Effect of Open-Arc Weld Overlay Process Parameters on Microstructure and Hardness of High-Chromium Alloy Cladding Layers
1. Definition and Technical Principles
High-chromium alloy open-arc weld overlay is a surface engineering technology in which a high-chromium alloy (typically Cr ≥ 26 wt%) is deposited onto a base substrate through arc welding processes to produce a surface layer with superior corrosion resistance, wear resistance, or both. The term "open-arc" (明弧) specifically denotes welding conducted in an ambient atmosphere without a full shielding enclosure, distinguishing it from inert-gas-shielded or submerged-arc methods. In practice, this approach often employs flux-cored or self-shielded consumables, or relies on the alloy's own chromium oxide formation to provide partial atmospheric protection during deposition.
The fundamental metallurgical principle underlying high-chromium alloy weld overlay is the formation of a Cr₂O₃ passive film on the deposited surface. When chromium content exceeds approximately 12 wt%, a continuous and stable chromium oxide layer forms in oxidizing environments, providing exceptional resistance to chemical attack. High-chromium alloys with chromium levels of 26–35 wt% exhibit martensitic, austenitic, or duplex microstructures depending on carbon content, nickel addition, and cooling rate—all of which are directly governed by welding process parameters.
The interplay between process parameters (current, voltage, travel speed, arc length, interpass temperature, and preheating) and the resulting microstructure is the central subject of this technical capability. These parameters determine the thermal cycle experienced by the deposited metal, which in turn controls solidification morphology, phase transformations, carbide precipitation, and ultimately the hardness profile of the overlay layer.
2. Category and Business Positioning
This technical capability falls squarely within the company's TIG/MIG weld overlay technology route, representing a core process development and parameter optimization competency. Within Cladding Technology Shanxi Co., Ltd's three-pronged technology portfolio—TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding—this entry addresses the weld overlay route, which is the most versatile and widely applicable method for producing corrosion- and wear-resistant surface layers on complex geometries.
The business positioning of this capability is threefold:
- Process Qualification and WPS Development: Systematic understanding of parameter-microstructure-hardness relationships enables the development of documented Welding Procedure Specifications (WPS) that meet customer and regulatory requirements.
- Product Performance Assurance: By establishing quantitative correlations between process variables and overlay properties, the company can guarantee hardness specifications (typically HRC 40–60 for high-chromium martensitic overlays) and microstructural integrity on delivered products.
- Technical Authority and Customer Confidence: Demonstrated expertise in high-chromium alloy metallurgy positions the company as a qualified supplier for demanding applications in mining, cement, power generation, and chemical processing industries.
3. Technical Purpose and Value
The primary technical purpose of this study is to establish a rigorous, data-driven framework for selecting and controlling open-arc weld overlay parameters to achieve target microstructural characteristics and hardness levels in high-chromium alloy cladding layers. The value delivered includes:
- Reduced trial-and-error: By understanding the fundamental relationships between process parameters and metallurgical outcomes, the company minimizes expensive trial welds and accelerates WPS qualification timelines.
- Consistent product quality: Parameter windows established through this study ensure that every production weld overlay achieves the specified hardness and microstructure, reducing rework and rejection rates.
- Extended service life of cladded products: Optimal microstructure—characterized by fine, uniformly distributed carbides in a tempered martensitic matrix—delivers superior wear resistance, directly extending the service life of cladded components by 3–10 times compared to uncladded carbon steel.
- Support for NDE and acceptance testing: Understanding the expected microstructural response to specific parameter sets enables more informed ultrasonic testing (UT) and magnetic particle inspection (MPI) acceptance criteria, as certain parameter combinations are known to produce porosity or cracking tendencies.
