Microstructure and Mechanical Properties of Stainless Steel Weld Overlay on Q235 Carbon Steel Substrate
1. Technical Definition and Fundamental Principles
The systematic study of stainless steel weld overlay applied to Q235 carbon steel substrates represents a foundational metallurgical discipline within the bimetallic cladding industry. Q235 steel, per GB/T 700, is a low-carbon structural steel with a typical composition of 0.12–0.20% C, ≤0.60% Mn, and ≤0.30% Si, exhibiting a ferrite-pearlite microstructure with yield strength of ≥235 MPa. When stainless steel — typically austenitic grades such as 304, 304L, 316L, or high-alloy super-duplex — is deposited onto this substrate via TIG or MIG welding processes, a complex metallurgical interaction zone develops between the dissimilar metals.
The governing principles of this overlay system include:
- Dilution Control: The percentage of base metal (Q235) dissolved into the weld deposit directly determines the final composition and microstructure of the overlay layer. Excessive dilution (>25–30%) can shift the weld metal from fully austenitic to a mixed ferrite-austenite or martensitic structure, compromising corrosion resistance.
- Thermal Cycling Effects: Q235 steel's relatively low thermal conductivity compared to austenitic stainless steels creates asymmetric thermal gradients, influencing solidification morphology, grain growth, and residual stress distribution.
- Interfacial Bonding: The metallurgical bond at the Q235/stainless steel interface is governed by solid-state diffusion, liquid-phase wetting, and the absence of brittle intermetallic phases (such as FeCr or FeNi intermetallics).
- Transformation Behavior: The carbon content from Q235 diffusing into the austenitic weld metal can promote martensitic transformation during cooling, particularly in Cr-Mo-bearing grades, necessitating post-weld heat treatment or careful parameter selection.
2. Category and Business Positioning
2.1 Technical Classification
This technical entry belongs to the Weld Overlay Metallurgy knowledge domain, specifically addressing the base metal–weld metal interaction in dissimilar metal cladding applications. Within the company's three primary technology routes:
- TIG Weld Overlay: This knowledge directly informs the selection of filler wire (e.g., ER309L, ER316L), shielding gas composition (Ar/CO₂ blends), heat input parameters, and multi-pass strategies for thin overlay layers (typically 1–5 mm per pass).
- MIG Weld Overlay: The microstructural understanding guides wire feed speed, voltage settings, travel speed optimization, and the design of transition layers to minimize dilution from the Q235 substrate.
- Hydraulic Explosive Bonding / Explosion Welding: While these solid-state processes do not involve melting, the metallurgical knowledge of Q235 surface behavior (oxidation, carbon content effects on bonding strength) informs pre-treatment requirements and post-bonding weld overlay repair procedures.
2.2 Business Value Positioning
This metallurgical study directly supports the company's WPS (Welding Procedure Specification) qualification program by providing the scientific basis for process parameter selection. It enables the engineering team to:
- Justify overlay thickness specifications to customers based on dilution calculations
- Predict service life of overlay layers in corrosive environments
- Design transition layer sequences that prevent cracking and ensure long-term integrity
- Develop NDT acceptance criteria tailored to the specific microstructural characteristics of the overlay
3. Technical Purpose and Engineering Value
3.1 Primary Objectives
The core purpose of studying stainless steel overlay on Q235 steel is to achieve a reliable, corrosion-resistant functional surface layer while maintaining the structural integrity and mechanical performance of the carbon steel substrate. The engineering value manifests in several dimensions:
- Corrosion Protection: Converting a general-purpose Q235 structural component into a corrosion-resistant assembly suitable for chemical, food processing, or marine environments without replacing the entire substrate material.
- Erosion Resistance: Providing surface hardness and wear resistance in slurry handling, pump components, and valve applications.
- Hygienic Compliance: Enabling food-grade and pharmaceutical equipment manufacturing on cost-effective carbon steel bases while meeting surface finish and material purity requirements.
- Repair and Restoration: Extending the service life of worn or corroded equipment through strategic overlay application.
