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Development of Large-Diameter UNS N08367 Super Austenitic Stainless Steel Seamless Pipes for Nuclear Power Plants
Manufacturing Process, Performance Analysis, and Corrosion Resistance Properties
Table of Contents
- Introduction to UNS N08367 Super Austenitic Stainless Steel
- Chemical Composition Control
- Manufacturing Process for Large-Diameter Seamless Pipes
- Heat Treatment Process
- Mechanical Properties and Test Results
- Metallographic Analysis
- Corrosion Resistance Performance
- Conclusions
- Frequently Asked Questions
1. Introduction to UNS N08367 Super Austenitic Stainless Steel
Over the past two decades, nuclear energy has experienced rapid development as a clean energy source. Nuclear power plants utilize large quantities of seawater as a cooling medium to dissipate heat from both the nuclear island and conventional island systems. However, the complex composition of seawater—with its high salt content, elevated chloride ion concentration, and abundant organic matter—causes severe corrosion to piping and equipment during long-term operation.
Addressing the corrosion resistance challenges of piping in nuclear power plant seawater systems while ensuring safe operation has been an international challenge. Advanced nuclear power plants overseas have traditionally used welded pipes made from AL-6XN super austenitic stainless steel for seawater systems. However, due to compositional and structural differences between the weld material and base metal, galvanic corrosion conditions are created, with corrosion typically initiating at the weld zones, thereby shortening the service life of welded pipes.
“UNS N08367 (AL-6XN) is a super corrosion-resistant stainless steel containing high levels of Cr, Ni, Mo, N, and Cu, offering significantly higher corrosion resistance than 316L, particularly in chloride-containing environments.”
UNS N08367, also known as “AL-6XN,” is a super corrosion-resistant stainless steel primarily containing chromium, nickel, and molybdenum, with additional nitrogen and copper. Compared to 316L, it exhibits significantly higher corrosion resistance, particularly in chloride-containing environments, demonstrating excellent resistance to pitting, crevice corrosion, intergranular corrosion, and stress corrosion cracking.
Seamless pipes made from super austenitic stainless steel effectively eliminate weld seams in the pipe body, significantly reducing heat-affected weakened zones in the base material piping. With only circumferential butt welds required for pipe connections, both stress corrosion and seawater corrosion resistance, as well as service life, are substantially improved. This article presents the development process for ∅711 mm × 9.53 mm specification UNS N08367 super austenitic stainless steel large-diameter seamless pipes for nuclear power plant seawater systems.
2. Chemical Composition Control
The fundamental cause of intergranular corrosion in UNS N08367 steel is the formation of Cr23C6 carbides at grain boundaries, where carbon combines with chromium. As these carbides continuously precipitate, localized chromium-depleted zones develop, reducing corrosion resistance and leading to intergranular corrosion.
By reducing the carbon content below its equilibrium saturation solubility in austenite, the precipitation of Cr23C6 carbides at grain boundaries is fundamentally prevented. Additionally, optimizing the proportions of Cr, Ni, Mo, N, Mn, Si, and Cu ensures that the steel meets conventional performance requirements while achieving freedom from intergranular corrosion.
Key Composition Control Strategies
- Molybdenum (Mo): Controlled at the lower limit of the standard range due to its high hot strength, which creates difficulties during hot piercing and rolling with high deformation resistance and cracking tendency.
- Manganese (Mn) and Nitrogen (N): Contents are reduced while maintaining nickel within the mid-range of specifications, as ambient temperature strength is easily achieved while corrosion resistance requires long-term environmental exposure.
- Carbon (C): Strictly controlled below 0.020% to prevent carbide precipitation at grain boundaries.