4. Key Process Parameters and Their Effects
4.1 Welding Current and Voltage
Welding current is the most influential parameter governing heat input and dilution in open-arc weld overlay. Higher currents increase the penetration depth, raising the dilution of base metal into the overlay layer. This dilution reduces the effective chromium content in the deposited metal, potentially shifting the microstructure from martensitic toward ferritic-austenitic or even fully austenitic compositions. Conversely, lower currents produce shallower welds with lower dilution but risk incomplete fusion and increased porosity.
| Parameter | Low Value | Optimal Range | High Value | Microstructural Effect |
|---|---|---|---|---|
| Welding Current (A) | < 100 | 120–200 | > 250 | Low: incomplete fusion, porosity; High: excessive dilution, reduced Cr content |
| Arc Voltage (V) | < 18 | 20–28 | > 32 | Low: narrow weld, high dilution ratio; High: wide weld, increased spatter, oxide inclusions |
| Travel Speed (mm/min) | < 50 | 60–150 | > 200 | Low: coarse grain, coarse carbides, lower hardness; High: under-deposited, incomplete fusion |
| Preheat Temperature (°C) | 0–50 | 100–250 | > 350 | Low: high cooling rate, untempered martensite, cracking risk; High: coarse grain, reduced hardness |
| Interpass Temperature (°C) | < 50 | 150–300 | > 400 | Low: thermal stress, cracking; High: carbide coarsening, reduced hardness |
4.2 Heat Input and Cooling Rate
Linear heat input (q = V × I / v, where V is voltage, I is current, and v is travel speed) is a composite parameter that integrates the individual effects of current, voltage, and speed. For high-chromium martensitic alloys, the cooling rate from the solidus temperature (approximately 1400°C) to 800°C is the critical determinant of microstructure:
- Fast cooling (V800 > 100°C/s): Produces untempered martensite with retained austenite. Hardness can reach HRC 55–62 but with high susceptibility to cracking and poor toughness.
- Moderate cooling (V800 = 30–100°C/s): Produces tempered martensite with fine carbide precipitation. Hardness of HRC 45–55 with acceptable toughness—the optimal target for most industrial applications.
- Slow cooling (V800 < 30°C/s): Produces ferrite-austenite duplex structures with coarse carbides. Hardness drops to HRC 30–40 with reduced wear resistance.
4.3 Layer Geometry and Build Strategy
The geometry of the overlay layer—single pass, multi-pass, or multi-layer multi-pass—significantly affects the thermal history and resulting microstructure. Multi-layer multi-pass strategies provide inherent tempering of previously deposited layers, reducing residual stress and cracking susceptibility. However, excessive interpass temperatures can cause carbide coarsening in lower layers, reducing hardness uniformity across the overlay thickness.
Recommended build strategy for high-chromium alloy overlays:
- Transition layer: One pass of a dilution-resistant alloy (e.g., 309L or equivalent) to reduce base metal dilution into the functional overlay.
- Build-up passes: 2–4 passes of high-chromium alloy with controlled interpass temperature of 150–250°C.
- Cap pass: Final pass with travel speed optimized for desired bead profile and hardness.
4.4 Microstructural Evolution with Parameter Variation
The microstructure of high-chromium alloy weld overlays is governed by the following phase transformation sequence:
During solidification, the austenite (γ) phase forms first, followed by eutectic solidification of austenite and carbides (M₇C₃, M₂₃C₆, or Cr₇C₃ depending on composition). Upon cooling below the Ms temperature (approximately 400–500°C for 26Cr alloys), martensite (α') forms. The final microstructure depends on whether post-weld tempering occurs (from interpass heating in multi-pass welds) or the weld remains in the as-deposited state.
| Process Condition | Resulting Microstructure | Typical Hardness (HRC) | Key Characteristics |
|---|---|---|---|
| Low heat input, fast cooling, single pass | Untempered martensite + retained austenite | 55–62 | High hardness, high residual stress, cracking-prone |
| Moderate heat input, multi-pass with interpass tempering | Tempered martensite + fine M₇C₃ carbides | 45–55 | Optimal wear resistance, good toughness, low cracking risk |
| High heat input, slow cooling, high dilution | Ferrite + austenite + coarse carbides | 30–40 | Reduced wear resistance, poor corrosion resistance if Cr < 12% |
| Very high interpass temperature (>400°C) | Coarse tempered martensite + spheroidized carbides | 35–42 | Significant hardness loss, carbide coarsening |
5. Applicable Standards and Acceptance Criteria
5.1 Welding Procedure and Performance Standards
- GB/T 985.1-2008: Welding procedure qualification test—Welding fillers, welding consumables, welding position, and welding equipment for ferrous metals.
