High-Pressure Liquid CO₂ Fracturing Equipment: Clad and Overlay Engineering for Underground Coal-Rock Permeability Enhancement

1. Definition and Technical Principles

High-pressure liquid CO₂ fracturing (also referred to as supercritical CO₂ hydraulic fracturing or liquid CO₂ hydraulic fracturing) is an advanced reservoir stimulation technique applied in coalbed methane (CBM) extraction and enhanced coalbed methane (ECBM) production. The technology involves injecting pressurized liquid carbon dioxide—typically maintained at pressures exceeding 7.5 MPa and temperatures above 31.1 °C (the critical point of CO₂)—into coal seams to generate complex fracture networks that enhance gas permeability and recovery rates. The "complete equipment set" (成套装备) referenced in this capability entry encompasses the full suite of ground and underground hardware required for this process, including high-pressure injection units, storage vessels, transport piping, control manifolds, and downhole tools.

The fundamental principle relies on the phase transition behavior of CO₂ under reservoir conditions. Liquid CO₂ injected into the formation expands dramatically upon depressurization (a volumetric expansion ratio of approximately 1:500 at standard conditions), generating sufficient pressure to propagate fractures through coal and surrounding rock. Unlike conventional water-based fracturing, CO₂ fracturing avoids water imbibition damage to coal matrix, reduces formation damage, and provides a dual benefit of carbon sequestration alongside gas recovery enhancement.

From a materials engineering perspective, the equipment must withstand a demanding combination of service conditions: sustained high-pressure operation (up to 35 MPa in some designs), cryogenic temperatures during CO₂ charging and transport (down to −40 °C for liquid CO₂ handling), cyclic thermal loading, and potential corrosion from trace impurities in the CO₂ stream (such as H₂S, CO₂ hydrate formation, or residual moisture-induced carbonic acid corrosion). This is precisely where the company's cladding and weld overlay expertise becomes critical to equipment integrity and service life.

2. Category and Business Positioning

This capability entry positions Cladding Technology Shanxi Co., Ltd. at the intersection of its core metallurgical expertise and the rapidly growing energy transition market. The business positioning spans three distinct value layers:

Equipment Manufacturing Layer: Supply of clad and overlay-fabricated high-pressure vessels, piping assemblies, and manifold components for CO₂ fracturing systems. This represents a direct product delivery capability aligned with the company's existing weld overlay and explosion welding production lines.
Engineering Consultancy Layer: Material selection, WPS (Welding Procedure Specification) qualification, and NDT (Non-Destructive Testing) certification services for equipment manufacturers and end-users in the CBM/ECBM sector. This builds the company's qualification portfolio in the energy equipment domain.
Technical Partnership Layer: Collaborative R&D with coal mining enterprises and equipment integrators to develop optimized material solutions for next-generation fracturing equipment. This strengthens the company's position as a materials solutions provider in the coal-to-clean-energy transition pathway.

Within Shanxi Province's coal industry ecosystem, this capability directly supports the provincial government's strategic objectives for CBM development, carbon neutrality, and coal mine safety enhancement. It represents a natural extension of the company's existing capabilities in high-pressure equipment cladding (particularly for petrochemical and power generation sectors) into the mining and gas extraction domain.

3. Technical Purpose and Value

The technical purpose of developing and applying clad/overlay technology to high-pressure liquid CO₂ fracturing equipment is multi-dimensional:

3.1 Corrosion and Wear Protection

High-pressure CO₂ in contact with moisture forms carbonic acid (H₂CO₃), creating a corrosive environment particularly aggressive to carbon steel and low-alloy steel base materials. Weld overlay and cladding with austenitic stainless steels (e.g., 309L, 316L, 321) or nickel-based alloys (e.g., Hastelloy C-276, Inconel 625) on pressure vessel shells, piping, and fittings provides the necessary corrosion resistance while maintaining the structural strength of the base material.

