Effect of CaCO₃ on Microstructure and Wear Resistance of High-Chromium Alloy Arc Surfacing Deposits

1. Definition and Technical Principles

The technical entry under review addresses the influence of calcium carbonate (CaCO₃) as a fluxing and deoxidizing agent incorporated into composite powder particles and solid welding wires used in open-arc surfacing (明弧堆焊) of high-chromium alloys. This subject falls squarely within the metallurgical science of weld overlay and surfacing consumable design, where the chemical composition of the filler metal and its flux coating directly governs the as-deposited microstructure, carbide morphology, and ultimately the tribological performance of the overlay layer.

High-chromium alloys—typically defined as alloys containing 10–30 wt% Cr—are widely used for wear-critical components in mining, cement, power generation, and material handling industries. The primary wear-resistance mechanism in these alloys is the presence of hard, thermodynamically stable chromium carbides (Cr₇C₃, Cr₃C, and Cr₂₃C₆) dispersed in a martensitic or austenitic matrix. The type, size, distribution, and volume fraction of these carbides are the dominant factors controlling abrasion resistance, and they are profoundly affected by the deoxidation chemistry of the consumable.

CaCO₃, when introduced into the consumable composition, undergoes thermal decomposition in the arc zone at temperatures above approximately 825°C, yielding CaO and CO₂. The resulting CaO acts as a powerful deoxidizer and slag-forming agent, while the CO₂ provides a diluting and inerting effect within the molten pool. The combined action of these decomposition products influences oxygen content in the weld metal, modifies the solidification sequence, and alters the nucleation and growth behavior of carbide phases. Specifically, CaO reduces the activity of oxygen at the melt-slag interface, suppressing unwanted oxide inclusions and promoting a cleaner, more homogeneous weld microstructure. Additionally, the basic nature of CaO (basicity ratio) can affect the solubility of various carbide species in the slag phase, thereby influencing the equilibrium partitioning of carbon and chromium between the weld metal and the slag.

1.1 Mechanism of Action

• Deoxidation: CaO preferentially combines with dissolved oxygen in the weld pool, reducing the oxygen potential and minimizing the formation of brittle iron oxide (FeO) and chromium oxide (Cr₂O₃) inclusions that would otherwise degrade mechanical properties and promote intergranular cracking.
• Slag Refining: The CaO-rich slag phase has a high affinity for sulfur and phosphorus, acting as a desulfurizer and dephosphorizer. This results in a cleaner weld metal with reduced hot cracking susceptibility.
• Carbon Activity Modification: By controlling the oxygen activity in the melt, CaO indirectly affects the carbon activity and the thermodynamic driving force for carbide precipitation. A lower oxygen activity environment favors the formation of chromium-rich carbides over iron-rich carbides, enhancing wear resistance.
• Microstructure Homogenization: The CO₂ released during decomposition provides a mild gas-shielding effect and can refine the grain structure by promoting heterogeneous nucleation, leading to a more uniform distribution of carbides throughout the deposit.

2. Category and Business Positioning

This technical entry belongs to the consumable metallurgy and process development category within Cladding Technology Shanxi Co., Ltd.'s capability portfolio. It is not a standalone manufacturing service but rather a foundational research and development activity that underpins the quality, reliability, and performance of the company's weld overlay products and services. The knowledge gained from studying CaCO₃ effects directly feeds into the design of proprietary composite powder formulations and solid welding wire specifications used in production.

In the company's business architecture, this entry serves as a technical qualification enabler. Understanding how fluxing agents affect overlay microstructure allows the company to:

• Develop and qualify new welding procedure specifications (WPS) for specific high-chromium alloy systems (e.g., Stellite-type, D2 tool steel, and Cr26 cast iron overlays).
• Demonstrate metallurgical competence to customers and certification bodies during third-party audits and qualification testing.
• Provide value-added technical consulting to customers who require custom overlay solutions for specialized wear environments.
• Reduce rework rates and warranty claims by ensuring consumable selection is metallurgically justified rather than empirically selected.

