Plasma Arc Weld Overlay of Q235 Electrolytic Shell-Breaking Hammer Heads: Microstructure, Performance, and Process Qualification
1. Definition and Technical Principles
Plasma Arc Weld Overlay (PAWO) is a specialized thermal spray process that uses a high-velocity, high-temperature plasma jet generated by a transferred or non-transferred arc to melt a consumable electrode and/or powder feedstock, depositing a cladding layer onto a base substrate. When applied to Q235 carbon steel shell-breaking hammer heads used in electrolytic copper production, PAWO serves to restore or enhance surface hardness, wear resistance, and corrosion resistance on a low-alloy structural steel that is inherently susceptible to rapid degradation under severe impact, abrasion, and electrolytic environment exposure.
The plasma arc operates at temperatures exceeding 10,000–20,000 K, providing a highly concentrated heat source with a narrow heat-affected zone (HAZ) compared to conventional arc welding methods. This thermal confinement is critical for Q235 substrates, which have low carbon content (0.12–0.20% C) and are prone to softening and loss of mechanical integrity under excessive thermal input. The plasma arc melts the cladding material and a controlled depth of the substrate surface, creating a metallurgical bond between the overlay and the base metal through controlled dilution.
For shell-breaking hammer heads, the primary failure modes include:
- Impact wear from repeated striking of copper cathode shells
- Corrosion from exposure to sulfuric acid electrolyte (typically 180–210 g/L H₂SO₄)
- Material loss due to erosion and abrasion
- Cracking from thermal cycling and residual stress accumulation
2. Category and Business Positioning
This technical capability falls squarely within the company's TIG/MIG Weld Overlay technology route, specifically under the plasma arc weld overlay sub-category. PAWO is distinguished from conventional TIG or MIG overlay by its use of ionized gas (typically argon or a mixture of argon and nitrogen) to generate a constricted plasma jet, offering superior process control, higher deposition efficiency, and reduced dilution rates compared to standard arc welding processes.
Within Cladding Technology Shanxi Co., Ltd.'s three technology routes:
- TIG/MIG Weld Overlay: PAWO is a premium variant offering higher precision and lower dilution, positioned for critical components requiring tight control over overlay composition and properties
- Hydraulic Explosive Bonding: Complementary for bulk cladding where high-integrity, crack-free interfaces are required without thermal distortion
- Explosion Welding: Used for large-area, high-volume cladding of structural components where speed and cost-effectiveness are prioritized
The shell-breaking hammer head application represents a high-value, niche market within the non-ferrous metals processing sector. Electrolytic copper refineries operate 24/7 with thousands of cathode shells, and the hammer heads used to break adherent copper shells experience extreme duty cycles. Restoring these components via PAWO rather than replacing them offers significant cost savings and supply chain continuity advantages for copper producers.
3. Technical Purpose and Value
3.1 Primary Technical Objectives
- Hardness Enhancement: Elevate surface hardness from the base Q235 level (typically 120–160 HV) to 400–600 HV or higher through deposition of hardfacing alloys (e.g., high-carbon martensitic, cobalt-based, or nickel-based compositions)
- Corrosion Resistance: Introduce alloying elements (Cr, Mo, Ni, Co) that form passive oxide films resistant to sulfuric acid environments
- Dimensional Restoration: Rebuild worn surfaces to original or specified dimensions, extending component service life by 3–5 cycles
- Impact Tolerance: Maintain adequate toughness in the overlay and HAZ to resist cracking under repeated impact loading
3.2 Economic and Operational Value
- Cost Reduction: PAWO restoration of hammer heads typically costs 30–50% less than procurement of new components, with lead times reduced from weeks to days
- Material Conservation: Minimizes consumption of alloy-bearing raw materials by selectively depositing hardfacing only on wear surfaces