3.2 Low-Temperature Toughness

Liquid CO₂ handling requires materials that maintain adequate impact toughness at temperatures as low as −40 °C to −60 °C. Overlay cladding with austenitic stainless steels ensures that the critical pressure-containing surfaces retain ductility and fracture resistance under cryogenic conditions, preventing brittle fracture in the event of rapid depressurization.

3.3 Cost Optimization

Full construction of high-pressure CO₂ equipment from expensive alloy materials is economically prohibitive. The hybrid approach—carbon steel or low-alloy steel base with a thin clad or overlay layer—achieves the required surface properties at 40–60% cost reduction compared to solid alloy construction, making large-scale equipment deployment economically viable.

3.4 Regulatory Compliance and Safety

Coal mine underground equipment must comply with stringent safety standards (including explosion-proof requirements, pressure vessel codes, and mine safety regulations). Properly qualified cladding and overlay processes with documented NDT and material certification ensure regulatory compliance and reduce the risk of catastrophic failure in confined underground environments.

4. Key Process and Implementation Points

4.1 Material Selection Matrix

Equipment Component	Base Material	Clad/Overlay Material	Process Route	Typical Clad Thickness	Design Pressure
CO₂ Storage Vessel (Shell)	16MnR / Q345R	309L + 316L (multi-layer)	Explosion Welding (exploded plate)	3–6 mm	35 MPa
High-Pressure Injection Piping	20# Carbon Steel / 15CrMo	309L (single layer)	TIG Weld Overlay (GTAW)	3–5 mm	35 MPa
Manifold / Control Valve Body	ASTM A350 LF2	316L (multi-pass)	MIG Weld Overlay (GMAW)	4–8 mm	25 MPa
Downhole Connector / Coupling	ASTM A105 / A350 LF3	Hastelloy C-276	TIG Weld Overlay (precision)	2–4 mm	40 MPa
Heat Exchanger Tubes	20# / 15CrMo	321 / Inconel 625	Hydraulic Explosive Bonding	1.5–3 mm	20 MPa
Pressure Relief Valve Seat	Carbon Steel	Stellite 6 / 309L	TIG Weld Overlay	2–3 mm	35 MPa

4.2 TIG/MIG Weld Overlay Implementation

For high-pressure CO₂ fracturing equipment, TIG (Gas Tungsten Arc Welding, GTAW) and MIG (Gas Metal Arc Welding, GMAW) weld overlay processes are the primary methods for applying corrosion-resistant cladding layers. The following implementation parameters are critical:

Preheat Temperature: 100–150 °C for carbon steel base materials; 150–250 °C for low-alloy steels. Preheating minimizes hydrogen-induced cracking in the heat-affected zone (HAZ) and reduces residual stress in high-pressure components.
Interpass Temperature: Maintained below 150 °C for austenitic overlay deposits to prevent sensitization and intergranular corrosion. For nickel-based overlays, interpass temperatures of 100–200 °C are specified.
Deposition Rate: 1.5–3.0 kg/h for TIG overlay (single wire); 5.0–10.0 kg/h for MIG overlay (flux-cored or solid wire). Deposition rate optimization balances productivity with dilution control.
Dilution Control: Base metal dilution must be controlled to ≤ 30% for single-layer TIG overlay and ≤ 40% for multi-pass MIG overlay. Multi-layer techniques (3–5 passes) progressively reduce dilution to achieve the required alloy composition in the final surface layer.
Wire Feed Parameters: TIG: 4.0–6.0 m/min at 180–260 A; MIG: 6.0–12.0 m/min at 160–240 A, depending on wire diameter (1.2–2.4 mm) and required deposition profile.
Shielding Gas: 100% Ar for TIG overlay on stainless/nickel-based materials; Ar + 5–10% CO₂ or pure Ar for MIG overlay, depending on wire type.

4.3 Hydraulic Explosive Bonding Implementation

Hydraulic explosive bonding (also known as explosive welding or explosion bonding) is employed for producing large-format clad plates used in CO₂ storage vessel fabrication. The process involves detonating a shaped explosive charge positioned between a flyer plate (clad material) and a base plate, generating a collision velocity of 2–3 km/s that creates a metallurgical bond through hydrodynamic jetting and interfacial turbulence.

