Friction Stir Welding Microstructure and Impact Performance Analysis of 9Cr-1.5W-0.15Ta Heat-Resistant Steel
1. Technical Overview and Background
1.1 Material Identification
The 9Cr-1.5W-0.15Ta heat-resistant steel (designated as 9Cr-1.5W in Chinese standard nomenclature, corresponding to ASTM A387 Gr.22 / SA387 Gr.22 / 9Cr1.5Mo1VNb / P91 in international nomenclature) is a 9% chromium martensitic heat-resistant steel widely employed in ultra-supercritical (USC) and supercritical power generation boilers, petrochemical reformer tubes, and high-temperature pressure vessels. Its nominal composition features 9.0 wt% Cr, 1.5 wt% W, and 0.15 wt% Ta as the primary alloying elements, supplemented by controlled levels of Mo, V, Nb, and Ni. The Ta addition specifically enhances high-temperature creep strength and stabilizes fine carbide precipitates (MC and M23C6) against coarsening during prolonged service exposure.
1.2 Friction Stir Welding (FSW) Principle
Friction Stir Welding is a solid-state joining process that achieves metallurgical bonding without melting the base material. A rotating non-consumable tool (typically tungsten carbide or tool steel with a shoulder and pin geometry) is plunged into the joint interface. Frictional heat generated at the tool-workpiece interface raises the material to a superplastic state (typically 0.5–0.7 Tm), enabling plastic flow and dynamic recrystallization. The material is displaced around the pin and consolidated under the shoulder's axial pressure, forming a defect-free weld upon tool withdrawal.
1.3 Technical Purpose and Value
The study of FSW weld microstructure and impact performance in 9Cr-1.5W-0.15Ta steel serves several critical purposes for Cladding Technology Shanxi Co., Ltd.:
- Process qualification intelligence — Understanding the solid-state weld zone microstructure provides foundational knowledge for selecting optimal joining strategies in dissimilar material assemblies involving 9Cr-1.5W components
- Impact toughness assurance — Establishing Charpy V-notch impact performance data at service-relevant temperatures (20°C, 100°C, 350°C, 450°C) directly supports qualification of repair and fabrication procedures
- Complementary technology development — FSW knowledge enriches the company's metallurgical expertise base for conventional fusion welding routes (TIG/MIG weld overlay) applied to the same material system
- Research credibility — Demonstrating advanced metallurgical analysis capability strengthens customer confidence in complex cladding and overlay projects
2. Microstructure Analysis of FSW Weld Zones in 9Cr-1.5W-0.15Ta Steel
2.1 Weld Zone Segmentation
Unlike fusion welding, which produces a distinct weld metal, heat-affected zone (HAZ), and base metal (BM), FSW produces a thermomechanically affected zone with distinct sub-regions characterized by different thermal histories and deformation intensities:
| Zone | Temperature Range | Deformation Intensity | Microstructural Characteristics |
|---|---|---|---|
| Weld Nugget (WN) / Thermo-Mechanically Affected Zone (TMAZ) | 600–800°C | Very high (plastic flow) | Equiaxed dynamic recrystallized grains (5–15 μm); fine MX-type carbides (NbC, VC); dissolved M23C6 carbides; lath martensite largely eliminated |
| Heat-Affected Zone (HAZ) / Thermo-Affected Zone (TAZ) | 500–600°C | Moderate | Partial recrystallization; retained lath martensite with coarsened prior austenite grain boundaries; M23C6 carbide precipitation at grain boundaries |
| Base Metal (BM) | Ambient (as-received) | None | Tempered lath martensite; M23C6 + MX carbides; prior austenite grain size 20–40 μm |
2.2 Key Microstructural Features
Dynamic Recrystallization: In the weld nugget, the intense plastic deformation combined with frictional heating drives complete dynamic recrystallization. The resulting equiaxed ferrite grains (typically 5–15 μm in 9Cr-1.5W steel) provide superior ductility and toughness compared to the coarse lath martensite of the base metal. The grain refinement is a direct consequence of the high dislocation density being consumed by nucleation and growth of new strain-free grains.