3. Technical Purpose and Value

3.1 Primary Objectives

The core purpose of investigating CaCO₃ effects is to optimize the balance between wear resistance and metallurgical soundness in high-chromium arc surfacing deposits. Specifically, the objectives include:

1. Maximize Cr₇C₃ and Cr₃C carbide volume fraction while minimizing the formation of brittle Cr₂₃C₆ and iron-rich carbides, thereby achieving the highest possible abrasion resistance.
2. Minimize intergranular cracking by controlling oxygen and sulfur content in the deposit through effective deoxidation and desulfurization.
3. Reduce oxide and slag inclusion populations to below the thresholds specified in relevant quality standards, ensuring that the overlay layer meets the acceptance criteria for critical service applications.
4. Establish quantitative relationships between CaCO₃ addition level (typically 0.5–5.0 wt% in composite powder formulations) and measurable microstructural and mechanical outcomes, enabling data-driven consumable design.

3.2 Value to the Company

The technical knowledge captured in this entry creates measurable value across the company's operations:

• Product Differentiation: Proprietary composite powder formulations with optimized CaCO₃ content yield overlay deposits with superior and more consistent wear performance compared to generic commercial consumables, providing a competitive advantage in bids and customer evaluations.
• WPS Qualification Efficiency: A deep understanding of the metallurgical effects of CaCO₃ reduces the number of trial runs required during WPS qualification, accelerating project timelines and reducing qualification costs.
• Customer Technical Confidence: The ability to present metallurgical data and analysis to customers—demonstrating that consumable selection is scientifically grounded—builds trust and supports long-term supplier relationships.
• Regulatory and Certification Compliance: Documented understanding of consumable chemistry and its effects on weld quality supports compliance with stringent industry standards and facilitates approval by classification societies, regulatory bodies, and end-user engineering teams.

4. Key Process and Implementation Points

4.1 Consumable Design Parameters

The following table summarizes the key parameters governing the use of CaCO₃ in high-chromium arc surfacing consumables:

Parameter	Typical Range	Effect on Deposit	Optimal Target
CaCO₃ addition level (wt%)	0.5 – 5.0	Higher levels increase slag volume and basicity; excessive levels promote CaO inclusion entrainment	1.5 – 3.0 wt% for most high-Cr systems
Slag basicity (CaO/SiO₂ ratio)	1.5 – 4.0	Higher basicity improves deoxidation and desulfurization but increases slag viscosity and spatter	2.5 – 3.5 for Stellite-type overlays
Weld pool oxygen content	0.005 – 0.030 wt%	Lower oxygen favors Cr₇C₃ formation; higher oxygen promotes Fe₃O₄ inclusions	< 0.010 wt% for wear-critical applications
Deposit carbon content	2.0 – 6.0 wt%	Higher carbon increases total carbide volume fraction; excess carbon promotes Cr₂₃C₆	3.5 – 5.0 wt% for balanced Cr₇C₃/Cr₃C
Cr content in deposit	20 – 30 wt%	Higher Cr increases carbide hardness but reduces ductility and increases cracking susceptibility	22 – 26 wt% for Cr26-class overlays
Particle size (composite powder)	0.15 – 0.45 mm	Finer particles improve arc stability and transfer efficiency; coarser particles increase dilution	0.25 – 0.35 mm (80–120 mesh equivalent)

4.2 Arc Surfacing Process Variables

The process parameters for open-arc surfacing of high-chromium alloys using composite powder or solid wire consumables containing CaCO₃ are as follows:

Process Variable	Typical Range	Notes
Shielding gas	Ar or Ar + 5% CO₂	Pure Ar preferred for low-oxygen deposits; CO₂ addition increases spatter with CaCO₃-containing consumables
Shielding gas flow rate	10 – 20 L/min	Higher flow rates needed for composite powder due to increased arc zone turbulence
Welding current (DCEN)	180 – 350 A	Depends on consumable diameter; higher current increases dilution and reduces carbide retention
Welding speed	150 – 400 mm/min	Faster speeds reduce heat input and dilution, preserving overlay composition
Travel angle / stickout	Stickout 8 – 12 mm	Optimized for powder transfer efficiency; excessive stickout causes erratic arc and porosity
Interpass temperature	≤ 150°C	Controlled to prevent excessive grain growth and Cr₂₃C₆ formation in subsequent passes
Number of passes	2 – 6	Multiple passes improve carbide distribution homogeneity; each pass should be ground flush before the next

4.3 Microstructural Characterization Protocol

To validate the effectiveness of CaCO₃ addition, the following characterization protocol should be implemented:

1. Optical Microscopy (OM): Examine deposit cross-sections at 100×–500× magnification to assess carbide morphology, distribution uniformity, and matrix structure. Quantify carbide volume fraction using image analysis software.
2. Scanning Electron Microscopy (SEM) with EDS: Identify individual carbide phases (Cr₇C₃, Cr₃C, Cr₂₃C₆, Fe₃C) and characterize oxide/slag inclusions. Map elemental distribution across the deposit to detect segregation.
3. X-Ray Diffraction (XRD): Quantify phase fractions of matrix (martensite, austenite, ferrite) and carbide phases. Establish the relationship between CaCO₃ level and phase evolution.
4. Hardness Testing (Vickers HV): Measure microhardness profiles across the deposit depth and at individual carbide particles. Typical targets: matrix HV 800–1000; Cr₇C₃ carbides HV 1800–2200.
5. Tribological Testing: Perform dry sliding wear tests (e.g., pin-on-disk per ASTM G99 or ASTM G213) against standardized counterface materials (SiC paper, alumina balls) to quantify volumetric wear rate and wear coefficient.
6. Impact/Bond Strength Testing: Conduct transverse impact or peening tests per ASTM A388 to verify overlay-to-substrate bond strength and assess cracking resistance.

5. Applicable Standards and Acceptance Criteria

5.1 Consumable Standards

• GB/T 12470 — Welding consumables for arc welding (Chinese national standard for welding wire and powder specifications)
• ASTM A5.21 — Specification for Filler Metals for Surfacing (covers Stellite-type and high-Cr overlay consumables)
• ISO 17641 — Classification and designation of welding consumables for arc welding
• GB/T 12467 — Classification and designation of welding consumables for arc welding (Chinese equivalent to ISO 17641)

5.2 Weld Overlay and Surfacing Standards

• GB/T 11350 — Classification and designation of welding consumables for arc welding of surfacing
• ASTM A388 — Standard Specification for Transverse Impact Test for Welds
• ASME Section IX, QW-451.4 — Qualification requirements for weld overlay (surfacing) procedures
• EN ISO 14555 — Welding and welding-related processes — Qualification rules
• NACE MR0175/ISO 15156 — Materials for use in H₂S-containing environments (relevant when overlays are used in oil and gas service)
• API 16C — Specification for Welded Steel Pipe for High Pressure Transmission Service (where clad pipe overlays are specified)

5.3 Non-Destructive Testing and Acceptance Standards

• GB/T 3323 — Radiographic testing of welds (acceptance for overlay deposits)
• GB/T 11345 — Ultrasonic testing of welds
• ASTM E709 / ASTM E165 — Magnetic particle and penetrant inspection for surface defect detection
• NB/T 47013 — Non-destructive testing methods for pressure vessels (Chinese petrochemical standard)
• ISO 17637 — Ultrasonic testing of welds — General rules

