Microstructure and Hardness Analysis of Stainless Steel Weld Overlay on Q235 Carbon Steel
1. Definition and Fundamental Principles
The study of joint microstructure and hardness distribution in stainless steel weld overlay deposited on Q235 carbon steel represents a foundational metallurgical investigation that underpins the entire weld overlay cladding business. Q235 steel, a Chinese standard (GB/T 700) low-carbon structural steel with a carbon equivalent of approximately 0.22%, serves as the substrate in numerous industrial applications. When stainless steel weld metal is deposited onto this substrate via arc welding processes, a complex metallurgical interaction occurs at the fusion boundary, producing a dilution zone, a heat-affected zone (HAZ), and a gradient of microstructural phases that directly governs the mechanical integrity and corrosion resistance of the resulting clad product.
The fundamental principles governing this interface include:
- Heat input and dilution control: The thermal cycle during welding determines the degree of substrate melting and mixing with weld metal, directly affecting the composition of the dilution zone. For Q235-to-stainless transitions, dilution rates between 10% and 30% are typical in the first pass.
- Solidification microstructure evolution: The rapid solidification of the weld pool produces columnar dendrites oriented perpendicular to the fusion boundary, with inter-dendritic phases (ferrite, martensite, or delta ferrite depending on composition) that influence hardness and crack susceptibility.
- Thermal residual stress development: Differential thermal expansion coefficients between Q235 (approximately 12×10⁻⁶/K) and austenitic stainless steel (approximately 17×10⁻⁶/K) generate residual stresses at the interface, which must be characterized and managed.
- Hardness gradient formation: A typical hardness profile across the overlay shows the base metal at 120–160 HV, the HAZ at 180–250 HV (due to tempering or partial hardening), the dilution zone at 200–320 HV, and the weld overlay itself at 180–280 HV depending on the stainless grade used.
2. Category and Business Positioning
This metallurgical study falls squarely within the company's core competence in WPS (Welding Procedure Specification) qualification and process development. It is not merely an academic exercise but a critical deliverable that supports:
- WPS qualification packages submitted to third-party inspection bodies (TÜV, DNV, Lloyd's Register, ABS)
- Product specification documentation for customer engineering reviews
- Internal training curricula for welders, inspectors, and metallurgists
- IP development for proprietary overlay procedures on dissimilar material combinations
Within the company's organizational structure, this knowledge base serves the TIG/MIG weld overlay production line as the primary technical reference, while also informing the design of transition layers for hydraulic explosive bonding and explosion welding interfaces where metallurgical compatibility must be verified.
3. Technical Purpose and Value
3.1 Purpose
The primary purpose of conducting microstructure and hardness analysis on Q235-to-stainless overlay joints is to establish a verified, repeatable relationship between welding parameters and resulting metallurgical quality. This enables the company to:
- Confirm that the overlay achieves the required corrosion resistance (by ensuring sufficient chromium and nickel content in the dilution zone)
- Verify that hardness values remain within acceptable ranges to prevent cracking during service or subsequent machining
- Demonstrate that the fusion bond is metallurgically sound with no interfacial defects
- Provide quantitative data for finite element analysis models used in design validation
3.2 Value to Customers
For end customers in chemical processing, petrochemical, power generation, and marine industries, the availability of documented microstructural and mechanical property data for specific overlay configurations provides:
- Reduced risk of premature failure due to undetected metallurgical defects
- Confidence in the durability and performance envelope of clad equipment
- Support for regulatory compliance (e.g., pressure vessel codes, marine classification society rules)
- Accelerated project timelines by eliminating the need for customers to commission independent metallurgical testing
4. Key Process and Implementation Points
4.1 Welding Parameters for Q235 to Stainless Overlay
| Parameter | Typical Range (TIG) | Typical Range (MIG) | Effect on Microstructure |
|---|---|---|---|
| Welding Current | 100–180 A | 120–250 A | Higher current increases dilution and HAZ width |
| Travel Speed | 40–80 mm/min | 200–500 mm/min | Slower speed increases heat input and grain coarsening |
| Heat Input | 0.8–2.5 kJ/mm | 1.5–4.0 kJ/mm | Controls dilution rate and HAZ microstructure |
| Shielding Gas | 100% Ar or Ar+2% O₂ | Ar+2% CO₂ or Ar+5% CO₂ | Affects arc stability and oxide inclusion formation |
| Interpass Temperature | ≤ 150°C | ≤ 100°C | Prevents excessive grain growth and residual stress buildup |
| Weld Wire/Consumable | ER309L, ER316L, ER308L | ER309L, ER316L, ER308L | Higher Cr/Ni content compensates for dilution |
4.2 Recommended Overlay Strategy
- Transition layer (Pass 1): Deposit a single pass of ER309L (high Cr-Ni austenitic) to bridge the composition gap between Q235 (Fe-C) and the final overlay grade. This pass accepts higher dilution (up to 30%) and produces a ferrite-austenite duplex structure that is crack-resistant.