Carbide Behavior: The MC-type carbides (TaC, NbC, VC) are stable up to 900°C and remain as fine precipitates within the recrystallized grains, contributing to solid-solution and precipitation strengthening. M23C6 carbides (Cr-rich) dissolve during the welding thermal cycle and may re-precipitate at grain boundaries during subsequent post-weld heat treatment (PWHT). The Ta addition specifically promotes the formation of (Nb,Ta)C, which pins grain boundaries and inhibits recrystallization grain growth, resulting in finer weld nugget grains.
Stir Zone Flow Patterns: The characteristic onion-ring flow lines visible in the cross-section of the stir zone reflect the material flow pattern around the rotating pin. These flow lines represent boundaries between material packets that have undergone different deformation histories, and they serve as preferred paths for crack initiation under cyclic loading.
2.3 Metallographic Examination Protocols
- Etching: 3–5% Nital (HNO₃ in ethanol) for ferrite grain structure; 10% picric acid in ethanol for carbide distribution
- SEM-EDS: For carbide identification and chemical composition mapping
- EBSD (Electron Backscatter Diffraction): For quantitative grain size, orientation, and recrystallization fraction analysis
- TEM (Transmission Electron Microscopy): For dislocation density measurement and fine precipitate characterization
3. Impact Performance Analysis
3.1 Test Methodology
Charpy V-notch (CVN) impact testing is the primary method for evaluating the fracture toughness and ductile-to-brittle transition behavior of 9Cr-1.5W-0.15Ta FSW welds. Testing follows ASTM E23 / GB/T 229 protocols:
| Parameter | Specification |
|---|---|
| Specimen Type | Standard 20 mm × 10 mm × 55 mm CVN (or sub-size 10 mm × 10 mm × 55 mm) |
| Test Temperatures | -20°C, 0°C, 20°C, 100°C, 200°C, 350°C, 450°C |
| Notch Orientation | Longitudinal (L), Transverse (T), Short Transverse (ST) per ASTM E23 |
| Impact Velocity | Per ASTM E23 (typically 3.2–4.0 m/s for 20 mm specimens) |
| Acceptance Criteria | ≥27 J at 20°C (per ASME BPV VIII Div.2); ≥41 J at 20°C (per GB/T 20878 for nuclear applications) |
3.2 Impact Energy Distribution Across Weld Zones
Impact test results in 9Cr-1.5W-0.15Ta FSW welds typically show the following distribution patterns:
| Zone | 20°C Impact Energy (J) | 350°C Impact Energy (J) | Fracture Mode |
|---|---|---|---|
| Base Metal (tempered) | 35–55 | 60–90 | Ductile (fibrous) |
| Weld Nugget (as-welded) | 25–45 | 50–75 | Mixed ductile/intergranular |
| HAZ (as-welded) | 15–30 | 35–55 | Intergranular (brittle) |
| Weld Nugget (after PWHT) | 30–50 | 65–95 | Ductile (fibrous) |
3.3 Post-Weld Heat Treatment (PWHT) Effects
The as-welded FSW microstructure in 9Cr-1.5W steel retains elevated dislocation density and carbide-free zones at grain boundaries, which can lead to susceptibility to intergranular cracking and reduced creep resistance. PWHT (typically 760°C × 2 h × air cool per ASTM A387 / GB/T 20878) is essential to:
- Re-precipitate M23C6 carbides at grain boundaries, restoring creep strength
- Temper the martensite in the HAZ, reducing hardness and improving toughness
- Relieve residual stresses (FSW residual stresses are significantly lower than fusion welding, typically 100–200 MPa vs. 300–500 MPa)
- Homogenize the microstructure between weld nugget and base metal
4. Connection to Company Technology Routes
4.1 Relevance to TIG/MIG Weld Overlay Operations
While FSW is a solid-state process distinct from the fusion-based TIG/MIG weld overlay routes employed by Cladding Technology Shanxi Co., Ltd., the metallurgical knowledge gained from FSW microstructure and impact performance analysis directly enhances the company's overlay capabilities:
- HAZ toughness management: Understanding the tempering response and carbide re-precipitation kinetics of 9Cr-1.5W steel informs preheat temperature selection (250–300°C per WPS), interpass temperature control (≤350°C), and PWHT parameters for TIG/MIG overlay welds on 9Cr-1.5W substrate
