Microstructural and Performance Characteristics of 310 Stainless Steel Weld Overlay on Q235 Carbon Steel
1. Definition and Technical Background
Weld overlay of austenitic stainless steel 310 (UNS S31008/S31009) onto carbon steel Q235 (GB/T 700) represents a classic dissimilar metal cladding application in which a highly corrosion-resistant and oxidation-resistant overlay layer is deposited onto an economical structural base plate. Q235 is a low-carbon structural steel with a typical carbon content of 0.12–0.20%, widely used in pressure vessels, structural supports, heat exchanger shells, and piping systems. 310 stainless steel, with approximately 24–26% Cr and 19–22% Ni, offers exceptional resistance to dry oxidation and carburization at temperatures up to 1150°C, making it the preferred overlay material for furnace components, radiant tubes, and high-temperature process equipment.
The study of microstructural evolution and mechanical performance of the 310-on-Q235 overlay system addresses a fundamental metallurgical challenge: the extreme dilution tendency between austenitic filler metal and ferritic-pearlitic base metal. Without proper process control, the heat-affected zone (HAZ) and dilution zone can develop brittle martensitic phases, excessive hardness, cracking susceptibility, and premature corrosion failure.
2. Category and Business Positioning
This technical entry falls squarely within the TIG/MIG weld overlay technology route of Cladding Technology Shanxi Co., Ltd. It represents the foundational metallurgical research and process development work that underpins the company's ability to deliver qualified overlay cladding solutions. Specifically, this work contributes to:
- WPS/PQR Development: Establishing the metallurgical basis for welding procedure specifications and procedure qualification records
- Customer Engineering Support: Providing material selection rationale and performance justification for overlay design
- Qualification Building: Supporting certification to national and international standards through documented metallurgical evidence
3. Technical Purpose and Value
The primary objective of studying the 310-on-Q235 overlay system is to establish a quantitative understanding of how welding parameters influence:
- The microstructural composition of the dilution zone and transition layer
- The mechanical properties (hardness, tensile strength, impact toughness) across the overlay/base interface
- The corrosion resistance and oxidation resistance of the final overlay surface
- The susceptibility to hot cracking, cold cracking, and solidification cracking
This knowledge directly translates into optimized welding procedures that minimize dilution, control grain structure, and ensure the overlay layer retains its designed 310-grade composition and performance characteristics.
4. Key Process Implementation Points
4.1 Base Metal Preparation
Q235 base plates must be pre-treated to ensure proper weldability and overlay adhesion:
- Surface cleaning: Removal of rust, scale, oil, and contaminants to bare metal using grinding or shot blasting per GB/T 8923.1
- Edge preparation: Single-V or double-V groove preparation with appropriate included angle (60°–70°) for overlay layers
- Preheating: Typically 100–150°C for plates ≥20 mm thickness to reduce hydrogen-induced cracking risk
- Flux selection: Basic fluxes (e.g., HJ431) for submerged arc processes, or no flux for TIG processes
4.2 Welding Process Parameters
| Parameter | TIG Overlay (GTAW) | MIG Overlay (GMAW) | Submerged Arc Overlay (SAW) |
|---|---|---|---|
| Welding Current | 120–180 A | 180–280 A | 300–500 A |
| Welding Voltage | 10–14 V | 20–26 V | 28–36 V |
| Travel Speed | 30–60 mm/min | 200–400 mm/min | 250–500 mm/min |
| Wire/Filler Diameter | φ2.4–3.2 mm | φ1.2–1.6 mm | φ2.0–2.4 mm |
| Shielding Gas | Ar (99.99%) or Ar+2%O₂ | Ar+2%CO₂ or Ar+5%CO₂ | Flux-covered (HJ431) |
| Interpass Temperature | ≤150°C | ≤200°C | ≤250°C |
| Typical Dilution Rate | 5–15% | 10–25% | 15–35% |
4.3 Multi-Layer Overlay Strategy
To achieve a pure 310-grade surface composition while controlling costs, a multi-layer approach is recommended:
- Layer 1 (Transition Layer): Deposit a 309L (UNS S30908) or 310L filler to buffer the dilution effect of Q235 base metal. This layer typically contains 20–30% dilution from the base.
- Layer 2 (Build-up Layer): Deposit 310 filler with reduced dilution (10–15%) to establish the bulk of the overlay.
- Layer 3 (Surface/Finish Layer): Deposit final 310 layer with minimal dilution (<5%) to ensure surface composition meets 310-grade specifications.
4.4 Heat Input Control
Heat input is the single most critical parameter governing dilution and microstructure. For TIG overlay:
- Low heat input (<0.8 kJ/mm): Minimizes dilution but risks incomplete fusion and porosity
- Optimal heat input (0.8–1.5 kJ/mm): Balances fusion quality with acceptable dilution
- Excessive heat input (>2.0 kJ/mm): Causes excessive dilution, grain coarsening, and potential cracking
5. Microstructural Analysis
5.1 Overlay Zone Microstructure
The weld metal in the 310 overlay layer should exhibit a fully austenitic microstructure (γ-phase) with δ-ferrite content ideally below 5%. The presence of δ-ferrite serves a beneficial role in mitigating hot cracking susceptibility, but excessive ferrite (>10%) indicates excessive dilution from the carbon steel base.
