Welding and Submerged Arc Strip Electrode Cladding of 2¼Cr1Mo1¼V and 3Cr1Mo1¼V Steels for Hydrocracking Reactors
1. Definition and Technical Principles
2¼Cr1Mo1¼V and 3Cr1Mo1¼V steels are high-strength, creep-resistant, low-alloy steels designed for the most demanding service conditions in hydrotreating and hydrocracking reactors. These materials are engineered to withstand prolonged exposure to elevated temperatures (typically 400–550 °C), high pressures (up to 250 bar or above), and aggressive hydrogen-containing environments. The addition of vanadium (¼V) to the chromium-molybdenum matrix provides exceptional resistance to hydrogen attack (HTHA) and creep rupture, making these grades the preferred choice for reactor shells, flanges, nozzles, and internal components in modern refinery hydrocracking units.
Submerged arc welding (SAW) with strip electrodes—referred to in Chinese technical terminology as "带极堆焊" (strip electrode surfacing)—is a highly productive welding process that uses a wide, flat copper-coated strip electrode (typically 15–25 mm wide) in combination with granular flux to deposit a thick, uniform weld metal layer at high travel speeds. This process is particularly suited for:
- Thick cladding layers on reactor shell interiors or repair areas where multiple TIG passes would be prohibitively time-consuming.
- Weld buildup and repair of heavily worn or corroded surfaces on existing reactor components.
- Transition layers between base steel and hardfacing or corrosion-resistant overlay alloys.
- Full-penetration butt welds in thick-section fabrication where productivity is critical.
The fundamental principle of strip electrode SAW relies on the arc generated between the wide strip electrode and the workpiece, with the arc being submerged under a layer of granular flux. The flux serves multiple functions: it shields the molten weld pool from atmospheric contamination, acts as a slag former to protect the solidifying weld metal, and provides alloying additions. The wide electrode geometry distributes heat over a larger area, reducing peak temperatures and minimizing dilution, while enabling deposition rates of 5–15 kg/h—significantly higher than conventional TIG or MIG processes.
2. Category and Business Positioning
This capability falls squarely within the TIG/MIG weld overlay and surfacing technology route, specifically in the advanced submerged arc surfacing (SAS) subcategory. Within Cladding Technology Shanxi Co., Ltd.'s three primary technology routes, this entry represents a high-productivity, heavy-section manufacturing capability that complements the precision TIG overlay work and the bulk bonding achieved through hydraulic explosive bonding and explosion welding.
The business positioning of this technology is as follows:
- Strategic asset for large-scale reactor fabrication: Hydrocracking reactors are among the largest and most expensive pressure vessels in the oil and gas industry, with wall thicknesses commonly ranging from 80 mm to 200 mm. Strip electrode SAW enables economical fabrication of these thick sections.
- Repair and maintenance value proposition: In-service reactors often develop localized corrosion, hydrogen blistering, or mechanical damage. Strip electrode surfacing provides a cost-effective repair method for restoring dimensional integrity and applying protective overlay layers.
- Qualification and IP development: Mastering the welding and cladding of 2¼Cr1Mo1¼V and 3Cr1Mo1¼V steels requires deep metallurgical understanding and extensive WPS/PQR qualification, positioning the company as a specialist in high-value alloy welding.
3. Technical Purpose and Value
3.1 Metallurgical Challenges of 2¼Cr1Mo1¼V and 3Cr1Mo1¼V Steels
These steels present unique metallurgical challenges that must be addressed through careful process design:
- High hardenability: The combination of chromium (2.25–3.0%), molybdenum (0.90–1.10%), and vanadium (0.15–0.25%) creates a steel with significant hardenability, making the heat-affected zone (HAZ) susceptible to excessive hardness and cracking.
- Carbon equivalent: The carbon equivalent (CE) of these steels, calculated per IIW or GB/T 1942 formulas, typically ranges from 0.45 to 0.60, indicating high preheat requirements.