- Process Reliability: The narrow HAZ and low dilution of PAWO minimize thermal distortion, ensuring dimensional accuracy without post-weld machining in many cases
- Environmental Benefit: Reduces scrap generation and associated environmental impact in copper refinery operations
4. Key Process and Implementation Points
4.1 Substrate Preparation
Q235 hammer heads must undergo rigorous pre-weld preparation to ensure reliable metallurgical bonding. The process sequence includes:
- Visual Inspection: Identify existing cracks, porosity, and severe wear areas using magnetic particle testing (MT) per GB/T 26952
- Machining/Grinding: Remove existing worn surfaces, paint, and surface contaminants to expose clean base metal; create a V-groove or U-groove preparation for overlay deposition where dimensional restoration is required
- Chemical Cleaning: Remove oil, grease, and oxide scales using alkaline degreasing followed by acid pickling; verify cleanliness by solvent wipe test
- Preheating: Apply localized or global preheat to 100–150°C to minimize thermal gradient and reduce the risk of hydrogen-induced cracking in the HAZ
4.2 Plasma Arc Weld Overlay Process Parameters
| Parameter | Typical Range | Notes |
|---|---|---|
| Plasma Gas | Argon (Ar) or Ar/He mixture | Purity ≥ 99.99%; flow rate 40–60 L/min |
| Arc Current | 150–350 A | Dependent on overlay thickness and electrode diameter |
| Arc Voltage | 20–30 V | Higher voltage increases arc length and dilution |
| Travel Speed | 100–300 mm/min | Higher speed reduces heat input and dilution |
| Welding Current Density | 200–400 A/mm² | Critical for plasma jet stability and penetration |
| Shielding Gas | Argon or Ar/CO₂ (90/10) | Flow rate 8–12 L/min; prevents atmospheric contamination |
| Preheat Temperature | 100–150°C | Interpass temperature maintained ≤ 200°C |
| Dilution Rate | 15–30% | Target ≤ 25% for optimal overlay properties |
| Overlay Thickness | 3–8 mm (single pass: 1–3 mm) | Multi-pass builds with interpass grinding |
| Electrode/ Powder | High-carbon martensitic, Co-based, or Ni-based | Selected per service condition (wear vs. corrosion) |
4.3 Cladding Material Selection
| Material Type | Composition (Typical) | Hardness (HV) | Primary Application |
|---|---|---|---|
| High-Carbon Martensitic | Cr 5–8%, C 1.5–2.5%, Mo 0.5–1.5% | 500–650 | Impact wear resistance; shell-breaking duty |
| Nickel-Based (Ni-Cr-Mo) | Ni bal., Cr 10–15%, Mo 2–4%, C 0.3–0.6% | 350–500 | Corrosion + moderate wear; acid environment |
| Cobalt-Based (Stellite-type) | Co bal., Cr 25–30%, W 5–10%, C 1.0–1.5% | 400–550 | Severe corrosion + wear; high-temperature service |
| Transition Layer (309L-type) | Cr 22–25%, Ni 12–15%, C ≤ 0.03% | 200–250 | Compatibility layer between Q235 and hardfacing |
4.4 Multi-Pass Overlay Strategy
For hammer head restoration requiring overlay thicknesses exceeding 3 mm, a multi-pass strategy is employed:
- Pass 1 – Bond/Transition Layer: Deposit a 1–2 mm layer of austenitic stainless steel (309L or equivalent) to ensure metallurgical compatibility between Q235 and the subsequent hardfacing. This layer acts as a buffer against cracking due to coefficient of thermal expansion (CTE) mismatch.
- Pass 2 – Intermediate Layer: Deposit a 1–2 mm layer of medium-carbon alloy to bridge hardness and composition between the transition layer and the final hardfacing.
- Pass 3+ – Functional Hardfacing: Deposit the final 1–3 mm layer of high-hardness alloy (martensitic, cobalt-based, or nickel-based) optimized for wear and corrosion resistance.
Interpass grinding between layers removes surface oxides and ensures intimate bonding. Interpass temperature must be maintained below 200°C to prevent softening of previously deposited layers and to minimize HAZ grain growth.
4.5 Post-Weld Heat Treatment
Following overlay completion, post-weld heat treatment (PWHT) is critical for stress relief and microstructure optimization:
- Stress Relief: Temper at 550–650°C for 1–2 hours in a furnace or by induction heating, then air cool. This reduces residual stresses from 200–400 MPa to below 50 MPa, minimizing the risk of delayed cracking.