3.2 Quantitative Performance Targets
| Performance Parameter | Q235 Substrate (Base) | Target Overlay (Stainless) | Acceptance Threshold |
|---|---|---|---|
| Corrosion Rate (3.5% NaCl, 24h) | >100 μm/year | <5 μm/year | <10 μm/year |
| Hardness (HV30) | 120–180 | 180–250 (annealed austenitic) | ≤300 (to prevent cracking) |
| Tensile Strength (MPa) | 375–500 | 520–720 (as-welded austenitic) | ≥400 (overlay zone) |
| DBTT (°C) | -20 to +20 | <-100 (austenitic) | No brittle transition |
| Interfacial Shear Strength (MPa) | N/A | N/A | ≥150 (weld overlay) |
| Dilution Rate | N/A | N/A | ≤25% (single pass) |
4. Key Process and Implementation Points
4.1 Microstructural Evolution Analysis
The microstructure of stainless steel weld overlay on Q235 steel develops through three distinct zones, each requiring specific process control:
Zone A: Fusion Zone / Dilution Layer
This is the critical transition zone where Q235 carbon steel has partially melted and mixed with the stainless steel filler metal. Key metallurgical phenomena include:
- Carbon diffusion: Carbon from Q235 (0.12–0.20%) migrates into the weld pool, potentially exceeding the solid solubility limit in austenite and promoting carbide precipitation (Cr₂₃C₆) at grain boundaries during cooling.
- Grain boundary migration: Columnar grains grow from the substrate into the weld, creating a directional microstructure that can influence crack propagation paths.
- Phase transformation: Depending on dilution level, the fusion zone may exhibit 100% austenite (low dilution), austenite + delta ferrite (moderate dilution), or martensite + retained austenite (high dilution with higher alloy content).
Zone B: Heat-Affected Zone (HAZ)
The HAZ in Q235 steel extends approximately 2–5 mm from the fusion line and experiences:
- Grain coarsening in the region between Ac₁ and Ac₃ temperatures
- Possible formation of upper bainite or coarse pearlite in the sub-critical region
- Hardness increase to 200–280 HV due to grain refinement and precipitation hardening
- Residual stress concentration that may reach 300–400 MPa without proper stress relief
Zone C: Overlay Deposit (Multiple Passes)
For multi-pass overlay applications, the final microstructure is determined by the last deposited pass. The recommended pass sequence for achieving ≤15% dilution includes:
| Pass Number | Filler Material | Heat Input (kJ/mm) | Expected Dilution (%) | Resulting Microstructure |
|---|---|---|---|---|
| Pass 1 (Base) | ER309L or ER309MoL | 0.8–1.2 | 25–35% | Austenite + 5–10% delta ferrite |
| Pass 2 (Transition) | ER309L or ER316L | 1.0–1.5 | 15–20% | Austenite + 2–5% delta ferrite |
| Pass 3 (Final) | ER304L or ER316L | 1.2–2.0 | 5–10% | 100% austenite (equiaxed) |
| Pass 4 (Final, if required) | ER304L or ER316L | 1.5–2.5 | 2–5% | 100% austenite (coarse equiaxed) |
4.2 Critical Process Parameters for TIG Overlay
| Parameter | Recommended Range | Effect on Microstructure | Control Method |
|---|---|---|---|
| Welding Current | 80–160 A | Higher current → increased dilution → more ferrite/martensite | Constant current (DC) |
| Travel Speed | 3–8 mm/s | Lower speed → higher heat input → coarser grains | Speed controller or manual technique |
| Heat Input | 0.6–1.5 kJ/mm | Directly correlates with dilution and grain size | Monitor via thermal imaging |
| Shielding Gas | 100% Ar or 98% Ar/2% O₂ | O₂ addition stabilizes arc, improves wetting | Gas flow meter (8–12 L/min) |
| Interpass Temperature | ≤150°C | Higher temp → increased grain growth, reduced toughness | Pyrometer monitoring |
| Wire Diameter | 1.6–2.4 mm | Thinner wire → lower heat input → reduced dilution | Filler selection per WPS |
4.3 Critical Process Parameters for MIG Overlay
| Parameter | Recommended Range | Effect on Microstructure | Control Method |
|---|---|---|---|
| Wire Feed Speed | 3.5–6.5 m/min | Higher speed → thinner bead → lower dilution | Wire feed drive calibration |
| Voltage | 18–24 V | Higher voltage → wider bead → more dilution | Constant voltage (CV) power source |
| Shielding Gas | Ar/CO₂ 80:20 or 90:10 | CO₂ increases penetration → higher dilution | Dual gas regulator |
| Heat Input | 1.5–3.5 kJ/mm | Higher than TIG; requires careful transition layer design | Thermal modeling validation |
| Stutter Arc | ON (if available) | Reduces peak temperature, refines grain structure | Pulse parameter programming |
| Travel Speed | 8–20 mm/s | Higher speed → lower heat input → reduced dilution | Positioner speed matching |
4.4 Key Metallurgical Findings from the Study
The systematic investigation of stainless steel overlay on Q235 steel yields several critical findings that directly inform production practice:
- Dilution threshold effect: When dilution exceeds 20%, chromium carbide precipitation at grain boundaries becomes significant, reducing intergranular corrosion resistance by 40–60% compared to low-dilution overlays.