- Creep sensitivity: The microstructure of the HAZ must be carefully controlled to ensure long-term creep resistance, which is critical for reactor components operating at elevated temperatures.
- Hydrogen embrittlement susceptibility: Residual hydrogen in the weld metal and HAZ can cause delayed cracking, particularly in high-strength steels.
- Temper embrittlement: Prolonged exposure in the temperature range of 370–570 °C can cause intergranular embrittlement, which must be mitigated through proper heat treatment.
3.2 Value Delivered
The successful execution of welding and strip electrode cladding on these materials delivers substantial value across multiple dimensions:
- Production efficiency: Strip electrode SAW achieves deposition rates 3–5 times higher than TIG welding, reducing fabrication time and cost for thick-section components.
- Weld quality consistency: The submerged flux environment provides superior protection against nitrogen and oxygen pickup, resulting in cleaner weld metal with fewer inclusions.
- Dimensional accuracy: The wide, flat bead profile of strip electrode SAW provides uniform buildup with minimal post-machining required.
- Extended component life: Properly applied overlay layers protect reactor internals from hydrogen attack, erosion, and corrosion, extending operational life between turnaround inspections.
4. Key Process and Implementation Points
4.1 Material Selection and Classification
| Parameter | 2¼Cr1Mo1¼V | 3Cr1Mo1¼V |
|---|---|---|
| Typical ASTM Grade | ASTM A387 Gr. 22 | ASTM A387 Gr. 22 (modified) |
| C (wt%) | 0.30–0.40 | 0.30–0.40 |
| Cr (wt%) | 2.20–2.50 | 2.80–3.20 |
| Mo (wt%) | 0.90–1.10 | 0.90–1.10 |
| V (wt%) | 0.15–0.25 | 0.15–0.25 |
| Carbon Equivalent (CE) | 0.45–0.55 | 0.48–0.58 |
| Typical Application | Hydrocracking reactor shell, flanges | High-pressure reactor internals |
4.2 Weld Consumable Selection
The selection of welding consumables is critical for ensuring metallurgical compatibility, mechanical properties, and long-term service performance:
| Welding Process | Consumable Type | Examples | Key Considerations |
|---|---|---|---|
| Strip Electrode SAW (Cladding) | Strip Electrode + Flux | Strip: 2½CrMoV equivalent; Flux: Low-alloy basic flux | Match base metal chemistry; control dilution; ensure low hydrogen content |
| SAW (Butt Weld) | Wire Electrode + Flux | Wire: AWS A5.17 E80T-6 equivalent; Flux: Low-hydrogen basic flux | Multi-pass sequence; interpass temperature control |
| TIG (Transition/Overlay) | Filler Wire | AWS A5.5 ER80S-Mo6V1 or equivalent | Low dilution; precise control for thin transition layers |
| MIG (Buildup) | Solid Wire | 2¼Cr1MoV equivalent solid wire | Shielding gas composition; wire feed speed control |
4.3 Critical Process Parameters
| Parameter | Typical Range | Notes |
|---|---|---|
| Preheat Temperature | 200–350 °C (minimum 200 °C for thicknesses >25 mm) | Controlled by CE value and joint thickness; verify with thermocouples |
| Interpass Temperature | 200–300 °C (maximum 300 °C) | Exceeding this range risks HAZ softening and reduced creep strength |
| Deposition Rate (Strip SAW) | 5–15 kg/h | Depends on strip width, current, and travel speed |
| Welding Current (Strip SAW) | 600–1200 A | Adjust based on strip width and desired penetration |
| Travel Speed (Strip SAW) | 200–500 mm/min | Coordinate with current for optimal bead geometry |
| Post-Weld Heat Treatment (PWHT) | 720–760 °C, hold 1–3 hours per 25 mm thickness | Essential for stress relief and microstructural stabilization |
| Cooling Rate (Post-PWHT) | Controlled furnace cooling or furnace hold | Avoid rapid cooling to prevent re-tempering embrittlement |
4.4 Strip Electrode Cladding Implementation Sequence
- Surface Preparation: Grind the base surface to a clean, uniform finish. Remove all contaminants, oxide, and scale. For cladding on existing reactor internals, remove hydrogen blisters and corroded material to sound metal.