- Martensitic Hardfacing: If high-carbon martensitic overlay is used, a tempering treatment at 400–500°C may be applied to convert brittle martensite to tempered martensite, improving toughness while maintaining acceptable hardness (450–550 HV).
- Slow Cooling: For cobalt-based overlays, furnace cool from PWHT temperature to prevent thermal shock cracking.
5. Microstructure Analysis and Performance Characterization
5.1 Expected Microstructural Features
Based on the study of plasma weld overlay on Q235 shell-breaking hammer heads, the following microstructural features are typically observed:
- Overlay Zone: Fine-grained martensitic or austenitic structure depending on alloy composition. High-carbon martensitic overlays exhibit lenticular or plate-like martensite with retained austenite (5–15%) providing toughness. Cobalt-based overlays show a solid solution matrix with M₇C₃ and M₂₃C₆ carbides providing wear resistance.
- Dilution Zone (Interface): A transitional region where base metal elements (Fe, C) diffuse into the overlay, creating a gradient in composition and hardness. Dilution typically extends 0.2–0.5 mm into the overlay.
- Heat-Affected Zone (HAZ): In Q235, the HAZ typically shows fine ferrite-pearlite structure with minimal grain growth due to the low carbon content and the concentrated plasma heat source. Hardness in the HAZ may increase slightly (to 180–220 HV) due to tempering effects but remains well below the overlay.
- Base Metal: Remains unaffected beyond the HAZ boundary; original ferrite-pearlite structure preserved.
5.2 Mechanical Property Benchmarks
| Property | Q235 Base Metal | Transition Layer (309L) | Hardfacing Overlay | Acceptance Criteria |
|---|---|---|---|---|
| Hardness (HV) | 120–160 | 200–250 | 400–650 | Overlay ≥ 400 HV; uniformity ± 50 HV across layer |
| Tensile Strength (MPa) | 375–500 | 550–620 | 600–900 (martensitic) | Overlay ≥ 600 MPa |
| Toughness (CVN, J) | 30–50 | 40–60 | 20–40 (tempered martensitic) | Overlay ≥ 15 J at room temperature |
| Corrosion Rate (mm/y in 20% H₂SO₄) | > 2.0 | < 0.5 | < 0.1 (Co-based) | Overlay < 0.5 mm/y |
| Wear Life (cycles vs. new) | 1.0× | — | 3.0–5.0× | ≥ 3.0× improvement over bare Q235 |
5.3 Non-Destructive Testing (NDT) Requirements
- Visual Inspection (VT): 100% inspection of all overlay surfaces for undercuts, excessive reinforcement, porosity, and surface defects per GB/T 3375
- Magnetic Particle Testing (MT): 100% inspection of overlay and HAZ for cracks, laps, and inclusions per GB/T 26952; acceptance per Level II indications only
- Hardness Mapping: Traverse hardness test across the overlay-to-base interface; verify monotonic transition without unexpected soft spots or hard bands
- Ultrasonic Testing (UT): Selected or 100% inspection for subsurface porosity and lack of fusion per GB/T 11345, using a phased array or contact probe with appropriate calibration
- Dimensional Verification: CMM or coordinate measurement to confirm restored geometry meets original drawing tolerances
6. Applicable Standards and Acceptance Criteria
6.1 Welding Procedure and Qualification Standards
- GB/T 985.1: Welding Procedure Specification (WPS) preparation and qualification requirements for arc welding processes
- GB/T 19418: Welding procedure qualification and performance qualification requirements
- GB/T 3375: Visual acceptance criteria for welds and welded joints
- GB/T 26952: Magnetic particle testing methods for ferromagnetic materials
- GB/T 11345: Ultrasonic testing of welds
- GB/T 6394: Metallographic preparation and examination
- GB/T 231.1: Vickers hardness testing
- GB/T 228.1: Tensile testing of metallic materials
- GB/T 229: Charpy V-notch impact testing
6.2 Material Standards
- GB/T 700: Carbon structural steel (Q235 base material specification)
- GB/T 17116: Hardfacing electrodes and wires (if applicable)
- ASTM A396: Castings, carbon steel, for pressure-containing parts (if applicable to hammer head casting)
- ASTM A889: Weld overlay deposits (material specifications)
- ASME Section IX: Qualification rules for welding, brazing, and fusing procedures