| Element | C | Mn | Si | Ni | Cr | Mo | N | Cu |
|---|---|---|---|---|---|---|---|---|
| Actual | 0.015 | 0.45 | 0.34 | 24.34 | 20.55 | 6.15 | 0.22 | 0.29 |
| ASTM B 690 | ≤0.030 | ≤2.00 | ≤1.00 | 23.5-25.5 | 20.0-22.0 | 6.0-7.0 | 0.18-0.25 | ≤0.75 |
| Internal Control | ≤0.020 | 0.40-1.00 | 0.30-0.40 | 24.0-25.0 | 20.0-21.5 | 6.20-6.80 | 0.20-0.23 | ≤0.50 |
Table 1: Chemical composition of ∅711 mm × 9.53 mm UNS N08367 seamless pipe (mass fraction, %)
3. Manufacturing Process for Large-Diameter Seamless Pipes
The production process for ∅711 mm × 9.53 mm specification UNS N08367 large-diameter seamless steel pipes involves a comprehensive workflow combining multiple hot and cold working stages:
Production Flow
Melting (30t EAF + LF + VOD) → Electroslag Remelting → Forging (∅470 mm billet) → Billet Heating → Hot Piercing (∅570 mm × 45 mm) → Hot Rolling (∅670 mm × 25 mm) → Reeling (∅760 mm × 15 mm) → Solution Heat Treatment (1110°C, 30 min) → First Cold Drawing (∅735 mm × 13 mm) → Solution Heat Treatment → Second Cold Drawing (∅711 mm × 9.53 mm) → Final Solution Heat Treatment → Finishing
Heating Process
Heating is performed in a ring furnace using natural gas fuel. Heating quality is a critical factor affecting wall thickness uniformity and surface quality. Due to the high Cr, Mo, and Ni content (total alloy content exceeding 50%), the heating temperature for austenitic stainless steel is limited by the formation temperature of high-temperature ferrite (α-phase).
- Temperature must be controlled below 1150°C to prevent excessive α-phase ferrite formation that reduces plasticity
- Slow heating below 800°C, then rapid heating through the 800-960°C sensitive temperature range to avoid harmful phase precipitation
- Soaking time: 150 minutes
- Direct flame impingement on billets must be strictly avoided to prevent carburization and grain boundary chromium depletion
Hot Piercing and Rolling
After heating, billets enter a two-roll skew rolling piercing mill. Due to the high rolling resistance of UNS N08367, roll speed is appropriately reduced and the plug forward reduction is adjusted to prevent motor overload during bite-in. The piercing and rolling temperature is controlled within 1050-1150°C to ensure smooth hot rolling with 6% molybdenum content providing high hot strength.
Cold Drawing Process
With a diameter-to-wall thickness ratio approaching 75, the ∅711 mm × 9.53 mm specification represents an extremely thin-walled pipe with exceptional manufacturing difficulty. A 2000-ton cold drawing machine is employed for two-pass final sizing:
- All internal and external surface defects are removed by grinding before cold drawing
- Graphite emulsion lubricant is applied uniformly to both surfaces
- Wall reduction per pass: ≤2 mm
- Cold drawing speed: ≤0.3 m/min
- Solution treatment required before each drawing pass at 1110°C, minimum 30 min, followed by water quenching
4. Heat Treatment Process
During heat treatment, austenitic stainless steel is highly sensitive and may precipitate σ-phase, which increases strength but progressively weakens toughness, corrosion resistance, and weldability. This is related to excessively high heat treatment temperature and holding time, causing excessive harmful phases to segregate at grain boundaries.
Solution Heat Treatment Parameters
Temperature
1140°C
Holding Time
30 min (min)
Temperature Tolerance
±10°C
Cooling Medium
Water
Precipitation Behavior During Aging
Research on aging precipitation behavior of UNS N08367 super austenitic stainless steel reveals:
- Below 550°C: No precipitates during extended holding
- At 600°C: M23C6 carbides precipitate at grain boundaries and triple junctions
- 650-750°C: Precipitates form at both grain boundaries and within grains, primarily σ-phase and Laves phase
Cooling Rate Control
A batch-type solution furnace is used with the time from furnace door opening to pipe immersion kept within 30 seconds. The cooling pool dimensions are 6 m × 14 m × 3 m, equipped with high-pressure water spray internal circulation for rapid, uniform heat dissipation. A single 12 m long pipe can be cooled below 550°C within 30 seconds. Infrared thermometry is used to monitor pipes 2 minutes after exiting the furnace.
“Through proper cooling rate control, optimal austenitic structure without grain boundary carbides is achieved, resulting in excellent corrosion resistance and comprehensive mechanical properties.”
