CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Hardness Analysis of Stainless Steel Weld Overlay on Q235 Steel
Literature Overview
This study examines the microstructural evolution and hardness distribution at the interface between a Q235 carbon steel substrate and a stainless steel weld overlay layer. The research addresses a fundamental engineering challenge encountered in bimetal pressure vessel fabrication and cladding operations: achieving a metallurgical bond with minimal dilution while ensuring adequate mechanical integrity across the dissimilar metal interface. The work is particularly relevant to engineers working on GB/T 150 and NB/T 47002 compliant vessels where carbon steel provides structural strength while the overlay layer delivers corrosion resistance.
Core Technical Findings
The study investigates the transition zone between the Q235 base metal and the stainless steel overlay, focusing on how the weld dilution rate affects the composition and microstructure of the fusion zone. Key observations include the formation of a distinct gradient in carbon and alloying element concentration across the interface, with the dilution rate typically ranging from 15% to 35% depending on the welding process parameters employed.
| Parameter | Typical Range | Engineering Significance |
|---|---|---|
| Dilution rate | 15%–35% | Controls corrosion resistance of overlay |
| Fusion zone width | 2–8 mm | Affects stress concentration |
| Hardness gradient | 120–380 HV | Indicates microstructural transitions |
| Carbon content at interface | 0.05%–0.45% | Influences carbide precipitation |
The hardness profile reveals a characteristic three-zone structure: the base metal zone (approximately 120–160 HV for Q235), the transition zone (180–280 HV), and the overlay zone (200–380 HV depending on the stainless grade). The transition zone exhibits martensitic and ferritic-austenitic microstructures due to the combined influence of the carbon diffusion from the Q235 substrate and the rapid solidification conditions near the fusion boundary.
Microstructural Analysis and Dilution Effects
The most critical finding relates to the relationship between dilution rate and the resulting microstructure in the transition zone. When the dilution rate exceeds 30%, the formation of hard and brittle martensite becomes more pronounced, which poses a risk of hydrogen-induced cracking under certain service conditions. The study demonstrates that controlling the dilution rate through process parameter optimization—specifically by adjusting welding current, travel speed, and layer thickness—can effectively mitigate this risk.
The intermetallic phases formed at the interface are predominantly Cr-rich carbides (Cr7C3 and Cr23C6), which provide beneficial hardening but may reduce ductility if excessively thick. The grain structure in the fusion zone shows columnar grains growing from the substrate into the overlay, with the grain boundary morphology directly correlating to the thermal cycle experienced during welding.
Engineering Practice Implications
For practical application in bimetal pressure vessel fabrication, the following process recommendations emerge from this research:
- Multi-pass welding with thin individual layers (1.5–3 mm) should be employed to minimize dilution and control the thermal input per pass.
- A transition layer approach—using a nickel-based or 309L filler for the first pass—can effectively buffer the compositional gradient between Q235 and the final stainless overlay.
- Post-weld heat treatment (PWHT) at 620–650°C for 1–2 hours can relieve residual stresses and temper any martensitic phases formed in the transition zone.
- The overlay layer thickness should be no less than 3 mm (with a minimum of 1.5 mm effective thickness after machining) to meet the requirements of NB/T 47002 for corrosion-resistant cladding.
Key Reflections
This research reinforces the principle that the dilution rate is the single most influential parameter governing the quality of a weld-overlay joint between dissimilar metals. In my engineering practice, I have observed that insufficient attention to dilution control is one of the primary causes of overlay failure in field service. The hardness gradient data presented here provides a useful benchmark for quality assessment: if the transition zone hardness exceeds 300 HV consistently, it indicates excessive dilution and potential cracking susceptibility that warrants additional inspection or rework.
The study also highlights the importance of process selection. Submerged arc welding (SAW) tends to produce higher dilution rates compared to gas metal arc welding (GMAW) or plasma transferred arc (PTA) processes, which must be considered when selecting the overlay method for critical applications. Engineers should always correlate hardness profiling with metallographic examination to develop a complete understanding of the joint quality, rather than relying on either method in isolation.