Collision Velocity: Optimized at 2.0–2.5 km/s for 316L-on-Q345R systems; 2.5–3.0 km/s for nickel-based clad systems.
Collision Angle: 15°–20° for stainless steel systems; 18°–25° for nickel alloy systems.
Bond Line Quality: Verified by macroscopic etching (4% HNO₃ + 5% HF solution) and microscopic examination per ASTM A497. A wavy bond line without voids, cracks, or unmelted inclusions indicates acceptable bonding quality.
Post-Bond Heat Treatment: Solution annealing at 1050–1100 °C for austenitic stainless clad plates to relieve residual stresses and restore full corrosion resistance. Subsequent pickling and passivation treatment.

4.4 Explosion Welding for Specialized Components

Explosion welding is also applied to produce clad pipe and pipe fittings for high-pressure CO₂ injection lines. The process is particularly advantageous for small-diameter tubing (OD ≤ 108 mm) where hydraulic explosive bonding equipment may not be available. Key considerations include:

Charge configuration optimization for cylindrical geometries to ensure uniform bond quality around the full circumference.
Post-explosion machining to achieve dimensional tolerances per ASME B16.9 (welded butt-weld fittings) or ASME B36.19M (welding fittings).
Full circumference NDT (RT + PT) of the bond line for pressure-containing applications.

4.5 Weld Procedure Qualification (WPQ) and WPS Development

Each overlay process applied to CO₂ fracturing equipment must be qualified through formal WPQ testing prior to production. The qualification program includes:

WPS Development: Documented welding procedure specifications covering base material, filler metal, process parameters, preheat, interpass temperature, and post-weld treatment.
Coupons Testing: Mechanical testing (tensile, bend, impact) of coupon welds per AWS D10.9M (Stainless Steel Clad Plate Welding) or AWS D16.1 (Specification for Welding Clad Plate).
Corrosion Testing: Salt spray testing per ASTM B117, CO₂ corrosion testing in simulated reservoir conditions (high-pressure CO₂ + H₂O at 60–120 °C), and intergranular corrosion testing per ASTM A262 Practice E.
NDT Requirements: 100% visual inspection (VT), 100% magnetic particle testing (MT) or penetrant testing (PT) of overlay surfaces, and radiographic testing (RT) or ultrasonic testing (UT) of the overlay-to-base interface for critical pressure components.

5. Applicable Standards and Acceptance Criteria

5.1 Pressure Vessel and Piping Codes

GB 150.1–150.4 — Pressure Vessels (Chinese national standard for design, materials, fabrication, inspection, and testing of pressure vessels)
TSG 21-2016 — Supervision and Inspection Rules for Fixed Pressure Vessels (Chinese regulatory standard)
ASME BPVC Section VIII Div. 1 & 2 — Construction Code for Pressure Vessels (for export or international projects)
ASME B31.3 — Process Piping (for high-pressure CO₂ piping systems)
GB/T 20801 — Pressure Piping (Chinese national standard for pressure piping design and fabrication)

5.2 Cladding and Overlay Standards

ASTM A497 — Standard Specification for Explosion Welding of Clad Plates
ASTM A240 — Standard Specification for Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip for Pressure Vessel Applications
AWS D10.9M — Specification for Welding Clad Plate (weld overlay procedure qualification and performance)
AWS D16.1 — Specification for Welding Clad Plate (alternative qualification standard)
GB/T 17748 — Steel Explosively Welded Clad Plates (Chinese standard for explosion-welded clad plate)
EN 12509 — Piping — Clad Pipes — Requirements and Testing (for clad piping applications)
ISO 19830 — Welding of Clad Materials (international standard for clad material welding)

