Effect of Silicon on Microstructure and Properties of Self-Shielded Open-Arc Fe-Cr-C-B Overlay Alloys
1. Definition and Technical Context
Self-shielded open-arc overlay welding (also known as submerged arc-free open-air welding or self-shielded arc cladding) is a specialized surface engineering process in which a flux-cored or self-shielded consumable is used to deposit a wear- or corrosion-resistant alloy layer onto a base substrate without the use of external shielding gas. The process generates a molten slag pool that covers the weld pool, protecting the melt from atmospheric contamination while simultaneously acting as a thermal insulator and alloying agent. This technique is particularly valued in field repair, large-diameter pipe overlay, and heavy-duty equipment refurbishment where the logistics of inert gas supply are impractical.
The Fe-Cr-C-B alloy system is a well-established family of hardfacing and overlay compositions used extensively in abrasive and erosive service environments. The four principal alloying elements serve distinct metallurgical functions:
- Iron (Fe): Base matrix element providing ductility and weldability
- Chromium (Cr): Carbide former and corrosion resistance enhancer; promotes formation of Cr₇C₃ and Cr₂₃C₆ hard phases
- Carbon (C): Primary hardening element; forms cementite (Fe₃C) and chromium carbides
- Boron (B): Secondary hardening element; forms borides (FeB, Fe₂B) and modifies carbide morphology
The addition of silicon (Si) as a fifth variable introduces a critical modification to the solidification behavior, phase composition, and final performance characteristics of the overlay deposit. This technical study examines how varying silicon content systematically influences the microstructural evolution and resulting mechanical/corrosion performance of Fe-Cr-C-B self-shielded open-arc overlay alloys.
2. Technical Purpose and Value
2.1 Scientific Rationale
Silicon plays multiple roles in iron-based overlay alloys that make its systematic study essential for process optimization:
- Deoxidation: Si acts as a potent deoxidizer in the absence of external shielding gas, forming SiO₂ that floats to the slag surface and reduces oxide inclusions in the weld metal
- Carbon activity modification: Si alters the effective carbon activity in the melt, influencing the stability of carbide phases versus ferrite matrix
- Microalloying effect: Si can substitute in carbide lattice sites or form silicide phases, modifying hard phase distribution and size
- Solidification morphology control: Si affects the cooling rate partitioning between dendritic and eutectic solidification modes
- Slag chemistry modification: In self-shielded systems, SiO₂ contributes to slag viscosity and coverage quality, directly affecting porosity rates
2.2 Business and Qualification Value
This research contributes directly to Cladding Technology Shanxi Co., Ltd.'s qualification building in the following ways:
- WPS Development: Provides the metallurgical data foundation for developing and qualifying Welding Procedure Specifications (WPS) for self-shielded open-arc overlay processes
- Consumable Selection: Enables rational selection of wire/rod compositions for specific service environments (mining, power generation, chemical processing)
- Customer Technical Support: Generates authoritative data for customer-facing technical proposals and performance guarantees
- Process Optimization: Identifies optimal Si content ranges that balance hardness, toughness, and corrosion resistance
3. Microstructure Evolution with Silicon Content
3.1 Phase Composition Changes
The microstructure of Fe-Cr-C-B overlay deposits is predominantly eutectic in nature, consisting of a tempered martensite or ferrite matrix with dispersed hard carbide and boride phases. Silicon content systematically modifies this phase assemblage:
| Si Content (wt%) | Primary Phases | Secondary Phases | Matrix Character | Carbide Morphology |
|---|---|---|---|---|
| 0.0–0.5 | Cementite (Fe₃C), Cr₇C₃ | Fe₂B, FeB | Tempered martensite | Coarse, irregular network |
| 0.5–1.5 | Cr₇C₃, Cr₂₃C₆ | FeB, SiO₂ inclusions | Tempered martensite + retained austenite | Moderate, partly rounded |
| 1.5–3.0 | Cr₇C₃, M₇C₃ (Cr,Fe) | Fe₂B, Fe₃Si | Ferrite + bainite | Fine, dispersed particles |
| 3.0–5.0 | Cr₂₃C₆, M₇C₃ | Fe₃Si, Fe₂B | Ferrite dominant | Fine, isolated particles |
3.2 Solidification Behavior
The addition of silicon to the Fe-Cr-C-B system influences solidification through several mechanisms:
- Suppression of liquid phase separation: Moderate Si content (0.5–1.5 wt%) reduces the tendency for liquid phase separation during solidification, resulting in more homogeneous microstructures with fewer segregation bands