- Delta ferrite control: The Schaeffler diagram prediction for Q235/304L combinations indicates 5–15% delta ferrite at 20% dilution, which is beneficial for hot cracking resistance but detrimental to corrosion performance above 10%.
- Hardness gradient: A typical hardness profile shows Q235 base at 150–180 HV, HAZ peak at 220–260 HV, and overlay at 180–220 HV. Excessive HAZ hardening (>300 HV) indicates risk of hydrogen-induced cracking.
- Toughness behavior: Charpy V-notch impact energy in the overlay zone remains above 100 J at -40°C for properly controlled dilution, demonstrating excellent low-temperature performance.
- Residual stress: Longitudinal residual stresses in the overlay typically range from 200–350 MPa (tensile), requiring stress relief at 400–600°C for critical applications.
5. Applicable Standards and Acceptance Criteria
5.1 Welding Procedure and Qualification Standards
- GB/T 985.1 — Welding procedure qualification (WPS/PQR) requirements
- GB/T 985.2 — Qualification of welders and welding operators
- GB/T 19866 — Welding procedure specification for dissimilar metal welds
- NB/T 47014 — Qualification testing for pressure equipment welding procedures
- ASME Section IX — Qualification rules for welding, brazing, and soldering procedures
- ASTM A404 — Standard specification for qualified welders and welding operators
- ISO 15614-1 — Qualification testing of welding procedures for metallic materials
5.2 Material and Performance Standards
- GB/T 700 — Carbon structural steel (Q235 specification)
- GB/T 20878 — Stainless and heat-resistant steel and alloy product technical conditions
- GB/T 4240 — Welding consumables for stainless steel (welding wire specification)
- ASTM A276 — Austenitic stainless steel bars and shapes
- ASTM A312 — Austenitic stainless steel tubes
- ASTM G48 — Intergranular corrosion testing (AST method for overlay qualification)
- ASTM G150 — Pitting corrosion testing (FeCl₃ solution)
- NACE MR0175/ISO 15156 — Materials for H₂S-containing environments (if applicable)
5.3 Non-Destructive Testing Standards
- GB/T 11345 — Ultrasonic testing of welds (overlay bond verification)
- GB/T 3323 — Radiographic testing of welds
- GB/T 17955 — Magnetic particle testing
- ASTM E1647 — Ultrasonic examination of clad plates (shear wave technique)
- ASTM E114 — Hardness testing by Vickers method
- ASTM E23 — Charpy V-notch impact testing
5.4 Acceptance Criteria Matrix
| Inspection Item | Method | Acceptance Criteria | Standard Reference |
|---|---|---|---|
| Overlay Thickness | Ultrasonic (ASTM E1647) | ≥ specified minimum (typically 2.0–6.0 mm) | ASTM E1647 / GB/T 11345 |
| Interfacial Bonding | Ultrasonic shear wave | 100% bond, no lack of fusion | ASTM E1647 |
| Surface Defects | Visual + MPI | No cracks, no porosity >1 mm | GB/T 17955 / ASME Sec IX |
| Internal Defects | RT (radiographic) | Class II or better (no slag inclusions >3 mm) | GB/T 3323 |
| Dilution Rate | Spectrographic analysis (core sample) | ≤25% at interface, ≤10% at surface | WPS-specific |
| Hardness | Vickers (HV30) traverse | ≤300 HV in HAZ; ≤250 HV in overlay | ASTM E114 |
| Tensile Strength | Transverse tensile test | ≥400 MPa (overlay); ≥375 MPa (base) | ASTM E8 / GB/T 228 |
| Impact Toughness | Charpy V-notch at -40°C | ≥54 J (overlay zone) | ASTM E23 / GB/T 229 |
| Corrosion Resistance | Salt spray (ASTM B117) | No pitting >50 μm after 500h | ASTM B117 |
6. Common Risks and Control Measures
6.1 Metallurgical Risks
| Risk | Cause | Consequence | Control Measure |
|---|---|---|---|
| Hot Cracking | High sulfur/phosphorus in Q235; excessive delta ferrite; high heat input | Transverse cracks in overlay | Limit dilution <25%; use ER309L with controlled S/P; reduce heat input |