5.3 Materials and Corrosion Standards

GB/T 24511 — Non-Destructive Testing of Welds in Metallic Materials (NDT methods and acceptance criteria)
GB/T 3323 — Non-Destructive Testing — Radiographic Examination of Welds
GB/T 11345 — Non-Destructive Testing — Ultrasonic Testing of Welds
ASTM B117 — Standard Practice for Salt Spray (Fog) Testing
NACE MR0175 / ISO 15156 — Materials for Use in H₂S-Containing Environments (applicable when CO₂ stream contains H₂S impurities)
GB/T 19079 — Welding Procedure Specification for Stainless Steel Overlay Welding

5.4 Coal Mine and Underground Equipment Standards

MT/T 1046 — Coal Mine Equipment — Explosion-Proof Requirements
GB 3836 — Electrical Equipment for Explosive Atmospheres (for associated electrical control systems)
ACQ/T — Coal Mine Safety Certification (mandatory safety approval for underground coal mine equipment in China)
API 562 — Specification for Hydrogen Service Seismic Resistant Bolting (reference for high-pressure bolted connections)

5.5 Acceptance Criteria Summary

Test/Inspection	Method Standard	Acceptance Level	Scope
Visual Inspection (VT)	GB/T 19866 / ISO 17637	No cracks, porosity > 0.5 mm, undercuts > 1 mm	100% of overlay surfaces
Magnetic Particle Testing (MT)	GB/T 26952 / ASTM E709	No linear indications; rounded indications ≤ 2 mm	100% of overlay welds
Radiographic Testing (RT)	GB/T 3323 / ASME V Art. 2	Level II: No cracks; porosity ≤ 1 mm (single), ≤ 4 mm (cluster)	100% for pressure components
Ultrasonic Testing (UT)	GB/T 11345 / ASME V Art. 4	No indications exceeding 20% DAC for planar defects	100% of clad interfaces
Hardness Testing	ASTM E92 / GB/T 231.1	Overlay: ≤ 30 HRC (stainless); ≤ 35 HRC (nickel-based)	Representative samples
Corrosion Testing	ASTM B117 / Custom CO₂ test	No pitting, intergranular corrosion, or uniform corrosion > 0.02 mm/yr	Qualification samples

6. Common Risks and Controls

6.1 Hydrogen-Induced Cracking (HIC) and Stress Corrosion Cracking (SCC)

Risk: Hydrogen atoms generated during the welding process can diffuse into the base metal, particularly in high-strength steels, causing delayed cracking. Additionally, CO₂ corrosion environments can induce SCC in sensitized austenitic overlays.

Controls: Strict preheat and interpass temperature control; low-hydrogen welding consumables; post-weld baking at 200–250 °C for 2 hours per 25 mm of weld metal thickness; avoidance of sensitization by controlling interpass temperature below 150 °C for austenitic overlays; optional stabilized grades (321, 347) for high-temperature service.

6.2 Excessive Dilution and Loss of Corrosion Resistance

Risk: High base metal dilution in overlay welds reduces the Cr and Ni content in the deposit, potentially falling below the minimum required for corrosion resistance in the CO₂ environment (typically Cr ≥ 18%, Ni ≥ 8%).

Controls: Multi-pass overlay technique with progressive dilution reduction; controlled first-pass geometry (shallow, wide profile); dilution monitoring through optical emission spectroscopy (OES) or lab analysis of representative coupons; WPS qualification with documented dilution limits.

6.3 Clad Delamination at Bond Interface

Risk: In explosion-welded clad plates, improper charge configuration or process parameters can result in incomplete bonding, voids, or weak interfaces susceptible to delamination under cyclic pressure loading.

Controls: Process parameter optimization through numerical simulation and test coupons; 100% UT inspection of bond interfaces; peel testing per ASTM A497 for qualification; post-weld stress relief heat treatment to minimize residual tensile stresses at the interface.

6.4 Cryogenic Brittle Fracture

Risk: Carbon steel base materials may lose impact toughness at liquid CO₂ temperatures (−40 °C to −60 °C), potentially leading to brittle fracture during rapid depressurization events.