- Impact qualification: The impact performance data from FSW studies provides baseline toughness values for the base metal and HAZ, enabling accurate interpretation of overlay weld impact test results
- Material flow understanding: Knowledge of solid-state plastic deformation behavior aids in understanding cold cracking susceptibility and hydrogen diffusion pathways in thick-section 9Cr-1.5W weld overlays
- Post-weld heat treatment optimization: The PWHT parameters validated through FSW research (760°C × 2 h) are directly applicable to overlay weld PWHT schedules
4.2 Relevance to Hydraulic Explosive Bonding (HEB)
Hydraulic explosive bonding involves high-strain-rate plastic deformation at the interface of dissimilar materials. The microstructural knowledge from FSW analysis — particularly regarding dynamic recrystallization, carbide stability, and impact energy distribution in heavily deformed zones — provides valuable insights for:
- Predicting interfacial microstructure in HEB joints involving 9Cr-1.5W and austenitic stainless steel cladding layers
- Assessing impact toughness of the severely deformed interface zone in HEB clad plates
- Optimizing bonding parameters (standoff distance, explosive charge configuration) to achieve target interfacial strain without excessive embrittlement
4.3 Relevance to Explosion Welding (EW)
Explosion welding of 9Cr-1.5W base plates with austenitic stainless steel (304L, 316L) or nickel-base alloy (Inconel 625) cladding layers is a core capability of the company. The FSW research contributes to EW operations through:
- Interfacial microstructure prediction: Both FSW and EW involve high-strain-rate solid-state deformation; the recrystallization and carbide behavior observed in FSW is analogous to the dynamic recrystallization at the EW interface
- Impact performance benchmarking: Understanding the minimum impact energy achievable in heavily deformed 9Cr-1.5W material helps set acceptance criteria for EW clad plate impact testing (typically requiring ≥27 J at 20°C for the interface zone per GB/T 13183)
- Defect identification: Knowledge of flow line patterns and potential cracking mechanisms in FSW welds aids in interpreting EW interface quality (wave amplitude, bonding ratio, void distribution)
5. Applicable Standards and Acceptance Criteria
| Standard | Scope | Key Requirements for 9Cr-1.5W |
|---|---|---|
| GB/T 20878-2007 | Stainless and heat-resistant steel for nuclear power | Impact ≥41 J at 20°C; Charpy at 20°C and 350°C |
| ASME BPV Section VIII Div.2 | Pressure vessels (alternative rules) | Impact ≥27 J at 20°C; FSW qualification per UW-4 |
| ASTM A387/A387M | Castings for pressure vessels | PWHT at 760°C; hardness ≤235 HBW |
| ASTM E23 | Impact testing methodology | Specimen preparation, test velocity, temperature control |
| NB/T 20264 | Welding procedure qualification for nuclear components | WPS qualification; impact testing at RT and service temperature |
| GB/T 13183 | Explosion-welded clad plates | Impact ≥27 J at 20°C; bonding ratio ≥95% |
| ASME BPV Section IX | Welding qualifications | PQR/WPS qualification; impact per QW-420/QW-430 |
| ISO 15614-2 | Qualification testing for arc welding | Procedure qualification; impact testing requirements |
| ASTM E10 / GB/T 231.1 | Hardness testing (Rockwell B / Vickers) | Hardness ≤235 HBW (base metal and weld metal) |
6. Common Risks and Controls
6.1 Technical Risks in 9Cr-1.5W Welding/Joining
- Cold cracking (hydrogen-induced): 9Cr-1.5W has high hardenability; residual stresses combined with diffusable hydrogen from welding moisture can cause delayed cracking. Control: Preheat to 250–300°C, limit interpass temperature to ≤350°C, post-weld bake at 150–200°C for 2 h to drive out hydrogen, use low-hydrogen consumables (E710T1, ER915)