- GB/T 19866-2005: Welding procedure qualification for ferrous metals—Qualification procedure for arc welding.
- ASTM A240: Standard specification for chromium and chromium-nickel stainless steel plate, sheet, and strip for pressure vessels (relevant for base metal substrate qualification).
- ASME Section IX, Part Q: Qualification of welding procedures, welders, and welding operators.
- EN ISO 15614-1:2017: Qualification testing for fusion welding procedures—Metallic materials—Part 1: Qualification procedure for arc and gas welding.
5.2 Overlay Hardness and Microstructure Acceptance
- GB/T 231.1-2018: Metallic materials—Brinell hardness test—Part 1: Test method (for overlay hardness verification where HRC testing is impractical on thin layers).
- ASTM A955/A955M-2018: Standard specification for wear-resistant steel plate with a hard surface by arc surfacing (defines hardness requirements for arc-surfaced wear plates).
- ISO 6508-1:2016: Metallic materials—Vickers hardness test—Part 1: Test method (for microhardness mapping across the overlay thickness).
- GB/T 13298-2015: Metallic materials—Microstructural examination of steels (for microstructure verification and carbide characterization).
5.3 Non-Destructive Testing Standards
- GB/T 3323.1-2017: Non-destructive testing of welds—Radiographic testing—Part 1: Film technique (for volumetric defect detection in thick overlay layers).
- GB/T 11345-2013: Non-destructive testing—Ultrasonic testing of welds—Manual technique (for detecting lack of fusion, cracks, and porosity at the overlay-base metal interface).
- ASTM E709/E709M-2018: Standard practice for magnetic particle testing (for surface-breaking crack detection in the overlay layer).
- NACE SP0169-2007: Corrosion control in underground or submerged piping systems (relevant for corrosion resistance verification of cladded pipelines).
5.4 Typical Acceptance Criteria
| Acceptance Parameter | Specification | Test Method | Standard Reference |
|---|---|---|---|
| Overlay hardness | HRC 45–58 (typical for 26Cr martensitic) | Rockwell C hardness test | ASTM A955 |
| Overlay thickness | ≥ 3 mm (minimum), typically 5–15 mm | Ultrasonic thickness measurement | GB/T 11345 |
| Crack-free zone | No cracks > 0.5 mm length in overlay or HAZ | MPI / Visual inspection | ASTM E709 |
| Porosity level | ≤ 1% of weld cross-section area | Radiographic testing | GB/T 3323.1 |
| Overlay-base metal dilution | ≤ 30% base metal dilution in first functional layer | Optical emission spectroscopy (OES) | ASTM A240 |
6. Common Risks and Controls
6.1 Cracking Risks
- Hot cracking (solidification cracking): High-chromium alloys are susceptible to hot cracking due to the wide freezing range of austenite-carbide eutectics. Control: Reduce sulfur and phosphorus content in consumables, increase current to reduce freezing range, and avoid high travel speeds that create narrow, deep welds.
- Cold cracking (hydrogen-induced cracking): Martensitic high-chromium alloys are highly susceptible to hydrogen cracking, particularly when deposited on low-alloy steel substrates. Control: Preheat to 200–300°C, maintain interpass temperature above 150°C, use low-hydrogen consumables, and apply post-weld stress relief at 550–650°C.
- Lamellar tearing: Occurs in the base metal when thick-section substrates with elongated inclusions are subjected to transverse restraint stresses. Control: Use low-sulfur base steels, preheat adequately, and limit weld sequence restraint.
6.2 Dilution and Composition Control
Dilution is the primary metallurgical risk in weld overlay. Excessive base metal dilution reduces the chromium content below the critical threshold for passivity formation, compromising corrosion resistance. For high-chromium overlays on carbon steel substrates, dilution in the first layer can easily reach 40–60%. Control measures:
- Apply a transition layer of 309L or 310 stainless steel before the functional high-chromium overlay.
