperature gradient in the welding area.

[0016] In some embodiments, the online short-time tempering in step (5) adopts a low-temperature short-time tempering method of 580–650℃ for 1.5–4 min to form a refined ferrite + bainite multiphase structure, reducing the amount of martensite by 20–40% and the area ratio of MA components by 30–60%; or the online short-time tempering in step (5) adopts a dual-time tempering system, which is used for pipeline steel with a wall thickness ≥20 mm, and performs 580℃×2 min and 620℃×1.5 min in sequence to make the structure more uniform and increase the elongation after fracture by 2–4% (for example, reaching more than 23.5%).

[0017] In some implementations, Nb, V, and Ti elements precipitate carbonitrides with a particle size of 10–60 nm during weld cooling, which act as dislocation pinning points and improve the toughness of the weld under -20°C and -40°C conditions.

[0018] In some embodiments, the contents of Mo and Cu microalloying elements are 0.15–0.25 wt.% and 0.15–0.30 wt.%, respectively, to delay temper embrittlement and improve the high-temperature stability of the weld microstructure.

[0019] In some implementations, the residual stress cloud map of the weld is transformed from the "high tensile concentration peak area" of the traditional weld to a "low stress uniform distribution area", in which the residual stress in the weld toe area decreases by 60–200 MPa.

[0020] Another aspect of the present invention provides a circumferential weld performance improvement structure obtained by the above method, wherein the weld has a refined ferrite + bainite composite structure, the MA component area ratio does not exceed 3.5%, and the residual tensile stress peak does not exceed 120 MPa.

[0021] The application of the circumferential weld performance improvement structure provided by another aspect of the present invention in the field of circumferential weld manufacturing in high-altitude and cold regions, high-pressure oil and gas pipelines, long-distance engineering pipelines, and station yards is also within the scope of the present invention. It is used to significantly reduce the residual tensile stress of circumferential welds and improve low-temperature impact toughness.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] (1) Significantly reduce residual tensile stress in circumferential welds: The “three-stage cooling + low heat input composite welding” proposed in this invention can reduce the residual tensile stress in welds from 300-500 MPa in traditional processes to 50-120 MPa, and form a slight compressive stress of −20 to −80 MPa in some areas, effectively inhibiting the initiation of stress corrosion cracking (SCC) and solving the problem of high residual stress that has long been unresolved in the industry.

[0024] (2) Significantly improved weld toughness: Through microstructure regulation and online tempering, the weld and HAZ region form a fine ferrite + bainite multiphase microstructure, the MA component area ratio is reduced by 30-60%, the impact absorption energy is increased by 30-60J at −20℃ (for example, reaching more than 110J), and still maintains a safe level of about 80J or more at −40℃ (for example, about 78J or more).

[0025] (3) Performance improvement can be achieved without overall post-weld heat treatment: This invention breaks through the bottleneck that traditional PWHT cannot be implemented for large long-distance pipelines. It replaces overall heat treatment with local online tempering, which significantly reduces construction costs by 40-70%, while achieving higher microstructure uniformity and more controllable residual stress field.

[0026] (4) The microalloying structure has a significant stabilizing effect: the combination of the strengthening effect of the precipitation of trace amounts of Nb, V and Ti and the controlled cooling effectively balances the dislocation density and grain size, improves the high-temperature stability and toughness of the weld zone, and makes the strength-toughness balance of the material under long-term service conditions significantly better than that of traditional processes.

[0027] (5) Universal process that adapts to the needs of engineering sites: This process does not rely on expensive equipment and can be implemented in medium and low temperature environments, large diameter steel pipes, field construction and other conditions. It has high engineering adaptability and large-scale application potential and can be widely used in the manufacturing of steel ring welds for the entire series of pipelines from X60 to X100. Detailed Implementation

[0028] The following is a detailed description of a production method for improving the residual stress and toughness of pipeline steel ring welds according to the present invention.

[0029] This invention begins with pre-welding preparation, ensuring consistent weld geometry through steps such as root cleaning of the circumferential welding area, beveling for uniformity, and standardizing weld dimensions. Furthermore, strict control over the cleanliness, moisture content, and oxide film on the welding wire surface effectively reduces hydrogen ingress into the weld. Simultaneously, by adjusting the welding machine parameters, a complete match between the main and auxiliary heat sources is achieved, ensuring the welding heat input is strictly controlled within the range of 0.8-1.5 kJ / mm. This provides a thermal cycle foundation for forming a refined weld microstructure and creates initial conditions for subsequent residual stress reduction.