- Preheat Application: Apply preheat uniformly across the entire work area using induction heating or propane gas torches. Verify preheat temperature with calibrated thermocouples or infrared pyrometers at multiple locations. Maintain preheat temperature throughout the welding operation.
- Flux Drying: Dry the granular flux in a dedicated flux dryer at 250–300 °C for a minimum of 4 hours. Use a flux hopper with controlled atmosphere to prevent moisture reabsorption. Monitor flux moisture content; it must remain below 0.2% for low-hydrogen performance.
- Welding Pass Execution: Perform the strip electrode SAW passes with consistent travel speed and current. For thick cladding layers, execute multiple passes with proper interpass temperature control. The first pass may require slightly higher current for adequate base metal fusion.
- Post-Weld Heat Treatment: Subject the welded/cladded component to PWHT at 720–760 °C. The soaking time should be calculated at approximately 1 hour per 25 mm of effective thickness (minimum 2 hours). Ramp rates should not exceed 150 °C/h during heating and 100 °C/h during cooling to prevent thermal stresses.
- Non-Destructive Testing (NDT): Perform NDT after PWHT, including ultrasonic testing (UT) for volumetric defects, magnetic particle inspection (MT) or liquid penetrant inspection (PT) for surface defects, and dimensional verification.
4.5 Multi-Pass Welding Strategy for Thick Sections
For reactor shell fabrication where wall thicknesses exceed 50 mm, a systematic multi-pass welding strategy is essential:
- Root Pass: TIG welding with ER80S-Mo6V1 filler wire for precise control of the root geometry and minimum dilution.
- Filler Passes: SAW with wire electrode (E80T-6 equivalent) for the bulk of the weld metal, using a multi-pass sequence that ensures uniform microstructure development.
- Cap Pass: TIG or SAW with wire electrode for the final cap, ensuring proper surface geometry and full fusion with the preceding passes.
- Cladding Passes (if applicable): Strip electrode SAW for applying overlay layers, with each pass providing 3–6 mm of buildup.
5. Applicable Standards and Acceptance Criteria
5.1 Welding Procedure Standards
- ASME Section IX: Governs qualification of welding procedures, welders, and welding operators. WPS and PQR must be qualified in accordance with QW-400 through QW-460 for SAW processes.
- GB/T 1942-2011: Chinese national standard for determining carbon equivalent in carbon and low-alloy steels, used to assess weldability and preheat requirements.
- NB/T 47014-2011: Chinese industry standard for qualification testing of welding procedures for pressure vessels and piping.
- ASME BPV Code Section VIII, Div. 1: Governs design, fabrication, and inspection of pressure vessels, including hydrocracking reactor shells.
- ASME BPV Code Section VIII, Div. 2: Alternative code case for pressure vessels with more detailed design requirements.
- API 579-1/ASME FFS-1: Fitness-for-service assessment standard, relevant for in-service repair and cladding of existing reactor components.
5.2 Material Standards
- ASTM A387 / A387M: Specification for Cr-Mo-V alloy steel plates for high-temperature service. Grade 22 corresponds to 2¼Cr1Mo1¼V.
- ASTM A217 / A217M: Specification for castings, iron-cast, for pressure-containing parts (for cast reactor components).
- ASME SA-387: ASME designation for Cr-Mo-V steel plates.
- GB/T 12229-2015: Chinese standard for Cr-Mo-V steel plates for high-temperature service.
5.3 Weld Consumable Standards
- AWS A5.17 / A5.17M: Specification for flux-cored and submerged arc welding electrodes. E80T-6 is the typical classification for 2¼Cr1MoV applications.
