CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Stainless Steel Overlay on Q235 Carbon Steel
Literature Overview
This 2014 study published in Welding Technology by Wu Zhisheng, Yun Hui, Liu Cuirong, Li Ke, and Quan Wanglin from Taiyuan University of Science and Technology investigates the microstructure and mechanical properties of stainless steel weld overlay deposited on Q235 carbon steel substrate. The research was funded by the Shanxi Provincial Science and Technology Program (Grant No. 20100321084), Taiyuan City Science and Technology Program (Grant No. 2011075), and Taiyuan University Student Innovation Fund (Grant No. 20121016). The work addresses a widely encountered industrial need: providing corrosion-resistant surfaces to low-cost carbon steel components without replacing the entire component with expensive stainless steel.
Core Technical Points
Q235 steel is the most widely used carbon structural steel in China, equivalent to ASTM A36 in many applications. Its widespread use stems from low cost and adequate mechanical properties, but its susceptibility to corrosion in aggressive environments limits its service life. Weld overlay with stainless steel provides an economical solution by depositing a corrosion-resistant layer on the surface while retaining the structural integrity and cost-effectiveness of the carbon steel base.
| Parameter | Q235 Base Steel | Stainless Overlay (Typical 304/316) |
|---|---|---|
| Carbon content | 0.12–0.20 wt% | 0.03–0.08 wt% |
| Yield strength | ≥ 235 MPa | 205–310 MPa |
| Corrosion resistance | Poor | Excellent |
| Thermal conductivity | 50 W/(m·K) | 16 W/(m·K) |
| Coefficient of thermal expansion | 12×10⁻⁶/K | 17×10⁻⁶/K |
| Typical overlay thickness | — | 2–6 mm |
The fundamental challenge in stainless steel overlay on carbon steel lies in the significant difference in thermal expansion coefficients between the two materials. During cooling, the stainless overlay contracts more than the carbon steel base, inducing tensile residual stresses at the overlay surface and compressive stresses at the interface. These residual stresses can lead to cracking, delamination, or reduced fatigue life if not properly managed.
Microstructural Analysis
The fusion zone between the Q235 base and the stainless overlay is the critical region where dilution occurs. The carbon steel base dilutes into the overlay melt, introducing carbon and manganese into the stainless steel deposit. This dilution effect transforms the microstructure from a pure austenitic structure (in the case of 304 or 316 overlay) to an austenite-ferrite dual-phase structure, with the ferrite fraction increasing with dilution level.
| Dilution Level | Ferrite Content (ASTM E490) | Microstructure | Corrosion Risk |
|---|---|---|---|
| Low (<15%) | 5–10% delta ferrite | Predominantly austenitic | Low |
| Moderate (15–30%) | 10–20% delta ferrite | Austenite-ferrite mixture | Moderate |
| High (>30%) | >20% delta ferrite | Significant ferrite | High (430-type) |
The Schaeffler diagram is a useful tool for predicting the weld metal composition and resulting microstructure based on the dilution ratio. Engineers should aim to keep the dilution below 25% to maintain adequate corrosion resistance in the overlay layer. This can be achieved through multi-pass welding with a high first-pass dilution followed by low-dilution subsequent passes, or by using a filler metal with higher alloy content to compensate for expected dilution.
Process Parameters and Their Influence
The welding process selection significantly affects the overlay quality. Submerged arc welding (SAW) and gas metal arc welding (GMAW) are the most common processes for this application. Each process offers different advantages in terms of dilution control, deposition efficiency, and microstructural refinement.
| Process | Typical Current | Voltage | Travel Speed | Dilution | Deposition Rate |
|---|---|---|---|---|---|
| SAW | 400–600 A | 28–36 V | 200–400 mm/min | 15–30% | High |
| GMAW | 150–250 A | 22–28 V | 300–600 mm/min | 10–25% | Moderate |
| FCAW | 200–350 A | 24–30 V | 250–500 mm/min | 12–28% | Moderate-High |
Preheating the Q235 base to 100–150 °C is recommended to reduce the cooling rate and minimize the risk of cracking in the heat-affected zone. The carbon equivalent of Q235 is relatively low (CE ≈ 0.35%), which makes it inherently less susceptible to hydrogen-induced cracking compared to high-strength steels. However, the thermal mismatch between the stainless overlay and carbon base can still generate significant residual stresses that must be managed through proper welding sequence, backing plate selection, and post-weld stress relief.
Engineering Practice Implications
For practical applications, the following recommendations emerge from this research:
- Use a 309L or 316L filler metal for the first pass to compensate for high dilution from the carbon steel base, followed by 304L or 316L for subsequent passes to achieve the desired corrosion resistance.
- Maintain interpass temperature below 200 °C to prevent excessive grain growth and minimize sensitization risk in austenitic stainless steel deposits.
- Implement a welding sequence that radiates outward from the center to minimize distortion in large overlay areas.
- Consider post-weld solution treatment at 1050–1100 °C for 1 hour followed by water quenching to dissolve carbides and restore full corrosion resistance, though this is often impractical for large components.
- Perform intergranular corrosion testing (ASTM A923 Practice A or GB/T 4334) on the overlay to verify that the dilution has not compromised the corrosion resistance below acceptable levels.
Key Questions and Reflections
A significant engineering question is the long-term performance of the overlay under cyclic thermal loading. The thermal expansion mismatch between the stainless overlay and carbon steel base means that repeated heating and cooling cycles can progressively fatigue the interface, potentially leading to delamination. This is particularly relevant for components such as heat exchanger tubesheets or furnace components where thermal cycling is inherent to the service condition.
Another reflection concerns the trade-off between corrosion resistance and mechanical properties. Higher dilution from the carbon steel base increases the hardness and strength of the overlay but reduces its corrosion resistance. Engineers must carefully balance these competing requirements based on the specific service environment, considering factors such as chloride concentration, temperature, and presence of reducing acids.
Summary
The study of stainless steel overlay on Q235 carbon steel provides essential guidance for engineers seeking cost-effective corrosion protection solutions. The understanding of dilution effects, microstructural evolution, and the resulting impact on corrosion resistance and mechanical properties is fundamental to successful overlay design. By carefully controlling welding parameters, filler metal selection, and welding sequence, it is possible to achieve a durable, corrosion-resistant overlay that extends the service life of carbon steel components significantly. This research underscores the importance of systematic approach to overlay welding, where each parameter must be considered in the context of the overall design objectives.