[0030] The welding stage employs the optimized dual-heat-source composite welding method of this invention. The main arc forms a stable molten pool, while the auxiliary arc reduces metal spatter and refines molten droplets, achieving an overall low-heat-input welding process. During welding, an infrared monitoring system tracks the weld bead temperature distribution in real time. Through intermittent welding, active air cooling, and forced cooling window control, the interpass temperature is strictly stabilized within the range of 100-180℃. This stable temperature range inhibits the expansion of the coarse-grained zone, reduces peak heat input, improves the cooling uniformity of the weld zone, reduces residual stress accumulation from the source, and improves weld refinement.

[0031] After welding, a 30-60 s delayed cooling window is introduced to allow the molten pool metal to undergo initial solidification and slow cooling, thereby reducing transient thermal stress. Subsequently, based on wall thickness and steel grade requirements, a multi-stage controlled cooling path—air cooling, atomized cooling, and water mist forced cooling—is employed to maintain an average cooling rate of 4-12℃ / s for the weld within the 500-200℃ range. This controlled cooling process avoids the brittle microstructure caused by rapid quenching and effectively inhibits martensite coarsening, promoting the formation of a more uniform ferrite + fine bainite microstructure in the weld and heat-affected zone (HAZ), thus significantly reducing the residual stress peaks in critical areas such as the weld toe and weld root.

[0032] After the weld cools to approximately 200-250°C, it immediately undergoes the online tempering process proposed in this invention, employing a short tempering regime of 580-650°C (1.5-4 min). This process promotes the recovery of coarse dislocation cells, reduces residual stress in the martensitic phase, and simultaneously induces the rapid precipitation of solid solution atoms such as Nb, V, Ti, and Mo to form refined carbonitride particles, thereby improving grain boundary stability and fracture resistance. This online tempering significantly reduces the area fraction of the martensitic phase (MA) by 30-60%, forming a uniform and tough multiphase microstructure, resulting in a significant improvement in the low-temperature impact toughness of the weld while maintaining excellent strength levels.

[0033] After the aforementioned integrated welding-temperature control-cooling-tempering process, the thermal cycle within the weld area is fully regulated, resulting in a refined and uniform microstructure, a reduced hardness gradient, and a significant decrease in residual stress peak (by 60-200 MPa), thus creating a uniform stress-microstructure distribution throughout the circumferential weld. This ultimately yields a refined ferrite + bainite composite microstructure with fewer and more stable MA components, and an improvement in low-temperature impact toughness of 30-70 J. The overall residual tensile stress of the weld is controlled within the range of 50-120 MPa, effectively avoiding brittle damage and stress corrosion cracking risks. This method achieves highly efficient stress control without PWHT and is suitable for large-scale welding applications in long-distance pipelines.

[0034] The present invention will be described in detail below through specific embodiments. These embodiments are intended to help understand the present invention and are not intended to limit the scope of the present invention.

[0035] Example 1:

[0036] This embodiment employs a low-heat-input dual-heat-source composite welding method. The main welding uses 320A / 26V (1.05kJ / mm heat input), while the auxiliary low-power heat source is controlled at 65A / 18V, resulting in stable weld formation and extremely low spatter. The bevel area is preheated to 80-100℃ before welding to prevent hydrogen-induced crack initiation.

[0037] During the welding process, the interpass temperature is strictly controlled between 120-150℃. After each layer is welded, the temperature is measured using an intelligent infrared instrument. When the temperature exceeds the control range, air cooling is immediately implemented to ensure that the microstructure obtains a fine mixed structure of ferrite and bainite, thus avoiding the appearance of coarse grain regions.

[0038] After welding, the weld enters the forced cooling section after a 40-second delay, using atomized cooling at 0.45 MPa to achieve a cooling rate of approximately 7°C / s to 230°C. This is followed by natural air cooling for 2 minutes, which significantly reduces the peak residual stress in the weld, lowering the tensile stress in the weld toe zone from 340-380 MPa to 80-110 MPa.

[0039] Subsequently, the weld was subjected to online tempering at 600℃ for 2 min using an annular tempering device, which caused a slight recovery of some martensite, reduced the MA component by 40%, and improved the low-temperature impact toughness to −20℃ / 115 J and −40℃ / 78 J.

[0040] The final weld has significantly refined equiaxed grains, no obvious coarsening in the HAZ transition zone, and the residual stress cloud map changes from a high tensile peak to a gentle distribution, with performance significantly better than traditional welding processes.