6.3 Industry-Specific Standards
- API 579-1/ASME FFS-1: Fitness-for-service assessment of repaired components (if hammer heads are pressure-containing or safety-critical)
- ISO 14555: Welding — Welding procedure qualification testing
- NACE MR0175/ISO 15156: Materials for use in H₂S-containing environments (if applicable to copper refinery off-gas environments)
6.4 Acceptance Criteria Summary
| Inspection Method | Acceptance Level | Reference Standard |
|---|---|---|
| Visual (VT) | No cracks, undercut ≤ 0.5 mm, reinforcement ≤ 3 mm | GB/T 3375 Level B |
| Magnetic Particle (MT) | No linear indications; round indications ≤ 2 mm | GB/T 26952 |
| Ultrasonic (UT) | No indications above reference block Level 1 | GB/T 11345 Level 1 |
| Hardness (HV) | Overlay ≥ 400 HV; HAZ ≤ 250 HV; uniformity ± 50 HV | GB/T 231.1 |
| Impact (CVN) | ≥ 15 J at 20°C (overlay material) | GB/T 229 |
| Macro/Micro Examination | No lack of fusion, centerline cracking, or excessive porosity | GB/T 6394 |
7. Common Risks and Controls
| Risk | Root Cause | Control Measure |
|---|---|---|
| Hot Cracking in Overlay | High sulfur/phosphorus in base metal; excessive dilution; improper alloy selection | Use low-dilution plasma parameters; deposit transition layer; control interpass temperature ≤ 200°C |
| Cold Cracking (Hydrogen-Induced) | Moisture in shielding gas or electrode; rapid cooling of HAZ | Dry shielding gas (dew point ≤ -40°C); preheat to 100–150°C; apply PWHT within 6 hours |
| Excessive Dilution | High arc current, slow travel speed, large electrode diameter | Optimize plasma parameters; use powder feed instead of electrode where possible; increase travel speed |
| Hardness Inhomogeneity | Uneven heat input; multi-pass thermal cycling | Maintain consistent travel speed and arc parameters; perform hardness mapping; adjust tempering if needed |
| Delamination | Inadequate surface preparation; poor wetting; CTE mismatch | Rigorous pre-weld cleaning; deposit compatible transition layer; control cooling rate |
| Thermal Distortion | Excessive heat input on thin or asymmetric sections | Use backer plates; apply backing welds; sequence welds to balance thermal input; use plasma's narrow HAZ advantage |
| Porosity | Moisture contamination; inadequate shielding; gas porosity from nitrogen/oxygen pickup | Maintain shielding gas purity ≥ 99.99%; ensure proper gas flow and nozzle positioning; clean surface thoroughly |
8. Application Across the Company's Three Technology Routes
8.1 TIG/MIG Weld Overlay (Primary Route for This Application)
Plasma arc weld overlay is a specialized subset of the TIG/MIG weld overlay route. The company's PAWO capability for hammer head restoration offers distinct advantages over conventional TIG or MIG overlay:
- Lower Dilution: Plasma arc achieves 15–25% dilution versus 30–50% for conventional TIG, preserving overlay composition and properties
- Narrower HAZ: Reduced thermal damage to Q235 substrate, preserving base metal mechanical properties
- Higher Deposition Rate: Powder-fed plasma arc can deposit 2–4 kg/h versus 0.5–1.5 kg/h for TIG, improving productivity for multi-pass builds
- Superior Process Control: Independent control of plasma gas flow, welding current, and arc voltage allows fine-tuning for different overlay materials and thicknesses
8.2 Hydraulic Explosive Bonding (Complementary Route)
While PAWO is the primary method for hammer head restoration, hydraulic explosive bonding (HEB) may be employed in related applications where:
- Large-area cladding of hammer head bodies is required (e.g., full-surface corrosion protection)
- Thick cladding layers (5–20 mm) are needed without the multi-pass approach of PAWO
- Zero dilution and perfect metallurgical bonding are critical for structural integrity
- Thermal distortion must be completely avoided (e.g., precision-machined hammer head heads)
HEB offers a cold-bonding alternative that complements PAWO by enabling bulk cladding of the hammer head body, while PAWO is used for localized wear-surface restoration.
