CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of the Fusion Zone in H1Cr24Ni13 Overlay Welding on Q235A Steel
Literature Overview
This research focuses on the metallurgical behavior at the fusion boundary when depositing H1Cr24Ni13 (equivalent to cast 310 stainless steel, containing approximately 24% Cr and 13% Ni) overlay weld metal onto Q235A carbon structural steel substrates. The fusion zone represents the most critical and challenging region in dissimilar metal weld overlay, as it is subject to severe compositional dilution, complex solidification behavior, and potential formation of brittle phases that compromise corrosion resistance and mechanical integrity.
Core Technical Analysis
Compositional Dilution and Phase Formation
The fundamental challenge in overlaying a high-chromium austenitic alloy onto low-carbon steel is the inevitable dilution of alloying elements at the fusion boundary. Q235A contains approximately 0.14-0.22% C, 0.30-0.65% Mn, and trace amounts of Si and P, with negligible Cr and Ni content. The dilution effect can reduce the local Cr and Ni content in the fusion zone by 40-60% compared to the base weld metal composition, potentially shifting the microstructure from fully austenitic to mixed austenite-ferrite or even martensitic.
| Parameter | H1Cr24Ni13 Base | Fusion Zone (Estimated) | Q235A Base |
|---|---|---|---|
| C (%) | 0.08-0.20 | 0.15-0.35 | 0.14-0.22 |
| Cr (%) | 23-25 | 10-16 | <0.30 |
| Ni (%) | 12-14 | 5-8 | <0.30 |
| Mn (%) | 1.0-2.0 | 1.5-3.0 | 0.30-0.65 |
| Ferrite content (% by volume) | 0-5 | 20-45 | 0 |
| Hardness (HV) | 180-220 | 250-380 | 120-160 |
Microstructure Characteristics
Metallographic examination reveals a gradient microstructure across the fusion zone. Near the weld metal side, the microstructure is predominantly austenitic with some delta ferrite retained from solidification. Moving toward the substrate side, the ferrite fraction increases significantly, and at the HAZ boundary, coarse grain austenite transforms to ferrite and pearlite with possible martensite formation due to the rapid cooling rates in thin overlay passes. The presence of sigma phase (Cr₂₃C₆) in the HAZ is a concern if the local Cr content exceeds 20% and the temperature remains in the 700-900°C range for extended periods.
Mechanical Properties
The fusion zone exhibits a hardness gradient that can create stress concentrations under cyclic loading. Typical hardness values:
- Weld metal (H1Cr24Ni13): 180-220 HV
- Fusion zone (diluted): 250-380 HV
- HAZ (Q235A): 180-280 HV
- Base metal (Q235A): 120-160 HV
The hardness mismatch between the fusion zone and base metal can be as high as 200 HV, which is significant for fatigue performance and residual stress distribution.
Process Parameters and Their Influence
The welding process significantly affects the dilution rate and consequently the fusion zone microstructure. Submerged arc welding (SAW) typically produces the highest dilution (40-60%), while gas tungsten arc welding (GTAW/TIG) produces the lowest (10-25%). Multi-pass overlay strategies are essential to minimize the dilution effect on subsequent passes.
| Process | Typical Dilution (%) | Heat Input (kJ/mm) | Recommended Application |
|---|---|---|---|
| GTAW (TIG) | 10-25 | 0.5-1.5 | Thin overlays, high precision |
| GMAW (MIG) | 20-40 | 1.0-3.0 | Medium thickness overlays |
| SAW | 35-60 | 2.0-5.0 | Thick overlays, large vessels |
| FCAW | 25-45 | 1.5-4.0 | Field repair, outdoor work |
| PTA | 15-30 | 0.8-2.5 | Critical corrosion service |
Engineering Practice and Defect Prevention
In practice, the following measures are recommended to control fusion zone quality:
- Preheating the Q235A substrate to 150-250°C to reduce cooling rate and minimize martensite formation in the HAZ
- Using a high-deposition-rate process for the first pass to establish a sufficient overlay thickness that subsequent passes can dilute less
- Employing at least 3-4 overlay passes for critical service to ensure the final surface composition approaches the nominal H1Cr24Ni13 composition
- Post-weld heat treatment (PWHT) at 1050-1100°C for 1 hour followed by air cooling to homogenize the fusion zone and dissolve brittle phases
The critical failure mode to prevent is intergranular corrosion (IGC) of the overlay layer due to chromium carbide precipitation at grain boundaries. This can be assessed through ASTM A262 Practice E testing, and the overlay layer should demonstrate resistance equivalent to or better than the base H1Cr24Ni13 material after appropriate PWHT.
Study Insights and Reflections
The most important takeaway from this study is that the fusion zone, though occupying only 0.1-0.3 mm of the total overlay thickness, governs the overall performance of the cladding system. A well-designed multi-pass strategy that progressively reduces dilution from the first pass outward is far more effective than attempting to achieve full composition in a single pass. Furthermore, the hardness gradient at the fusion zone interface represents a potential fatigue crack initiation site under cyclic loading, which is particularly relevant for pressure vessels and rotating equipment subject to pressure cycling.