[0041] Example 2:

[0042] This embodiment is applicable to pipeline steel ring welds of higher strength grades (such as X80 / X90). The main welding current is controlled at 280A / 27V, with a linear energy of approximately 0.85 kJ / mm, which is the lower limit of the low heat input of this invention. To prevent unstable weld formation, a highly stable pulsed arc is used to enhance the droplet transfer pattern, resulting in a smooth weld surface.

[0043] The interpass temperature is strictly controlled at 100–120℃, and active air cooling is used during welding intervals to ensure optimal phase transformation conditions for the microstructure. After a 30-second delay following welding, a 0.6 MPa water mist is applied for strong cooling to achieve a cooling rate of 10–12℃ / s, rapidly penetrating the bainite nose region and significantly reducing coarse lath martensite.

[0044] After cooling to 210℃, online tempering is immediately carried out. The tempering regime is 630℃×1.8 min, which causes a small amount of aggregation and homogenization of the high dislocation density martensite lamellars, further improving toughness.

[0045] The residual tensile stress peak obtained in this embodiment is approximately 50–90 MPa, the lowest among all embodiments, belonging to a low residual stress or even local compressive stress distribution. The weld impact toughness at −40℃ reaches 90 J, and there is no tendency for crack initiation in the weld toe area, making it suitable for high-stress pipeline projects in frigid regions.

[0046] Example 3:

[0047] This embodiment is applied to the welding of pipeline steel with a large wall thickness (≥ 20 mm). The main welding heat input is controlled at 1.20 kJ / mm, and the auxiliary heat source is maintained at 65A / 17V. The welding process is stable and the weld formation is good.

[0048] Due to the large wall thickness, this embodiment uses a relatively high interpass temperature of 150–180℃, but introduces staged air cooling to prevent the temperature from remaining in the tempering embrittlement sensitive range of 300–450℃ for an extended period. A 60-second delay is made after welding before entering the composite cooling stage. Air cooling is used for 20 seconds to stabilize the temperature, followed by intermittent atomized cooling (5 seconds cooling / 5 seconds stopping) to maintain an average cooling rate of 6℃ / s, which is beneficial for temperature uniformity within the thick weld.

[0049] After cooling to 250℃, a two-stage tempering process is employed: 580℃×2 min (first time) to promote the precipitation of fine carbides; 620℃×1.5 min (second time) to improve plasticity and reduce temper embrittlement sensitivity.

[0050] The resulting weld has a uniform microstructure, residual tensile stress reduced to 60–120 MPa, and toughness close to that of the base material, making it suitable for long-distance high-voltage substations.

[0051] Comparative Example 1:

[0052] Traditional welding employs a high heat input of 1.8–2.4 kJ / mm, with interpass temperatures reaching 200–260℃. Direct natural cooling after welding results in a weld cooling rate of only 1–3℃ / s, leading to coarse grains in the heat-affected zone and residual tensile stress exceeding 320–380 MPa in the weld toe zone.

[0053] The impact toughness at -20℃ is only 40–55 J, the fracture surface shows obvious cleavage cracks, and the area ratio of MA component is as high as 6–8%, which is far from meeting the requirements of pipelines in cold regions.

[0054] Comparative Example 2:

[0055] Forced air cooling with a blower was used after welding, resulting in a rapid cooling rate (6–7℃ / s). However, online tempering was not employed, leading to a brittle mixed structure of martensite and low-temperature tempered sorbite. Residual tensile stress was reduced, but toughness was severely insufficient.

[0056] The impact toughness at -40℃ is only 35–45 J, and fine crack initiation occurs in the HAZ region, which cannot meet the engineering requirements.

[0057] Comparative Example 3:

[0058] PWHT is heated to 650℃ and held for 60 minutes. Due to the large heat capacity of large-diameter pipelines and uneven heating, the weld seam may experience "over-tempering," resulting in the complete disappearance of MA (magnetic alloy strength) but a significant reduction in material strength.

[0059] The residual stress is significantly reduced (60–120 MPa), but the impact toughness at −20℃ only increases to 70–80 J, and the strength decreases by about 60–80 MPa, making it unsuitable for high-strength pipelines.