8.3 Explosion Welding (Volume Production Route)
For high-volume production of clad hammer heads or for cladding of other electrolytic copper production components (e.g., cathode support rails, transfer tongs), explosion welding provides:
- Rapid cladding of large surface areas (up to several square meters per detonation)
- Consistent interface quality with wave-like bonding patterns verified by NDT
- Cost-effectiveness for standardization of clad hammer head designs
- Ability to clad dissimilar material combinations (e.g., Q235/Cr-Ni alloy) without thermal concerns
9. Contribution to Qualification Building, Product Delivery, and Customer Value
9.1 Qualification Building
The plasma weld overlay study on Q235 shell-breaking hammer heads contributes directly to the company's qualification portfolio in the following ways:
- WPS Qualification: Establishes documented welding procedure specifications for PAWO of Q235 substrates with various hardfacing alloys, satisfying GB/T 19418 and ASME Section IX requirements
- Material Qualification: Generates metallurgical and mechanical property databases for specific overlay compositions on Q235, supporting material selection for future projects
- Personnel Qualification: Provides training and experience for welders and inspectors on plasma arc overlay techniques, maintaining certifications under GB/T 15169
- Equipment Qualification: Validates plasma arc welding equipment capabilities and parameters, supporting equipment registration and periodic requalification
9.2 Product Delivery Enhancement
- Standardized Procedures: The study enables development of repeatable, documented procedures that reduce batch-to-batch variability and improve first-pass yield
- Faster Turnaround: Optimized plasma parameters and multi-pass strategies reduce total processing time per hammer head, enabling faster delivery to copper refinery customers
- Quality Consistency: Defined acceptance criteria and NDT protocols ensure every restored hammer head meets performance specifications, reducing field failures and warranty claims
- Scalability: The qualification framework established for hammer heads can be extended to other Q235 components in copper production (e.g., shell carriers, transfer tools, electrode holders)
9.3 Customer Value Creation
- Extended Component Life: PAWO-restored hammer heads achieve 3–5× the service life of bare Q235, reducing customer maintenance frequency and downtime
- Cost Savings: Restoration costs 30–50% less than new component procurement, with additional savings from reduced inventory and logistics
- Performance Guarantee: Documented microstructure and property data provide customers with confidence in overlay performance, supported by traceable quality records
- Technical Partnership: The depth of metallurgical understanding demonstrated in this study positions the company as a technical partner rather than a simple service provider, enabling collaborative development of optimized overlay solutions for specific customer applications
- Supply Chain Security: Localized restoration capability reduces customers' dependence on imported replacement hammer heads, mitigating supply chain risks
10. Conclusion and Recommendations
The plasma arc weld overlay of Q235 electrolytic shell-breaking hammer heads represents a technically sophisticated and commercially valuable application of the company's TIG/MIG weld overlay route. The study of overlay microstructure and performance provides the metallurgical foundation for developing robust, repeatable welding procedures that deliver consistent, high-performance cladding layers.
Key recommendations for advancing this capability include:
- Expand alloy database: Systematically qualify additional hardfacing compositions (tungsten carbide-cermet, chromium carbide, nickel-aluminum bronze) for specific wear and corrosion conditions encountered in copper refineries
- Automate process monitoring: Implement real-time arc parameter monitoring and data logging to enable process traceability and statistical process control (SPC)
- Develop digital twin models: Create finite element models of the PAWO process to predict dilution, residual stress, and microstructure evolution, reducing trial-and-error qualification cycles
- Pursue international certifications: Seek AWS D1.1 or ISO 3834 certifications for plasma arc weld overlay procedures to access international copper production markets
- Cross-route integration: Develop hybrid approaches combining explosion welding for bulk cladding with PAWO for localized wear-surface enhancement, offering customers optimized solutions across the full spectrum of cladding requirements
By leveraging the metallurgical insights gained from this study and integrating them into the company's broader technology portfolio, Cladding Technology Shanxi Co., Ltd. can establish a dominant position in the electrolytic copper production cladding market, delivering superior value through technically differentiated, standards-compliant, and economically advantageous solutions.