- Tempered martensite embrittlement (TME): Exposure to 475–600°C during PWHT or service can cause irreversible embrittlement. Control: Limit PWHT time at 760°C to ≤4 h; avoid prolonged exposure in the 475–600°C range; rapid cool from PWHT temperature
- Creep rupture at high temperature: Carbide-free zones at grain boundaries after FSW can reduce creep life. Control: Mandatory PWHT to re-precipitate M23C6; verify carbide distribution by metallography post-PWHT
- Weld nugget grain coarsening (FSW-specific): Excessive welding speed or insufficient axial force can result in incomplete recrystallization and coarse grains. Control: Optimize tool rotation speed (1000–2000 rpm), traverse speed (20–50 mm/min), and axial force (20–40 kN) for 9Cr-1.5W
6.2 Quality Control Measures
- Pre-weld: Verify base material chemistry (9.0±0.4% Cr, 1.4±0.1% W, 0.12±0.03% Ta); confirm PWHT condition; perform hardness mapping (≤235 HBW)
- In-process: Monitor preheat temperature (thermocouple on joint); record welding parameters (FSW: speed, rotation, force; TIG: current, voltage, travel speed, gas flow)
- Post-weld: PWHT per WPS; NDT (UT per ASTM E164 / GB/T 11345, RT per ASTM E94 / GB/T 3323); impact testing per ASTM E23; hardness survey per ASTM E10
7. Application Scenarios and Customer Value
7.1 Power Generation Industry
9Cr-1.5W-0.15Ta steel is the primary material for USC boiler main steam piping (operating at 600–650°C, 25–31 MPa) and reheater tubing. Cladding Technology Shanxi Co., Ltd. provides:
- TIG weld overlay repair of 9Cr-1.5W main steam piping with corrosion-resistant cladding layers (Inconel 625, 321 stainless steel) at weld joints and erosion zones
- Explosion-welded clad plates (9Cr-1.5W + 304L/316L) for boiler economizer and air preheater components
- Hydraulic explosive bonded pipe assemblies for high-temperature steam lines requiring internal corrosion protection
7.2 Petrochemical and Refining
In hydrogen service and high-temperature reforming applications, 9Cr-1.5W components require overlay protection against sulfidation, oxidation, and hydrogen attack. The company delivers:
- MIG/TIG weld overlay of 309L/310S transition layers on 9Cr-1.5W reactor tubes and heat exchanger tubesheets
- Explosion-welded clad pipe (9Cr-1.5W base + Hastelloy C-276 cladding) for hydrocracker reactors
- Repair overlay welds on in-service 9Cr-1.5W components with qualified WPS and PQR documentation
7.3 Nuclear and Special Applications
For nuclear-grade 9Cr-1.5W components (per NB/T 20264), the metallurgical knowledge from FSW research supports:
- Development of qualified overlay welding procedures with verified impact performance at 20°C and service temperature
- Documentation and traceability of microstructural quality for nuclear regulatory inspections
- Explosion-welded clad components meeting GB/T 13183 bonding ratio requirements with verified interface toughness
8. Contribution to Qualification Building and Product Delivery
8.1 WPS Qualification Enhancement
The microstructure and impact performance data from FSW research directly supports the development and qualification of TIG/MIG overlay welding procedures (WPS) for 9Cr-1.5W substrate applications. Specifically:
- Preheat temperature justification: FSW research demonstrates that 9Cr-1.5W retains ductility above 600°C, supporting the selection of 250–300°C preheat for TIG overlay (sufficient to prevent cold cracking while avoiding TME)
- PWHT parameter validation: The 760°C × 2 h schedule validated through FSW microstructure studies is directly transferable to overlay weld PWHT
- Impact acceptance criteria: Baseline impact energy data enables defensible acceptance criteria setting in WPS qualification
8.2 Customer Technical Documentation
For each overlay or clad plate delivery, the company provides comprehensive technical documentation including:
- WPS/PQR with verified impact performance data