- AWS A5.5 / A5.5M: Specification for welding electrodes for TIG and MIG. ER80S-Mo6V1 is specified for 2¼Cr1MoV applications.
- EN ISO 14343: European standard for submerged arc welding consumables.
- GB/T 12470-2008: Chinese standard for submerged arc welding flux.
5.4 Inspection and Acceptance Criteria
| Inspection Method | Standard | Acceptance Criteria |
|---|---|---|
| Ultrasonic Testing (UT) | ASME BPV Code Sec. V, Art. 4; NB/T 47013.3 | Level 2 acceptance per ASME; no cracks, laminations, or slag inclusions exceeding specified limits |
| Magnetic Particle Inspection (MT) | ASME BPV Code Sec. V, Art. 7; NB/T 47013.4 | No linear indications; rounded indications limited to 3 mm length |
| Visual Inspection (VT) | ASME BPV Code Sec. V, Art. 9 | No cracks, undercut, porosity, or excessive reinforcement |
| Hardness Testing | ASME BPV Code Sec. V, Art. 22 | HAZ hardness ≤ 350 HV10 (or per material specification); uniform hardness profile |
| Hydrostatic Testing | ASME BPV Code Sec. VIII, Div. 1, UG-99 | No leakage at 1.3× MAWP for minimum 30 minutes |
| Flaw Size Limit | ASME BPV Code Sec. VIII, Div. 1, UW-51 | Slag inclusions ≤ 1/8 wall thickness; porosity ≤ 1/16 wall thickness |
6. Common Risks and Controls
6.1 Hydrogen-Induced Cracking (HIC)
Risk: Residual hydrogen trapped in the weld metal and HAZ during cooling can cause delayed cracking, particularly in high-strength steels like 2¼Cr1Mo1¼V. This is the most critical failure mode for these materials.
Controls:
- Use low-hydrogen welding consumables (diffusible hydrogen content ≤ 5 ml/100g for SAW flux).
- Thoroughly dry flux at 250–300 °C for minimum 4 hours before use; store in heated hopper.
- Apply adequate preheat (minimum 200 °C) to slow cooling rate and allow hydrogen diffusion.
- Control interpass temperature to maintain above 200 °C until the entire weld is complete.
- Perform hydrogen bake-out at 250 °C for 1–2 hours immediately after welding, if required by the WPS.
- Conduct delayed UT (24–72 hours after welding) to detect any delayed cracking.
6.2 Excessive HAZ Hardness
Risk: The high hardenability of 2¼Cr1Mo1¼V and 3Cr1Mo1¼V steels can result in HAZ hardness exceeding 350 HV10, creating a susceptible zone for hydrogen cracking and reduced ductility.
Controls:
- Apply preheat at the minimum specified temperature (200–350 °C) to reduce cooling rates.
- Limit heat input per pass to avoid excessive grain growth in the HAZ.
- Use consumables with appropriate alloy composition to promote tempered microstructure in the weld metal.
- Perform PWHT at 720–760 °C to temper the HAZ and reduce hardness.
- Verify HAZ hardness with a traverse survey per ASME Sec. V, Art. 22.
6.3 Temper Embrittlement
Risk: Prolonged exposure in the temperature range of 370–570 °C during PWHT or service can cause intergranular embrittlement, significantly reducing fracture toughness.
Controls:
- Minimize time spent in the embrittlement range during PWHT by using controlled ramp rates.
- Use furnace cooling from PWHT temperature rather than air cooling to avoid passing through the critical range too quickly.
- Consider adding small amounts of niobium or titanium to the base material (if specified) to mitigate temper embrittlement.
- Perform Charpy V-Notch (CVN) testing at the minimum service temperature to verify adequate toughness.
6.4 Dilution and Microsegregation in Strip Electrode Cladding
Risk: In strip electrode SAW cladding, the wide bead geometry can result in significant dilution between the base metal and the overlay layer, potentially altering the chemical composition and mechanical properties of the cladding.