5.2 Dilution Zone and HAZ
The transition zone between the overlay and Q235 base metal is the most critical region from a metallurgical standpoint:
- Dilution Zone: Characterized by a gradient from austenite (310 side) to martensite/ferrite (Q235 side). Hardness can reach 350–450 HV in the dilution zone.
- Heat-Affected Zone: Exhibits grain coarsening and potential formation of upper bainite or martensite depending on cooling rate. Hardness typically 200–300 HV.
5.3 Cracking Susceptibility Assessment
The 310-on-Q235 system is susceptible to several cracking modes:
- Hot Cracking (Solidification Cracking): Occurs in the dilution zone due to low melting point intermetallics (Ni-Cr-Si phases) segregating at grain boundaries. Controlled by maintaining δ-ferrite at 3–8%.
- Cold Cracking (Hydrogen-Induced Cracking): Occurs in the Q235 HAZ due to high carbon equivalent and hydrogen pickup. Controlled by preheating and low hydrogen processes.
- Lamellar Tearing: Occurs in rolled Q235 plates with high sulfur inclusions. Controlled by base plate selection and proper groove orientation.
6. Performance Testing and Acceptance Criteria
6.1 Mechanical Properties
| Property | 310 Overlay (Target) | Q235 Base Metal | Acceptance Standard |
|---|---|---|---|
| Hardness (HV) | 120–180 HV | 120–160 HV | Gradient ≤100 HV/mm |
| Tensile Strength (MPa) | ≥520 MPa | ≥370 MPa | ASTM A376 / GB/T 228 |
| Impact Energy (J @ RT) | ≥50 J | ≥27 J | GB/T 229 / ASTM E23 |
| δ-Ferrite Content | 3–8% | N/A | ASTM E1092 |
6.2 Corrosion and Oxidation Testing
- Potential Difference Test (PDT): Per NACE SP0775 or GB/T 19426, the overlay surface must be noble relative to the base metal to ensure cathodic protection of the base
- Intergranular Corrosion: Per ASTM A262 Practice A/E, the overlay must resist sensitization-induced intergranular attack
- Oxidation Resistance: Weight gain testing at 1000–1100°C in air per ASTM G93 or equivalent
- Potentiodynamic Polarization: Per ASTM G5, measuring corrosion potential and passivation behavior
6.3 NDT Requirements
- Visual Inspection: Per GB/T 3323 or ASME V Article 1, no cracks, undercut, or excessive reinforcement
- Penetrant Testing (PT): Per GB/T 18851 or ASME V Article 6, for surface-breaking defects
- Ultrasonic Testing (UT): Per GB/T 11345 or ASME V Article 5, for volumetric defects and overlay thickness verification
- Hardness Mapping: Transverse hardness traverse across the overlay/base interface per ASTM B231
7. Applicable Standards
| Standard | Scope |
|---|---|
| GB/T 12467 | Welding procedure qualification for steels (Chinese equivalent to ISO 15614) |
| GB/T 19426 | Corrosion resistance requirements for weld overlay |
| NB/T 47014 | Welding procedure qualification for pressure vessels (China) |
| ASME BPVC Section IX | Welding and Brazing Qualifications (USA) |
| ASTM A376 | Weld overlay requirements for carbon and low-alloy steels |
| ASTM A240 | Stainless steel plate/sheet/bar (310 grade specification) |
| GB/T 700 | Carbon and low-alloy structural steel (Q235 specification) |
| ISO 15614-1 | Welding procedure qualification for metallic materials |
| NACE SP0775 | Weld overlay for corrosion resistance |
8. Common Risks and Controls
| Risk | Cause | Control Measure |
|---|---|---|
| Excessive dilution | High heat input, insufficient layers | Reduce heat input, add transition layer, use multi-pass technique |
| Hot cracking in dilution zone | Low δ-ferrite, high sulfur | Use 309L transition layer, control S ≤0.015%, maintain δ-ferrite 3–8% |
| Hydrogen-induced cold cracking | High CE of Q235, moisture in flux | Preheat 150°C, use low-hydrogen flux, post-weld bake 200°C/2h |
| Poor corrosion resistance | Dilution below 310 composition | Ensure ≥3 overlay layers, verify surface composition by optical emission spectrometry |
| Undercut at overlay edge | Excessive travel speed, improper technique | Reduce travel speed, use backing bar or backing strip |
| Lamellar tearing in base | High S inclusions in rolled Q235 | Select through-thickness plate, orient groove parallel to rolling direction |
9. Application Scenarios Across Technology Routes
9.1 TIG/MIG Weld Overlay Route
The 310-on-Q235 overlay system is most commonly produced via TIG or MIG welding for the following applications:
- Furnace Radiant Tubes and Burner Supports: Q235 structural tubes with 310 overlay on the heated surface for oxidation resistance at 1000–1150°C
- Heat Exchanger Tubesheets: Q235 base with 310 overlay on the process fluid side for high-temperature corrosion resistance
- Boiler Components: Superheater tubes and air preheater elements requiring oxidation resistance
- Petrochemical Reactor Linings: Carbon steel vessels with 310 overlay for carburization resistance
- Repair and Restoration: Field repair of worn or corroded 310 overlay surfaces on existing equipment
9.2 Hydraulic Explosive Bonding Route