- Build-up layers (Passes 2–4): Deposit ER316L or ER308L to achieve the final corrosion-resistant composition with dilution reduced below 15% in subsequent passes.
- Surface finish pass: A final low-heat-input pass to refine surface grain structure and reduce surface hardness for machining compatibility.
4.3 Metallurgical Examination Protocol
- Sample preparation: Transverse and longitudinal sections cut perpendicular to the weld axis, mounted, ground (SiC 120–2000 grit), polished, and etched with appropriate reagents (Villemenne's solution for ferrite/austenite identification, Nital for martensite detection).
- Microstructural evaluation: Optical microscopy at 100×–500× magnification to characterize grain size, phase distribution, and any interfacial defects. Metallographic examination per ASTM E3 and ASTM E112.
- Hardness mapping: Vickers hardness traverses at 10 HV or 20 HV load, with measurements taken at 0.5 mm intervals from the substrate through the overlay. Hardness testing per ASTM E92 or GB/T 4341.
- Chemical analysis: EDS (Energy Dispersive Spectroscopy) or optical emission spectrometry to verify dilution rates and confirm that the overlay composition meets the specified grade requirements per ASTM A240 or GB/T 3280.
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
| Standard | Scope of Applicability |
|---|---|
| GB/T 700 | Chemical composition and mechanical properties of Q235 substrate steel |
| GB/T 12469 | Stainless steel plate specifications for overlay consumables reference |
| ASTM A240 | Stainless steel plate/sheet specifications (overlay composition verification) |
| ASTM E92 | Hardness testing method (Vickers) |
| ASTM E3 | Standard practices for preparation of metallographic specimens |
| ASTM E112 | Determination of average grain size |
| ASME Section IX | Welding procedure qualification requirements (QW-425 for dissimilar welds) |
| NB/T 47014 | Chinese standard for welding procedure qualification in pressure equipment |
| GB/T 8165 | Welding procedure qualification rules for steel |
| NACE MR0175/ISO 15156 | Materials for H₂S-containing environments (if applicable) |
| ISO 9001:2015 | Quality management system requirements for documentation and traceability |
5.2 Acceptance Criteria
- Hardness: Maximum hardness in the dilution zone shall not exceed 350 HV (to prevent cracking susceptibility per NACE MR0175 guidance for sour service applications). Overlay hardness shall conform to the specified grade (typically 150–250 HV for austenitic grades).
- Microstructure: No continuous grain boundary carbide precipitation, no interfacial cracking, and no excessive delta ferrite (>20% volume fraction in 300-series welds per ASTM A240 guidance).
- Penetration: Full fusion at the interface with no lack of fusion, porosity, or cracking detected by macrographic examination.
- NDT: Visual inspection (VT) per ISO 17637, ultrasonic testing (UT) per ISO 17640 for overlay thickness and defect detection, and magnetic particle inspection (MT) per ISO 17638 for surface-breaking defects on the overlay.
6. Common Risks and Controls
| Risk | Root Cause | Control Measure |
|---|---|---|
| Cracking in dilution zone | Excessive carbon dilution from Q235 forming hard martensite; high heat input causing coarse grains | Use high-Ni transition consumable (ER309L); limit heat input to ≤2.5 kJ/mm; apply interpass temperature control |
| Insufficient corrosion resistance | Excessive dilution reducing Cr/Ni below passive threshold | Multi-pass strategy with increasing Cr/Ni content; verify dilution by EDS; ensure final pass has <10% dilution |
| High residual stress leading to distortion | Thermal mismatch and constrained cooling | Weld sequencing strategy (back-step or symmetric); post-weld stress relief at 420–540°C (for martensitic substrates) or 620–680°C (for austenitic overlay) |
| Intergranular corrosion in HAZ | Sensitization at 450–850°C due to slow cooling rates | Use low-carbon consumables (ER309L, ER316L with C≤0.03%); consider stabilized grades (ER347) for higher heat input applications |
| Porosity at fusion boundary | Hydrogen absorption from moisture; inadequate gas shielding | Preheat substrate to 100–150°C; use high-purity shielding gas (>99.99% Ar); ensure proper gas flow (15–20 L/min) |
| Delamination during machining | Weak metallurgical bond; residual stress release during cutting | Verify full fusion by macrograph; apply stress relief before machining; use progressive machining depths |
7. Application Across the Company's Three Technology Routes
7.1 TIG/MIG Weld Overlay Route
The microstructure and hardness analysis directly governs the WPS qualification for the company's primary TIG/MIG weld overlay operations. Every WPS developed for Q235-to-stainless applications must be supported by metallurgical verification demonstrating:
- Compliance with ASME Section IX QW-425 requirements for dissimilar metal weld qualification
- Hardness profiles that satisfy the acceptance criteria in NB/T 47014 for pressure equipment
- Documented dilution rates that ensure the final overlay composition meets the specified stainless grade
This analysis feeds directly into the company's product delivery documentation package, providing customers with confidence that the overlay will perform as specified in service. For large-scale production runs (e.g., chemical reactor internals, heat exchanger tubesheets), the metallurgical data enables statistical process control (SPC) of welding parameters to maintain consistent quality.