Controls:
- Use a strip electrode composition that is slightly enriched in alloying elements to compensate for dilution.
- Perform chemical analysis on the first cladding pass to verify dilution level; adjust composition for subsequent passes.
- Limit the number of cladding passes to the minimum required for the specified overlay thickness.
- Consider using a TIG-welded transition layer as the first pass to establish a controlled dilution zone before strip electrode SAW cladding.
6.5 Residual Stress and Distortion
Risk: The high heat input of strip electrode SAW can generate significant residual stresses and distortion, particularly in thick-section reactor components.
Controls:
- Use a symmetric welding sequence to balance thermal input across the component.
- Apply back-heat or backing bars to control cooling rates and reduce residual stress.
- Perform PWHT to relieve residual stresses to below 20% of yield strength.
- Implement distortion monitoring with dial gauges or laser trackers during fabrication.
- Use jigs and fixtures to constrain the component during welding and minimize distortion.
7. Application Scenarios Across the Three Technology Routes
7.1 TIG/MIG Weld Overlay Route
The TIG/MIG weld overlay route is the primary technology platform for this capability. The following application scenarios are typical:
- Reactor Shell Fabrication: Full-scale fabrication of hydrocracking reactor shells using multi-pass SAW for butt welds, with TIG for root and cap passes. Strip electrode SAW for any required overlay layers on internal surfaces.
- Nozzle and Flange Cladding: Application of corrosion-resistant overlay layers on reactor nozzles and flanges using TIG welding for precision control in tight geometries.
- Transition Layer Welding: TIG welding of transition layers between dissimilar materials (e.g., between 2¼Cr1Mo1¼V shell and 309L/310L stainless steel internal components).
- Repair and Restoration: TIG welding for localized repairs on in-service reactor components where access is limited and precision is required.
- Buildup Welding: MIG welding for restoring worn surfaces on reactor internals such as distribution cones, support plates, and catalyst baskets.
7.2 Hydraulic Explosive Bonding Route
The hydraulic explosive bonding route is less directly applicable to the welding of 2¼Cr1Mo1¼V and 3Cr1Mo1¼V steels, but the following synergistic applications exist:
- Clad Plate Production: Hydraulic explosive bonding can be used to produce clad plates with a 2¼Cr1Mo1¼V base layer and a stainless steel or nickel-based overlay, which are then fabricated into reactor components using the welding capabilities described in this entry.
- Pre-Fabricated Clad Components: Hydraulic explosive bonding can create pre-clad pipe sections and flanges that are then welded into reactor assemblies, reducing the amount of in-situ cladding required.
- Material Development: Understanding the welding behavior of 2¼Cr1Mo1¼V steels informs the design of clad plate configurations, ensuring that the base material is suitable for subsequent welding operations.
7.3 Explosion Welding Route
Explosion welding is primarily a bonding technology for creating clad plates and pipes, and its relationship to this entry is primarily in the upstream supply of clad materials:
- Clad Plate Supply for Reactor Fabrication: Explosion-welded clad plates (e.g., 2¼Cr1Mo1¼V base with 304L or 316L overlay) can be supplied to reactor fabrication operations, where the welding capabilities described in this entry are used to join these clad plates into pressure-retaining assemblies.
- Explosion-Welded Pipe Cladding: Explosion-welded clad pipes for high-pressure hydrogen service can be welded into reactor internals using qualified WPS procedures for 2¼Cr1Mo1¼V steels.
- Material Compatibility Studies: The welding qualification work performed for 2¼Cr1Mo1¼V and 3Cr1Mo1¼V steels provides valuable data on weldability, which informs the selection of base materials for explosion welding operations.