Controls: Charpy V-notch impact testing of base material at minimum design temperature (MDD) per ASME VIII Div. 1 UG-20; selection of impact-tested base materials (e.g., 16MnDR, A333 Gr.6); overlay with austenitic materials that maintain ductility at cryogenic temperatures; avoidance of thick-section welds without post-weld heat treatment.

6.5 CO₂ Hydrate Formation

Risk: In the presence of water, CO₂ can form solid hydrates that block piping and equipment, creating localized pressure buildup and potential mechanical damage to clad surfaces.

Controls: Thorough drying of equipment before commissioning; continuous temperature monitoring above hydrate formation temperature; periodic flushing with methanol or MEG (monoethylene glycol) inhibitors; overlay materials selected for resistance to thermal cycling from hydrate formation/dissolution cycles.

7. Application Across the Company's Three Technology Routes

7.1 TIG/MIG Weld Overlay Applications

TIG (GTAW) and MIG (GMAW) weld overlay is the primary process route for CO₂ fracturing equipment components that require localized cladding of specific surfaces, repairs, or small-diameter piping. Applications include:

High-pressure injection pump cavities: Multi-pass 316L TIG overlay on 15CrMo pump bodies to resist CO₂ corrosion while maintaining high-strength base material for pressure containment.
Valve seats and trim: Precision TIG overlay of Stellite 6 or 309L on valve seats and stems for wear and corrosion resistance in high-cycle-actuation conditions.
Piping repairs and extensions: Field-applicable MIG overlay for repair of corroded sections in installed CO₂ transport piping, minimizing downtime.
Downhole tool protection: TIG overlay of nickel-based alloys on downhole connector threads and seals to resist CO₂ corrosion and mechanical wear during fracturing operations.
Heat exchanger tube sheets: MIG overlay of 321 stainless steel on carbon steel tube sheets for CO₂ cooling/condensation heat exchangers, providing corrosion resistance at tube-to-tubesheet joints.

Technical Advantage: Flexibility in application geometry, ability to apply to both new fabrication and in-service repair, and compatibility with tight spatial constraints typical of underground mine environments.

7.2 Hydraulic Explosive Bonding Applications

Hydraulic explosive bonding is employed for producing large-format clad plates used in the fabrication of CO₂ storage vessels, surge tanks, and heat exchanger shells. Applications include:

Large-format storage vessel cladding: Production of 316L/Q345R clad plates (dimensions up to 3000 mm × 6000 mm) for vessel shells and heads. The explosion bonding process provides uniform bond quality over large areas, superior to mechanical cladding methods.
Heat exchanger shell cladding: Production of Inconel 625/Q345R clad plates for CO₂ condenser and evaporator shells operating under cyclic thermal loading.
Pressure vessel end caps: Custom-shaped clad plates for hemispherical or torispherical heads, with post-explosion forming and machining to final geometry.
Manifold blocks: Clad plate sections machined into complex manifold geometries with multiple ports and connections, combining corrosion-resistant surfaces with structural strength.

Technical Advantage: Superior bond quality and uniformity over large areas; ability to bond dissimilar material combinations that are not weldable; production of clad plates with thickness ratios optimized for cost-efficiency; consistent quality suitable for critical pressure-containing applications.

7.3 Explosion Welding Applications

Explosion welding (solid-state explosive welding) is applied for specialized components in the CO₂ fracturing equipment suite where cylindrical or complex geometries require clad construction. Applications include:

Clad piping for high-pressure injection lines: Production of 316L/20# clad pipe (OD 57–219 mm) for high-pressure CO₂ transport from surface to downhole injection points. The explosion welding process creates a metallurgical bond along the full pipe circumference.
Clad pipe fittings: Production of clad elbows, tees, reducers, and flanges per ASME B16.9/B16.5 dimensions, explosion-welded and machined to specification.
Specialized downhole tools: Explosion-welded clad components for downhole pressure control devices and safety valves operating under extreme conditions.
Research and development components: Custom clad samples for materials research on CO₂ corrosion mechanisms and overlay performance optimization.