7.2 Hydraulic Explosive Bonding Route
While hydraulic explosive bonding (HEB) produces a solid-state metallurgical bond without melting, the principles learned from weld overlay microstructure analysis remain relevant in the following ways:
- Interface characterization: The understanding of how microstructural phases develop at dissimilar material interfaces informs the interpretation of HEB bondline examination results. The wavy interface characteristic of HEB is analyzed using similar metallographic techniques.
- Hardness gradient correlation: Hardness traverses across HEB interfaces reveal work-hardening effects in the deformation zone. The metallurgical expertise developed through weld overlay analysis enables proper interpretation of these results and prediction of mechanical performance.
- Substrate preparation: Knowledge of Q235 microstructure and its response to thermal cycling informs the selection of pre-treatment methods (shot peening, annealing) before HEB processing.
7.3 Explosion Welding Route
In explosion welding, the extreme plastic deformation at the collision interface produces a unique metallurgical structure that shares conceptual similarities with weld overlay interfaces:
- Collision zone analysis: The high-strain-rate deformation zone in explosion welding produces dynamic recrystallization and grain refinement, analogous to the rapid solidification zones in weld overlays. The analytical methods (optical microscopy, SEM, hardness mapping) are directly transferable.
- Material compatibility assessment: The understanding of how Q235 behaves at the interface with stainless steel under welding conditions provides baseline data for evaluating explosion welding parameters (velocity ratio, collision angle) that produce optimal bonding.
- Post-weld treatment: If explosion-welded clad plates require post-weld stress relief or solution treatment, the metallurgical knowledge of phase stability and transformation temperatures directly informs the heat treatment specification.
8. Contribution to Qualification Building and Customer Value
8.1 Qualification Building
The documented microstructure and hardness analysis for Q235-to-stainless overlay joints constitutes a critical building block in the company's qualification portfolio. Specifically:
- WPS qualification packages: Each analysis report supports a specific WPS, enabling the company to demonstrate to customers and third-party inspectors that procedures have been qualified to recognized standards (ASME IX, NB/T 47014, ISO 15614).
- Material combination matrix: The systematic accumulation of metallurgical data across different stainless grades (304, 304L, 316, 316L, 321, 347) deposited on Q235 builds a comprehensive material combination database that differentiates the company from competitors.
- Regulatory compliance: For products destined for pressure vessel applications, the metallurgical documentation satisfies the requirements of TSG 21 (China's pressure vessel safety regulation) and equivalent international codes.
8.2 Product Delivery Enhancement
For each production order, the company can provide:
- A metallurgical verification report confirming that the delivered product meets the specified overlay composition and hardness requirements
- Hardness mapping data demonstrating uniformity across the overlay area
- Macrographic examination results confirming full fusion and absence of interfacial defects
- NDT reports (UT, MT, PT) confirming the absence of volumetric and surface defects
8.3 Customer Value Proposition
The depth of metallurgical understanding demonstrated through this analysis translates into tangible customer benefits:
- Risk reduction: Customers receive products with documented metallurgical quality, reducing the probability of in-service failure and associated production losses.
- Design flexibility: The company can advise customers on optimal overlay configurations for specific service conditions (temperature, pressure, corrosive media), leveraging the metallurgical knowledge base.
- Cost optimization: By understanding the dilution-hardness relationship, the company can minimize the number of overlay passes required, reducing material and labor costs while maintaining quality.
- Accelerated approval: Pre-qualified WPS packages with supporting metallurgical data reduce the time required for customer and regulatory approval, accelerating project timelines.
9. Continuous Improvement and Knowledge Management
The "learning reflection" (学习心得) format of this technical entry indicates the company's commitment to continuous knowledge accumulation. Each metallurgical analysis contributes to:
- An internal technical library that grows with every project
- Training materials for new metallurgists and welding engineers
- Proprietary know-how that constitutes intellectual property
- Improved prediction models for welding outcomes based on accumulated empirical data
This systematic approach to knowledge management ensures that the company's metallurgical competence compounds over time, creating an increasingly robust foundation for product development, quality assurance, and customer service.
10. Conclusion
The microstructure and hardness analysis of stainless steel weld overlay on Q235 carbon steel is not merely a metallurgical study—it is the technical backbone of the company's weld overlay qualification system, product delivery documentation, and customer trust. By rigorously characterizing the metallurgical interface, establishing hardness profiles, and correlating these with welding parameters, the company demonstrates engineering rigor that meets the highest international standards. This knowledge directly supports all three production technology routes (TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding) and forms an indispensable component of the company's competitive positioning in the global cladding technology market.