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
Mastering the welding and strip electrode cladding of 2¼Cr1Mo1¼V and 3Cr1Mo1¼V steels represents a significant qualification milestone for Cladding Technology Shanxi Co., Ltd. The following qualifications are established or enhanced:
- WPS Qualification: Development and qualification of multiple WPS for SAW (wire and strip electrode), TIG, and MIG processes on 2¼Cr1Mo1¼V and 3Cr1Mo1¼V steels, covering a wide range of thicknesses, joint geometries, and positions.
- Welder Qualification: Training and qualification of welders on these high-alloy materials, ensuring a skilled workforce capable of producing code-compliant welds.
- Procedure Qualification Records (PQR): Accumulation of PQR data including mechanical testing (tensile, Charpy, hardness), chemical analysis, and NDT results, providing a robust database for future WPS development.
- Code Case Eligibility: Familiarity with ASME and NB code requirements enables the company to pursue code case approvals for novel welding procedures.
8.2 Product Delivery
The technical capabilities described in this entry directly enable the delivery of the following products:
- Hydrocracking Reactor Shells: Fabrication of complete reactor shells with qualified welds meeting ASME BPV Code requirements.
- Reactor Flanges and Nozzles: Manufacturing of high-pressure flanges and nozzles with corrosion-resistant overlay layers.
- Repair and Overhaul Services: In-situ repair of existing reactor components, including cladding of worn or corroded surfaces, restoration of dimensional integrity, and replacement of damaged sections.
- Clad Components for Hydrogen Service: Production of clad plates, pipes, and fittings suitable for high-pressure hydrogen-containing environments.
8.3 Customer Value
The value proposition to customers is multi-faceted:
- Cost Reduction: Strip electrode SAW reduces fabrication time and labor costs compared to conventional TIG welding, translating to lower project costs for reactor fabrication.
- Quality Assurance: Extensive WPS qualification and adherence to international standards (ASME, NB, ASTM) provide customers with confidence in weld quality and long-term service performance.
- Extended Asset Life: Properly applied overlay layers protect reactor components from hydrogen attack and corrosion, extending operational life and reducing the frequency of major overhauls.
- Technical Expertise: Deep metallurgical understanding of 2¼Cr1Mo1¼V and 3Cr1Mo1¼V steels enables the company to provide technical consulting services, including weld procedure development, failure analysis, and repair recommendations.
- Regulatory Compliance: Ability to produce code-stamped products meeting ASME BPV Code, NB/T standards, and API requirements, enabling customers to meet regulatory and insurance requirements.
9. Summary and Recommendations
The welding and submerged arc strip electrode cladding of 2¼Cr1Mo1¼V and 3Cr1Mo1¼V steels for hydrocracking reactors represents a high-value, technically demanding capability that positions Cladding Technology Shanxi Co., Ltd. as a specialist in high-alloy steel fabrication for the oil and gas industry. The key recommendations for continued development of this capability are:
- Expand WPS Database: Develop and qualify additional WPS covering a broader range of thicknesses, joint configurations, and welding positions to maximize flexibility in customer projects.
- Invest in Advanced NDT: Equip the facility with phased array ultrasonic testing (PAUT) and time-of-flight diffraction (TOFD) capabilities for enhanced defect detection in thick-section welds.
- Develop Digital Welding Monitoring: Implement real-time welding parameter monitoring and data logging systems to ensure process consistency and traceability.
- Strengthen Metallurgical Laboratory: Invest in metallographic and mechanical testing capabilities to support PQR development and failure analysis.
- Pursue International Certifications: Obtain ASME "U" stamp and "R" stamp certifications to enable global market access, and pursue ISO 3834-2 welding quality management certification.
- Develop Technical Publications: Publish technical papers and present at industry conferences to establish thought leadership in high-alloy steel welding and cladding.
By continuing to invest in this capability, Cladding Technology Shanxi Co., Ltd. can solidify its position as a leading provider of welding and cladding solutions for the world's most demanding pressure vessel applications, delivering superior quality, cost efficiency, and technical expertise to its global customer base.