[0060] Table 1: Comparison of Chemical Composition of Examples and Comparative Examples (mass percentage, %)

[0061]

[0062] Table 2: Comparison of process parameters between the examples and comparative examples

[0063]

[0064] Table 3: Performance Comparison Table of Examples and Comparative Examples

[0065]

[0066] The present invention has the following advantages: (1) The residual stress of traditional circumferential welds is difficult to reduce through natural cooling. However, the present invention achieves plastic coordination of the internal structure of the weld during the shrinkage stage through the innovative path of "delayed + controlled strong cooling + online tempering", which greatly reduces the peak value of residual stress and is a significant technological advancement. (2) The present invention combines cooling rate with microstructure control for the first time, achieving simultaneous improvement of residual stress and toughness. (3) Through the synergistic effect of low heat input welding, intelligent interpass temperature control, microalloying stabilization, and short-time online tempering, the weld forms a refined multiphase structure and a highly uniform low residual stress field, which is a comprehensive technical effect that cannot be achieved by any single measure.

[0067] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A production method for improving the residual stress and toughness of pipeline steel ring welds, characterized in that, Includes the following steps: (1) A low-heat-input dual-heat-source composite welding method is adopted to achieve a weld line energy of 0.8-1.5 kJ / mm; (2) The interlayer temperature is stabilized in the range of 100–180℃ by infrared thermometry and air cooling; (3) After welding, controlled cooling is carried out after a 30–60 s delay cooling, so that the temperature of the weld is reduced from high temperature to 200–250℃ at a rate of 4–12℃ / s; (4) After cooling, perform online short-time tempering at 580–650℃ for 1.5–4 min to allow the weld microstructure to recover substructure. (5) Grain boundary stability is improved by regulating the solid solution-precipitation of microalloying elements Nb, V, Ti, Mo, and Cu; and (6) Ultimately, the residual tensile stress of the weld is reduced to 50–120 MPa, and the impact toughness at −20℃ is increased by 30–70 J.

2. The method according to claim 1, wherein the dual-heat source composite welding method in step (1) includes: The main electric arc is used for welding at 260–350 A and 24–30 V, while the auxiliary heat source is used at 50–80 A and 16–20 V to stabilize the droplet transfer and suppress the formation of coarse grains in the weld metal.

3. The method according to claim 1, wherein the interpass temperature in step (2) is controlled by "welding intermittent heat management + air cooling regulation", and its upper limit is not more than 180°C, in order to avoid excessive transformation of bainite to tempered sorbite and improve the low-temperature toughness of HAZ.

4. The method according to claim 1, wherein the controlled cooling in step (3) adopts air cooling, atomized cooling, water mist strong cooling or a combination thereof, so that the average cooling rate in the initial cooling stage (500–300℃) is maintained at 6–12℃ / s, so as to plastically coordinate shrinkage deformation and reduce residual stress peak. Optionally, the pressure of the atomization cooling is 0.3–0.6 MPa, so that the time for the weld to cool from 500°C to 230°C is controlled within 15–35 seconds, to ensure a uniform temperature gradient in the welding area.

5. The method according to claim 1, wherein the online short-time tempering in step (5) adopts a low-temperature short-time tempering method of 580–650℃ for 1.5–4 min to form a refined ferrite + bainite multiphase structure, reducing the martensite content by 20–40% and the MA component area ratio by 30–60%; or The online short-time tempering in step (5) adopts a dual-time tempering system, which is used for pipeline steel with a wall thickness ≥ 20 mm. It is carried out at 580℃ for 2 min and 620℃ for 1.5 min in sequence to make the microstructure more uniform and increase the elongation after fracture by 2–4%.

6. The method according to claim 1, wherein Nb, V, and Ti elements precipitate carbonitrides with a particle size of 10–60 nm during the weld cooling process, serving as dislocation pinning points to improve the toughness of the weld under conditions of −20℃ and −40℃.

7. The method according to claim 1, wherein the contents of Mo and Cu microalloying elements are 0.15–0.25 wt.% and 0.15–0.30 wt.%, respectively, to delay temper embrittlement and improve the high-temperature stability of the weld structure.

8. The method according to claim 1 transforms the residual stress cloud map of the weld from the "high tensile concentration peak area" of the traditional weld to the "low stress uniform distribution area", wherein the residual stress in the weld toe area decreases by 60–200 MPa.

9. A structure for improving the performance of a circumferential weld obtained by the method of any one of claims 1-8, characterized in that, The weld has a refined ferrite + bainite composite structure, the MA component area ratio does not exceed 3.5%, and the residual tensile stress peak does not exceed 120 MPa.

10. The application of the circumferential weld performance improvement structure of claim 9 in the field of circumferential weld manufacturing in cold regions, high-pressure oil and gas pipelines, long-distance pipeline projects, and stations, which is used to significantly reduce the residual tensile stress of circumferential welds and improve low-temperature impact toughness.

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