While 310 stainless steel is not typically bonded via explosive cladding due to its high density and ductility (which can prevent proper wave interaction), the metallurgical knowledge from the weld overlay study informs:
- Post-bonding overlay: When explosive bonding produces a thinner-than-required cladding, TIG/MIG overlay with 310 filler can build up the cladding thickness
- Repair welding: Welding across explosive bond interfaces requires understanding of the bond zone microstructure, informed by the dilution zone studies
- Transition layer design: The transition layer concepts developed for weld overlay apply to the bonding interface design in hybrid clad structures
9.3 Explosion Welding Route
Similar to hydraulic explosive bonding, explosion welding of 310 onto Q235 is limited by metallurgical compatibility. However, the research contributes to:
- Composite plate qualification: Understanding the weldability of explosion-welded composite plates when subsequent welding operations are required
- Weld overlay on explosion-welded cladding: Building additional overlay thickness on top of explosion-welded cladding for thickness-critical applications
- Process development: Informing the selection of intermediate layers for explosion welding when direct 310/Q235 bonding is not feasible
10. Contribution to Qualification Building and Customer Value
10.1 Qualification Building
This technical study directly supports the company's qualification portfolio by:
- WPS Development: Providing the metallurgical data required to develop qualified welding procedure specifications for 310 overlay on carbon steel
- PQR Documentation: Generating the procedure qualification records with documented mechanical properties, microstructural analysis, and NDT results
- Standard Compliance: Demonstrating compliance with GB/T 12467, NB/T 47014, and ASME Section IX requirements for dissimilar metal weld qualification
- Third-Party Certification: Providing the technical evidence base for certification bodies to approve the company's overlay capabilities
10.2 Product Delivery
The metallurgical understanding gained from this study enables:
- Process Optimization: Reducing dilution rates from >25% to <10% through optimized parameter selection
- Quality Assurance: Establishing in-process monitoring criteria (interpass temperature, dilution rate, hardness mapping) for consistent product quality
- Defect Reduction: Minimizing hot cracking, cold cracking, and porosity through informed process design
- Cost Efficiency: Optimizing the number of overlay layers and filler metal consumption while maintaining performance requirements
10.3 Customer Value
This technical capability translates directly into customer benefits:
- Extended Equipment Life: 310 overlay on Q235 provides oxidation and corrosion resistance at temperatures where base carbon steel would fail in hours
- Reduced Maintenance: Properly executed overlay with controlled dilution reduces unplanned shutdowns for repair
- Material Cost Savings: Using economical Q235 base with expensive 310 overlay achieves 310-grade performance at a fraction of the cost of solid 310 components
- Technical Confidence: Providing customers with documented metallurgical evidence, NDT reports, and qualification records that support their regulatory and quality requirements
11. Recommended Testing Protocol
For a complete qualification of the 310-on-Q235 overlay system, the following testing protocol is recommended:
- Visual Inspection: 100% visual examination per GB/T 3323 or ASME V Article 1
- Penetrant Testing: 100% PT on all overlay surfaces per GB/T 18851
- Ultrasonic Testing: 100% UT for volumetric defects per GB/T 11345 or ASME V Article 5
- Hardness Mapping: Transverse hardness traverse across overlay/base interface per ASTM B231
- Metallographic Examination: Cross-section preparation, etching, and microstructural analysis per ASTM E3 and ASTM E4
- δ-Ferrite Measurement: Per ASTM E1092 on dilution zone and overlay weld metal
- Chemical Analysis: Surface composition verification by optical emission spectrometry (OES) to confirm 310-grade composition
- Corrosion Testing: Potential difference test per NACE SP0775 or GB/T 19426
- Mechanical Testing: Tensile and impact testing per GB/T 228 and GB/T 229 if required by the applicable code
12. Conclusion
The study of 310 stainless steel weld overlay on Q235 carbon steel represents a foundational metallurgical investigation that underpins the company's capability to deliver high-performance dissimilar metal cladding solutions. By systematically understanding the microstructural evolution, dilution behavior, mechanical properties, and corrosion performance of this overlay system, the company can develop qualified welding procedures, ensure consistent product quality, and provide customers with technically sound solutions for high-temperature and corrosion-resistant applications. This knowledge base is essential for maintaining competitive positioning in the weld overlay market and for supporting the company's broader qualification and certification objectives.