CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Alloying Elements on Overlay Weld Hardness
Literature Overview
This study by Zhang Yuanbin and Ren Dengyi from the School of Materials Science and Engineering at Shandong University, published in 2003 in Hot Working Technology, investigates the systematic influence of alloying elements on the hardness of weld overlay deposits. Funded by the Shandong Provincial Natural Science Foundation (Grant Y99F01), the research addresses a fundamental yet persistently challenging question in overlay welding technology: how specific alloy additions modify the microhardness distribution across multi-layer overlay welds. The work is particularly relevant to engineers designing wear-resistant and corrosion-resistant overlay systems for pressure vessels, heat exchangers, and rotating equipment where surface hardness directly governs service life.
Core Technical Findings
The authors examined a range of alloying elements including Cr, Mo, V, W, and Mn, deposited via submerged arc welding (SAW) and gas metal arc welding (GMAW) onto low-carbon steel substrates. The key finding is that hardness in overlay welds is not a simple function of bulk alloy composition but is governed by the interplay between carbide precipitation, solid solution strengthening, and the thermal cycling history of multi-pass deposition.
| Alloying Element | Typical Addition Range (wt%) | Hardness Contribution Mechanism | Typical Hardness Range (HV30) |
|---|---|---|---|
| Cr | 8–28 | Carbide formation (Cr7C3, Cr23C6), solid solution | 300–650 |
| Mo | 1–6 | M7C3, Mo2C precipitation, solid solution | 400–700 |
| V | 1–4 | VC, V4C3, V8C7 formation | 500–900 |
| W | 2–8 | WC, W2C precipitation | 600–1000 |
| Mn | 1–5 | Mn3C, Mn23C6, austenite stabilization | 250–500 |
Carbide Formation and Hardness Hierarchy
The study confirms the well-established carbide hardness hierarchy: WC > VC > Mo2C > Cr7C3 > Mn3C. However, a critical engineering insight emerges from the multi-layer deposition context. As the number of overlay passes increases, the cumulative thermal input causes progressive dissolution and coarsening of primary carbides. The authors demonstrate that the first 1–2 passes often exhibit peak hardness due to rapid solidification and fine carbide distribution, while subsequent passes show a measurable hardness decline of 5–15% per additional pass due to thermal softening.
Solid Solution Strengthening
Beyond carbide precipitation, the study quantifies the contribution of interstitial and substitutional solid solution strengthening. Chromium and molybdenum atoms in solution within the austenitic or martensitic matrix provide a baseline hardness increment of approximately 10–20 HV per weight percent. This contribution, while smaller than carbide precipitation, is thermally stable and does not degrade with thermal cycling, making it particularly valuable for overlay systems subjected to repeated heating during vessel fabrication or service.
Engineering Practice Implications
For bimetal pressure vessel fabrication, this research directly informs the selection of overlay consumables and process parameters. When designing a corrosion-resistant overlay on a carbon steel hydrogenation reactor, an engineer must balance:
- Hardness requirement – Higher hardness generally correlates with better abrasive wear resistance but may compromise ductility and stress corrosion cracking resistance.
- Thermal cycling tolerance – Multi-pass overlay on thick-section vessel walls introduces significant heat input; consumables with carbide systems that are thermally stable (e.g., Mo2C rather than Mn3C) are preferred.
- Bond layer compatibility – The first pass (bond layer) must have adequate ductility to accommodate residual stresses, typically achieved by reducing carbon content and carbide-forming elements.
A practical design approach derived from this study is the graded alloy strategy: use a low-carbon, low-alloy bond layer (e.g., 309L-type with reduced C and Cr) for the first pass, followed by progressively higher-alloy face layers with increasing carbide-former content. This approach achieves the target surface hardness while maintaining interfacial toughness.
Key Reflections
The 2003 publication predates modern computational thermodynamic modeling (CALPHAD-based approaches) that can now predict carbide precipitation sequences with greater accuracy. Nevertheless, the empirical data presented remains highly valuable as a validation dataset. The study's emphasis on the thermal history effect during multi-pass deposition is a lesson that is still under-appreciated in industry practice, where engineers often select consumables based solely on as-welded hardness specifications without considering the degradation that occurs during multi-pass buildup. This represents a significant gap between laboratory characterization and field performance.