5. Mechanical Properties and Test Results
Specimens were taken from the head, middle, and tail sections of the UNS N08367 seamless pipe in both longitudinal and transverse directions. Room temperature tensile testing was performed according to ASTM E8/E8M-2022 on a CMT5105 computer-controlled universal testing machine. Low-temperature impact testing was conducted at -46°C according to ASTM E23-2023 using standard V-notch specimens (55 mm × 7.5 mm × 10 mm).
| Location | Direction | Rp0.2 (MPa) | Rm (MPa) | A (%) | Impact Energy at -46°C (J) |
|---|---|---|---|---|---|
| Head | Longitudinal | 326 | 735 | 65.0 | 290 / 313 / 310 |
| Head | Transverse | 321 | 739 | 60.0 | 301 / 290 / 307 |
| Middle | Longitudinal | 339 | 743 | 55.5 | 201 / 281 / 302 |
| Middle | Transverse | 327 | 735 | 56.5 | 301 / 281 / 302 |
| Tail | Longitudinal | 356 | 753 | 60.5 | 303 / 287 / 301 |
| Tail | Transverse | 335 | 760 | 59.5 | 257 / 287 / 301 |
| ASTM B 690 Requirement | ≥310 | ≥655 | ≥30 | – | |
Table 2: Room temperature tensile and low-temperature impact test results for UNS N08367 seamless pipe
Test results demonstrate that the head, middle, and tail sections of the sample pipe exhibit uniform and excellent properties throughout, meeting all relevant standard requirements.
6. Metallographic Analysis
Microstructural and grain size examinations were performed on full-thickness longitudinal samples taken from the head, middle, and tail sections of the UNS N08367 seamless steel pipe. Samples were etched with aqua regia (one volume concentrated nitric acid slowly poured into three volumes concentrated hydrochloric acid), allowed to stand for 2 hours, then etched for 15 seconds.
Microstructure Results
- Structure: Fully austenitic with no grain boundary carbides
- Grain Size: Grade 4.0 throughout (per ASTM E112-2013)
- Uniformity: Consistent microstructure from head to tail
Non-Metallic Inclusion Assessment
Non-metallic inclusion assessment was performed according to ASTM E45-2018(R2023):
| Sample Location | Type B (Alumina) – Fine | Type B (Alumina) – Coarse | Type D (Globular Oxide) – Fine | Type D (Globular Oxide) – Coarse |
|---|---|---|---|---|
| Head | 0 | 0 | 0 | 0.5 |
| Middle | 0.5 | 0 | 0 | 1.0 |
| Tail | 0.5 | 0 | 0 | 0.5 |
Table 3: Non-metallic inclusion ratings (Type A Sulfides, Type C Silicates, and DS Single Globular categories all rated 0 for both fine and coarse series)
7. Corrosion Resistance Performance
Intergranular Corrosion Resistance
Full-thickness samples (25 mm × 50 mm × 3 mm) from the head, middle, and tail sections were prepared by grinding to 600 grit, ultrasonically cleaned in alcohol, and dried. Testing was performed according to ASTM A262-2015(R2021) Method C after sensitization treatment (675°C ± 5°C, 1 hour hold, air cooled). The immersion test was conducted over 5 periods with weight measurement and corrosion rate calculation after each period.
| Location | Period 1 | Period 2 | Period 3 | Period 4 | Period 5 | Average |
|---|---|---|---|---|---|---|
| Head | 0.150 | 0.160 | 0.146 | 0.107 | 0.157 | 0.155 |
| Middle | 0.130 | 0.134 | 0.125 | 0.099 | 0.132 | 0.136 |
| Tail | 0.129 | 0.126 | 0.127 | 0.094 | 0.139 | 0.140 |
Table 4: Intergranular corrosion immersion test results – Corrosion rate (mm/year)
All three sample locations demonstrated excellent intergranular corrosion resistance. While the head sample showed slightly higher corrosion rates than other locations, all corrosion rates remained at very low levels overall.
Pitting Corrosion Resistance
Samples (30 mm × 20 mm × 3 mm) from head, middle, and tail sections were ground to 600 grit, ultrasonically cleaned in alcohol, and dried. Testing followed ASTM G48-2011(R2020) Method E with immersion in 6% acidified FeCl3 solution at 50°C for 72 hours.