- Use high current with low travel speed to create a broad, shallow bead with lower dilution ratio.
- Verify dilution by OES or wet chemical analysis of cross-sections after WPS qualification.
- Design overlay geometry to minimize the ratio of base metal area to deposited metal area in each pass.
6.3 Hardness Uniformity
Non-uniform hardness across the overlay thickness—soft at the interface due to dilution, hard at the surface—can cause premature failure at the interface during service. Control measures:
- Implement multi-layer multi-pass builds with controlled interpass temperatures.
- Perform hardness traverse testing (from base metal through overlay) on qualification coupons.
- Specify minimum overlay thickness to ensure the functional zone is sufficiently far from the dilution-affected interface.
7. Application Across the Company's Three Technology Routes
7.1 TIG/MIG Weld Overlay Route (Primary Application)
This technical capability is most directly applicable to the TIG/MIG weld overlay route. The parameter-microstructure-hardness relationships established through this study form the scientific basis for WPS development in the following application areas:
- Wear-resistant surfacing on mining equipment: Bucket teeth, crusher hammers, and conveyor rollers clad with high-chromium martensitic overlays (HRC 50–58) for extended service life in abrasive environments.
- Corrosion-resistant overlay on chemical processing equipment: Pumps, valves, and heat exchanger tubes clad with high-chromium austenitic or duplex overlays for resistance to aggressive chemical media.
- Transition layer deposition: TIG welding of 309L transition layers beneath high-chromium functional overlays to control dilution and prevent cracking at the overlay-base metal interface.
- Repair and maintenance cladding: On-site MIG weld overlay of worn or corroded components, leveraging the parameter optimization knowledge to ensure rapid, reliable repair with guaranteed hardness.
The open-arc welding aspect of this study is particularly relevant to field applications where inert gas shielding is impractical. Understanding how atmospheric exposure affects the microstructure and hardness of high-chromium overlays enables the company to specify appropriate consumables (flux-cored, self-shielded) and post-weld treatments for outdoor and remote site work.
7.2 Hydraulic Explosive Bonding Route
While this technical capability primarily addresses weld overlay, the understanding of high-chromium alloy microstructures and hardness profiles is valuable in the hydraulic explosive bonding route in the following ways:
- Post-bonding weld overlay: Hydraulic explosive bonding produces metallurgical bonds between dissimilar metals (e.g., carbon steel and 316L stainless steel). Where additional wear or corrosion resistance is required beyond the bonded layer, high-chromium weld overlay can be applied on top of the bonded clad plate. The parameter optimization knowledge ensures that the weld overlay does not compromise the explosive-bonded interface.
- Thermal management: Understanding heat input effects on microstructure helps define maximum allowable heat input when welding onto explosively bonded clad plates, preventing delamination or interface degradation.
- Hardness matching: The hardness profile knowledge enables selection of overlay compositions that match or complement the hardness of the explosively bonded layers, ensuring uniform wear behavior across the entire cladding system.
7.3 Explosion Welding Route
Explosion welding produces clad plates and pipes through high-velocity impact bonding. The technical knowledge from this entry contributes to the explosion welding route through:
- Weld overlay on explosion-welded components: Explosion-welded pipes and plates often require additional surface protection. High-chromium weld overlay applied to explosion-welded products leverages the microstructure-hardness knowledge to ensure compatibility and performance.
- Hardness and microstructure characterization: The analytical methods developed for weld overlay microstructure characterization (metallographic examination, hardness traverse, carbide identification) are equally applicable to characterizing the explosion-welded interface and the deformation zones in explosion-clad products.
- Post-weld heat treatment optimization: Understanding how heat input affects high-chromium alloy microstructures informs the design of post-weld stress relief and tempering cycles for explosion-welded components that subsequently receive weld overlay treatment.
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
This technical capability directly supports the company's qualification program in the following ways:
- WPS Development: The parameter-microstructure-hardness database enables rapid development of qualified welding procedure specifications for new high-chromium alloy overlays, reducing qualification lead time from weeks to days.