5.4 Acceptance Criteria Summary

Acceptance Parameter	Typical Criteria	Governing Standard
Overlay bond strength (transverse impact)	Energy ≥ 34 J (or as specified in WPS)	ASTM A388
Surface hardness (Vickers HV10)	≥ 800 HV for Cr26-class; ≥ 450 HB for Stellite-type	ASTM A5.21 / Customer spec
Porosity (visual/MT)	No individual pore > 1.0 mm; no cluster porosity	GB/T 11345 / ISO 17637
Undercut depth	≤ 0.5 mm (or as specified in WPS)	ASME Section IX
Overlay thickness uniformity	Within ±10% of nominal specified thickness	Customer specification
Crack-free requirement	Zero cracks in overlay or at overlay-substrate interface	NB/T 47013 / GB/T 3323

6. Common Risks and Controls

6.1 Metallurgical Risks

Risk	Cause	Control Measure
Intergranular cracking in overlay	Excessive CaCO₃ leading to high slag inclusion density; inadequate interpass temperature control	Limit CaCO₃ to ≤ 3.0 wt%; maintain interpass temperature ≤ 150°C; use low-sulfur base metal
Excessive Cr₂₃C₆ formation	Low welding speed / high heat input; low carbon activity in melt	Increase travel speed; ensure adequate carbon content in consumable; optimize CaCO₃ level for proper deoxidation
Oxide inclusion enrichment at overlay-substrate interface	Inadequate surface preparation; insufficient deoxidation	Grind substrate surface to bare metal; verify CaCO₃ deoxidation effectiveness through oxygen analysis
Spatter-induced surface roughness	High CaCO₃ level increasing slag viscosity and spatter	Optimize stickout and gas flow; use Ar shielding instead of CO₂ mix; post-grind if necessary
Hot cracking in overlay	High sulfur content in base metal or consumable; unfavorable solidification range	Use low-sulfur consumables (S ≤ 0.010%); leverage CaO desulfurization capacity; add trace Ti or Al as grain refiners

6.2 Process Risks

• Inconsistent powder feed rate: Composite powder containing CaCO₃ may exhibit different flow characteristics compared to standard powder due to the lighter density of the carbonate. Control: calibrate powder feeder regularly; monitor feed rate with a load cell or optical sensor; conduct daily flow rate verification tests.
• Arc instability: CaCO₃ decomposition products (CO₂ gas) can cause arc length fluctuations. Control: use a stable DCEN polarity; maintain consistent stickout; use a powder feeder with a narrow particle size distribution (0.25–0.35 mm).
• Operator variability: Open-arc surfacing is highly operator-dependent. Control: certify operators per ASME Section IX QW-301; implement standardized WPS with fixed parameters; conduct periodic performance audits.

7. Application Across the Company's Three Technology Routes

7.1 TIG/MIG Weld Overlay Route

The CaCO₃ metallurgical knowledge is most directly applicable to the TIG/MIG weld overlay route, which constitutes the primary production method for the company's surfacing services. Specific applications include:

• Custom WPS Development: When qualifying a new WPS for a high-chromium overlay on a specific substrate (e.g., Cr26 overlay on Q345B carbon steel, or Stellite 6 overlay on 304 stainless steel), the CaCO₃ effects data informs consumable selection and parameter optimization. The knowledge of how CaCO₃ influences carbide formation allows the metallurgical engineer to predict and control the final microstructure.
• Transition Layer Design: For dissimilar material weld overlays (e.g., high-Cr overlay on low-alloy steel), the CaCO₃ data helps design intermediate transition layers (typically 309L or 310 stainless) that manage dilution and thermal expansion mismatch. Understanding oxygen control through CaCO₃ is critical for preventing intergranular corrosion in the transition zone.
• Multi-Pass Overlay Optimization: The company routinely performs multi-pass (2–6 pass) overlays for thick wear layers. CaCO₃ effects on interpass carbide evolution guide the decision on whether to maintain the same consumable for all passes or switch to a lower-CaCO₃ consumable for final passes to minimize slag inclusion entrapment.
• WPS Qualification Packages: The metallurgical analysis supporting CaCO₃ selection forms part of the technical justification submitted to customers, certification bodies, and regulatory authorities during WPS qualification. This documentation demonstrates that the overlay procedure is scientifically validated, not merely empirically derived.