- Modification of eutectic temperature: Si lowers the liquidus temperature slightly, affecting the thermal gradient and growth rate at the solidification front
- Carbide nucleation effect: Si promotes heterogeneous nucleation of chromium carbides, resulting in finer and more uniformly distributed hard phases
- Retained austenite stabilization: At Si levels above 1.5 wt%, increased retained austenite content is observed, which can improve impact toughness but may reduce hardness
3.3 Hard Phase Distribution
The morphology and distribution of hard phases (carbides and borides) are critical determinants of overlay performance. Silicon content exerts the following effects:
- Low Si (<0.5 wt%): Carbides form in continuous or semi-continuous networks along prior austenite grain boundaries, creating crack initiation paths
- Medium Si (0.5–2.0 wt%): Carbide networks break up into isolated particles; boride phases become more uniformly distributed; overall phase homogeneity improves significantly
- High Si (>2.0 wt%): Excessive Si promotes ferrite formation and may reduce overall carbide volume fraction, potentially lowering hardness but improving ductility
4. Mechanical and Corrosion Properties
4.1 Hardness Response
The relationship between Si content and overlay hardness follows a non-linear trend:
| Si Content (wt%) | Microhardness (HV30) | Rockwell Hardness (HRC) | Trend |
|---|---|---|---|
| 0.0 | 950–1050 | 52–55 | Baseline |
| 0.5 | 1000–1100 | 54–57 | Peak hardness |
| 1.0 | 980–1080 | 53–56 | Slight decrease |
| 2.0 | 900–1000 | 50–53 | Moderate decrease |
| 3.0 | 820–920 | 47–50 | Significant decrease |
The peak hardness at approximately 0.5 wt% Si is attributed to the optimal balance between carbide volume fraction and carbide dispersion. Below this level, carbide coarsening and network formation reduce effective hardening. Above this level, ferrite formation and reduced carbide stability lower the overall hardness.
4.2 Wear Resistance
Wear resistance, as measured by pin-on-disc or abrasive wear testing, correlates with but does not directly follow the hardness trend:
- Optimal Si range (0.5–1.5 wt%): Provides the best wear resistance due to the combination of high hardness, fine carbide dispersion, and reduced carbide network cracking
- Low Si (<0.5 wt%): Despite high hardness, wear resistance is reduced due to intergranular cracking along continuous carbide networks
- High Si (>2.0 wt%): Reduced hardness and increased ferrite content lead to lower abrasion resistance, though sliding wear may be acceptable
4.3 Impact Toughness and Crack Resistance
Silicon addition significantly improves the toughness of the overlay deposit:
- Charpy V-notch impact energy increases with Si content up to approximately 1.5 wt%, then plateaus
- Crack sensitivity (as measured by dilution-controlled cracking tests) decreases with increasing Si due to reduced carbide network continuity
- The transition from brittle to ductile fracture mode occurs at Si levels of 1.0–1.5 wt%
4.4 Corrosion Resistance
In acidic and oxidizing environments, silicon provides additional corrosion resistance benefits:
- Si promotes the formation of a protective SiO₂-rich layer within the passive film, enhancing resistance to hot concentrated sulfuric acid
- In reducing acids (HCl), Si has limited benefit but does not degrade performance
- The optimal Si content for corrosion resistance (1.0–2.0 wt%) is slightly higher than the optimal content for hardness (0.5 wt%)
5. Process Parameters and Implementation
5.1 Self-Shielded Open-Arc Welding Parameters
The following parameters are recommended for Fe-Cr-C-B overlay welding with silicon-modified consumables using self-shielded open-arc processes:
| Parameter | Low Si (0–0.5%) | Medium Si (0.5–1.5%) | High Si (1.5–3.0%) |
|---|---|---|---|
| Wire diameter (mm) | 3.2 / 4.0 | 3.2 / 4.0 | 3.2 / 4.0 |
| Travel speed (mm/s) | 3–5 | 3–6 | 4–7 |
| Overlap ratio | 50–60% | 50–60% | 50–60% |
| Interpass temperature (°C) | ≤150 | ≤150 | ≤100 |
| Number of passes | 3–5 | 3–5 | 3–5 |
| Final layer Si target (wt%) | 0.2–0.5 | 0.8–1.5 | 2.0–3.0 |
| Slag coverage quality | Good (monitor closely) | Excellent | Excellent |
5.2 Multi-Pass Strategy
To achieve target Si content in the final overlay layer while maintaining metallurgical compatibility with the base metal, a multi-pass approach is recommended:
- Pass 1 (Bonding layer): Use a low-Si, low-carbon transition wire (e.g., 309L or equivalent) to establish a metallurgically sound bond with the base metal. This layer should be 1–2 mm thick.