- WPQ Validation: Understanding the parameter windows ensures that Welder Performance Qualifications (WPQ) are conducted under conditions representative of production, ensuring qualified welders can produce acceptable work.
- Customer-Specific Qualifications: When customers require specific hardness ranges (e.g., HRC 50±3 for a particular mining application), the parameter optimization knowledge enables targeted WPS development to meet exact specifications.
- Standard Compliance: The systematic approach to parameter optimization and microstructural verification aligns with qualification requirements under GB/T 19866, ASME Section IX, and EN ISO 15614-1.
8.2 Product Delivery
- Reduced Rework: By applying optimized parameters from the start, the company minimizes the need for rework due to hardness non-conformance, cracking, or excessive dilution.
- Consistent Quality: Parameter control ensures that every batch of cladded products meets the specified hardness and microstructural requirements, supporting consistent product quality across production runs.
- Accelerated Production: Optimized parameters reduce the number of qualification trials, enabling faster turnaround on new product development and customer-specific cladding jobs.
- Traceability: Documented parameter-microstructure relationships provide a traceable link from welding parameters to final product properties, supporting quality traceability and customer audits.
8.3 Customer Value
- Extended Equipment Life: Optimally parameterized high-chromium overlays deliver 3–10× the service life of unclad components, reducing customer downtime and replacement costs.
- Technical Consultation Capability: The company can advise customers on optimal overlay specifications for their specific application, providing added value beyond simple fabrication.
- Risk Mitigation: Understanding cracking, dilution, and hardness uniformity risks enables the company to proactively address potential failure modes, protecting the customer's operational reliability.
- Competitive Differentiation: Demonstrated expertise in high-chromium alloy weld overlay metallurgy distinguishes the company from competitors who may lack systematic parameter optimization capabilities.
9. Implementation Recommendations
9.1 Process Control Checklist
- Verify base metal composition and preheat requirements per WPS.
- Confirm consumable type, composition, and hydrogen content per specification.
- Set welding parameters (current, voltage, travel speed) within the qualified WPS window.
- Monitor and record preheat temperature and interpass temperature throughout the build.
- Perform visual inspection of each pass for surface defects before proceeding to the next layer.
- Apply post-weld stress relief or tempering treatment as specified in the WPS.
- Perform hardness testing on the completed overlay per the applicable acceptance criteria.
- Conduct NDT (MPI, UT, or RT) per the applicable standard and acceptance level.
- Document all parameters, temperatures, and test results in the production record.
9.2 Parameter Optimization Strategy for New Applications
- Define target properties: Specify required hardness range, microstructure type, and corrosion/wear resistance requirements.
- Select alloy composition: Choose high-chromium alloy grade (e.g., 26Cr-0.5C for martensitic, 26Cr-1Ni for austenitic) based on target properties.
- Establish parameter window: Based on the parameter-microstructure-hardness database, select initial parameters targeting the desired cooling rate and dilution level.
- Qualification welding: Perform qualification welds on representative base metal and geometry.
- Property verification: Perform hardness traverse, microstructural examination, and NDT on qualification samples.
- Iterate if necessary: Adjust parameters and repeat until all acceptance criteria are met.
- Document and qualify: Finalize WPS and submit for customer approval and/or third-party qualification.
10. Conclusion
The systematic study of open-arc weld overlay process parameters and their effects on high-chromium alloy microstructure and hardness represents a foundational technical capability for Cladding Technology Shanxi Co., Ltd. This knowledge base directly enables the development of qualified welding procedures, ensures consistent product quality, and provides the technical authority to deliver high-performance cladded components across diverse industrial applications. By integrating this capability across all three technology routes—TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding—the company maximizes the value of this metallurgical expertise and positions itself as a technically competent supplier in the surface engineering and cladding industry.
The actionable outcomes of this technical capability include reduced qualification timelines, lower rework rates, consistent hardness delivery, and enhanced customer confidence—all of which contribute directly to the company's competitive position and long-term business sustainability in the cladding technology market.