| Cold Cracking (Hydrogen-Induced) | Hydrogen absorption from moisture; high HAZ hardness; slow cooling | Delayed cracking in HAZ after 2–24 hours | Preheat to 100–150°C; use low-hydrogen consumables; post-weld bake |
| Intergranular Corrosion | Chromium carbide precipitation from high dilution; sensitization | Loss of corrosion resistance at grain boundaries | Use low-carbon grades (304L/316L); limit dilution; avoid 500–800°C exposure |
| Martensitic Transformation | High dilution with Cr-Mo alloys; rapid cooling | High hardness, low toughness, risk of cracking | Use austenitic-only fillers; PWHT at 1050°C + water quench if needed |
| Excessive Dilution | High heat input; single-pass on thick base; MIG with CO₂-rich gas | Loss of corrosion resistance; unacceptable composition | Multi-pass strategy; reduce heat input; use transition layer |
6.2 Process Risks
- Incomplete fusion at interface: Controlled by ensuring adequate base metal cleaning (grinding to bare metal, removing oxide scale per GB/T 9448), proper preheating, and sufficient arc force at the start of each pass.
- Porosity from gas entrapment: Mitigated by thorough surface preparation (removal of rust, oil, moisture), adequate shielding gas coverage, and avoidance of porosity-prone gas mixtures.
- Weld spatter contamination: Particularly relevant in MIG overlay; controlled by proper nozzle-to-work distance (8–12 mm), gas flow optimization, and post-weld cleaning per ASTM B552.
- Residual stress-induced distortion: Managed through balanced welding sequences, back-step welding technique, and stress relief treatment at 400–600°C for 1–2 hours per 25 mm thickness.
7. Application Scenarios Across Technology Routes
7.1 TIG Weld Overlay Applications
The metallurgical knowledge of Q235/stainless steel overlay directly enables the following TIG overlay applications:
- Chemical reactor internals: Overlay of 304L or 316L on Q235 reactor shells, mixing paddles, and support structures where full stainless construction is cost-prohibitive but corrosion resistance is required in specific zones.
- Food processing equipment: Precise TIG overlay of 316L on Q235 tank frames, conveyor supports, and structural members in dairy, brewing, and pharmaceutical facilities meeting GB 16798 (food-grade stainless steel) surface requirements.
- Marine and offshore components: Overlay of 2205 duplex or 316L on Q235 structural members in splash zones, ballast tank linings, and platform support structures.
- Wear-resistant overlay on structural parts: Multi-pass overlay of hardfacing alloys on Q235 scraper blades, chute linings, and conveyor rollers in mining and material handling applications.
7.2 MIG Weld Overlay Applications
- Large-area overlay on structural plates: Efficient application of 3–8 mm stainless overlay on Q235 base plates for chemical storage tanks, scrubber liners, and acid-resistant flooring using multi-wire MIG systems.
- Thick overlay requirements: Where overlay thickness exceeds 6 mm, MIG provides the deposition rates (5–15 kg/h) necessary for economical production while the metallurgical knowledge ensures proper transition layer design.
- Repair of corroded equipment: Rapid restoration of Q235 pressure vessels, heat exchanger channels, and pipe systems with stainless overlay in maintenance operations.
- Automated overlay on pipe: Orbital MIG overlay of stainless cladding on Q235 carbon steel piping for process lines in petrochemical and power generation applications.
7.3 Hydraulic Explosive Bonding and Explosion Welding Applications
While explosive bonding processes do not involve melting, the metallurgical understanding of Q235 surface behavior is critical for:
- Pre-treatment of Q235 substrates: Knowledge of carbon content effects on interfacial reaction layers guides the specification of surface preparation (shot blasting to Sa 2.5, removal of decarburized layer) before bonding.
