CLADDING TECHNOLOGY SHANXI CO., LTDWelding Defect Analysis and Improvement Measures for Austenitic Stainless Steel Overlay Layers
Literature Overview
This 2023 study by Wang Longhu, Guo Ning, Chen Derun, and Ma Jin from Gongxiang Cast Steel Co., Ltd. addresses the critical issue of welding defects in austenitic stainless steel overlay layers on cast steel substrates. Funded under the Ningxia Hui Autonomous Region Key R&D Plan (2023BDE92003), this research is particularly relevant to the manufacturing of cast and welded structures where corrosion-resistant overlay layers are applied to carbon steel castings.
Core Technical Content
Austenitic stainless steel overlay welding on cast steel substrates presents unique challenges arising from the large difference in thermal conductivity, thermal expansion coefficient, and solidification behavior between the two materials. The study systematically identifies and categorizes the primary defect types encountered in industrial practice, then proposes specific improvement measures for each.
Classification of Welding Defects
| Defect Category | Specific Defect | Occurrence Rate | Severity Level |
|---|---|---|---|
| Cracking | Hot cracking (solidification) | 25–35% | Critical |
| Cracking | Cold cracking (hydrogen-induced) | 10–15% | Critical |
| Cracking | Intergranular cracking in HAZ | 5–10% | Critical |
| Porosity | Gas porosity (N₂, O₂, H₂) | 15–20% | Major |
| Porosity | Slag inclusion | 8–12% | Moderate |
| Fusion defects | Incomplete fusion | 10–15% | Major |
| Fusion defects | Lack of penetration | 5–8% | Moderate |
| Surface defects | Crater cracks | 12–18% | Moderate |
| Surface defects | Excessive dilution | 20–30% | Major |
Root Cause Analysis
Hot cracking is the most prevalent and severe defect in austenitic stainless steel overlay welding. The root causes include:
- Sulfur and phosphor segregation at grain boundaries during solidification, creating low-melting eutectic films that crack under thermal stress.
- Columnar grain structure with impurity segregation at grain boundaries, particularly in the dilution zone near the bond line.
- Excessive restraint from the thick cast steel substrate, which creates high tensile stresses in the cooling overlay layer.
Porosity in austenitic stainless steel overlay deposits is primarily caused by nitrogen pickup from the atmosphere, as austenitic steels have a very high solubility for nitrogen at welding temperatures. The study identifies inadequate shielding gas coverage, excessive arc length, and contamination of the base metal surface as the primary contributing factors.
Excessive dilution is particularly problematic in cast steel applications because the high carbon content of cast steel dilutes into the overlay layer, promoting martensite formation and chromium carbide precipitation that compromises corrosion resistance.
Improvement Measures
| Defect Type | Primary Improvement Measure | Secondary Measure | Verification Method |
|---|---|---|---|
| Hot cracking | Use Ni-rich filler (ERNiCrMo-3) for first pass | Increase preheat to 200–300°C | UT + MT inspection |
| Hot cracking | Add 5–10% Ni to subsequent passes | Reduce travel speed for wider beads | Metallographic examination |
| Porosity | Use high-purity Ar+5%CO₂ shielding | Maintain arc length < 3 mm | RT or UT inspection |
| Porosity | Thoroughly clean base metal surface | Use back-gas shielding for thin sections | Visual + PT inspection |
| Excessive dilution | Use thin first pass with low current | Apply 2+ passes to dilute carbon content | Spectrochemical analysis |
| Cold cracking | Strict hydrogen control in filler metal | Post-weld heat treatment at 350°C | Hydrogen content measurement |
Process Optimization for Cast Steel Substrates
The study recommends a multi-pass welding strategy for overlaying austenitic stainless steel on cast steel:
- Preparation pass: Machine the substrate surface to remove scale, sand inclusion, and other defects. Grind to a smooth finish with a maximum surface roughness of Ra 12.5 μm.
- First pass (transition pass): Use a nickel-based filler metal (such as ERNiCrMo-3 or ERNiClad-3) with low heat input to create a nickel-rich transition zone that acts as a barrier against carbon dilution.
- Second pass (build-up pass): Use the target austenitic stainless steel filler (such as ER308L or ER316L) with controlled heat input to build up the required overlay thickness.
- Final pass (capping pass): Use the same filler as the second pass with optimized parameters to achieve a smooth, defect-free surface suitable for machining.
The interpass temperature should be maintained between 150°C and 250°C throughout the welding sequence. Post-weld heat treatment at 350°C for 2 hours per 25 mm of overlay thickness is recommended to relieve residual stresses and reduce the risk of delayed cracking.
Study Insights
This research is particularly valuable for the foundry and pressure vessel industries where cast steel components require corrosion-resistant overlay protection. The systematic defect analysis approach, combined with specific improvement measures for each defect type, provides a practical framework that can be directly implemented in production environments. The emphasis on the first-pass transition layer using nickel-based filler metal is a critical insight that has been validated in my own engineering practice with clad pressure vessels, where the dilution zone is consistently the weakest link in terms of both mechanical properties and corrosion resistance.