7.2 Hydraulic Explosive Bonding Route

While hydraulic explosive bonding is a solid-state joining process that does not involve melting or consumables, the CaCO₃ metallurgical knowledge contributes indirectly through:

• Post-Bonding Surface Preparation: Hydraulic explosive bonded clad plates often require surface treatment before delivery. Understanding high-chromium metallurgy ensures that any post-bonding machining, grinding, or heat treatment does not compromise the bond interface. The CaCO₃ study provides insight into how oxygen exposure during machining affects the near-surface carbide stability of the clad layer.
• Clad Layer Selection for Hybrid Products: When the company produces hybrid products combining explosive-bonded clad plates with welded overlay repairs, the CaCO₃ knowledge ensures metallurgical compatibility between the bonded overlay and the welded repair overlay. For example, a Cr26 layer produced by hydraulic explosive bonding can be locally repaired with a CaCO₃-optimized composite powder overlay without creating a metallurgical discontinuity.
• Quality Assurance of Bonded Clad Layers: The microstructural characterization techniques developed for CaCO₃ studies (SEM, EDS, XRD, hardness mapping) are directly transferable to the quality assurance of hydraulic explosive bonded interfaces. These techniques verify bond integrity, detect unbonded areas, and characterize the deformed layer structure at the bond interface.

7.3 Explosion Welding Route

Similar to hydraulic explosive bonding, explosion welding is a solid-state process, but the CaCO₃ metallurgical knowledge supports the following activities:

• Explosion-Welded Pipe and Plate Surface Treatment: Explosion-welded clad pipes and plates often undergo post-welding overlay to achieve specific thickness requirements or to repair localized defects. The CaCO₃ knowledge ensures that the overlay consumable is metallurgically compatible with the explosion-welded clad layer, maintaining consistent hardness, carbide distribution, and wear performance across the entire surface.
• Material Compatibility Assessment: When selecting the clad material for explosion welding (e.g., Cr26, Stellite, or high-Cr cast iron on carbon steel or stainless steel), the CaCO₃ research data informs the understanding of the clad material's metallurgical behavior under thermal cycling. This is relevant because explosion-welded products may subsequently undergo thermal processing (stress relief, annealing) that affects the clad layer microstructure.
• Customer Technical Documentation: For customers requiring explosion-welded products with specified overlay properties, the CaCO₃ metallurgical data supports the preparation of technical dossiers that document the relationship between clad material composition, processing history, and final performance. This documentation is essential for regulatory approval in petrochemical, nuclear, and offshore applications.

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

8.1 Qualification Building

This technical entry directly supports the company's qualification portfolio in the following ways:

• ASME Section IX WPS Qualification: The metallurgical understanding of CaCO₃ effects enables the company to develop and qualify WPS packages that meet ASME Section IX requirements for weld overlay procedures. The qualification records include consumable specifications with documented CaCO₃ content, process parameters, and mechanical/metallurgical test results.
• API 925 / API 16C Qualification: For clad pipe products, API standards require demonstration of overlay performance. The CaCO₃ research data provides the technical basis for meeting API-specified hardness, bond strength, and wear performance requirements.
• NB/T 47013 Compliance: For pressure vessel and piping applications governed by Chinese petrochemical standards, the company must demonstrate NDT capability and overlay qualification. The CaCO₃ knowledge supports the development of NDT procedures that account for the specific microstructural features of CaCO₃-modified overlays.
• ISO 9001 / ISO 3834 Quality Management: The documented technical understanding of consumable chemistry and its effects on weld quality is integral to the company's quality management system, demonstrating process control and technical competence to auditors and customers.