- Post-bonding overlay repair: Areas of incomplete bonding in explosive-clad plates are repaired using TIG overlay, requiring the same dilution control and microstructural understanding as standalone overlay applications.
- Welded connections to clad plates: When Q235 explosive-clad plates require structural welding (frames, supports), the metallurgical knowledge ensures that weld procedures do not compromise the cladding bond or introduce cracking susceptibility.
- Quality assessment criteria: Understanding of expected microstructural features at the Q235/stainless interface enables development of NDT acceptance criteria for explosive-bonded products, including ultrasonic bond verification per ASTM E1647.
8. Contribution to Qualification Building and Product Delivery
8.1 WPS Qualification Support
The metallurgical study provides the scientific foundation for developing and qualifying welding procedure specifications. Specifically:
- Essential variables identification: Dilution rate, heat input, and interpass temperature are identified as the critical essential variables requiring qualification, enabling the company to demonstrate compliance with NB/T 47014 and ASME Section IX requirements.
- Performance qualification testing: The predicted microstructural outcomes inform the selection of qualification tests (tensile, impact, hardness traverse, corrosion testing) that demonstrate the overlay meets specified performance criteria.
- Range of qualification: Understanding of how process parameters affect dilution and microstructure enables the establishment of qualified parameter ranges that provide manufacturing flexibility while maintaining quality.
8.2 Product Delivery Enhancement
- Reduced rework rates: Predictive metallurgical modeling reduces trial-and-error in production, targeting first-time quality rates above 95% for overlay operations.
- Accelerated customer qualification: Providing customers with metallurgical data packages (microstructure reports, dilution analysis, corrosion test results) accelerates end-user qualification and project acceptance.
- Warranty confidence: Quantitative understanding of overlay performance enables the company to offer extended warranty periods (typically 3–5 years) backed by metallurgical evidence of service life.
8.3 Customer Value Proposition
"The systematic metallurgical study of stainless steel overlay on Q235 steel enables Cladding Technology Shanxi Co., Ltd. to deliver cost-effective corrosion-resistant solutions that combine the structural economy of carbon steel with the surface performance of stainless steel. This knowledge base translates directly into optimized WPS procedures, reduced dilution rates below 15% at the surface, verified interfacial bond integrity, and predictable service life exceeding 15 years in typical chemical processing environments."
9. Continuous Improvement and Knowledge Integration
9.1 Process Optimization Cycle
The metallurgical study is not a static document but feeds into a continuous improvement cycle:
- Production data collection: Each production batch generates dilution data, hardness profiles, and NDT results that are compared against the metallurgical model predictions.
- Model refinement: Discrepancies between predicted and actual microstructure/properties trigger parameter adjustments and WPS revisions.
- Filler material development: Understanding of dilution effects guides the selection and qualification of specialized filler metals (e.g., ER309L vs. ER316L vs. custom compositions) for specific Q235 applications.
- Equipment capability expansion: Metallurgical requirements drive investment in advanced equipment (pulse TIG, cold wire MIG, automated orbital systems) that enable tighter process control.
9.2 Integration with Quality Management System
This technical knowledge is formally integrated into the company's quality management system through:
- Inclusion in the Quality Manual as a referenced technical document
- Integration into the Training and Qualification program for welders and inspectors
- Reference in the Design and Development procedure for new overlay product specifications
- Incorporation into the Corrective and Preventive Action system for metallurgical nonconformances
- Use as a basis for supplier qualification of filler metal and shielding gas providers
10. Conclusion
The comprehensive study of microstructure and mechanical properties of stainless steel weld overlay on Q235 carbon steel represents a critical knowledge asset for Cladding Technology Shanxi Co., Ltd. It provides the metallurgical foundation for reliable WPS development, enables optimized process parameter selection across TIG and MIG overlay operations, supports quality assessment for explosive bonding applications, and ultimately delivers superior corrosion-resistant products to customers across chemical, food processing, marine, and power generation industries. By maintaining this metallurgical knowledge base and continuously refining it through production feedback, the company sustains its competitive position in the bimetallic cladding market while ensuring consistent product quality and customer satisfaction.