- NDT reports (UT, RT, MPI) per applicable codes
- Material traceability certificates (mill test reports, chemical analysis, mechanical properties)
- Microstructural examination reports for critical applications
- Hardness survey maps confirming ≤235 HBW compliance
8.3 Competitive Differentiation
The advanced metallurgical research capability demonstrated through FSW microstructure and impact performance analysis positions Cladding Technology Shanxi Co., Ltd. as a technically sophisticated provider capable of:
- Addressing complex dissimilar material joining challenges (9Cr-1.5W + austenitic stainless steel + nickel-base alloys)
- Providing root-cause analysis for field failures involving 9Cr-1.5W weld overlays
- Developing custom overlay solutions with verified performance data
- Meeting the most stringent qualification requirements (nuclear, aerospace, USC power generation)
9. Key Process Parameters Summary
| Parameter | FSW (9Cr-1.5W) | TIG Overlay (on 9Cr-1.5W) | MIG Overlay (on 9Cr-1.5W) |
|---|---|---|---|
| Preheat Temperature | 100–150°C (optional) | 250–300°C | 250–300°C |
| Interpass Temperature | N/A (single pass typical) | ≤350°C | ≤350°C |
| Welding Current | N/A (frictional heating) | 80–150 A (DCEN) | 150–250 A (short arc) |
| Travel/Traverse Speed | 20–50 mm/min | 300–600 mm/min | 500–1000 mm/min |
| Shielding Gas | N/A (or light He purge) | 100% Ar (8–12 L/min) | Ar + 2% O₂ (15–20 L/min) |
| PWHT | 760°C × 2 h × air cool | 760°C × 2 h × air cool | 760°C × 2 h × air cool |
| Post-Weld Bake | 150°C × 2 h (H₂ relief) | 150–200°C × 2 h | 150–200°C × 2 h |
| Hardness Limit | ≤235 HBW | ≤235 HBW (weld + HAZ) | ≤235 HBW (weld + HAZ) |
| Impact Requirement | ≥27 J @ 20°C | ≥27 J @ 20°C | ≥27 J @ 20°C |
10. Conclusions and Recommendations
The study of 9Cr-1.5W-0.15Ta heat-resistant steel FSW weld microstructure and impact performance provides Cladding Technology Shanxi Co., Ltd. with critical metallurgical intelligence that directly enhances the company's core capabilities in TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding. Key conclusions include:
- Microstructural control is paramount: The transition from tempered lath martensite (base metal) to equiaxed recrystallized ferrite (weld/overlay HAZ) fundamentally alters mechanical behavior, and PWHT is non-negotiable for restoring creep resistance and carbide stability.
- Impact performance is achievable but requires disciplined execution: With proper preheat, interpass control, and PWHT, 9Cr-1.5W weld overlays routinely achieve ≥27 J at 20°C, meeting ASME and GB/T requirements.
- The Ta addition is beneficial: The 0.15% Ta stabilizes fine MC carbides, promotes finer recrystallized grains, and enhances high-temperature toughness — making 9Cr-1.5W-0.15Ta a superior choice for USC applications.
- Cross-technology knowledge transfer is valuable: Solid-state joining metallurgy (FSW, EW, HEB) and fusion welding metallurgy (TIG, MIG) share fundamental principles regarding carbide behavior, phase transformations, and residual stress management.
- Documentation and traceability are competitive advantages: Comprehensive metallurgical data packages (microstructure, impact, hardness, NDT) provide customers with the confidence and regulatory compliance evidence required for critical applications.
Recommendations for ongoing capability development:
- Establish a dedicated metallurgical laboratory for in-house microstructural characterization and impact testing
- Develop a database of qualified WPS/PQR for all 9Cr-1.5W overlay configurations (309L, 310S, 321, Inconel 625, Hastelloy C-276)
- Pursue ASME "R" stamp and NB nuclear welding qualification to access the full range of USC and nuclear power generation projects
- Invest in advanced NDT capabilities (phased array UT, TOFD) for reliable detection of overlay weld defects in thick-section 9Cr-1.5W components