CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Stainless Steel Cladding on Q235 Steel
Literature Overview
This study investigates the microstructural evolution and mechanical properties of stainless steel overlay welds deposited on Q235 carbon structural steel substrates. The work addresses a practical and widely encountered engineering scenario in pressure vessel and piping fabrication, where corrosion-resistant cladding is applied to low-carbon steel components to achieve a cost-effective bimetallic solution. The research examines dilution effects at the fusion boundary, the formation of intermetallic compounds, hardness distribution through the cladding layers, and the resulting mechanical performance under various welding parameters.
Core Technical Findings
The study reveals that the dilution rate at the substrate-overlay interface is the single most critical factor governing the final cladding performance. When the dilution exceeds approximately 30%, the resulting fusion zone microstructure transitions from a predominantly austenitic phase to a mixed ferrite-austenite structure, significantly degrading corrosion resistance. The hardness profile typically shows a gradient: the substrate region maintains 120–140 HV, the dilution zone reaches 200–280 HV due to carbide precipitation and phase transformation, and the outer cladding layers stabilize at 160–200 HV for 304-grade stainless steel deposits.
Key microstructural observations include:
- Ferrite formation in the heat-affected zone (HAZ) of the Q235 substrate, with acicular ferrite and martensite appearing in regions experiencing rapid cooling rates above 150 °C/s.
- Chromium carbide precipitation (M23C6 and Cr7C3) at grain boundaries in the dilution zone when chromium content drops below the critical threshold for full austenite stabilization.
- Intermetallic phases such as FeCr and FeCr2 at the fusion boundary when excessive heat input promotes diffusion bonding.
Process Parameters and Their Influence
The following table summarizes the typical process windows identified in the study:
| Parameter | Recommended Range | Effect on Dilution | Effect on Microstructure |
|---|---|---|---|
| Welding current (SAW) | 280–350 A | Higher current increases dilution | Coarser grain in dilution zone |
| Travel speed | 200–350 mm/min | Higher speed reduces dilution | Finer grain but potential lack of fusion |
| Heat input | 1.5–3.0 kJ/mm | Directly proportional to dilution | Controls phase fraction |
| Electrode diameter | 3.2–4.0 mm | Larger electrode increases dilution | Broader fusion zone |
| Preheat temperature | 50–100 °C | Reduces cracking tendency | Slower cooling rate |
The study demonstrates that a multi-pass strategy with a low-dilution first pass followed by higher-deposition-rate subsequent passes achieves the optimal balance between bond strength and corrosion performance.
Engineering Practice Implications
In pressure vessel fabrication governed by GB/T 150 and ASME VIII Div. 1, the cladding layer thickness must typically be no less than 3 mm for stainless steel overlays on carbon steel. The study's findings directly support the requirement for bond strength testing per ASTM A263, where the minimum tensile strength of the cladding layer must exceed 450 MPa and the dilution zone must not exhibit intergranular corrosion after the 65% acid test.
A critical practical insight from this literature is that the welding sequence matters substantially. When cladding a cylindrical vessel, starting from the bottom and progressing upward with overlapping passes by 50–70% of the electrode width minimizes residual stress concentration and reduces the risk of cracking in the dilution zone.
Key Questions and Reflections
One question that arises from this study is whether the observed dilution behavior scales linearly with plate thickness. In thin plates (below 6 mm), the thermal mass is insufficient to absorb the heat input, potentially leading to burn-through and excessive dilution. Conversely, in thick plates (above 30 mm), the thermal gradient is steep, and the dilution zone may be narrow but highly stressed due to differential cooling rates.
Another reflection concerns the long-term performance under cyclic thermal loading. The study focuses on as-welded condition properties, but in hydrogenation reactor service or high-temperature exchangers, the dilution zone may experience sensitization over time, leading to intergranular corrosion that was not present in the as-deposited state.
Study Insights and Implications
This literature provides a solid foundation for understanding the metallurgical challenges of stainless steel cladding on Q235 steel. The emphasis on dilution control as the primary design variable aligns with industry practice, where the first pass is often deposited with reduced parameters or using a specially formulated low-dilution electrode such as E309L to create a transition layer that buffers the substrate from the subsequent 304 or 316L cladding passes. The research reinforces the importance of rigorous non-destructive testing, particularly ultrasonic testing (UT) per JB/T 4730, to detect subsurface cracks and lack of bond in the dilution zone. Engineers working on clad plate pressure vessels should treat the dilution zone as a critical inspection region and ensure that all applicable standards—NB/T 47014 for procedure qualification and ASTM A263 for material acceptance—are fully satisfied through systematic testing protocols.