Pitting Corrosion Test Results
- No visible surface changes before and after immersion testing
- No macroscopic corrosion traces observed
- No evidence of stable pitting initiation at the microscopic level
- Mass change before and after immersion within 0.001 g, indicating extremely low corrosion rate
Crevice Corrosion Resistance
Samples (25 mm × 50 mm × 3 mm) with center holes (8.5 mm ± 0.5 mm diameter) were ground to 2000 grit and polished with 2.5 μm diamond paste. Testing followed ASTM G48-2011(R2020) Method F for critical crevice corrosion temperature testing in 6% acidified FeCl3 solution at 29°C for 72 hours.
Crevice Corrosion Test Results
- No visible surface changes before and after immersion testing
- Weight loss before and after immersion within 0.001 g, indicating extremely low corrosion rate
All three sample locations demonstrated excellent crevice corrosion resistance, confirming the suitability of this material for nuclear power plant seawater system applications.
8. Conclusions
- Successful Development: Large-diameter thin-walled UNS N08367 seamless pipes have been successfully developed using a combined process of skew rolling hot piercing + hot rolling + reeling + two-pass cold drawing. Scientific optimization of chemical composition and billet heating schedules prevents excessive α-phase ferrite formation. Strict control of hot rolling temperature range and deformation prevents inhomogeneous carbonitride precipitation, solving the problems of high deformation resistance and cracking tendency in super austenitic stainless steel.
- Optimized Heat Treatment: Through analysis of heat treatment parameter effects on UNS N08367 seamless pipe microstructure, a rational heat treatment process has been designed. Strict control of heat treatment temperature, holding time, and cooling rate prevents harmful phase precipitation, thereby improving the corrosion resistance of super austenitic stainless steel.
- Performance Verification: Room temperature tensile, low-temperature impact, metallographic, non-metallic inclusion, and corrosion performance analyses of the ∅711 mm × 9.53 mm UNS N08367 seamless pipe confirm that all performance indicators of the large-diameter super austenitic stainless steel UNS N08367 seamless pipe meet or exceed ASTM B 690 standard requirements, making it suitable for pressurized water reactor nuclear power plant service water systems.
9. Frequently Asked Questions
What is UNS N08367 stainless steel?
UNS N08367, also known as AL-6XN, is a super austenitic stainless steel containing high levels of chromium (20-22%), nickel (23.5-25.5%), molybdenum (6-7%), and nitrogen (0.18-0.25%). It offers exceptional resistance to pitting, crevice corrosion, intergranular corrosion, and stress corrosion cracking, especially in chloride-containing environments such as seawater systems.
Why are seamless pipes preferred over welded pipes for nuclear power plant seawater systems?
Seamless pipes eliminate weld seams in the pipe body, which significantly reduces the risk of galvanic corrosion that typically occurs at weld zones due to compositional and structural differences between weld material and base metal. With only circumferential butt welds required for pipe connections, both stress corrosion resistance and seawater corrosion resistance are substantially improved, extending the service life of the piping system.
What is the optimal heat treatment temperature for UNS N08367 seamless pipes?
The solution heat treatment temperature for UNS N08367 seamless pipes is typically 1110-1140°C with a temperature tolerance of ±10°C. The holding time should be at least 30 minutes (minimum 2 min/mm of wall thickness), followed by rapid water quenching within 30 seconds to prevent precipitation of harmful phases such as σ-phase, Laves phase, and M23C6 carbides.
What are the key challenges in manufacturing large-diameter UNS N08367 seamless pipes?
Key challenges include: (1) controlling α-phase ferrite formation during heating by maintaining temperature below 1150°C, (2) managing high deformation resistance due to elevated molybdenum content (6%), (3) preventing precipitation of inhomogeneous carbonitrides during rolling by strictly controlling the 1050-1150°C temperature range, and (4) achieving extremely thin walls with diameter-to-wall thickness ratios exceeding 70 through precise multi-pass cold drawing.
What corrosion tests are performed on UNS N08367 seamless pipes?
Standard corrosion tests include: (1) intergranular corrosion testing per ASTM A262 Method C with sensitization at 675°C followed by 5-period immersion testing, (2) pitting corrosion testing per ASTM G48 Method E in 6% acidified FeCl3 solution at 50°C for 72 hours, and (3) crevice corrosion testing per ASTM G48 Method F in 6% acidified FeCl3 solution at 29°C for 72 hours.
Category: Technical Blog | Super Austenitic Stainless Steel
Published: December 2024
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