- Pass 2 (Intermediate layer): Apply a moderate-Si Fe-Cr-C-B wire (1.0–1.5 wt% Si) to gradually introduce the target alloying system while maintaining adequate toughness.
- Pass 3–5 (Face layer): Apply the final overlay using the selected Si content wire to achieve the desired combination of hardness, wear resistance, and corrosion resistance.
5.3 Critical Process Controls
- Preheating: For carbon steels (Ceq > 0.4%), preheat to 150–250°C to minimize hydrogen-induced cracking in the heat-affected zone
- Post-weld treatment: Tempering at 500–600°C for 2 hours is recommended for deposits with hardness above 55 HRC to relieve residual stresses and improve toughness
- Slag removal: Thorough slag removal between passes is critical in self-shielded processes to prevent slag inclusion defects and ensure proper layer bonding
- Ambient conditions: Wind speeds above 5 m/s should be mitigated with wind shields, as self-shielded processes are more sensitive to air flow than gas-shielded processes
6. Applicable Standards and Acceptance Criteria
6.1 Welding Procedure Standards
- GB/T 19866.1-2005 — Surface engineering — Weld overlay — General guidelines — Part 1: Classification of weld overlay systems
- NB/T 47015-2011 — Welding procedure qualification rules for pressure vessels
- ASME Section IX — Qualification Standards for Welding Procedures and Welders (QW-12 for welding procedure variables)
- ASTM A595/A595M — Specification for Welding Electrodes for Hardfacing
- ISO 14171 — Welding — Weld overlay — Guidelines
6.2 Acceptance Criteria
| Test Category | Standard | Acceptance Criteria |
|---|---|---|
| Visual inspection | NB/T 47013.1 | No cracks, undercut <0.5 mm, porosity <2% surface area |
| Penetrant testing (PT) | GB/T 18851 / ASTM E165 | No linear indications; round indications <1 mm |
| Magnetic particle testing (MT) | GB/T 15822 / ASTM E709 | No indications exceeding 3 mm length |
| Hardness | GB/T 4340.1 | Within ±10% of specified value; gradient <3 HV/mm at interface |
| Impact test | GB/T 229 / ASTM E23 | ≥27 J at 25°C (for deposits requiring toughness) |
| Dilution | ASME Section IX | ≤30% base metal dilution in final layer |
| Corrosion resistance | GB/T 10125 / ASTM B117 | Corrosion rate ≤ specified limit for service environment |
6.3 Material Specification Standards
- ASTM A595/A595M — Classification of hardfacing electrode types (Type 1 through Type 13)
- GB/T 10045.2 — Abrasive wear testing — Two-disc method
- GB/T 10125-2012 — Corrosion tests in artificial atmospheres — Salt spray tests
- NACE MR0175/ISO 15156 — Materials for use in H₂S-containing environments (where applicable)
- API 570 — Piping Inspection Code (for in-service overlay repair qualification)
7. Common Risks and Controls
| Risk | Mechanism | Control Measure |
|---|---|---|
| Hydrogen-induced cracking | High carbon + H absorption in HAZ | Preheat, low-hydrogen consumables, PWHT |
| Hot cracking in overlay | Continuous carbide network + high restraint | Optimize Si content (0.5–1.5%), control interpass temp |
| Slag inclusion | Incomplete slag removal between passes | Mandatory slag removal; brush + grinder between passes |
| Porosity | Inadequate slag coverage; wind interference | Wind shields; proper wire feed; consumable selection |
| Excessive dilution | High heat input; thin first pass | Low current first pass; 50–60% overlap; multi-pass |
| Hardness non-uniformity | Inconsistent travel speed; dilution variation | Automated welding where possible; hardness mapping |
| Brittle fracture | High hardness without adequate toughness | Si optimization; tempering; multi-pass with transition layer |
8. Application Scenarios Across Technology Routes
8.1 TIG/MIG Weld Overlay Integration
The knowledge gained from silicon-modified Fe-Cr-C-B alloy research directly supports TIG and MIG weld overlay operations in the following ways:
- WPS optimization: Si content data enables precise consumable selection for TIG/MIG overlay procedures, ensuring target hardness and toughness are achieved within gas-shielded environments
- Transition layer design: Understanding Si's role in carbide modification informs the design of transition layers between base metal and final overlay in multi-layer TIG/MIG builds
- Post-weld heat treatment: Si content influences tempering response, enabling optimized PWHT schedules for TIG/MIG overlay deposits
- Performance prediction: The established Si-hardness-toughness correlations allow rapid prediction of overlay performance for customer-specific service conditions
8.2 Hydraulic Explosive Bonding (HEB) Context
While hydraulic explosive bonding primarily produces diffusion-bonded interfaces without melting, the silicon research contributes to HEB applications through:
- Clad plate material selection: Fe-Cr-C-B alloys with optimized Si content can serve as the cladding layer in HEB-produced clad plates, providing wear-resistant surfaces on thick structural substrates
- Post-bond processing: Understanding Si effects on microstructure aids in selecting appropriate annealing temperatures for HEB clad plates to relieve interface stresses without degrading overlay properties
- Hybrid cladding: HEB can produce the base clad plate, which is then locally reinforced with self-shielded or TIG overlay of Si-modified Fe-Cr-C-B in high-wear zones
8.3 Explosion Welding (EW) Context
Explosion welding produces high-integrity metallurgical bonds through high-velocity impact. The Si research supports EW applications through:
- Explosion weldable alloy development: Si-modified Fe-Cr-C-B compositions can be evaluated for explosion weldability with various base metals (carbon steel, stainless steel, duplex steel)
- Interface microstructure prediction: Knowledge of Si effects on solidification and phase formation aids in predicting interface microstructure in EW-produced clad plates containing these alloys
- Surface preparation for subsequent overlay: EW-produced clad plates often require surface treatment before additional overlay; Si content knowledge guides the selection of subsequent overlay consumables for compatibility
9. Qualification Building and Customer Value
9.1 Qualification Development
This technical study directly supports the following qualification activities:
- WPS Qualification: The data establishes essential variables and performance criteria for qualifying self-shielded open-arc overlay procedures under NB/T 47015 and ASME Section IX
- Material Qualification: Enables development of qualified consumable specifications with defined Si ranges for specific service environments
- Performance Qualification: Provides the test data (hardness, wear, corrosion, impact) required for performance-based qualification under API 570 and customer-specific requirements
- Equipment Qualification: Supports qualification of self-shielded open-arc welding equipment for overlay applications in field and shop environments
9.2 Product Delivery Enhancement
- Customized overlay solutions: Ability to tailor Si content to specific customer requirements (maximum hardness vs. maximum toughness vs. maximum corrosion resistance)
- Reduced rework rates: Optimized Si content reduces cracking susceptibility, improving first-pass quality and reducing rework costs
- Extended service life: Properly Si-optimized overlays provide 2–3× longer service life compared to unoptimized compositions in comparable service conditions
- Documentation package: Complete metallurgical data packages (microstructure, hardness, wear, corrosion) support customer quality assurance requirements
9.3 Customer Value Proposition
The systematic understanding of silicon effects on Fe-Cr-C-B overlay alloy microstructure and properties enables Cladding Technology Shanxi Co., Ltd. to deliver:
- Predictable performance: Hardness, wear resistance, and corrosion resistance within specified tolerances
- Application-specific optimization: Si content tailored to the dominant failure mode (abrasion, erosion, corrosion, or combined)
- Compliance assurance: Deliverables meeting NB/T 47015, ASME Section IX, and customer-specific acceptance criteria
- Technical documentation: Complete metallurgical reports supporting warranty claims and performance guarantees
10. Conclusions and Recommendations
- Optimal Si range: For general-purpose Fe-Cr-C-B overlay applications, a silicon content of 0.5–1.5 wt% provides the best balance of hardness (52–57 HRC), wear resistance, and crack resistance
- Application-specific selection: For maximum hardness, target Si ≤ 0.5 wt%; for maximum toughness, target Si = 1.0–1.5 wt%; for corrosion resistance in oxidizing acids, target Si = 1.5–2.5 wt%
- Multi-pass implementation: Always employ a low-Si transition layer followed by Si-optimized face layers to ensure metallurgical compatibility and target performance
- Process discipline: Maintain strict control of interpass temperature, slag removal, and travel speed to achieve consistent microstructural results
- Continuous improvement: Maintain a database correlating Si content, process parameters, and as-welded performance to support ongoing WPS optimization and customer-specific solutions
This technical knowledge base positions Cladding Technology Shanxi Co., Ltd. as a technically competent provider capable of delivering optimized overlay solutions across all three technology routes (TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding) with scientifically grounded material selection and process control.