Technical Advantage: Ability to produce clad products in complex geometries (tubes, fittings); solid-state bonding eliminates intermetallic compound formation; suitable for producing small-quantity specialized components; metallurgical bond quality independent of component size.

8. Contribution to Qualification Building, Product Delivery, and Customer Value

8.1 Qualification Building

This capability entry significantly advances the company's qualification portfolio in several dimensions:

Industry qualification: Establishes the company as a qualified supplier in the coalbed methane and enhanced oil/gas recovery sector, complementing existing qualifications in petrochemical and power generation.
Process qualification: Development and certification of WPS/WPQ packages specific to CO₂ service conditions, including cryogenic impact testing and CO₂ corrosion testing, which can be leveraged across multiple customer projects.
Standard compliance: Accumulation of NDT records, material certifications, and inspection documentation that build a track record of quality and compliance with GB, ASME, and AWS standards.
Patent and IP generation: Novel process parameters, material combinations, and equipment designs developed for CO₂ fracturing applications can be protected through patents and trade secrets, creating intellectual property assets.

8.2 Product Delivery

The capability directly enables product delivery in the following formats:

Turnkey equipment packages: Complete CO₂ fracturing equipment sets with clad/overlay components, including storage vessels, injection systems, piping, and control manifolds, delivered as integrated systems ready for underground deployment.
Component supply: Individual clad vessels, piping assemblies, fittings, and valve components supplied to equipment integrators and EPC contractors.
Repair and maintenance services: Overlay repair of corroded or damaged components in-service, extending equipment life and reducing unplanned downtime in underground mining operations.
Custom fabrication: Made-to-specification clad components based on customer design requirements, supported by engineering consultation on material selection and process optimization.

8.3 Customer Value

The value proposition delivered to customers through this capability includes:

Extended equipment life: Clad and overlay components resist CO₂ corrosion, extending service life by 3–5× compared to unprotected carbon steel components, reducing replacement frequency and lifecycle costs.
Enhanced safety: Properly qualified clad components maintain structural integrity under high-pressure and cryogenic conditions, reducing the risk of catastrophic failure in underground environments where emergency response is limited.
Cost efficiency: Hybrid clad construction achieves the required performance at 40–60% lower material cost than solid alloy construction, enabling more economical deployment of CO₂ fracturing technology across multiple coal mines.
Regulatory compliance: Full documentation, testing, and certification packages ensure compliance with coal mine safety regulations (ACQ/T certification), pressure vessel codes (GB 150, TSG 21), and international standards, facilitating equipment approval and deployment.
Technical partnership: Ongoing engineering support, materials selection guidance, and performance monitoring services establish long-term customer relationships and create opportunities for repeat business and technology upgrades.

9. Conclusion and Strategic Outlook

The development and application of clad and overlay technology to high-pressure liquid CO₂ fracturing equipment represents a strategically significant capability for Cladding Technology Shanxi Co., Ltd. It leverages the company's core metallurgical expertise—weld overlay, explosive bonding, and explosion welding—in a rapidly growing market driven by China's coalbed methane development targets, carbon neutrality commitments, and coal mine safety modernization initiatives.

The technology contributes to the company's growth through multiple pathways: direct product sales of clad equipment components, engineering services and qualification support, technical partnerships with coal mining enterprises, and knowledge accumulation that strengthens the company's position as a materials solutions provider in the energy transition sector. The capability also provides a platform for future expansion into related applications such as supercritical CO₂ power cycles, carbon capture and storage (CCS) equipment, and enhanced geothermal systems, all of which require high-pressure CO₂ handling equipment with corrosion-resistant surfaces.

By maintaining rigorous adherence to applicable standards (GB 150, TSG 21, ASME BPVC, ASTM A497, AWS D10.9M, NACE MR0175), investing in continuous process improvement, and building a comprehensive qualification portfolio, the company is well-positioned to capitalize on the growing demand for reliable, cost-effective high-pressure CO₂ equipment in China's coal and energy sectors.