8.2 Product Delivery

The practical impact of CaCO₃ metallurgical knowledge on product delivery includes:

• Reduced Rework Rate: By selecting the optimal CaCO₃ level for each consumable formulation, the company can minimize the incidence of overlay defects (cracking, excessive porosity, hardness non-uniformity) that require rework. This directly improves production efficiency and on-time delivery rates.
• Consistent Product Quality: Standardized consumable formulations with documented CaCO₃ content ensure batch-to-batch consistency in overlay performance, which is critical for repeat orders and long-term customer relationships.
• Faster Qualification Turnaround: With a pre-established understanding of CaCO₃ effects, the company can predictively select process parameters and consumable compositions, reducing the number of trial coupons required during WPS qualification and accelerating project timelines.

8.3 Customer Value

"The ability to provide metallurgically justified, data-driven overlay solutions is a key differentiator in the competitive landscape of weld overlay and cladding services. Understanding the role of CaCO₃ in high-chromium alloy surfacing allows Cladding Technology Shanxi Co., Ltd. to deliver overlay products with superior and more predictable wear performance, extended service life, and reduced lifecycle costs for the end user."

Specific customer value propositions include:

• Extended Component Service Life: Optimized CaCO₃ levels in overlay consumables produce deposits with higher Cr₇C₃ carbide volume fractions and more uniform distribution, translating directly to longer service intervals for wear components such as ball mill liners, crusher hammers, and pump impellers.
• Reduced Total Cost of Ownership: Although the consumable cost may be marginally higher with optimized CaCO₃ formulation, the extended service life and reduced downtime result in significant savings in total cost of ownership for the customer.
• Technical Consultancy: The company can offer customers metallurgical consultation services, providing tailored overlay solutions based on the specific wear mechanisms (abrasive, erosive, adhesive, impact) and environmental conditions (temperature, chemical exposure) of the application.
• Warranty Confidence: With documented metallurgical validation of overlay performance, the company can offer performance warranties with greater confidence, reducing customer risk and strengthening commercial relationships.

9. Conclusion and Recommendations

The study of CaCO₃ effects on high-chromium alloy arc surfacing deposits represents a foundational element of the company's technical competence in weld overlay metallurgy. The knowledge gained from this research is not merely academic—it directly informs consumable design, WPS qualification, quality control, and customer technical support across all three of the company's technology routes.

The following recommendations are proposed to maximize the value of this technical entry:

1. Establish a Consumable Formulation Database: Systematically document the CaCO₃ content, slag basicity, and resulting microstructural/mechanical properties for all proprietary composite powder and solid wire formulations. This database should be maintained as a living document and updated with each new WPS qualification.
2. Integrate CaCO₃ Analysis into WPS Development Workflow: Make CaCO₃ effects assessment a mandatory step in the WPS development process for all high-chromium overlay applications. This ensures that consumable selection is metallurgically justified and documented.
3. Conduct Periodic Metallurgical Audits: Perform periodic metallurgical audits on production overlays to verify that the as-deposited microstructure matches the expected outcomes predicted by the CaCO₃ formulation model. Deviations should trigger root cause analysis and corrective action.
4. Publish Technical White Papers: Develop customer-facing technical white papers that explain the metallurgical basis for the company's overlay solutions, highlighting the role of CaCO₃ optimization. This positions the company as a technical leader and differentiates it from competitors who rely on generic consumables.
5. Cross-Train Personnel Across Technology Routes: Ensure that metallurgical knowledge from the CaCO₃ research is shared with teams responsible for hydraulic explosive bonding and explosion welding, enabling holistic technical support for customers who require integrated solutions combining multiple technology routes.

By embedding the CaCO₃ metallurgical knowledge into the company's operational processes, qualification systems, and customer engagement strategies, Cladding Technology Shanxi Co., Ltd. can sustain and enhance its competitive position in the high-chromium weld overlay market, delivering products and services that are metallurgically superior, consistently qualified, and demonstrably valuable to end users.