Zhuojin Pipe Fitting Co., LtdMicrostructure and Mechanical Properties of Austenitic Stainless Steel Surfacing Layer on Low Alloy Steel
Literature Overview
The research conducted by Wang Zhiling and Yu Genxi, published in Foundry Technology in 2016, addresses the metallurgical compatibility and mechanical performance of austenitic stainless steel surfacing layers deposited on Q345 low alloy structural steel using the MAG (Melted Active Gas) welding process. The study employs a two-layer surfacing strategy with ER309L as the transition layer and ER347L as the corrosion-resistant top layer. This layered approach is a standard industrial practice for achieving metallurgical compatibility between dissimilar materials while providing the desired corrosion resistance in the final surface layer.
Welding Process and Material Selection
The substrate material Q345 is a low alloy high-strength structural steel with a yield strength of 345 MPa, widely used in pressure vessels, pipelines, and structural applications. The surfacing application is typically motivated by the need to provide corrosion resistance to a carbon steel or low alloy steel component without the cost of fabricating the entire component from stainless steel.
The selection of ER309L for the transition layer is based on its composition, which contains approximately 23–27 wt% Cr and 12–14 wt% Ni. This composition ensures that the weld metal has sufficient nickel content to stabilize austenite in the weld zone, even with significant dilution from the carbon steel base metal. The low carbon content (below 0.03 wt%) minimizes the risk of intergranular corrosion.
The ER347L top layer contains approximately 19–22 wt% Cr, 9–13 wt% Ni, and 0.5–1.0 wt% Nb. The niobium addition forms NbC precipitates that tie up carbon, preventing chromium carbide precipitation at grain boundaries and thereby enhancing intergranular corrosion resistance.
| Material | Composition (wt%) | Primary Function |
|---|---|---|
| Q345 substrate | C: 0.12–0.20, Mn: 1.0–1.6, Si: 0.17–0.37 | Structural base material |
| ER309L transition layer | Cr: 23–27, Ni: 12–14, C: ≤0.03 | Metallurgical transition, dilution buffer |
| ER347L top layer | Cr: 19–22, Ni: 9–13, Nb: 0.5–1.0, C: ≤0.03 | Corrosion resistance |
Microstructural Analysis
The microstructure of the surfacing layers was examined using metallographic techniques. The transition layer (ER309L) exhibited a mixed microstructure of austenite and delta ferrite. The delta ferrite content is typically in the range of 5–15% for ER309L weld metal, which serves a beneficial function by providing resistance to hot cracking. The austenite phase, being FCC, provides the primary corrosion resistance through the chromium-rich passive film.
The top layer (ER347L) showed a predominantly austenitic microstructure with some delta ferrite. The presence of NbC precipitates was identified, which effectively pin carbon atoms and prevent the formation of Cr-rich carbides (such as Cr23C6) at grain boundaries. This is critical for maintaining intergranular corrosion resistance, particularly in applications involving sensitization temperatures (450–850°C).
A notable observation was the presence of trace amounts of chromium carbide brittle phases within the surfacing layers. These phases, while present in small quantities, can potentially act as crack initiation sites under cyclic or impact loading. However, their limited volume fraction means they do not dominate the mechanical behavior of the surfacing layers.
Mechanical Performance
The bending test results demonstrated that the two-layer surfacing design significantly improved the bendability of the deposited layers. The transition layer accommodates the strain mismatch between the austenitic top layer and the ferritic base metal, preventing cracking during bending. The top layer alone, without a transition layer, would likely exhibit cracking during bending due to the high strain incompatibility between the austenitic weld metal and the carbon steel substrate.
The shear test results confirmed good metallurgical bonding between the layers and between the surfacing layers and the base metal. The shear strength values were consistent with the expected performance of dissimilar metal welds with appropriate transition layers.
The shear fracture morphology analysis revealed predominantly ductile fracture features characterized by dimple formation. This indicates that the failure mechanism was through microvoid coalescence rather than brittle intergranular fracture. The presence of dimples on the fracture surface is a positive indicator, suggesting that the surfacing layers possess adequate ductility and toughness despite the presence of some brittle carbide phases.
Engineering Practice Considerations
In engineering practice, the two-layer surfacing approach described in this study is widely applied in the fabrication of corrosion-resistant components where the bulk material is carbon steel or low alloy steel. Typical applications include:
- Corrosion-resistant linings on carbon steel pressure vessels
- Surface hardening and corrosion protection on pump impellers and valve components
- Repair of worn or corroded carbon steel equipment in chemical processing environments
The MAG welding process is preferred for this application due to its high deposition rate, good weld appearance, and low spatter. The shielding gas composition is typically 80% Ar + 20% CO2 or 75% Ar + 25% CO2, which provides adequate arc stability and penetration while maintaining acceptable weld bead profiles.
A critical process parameter is the heat input. Excessive heat input can lead to excessive dilution, carbide precipitation, and potential sensitization of the austenitic layers. Typical heat input values for this application should be maintained in the range of 0.5–1.5 kJ/mm to ensure proper weld quality without excessive thermal effects.
The interlayer temperature should also be carefully controlled. For the transition layer, interlayer temperatures above 150°C should be avoided to prevent excessive grain growth and potential cracking. For the top layer, interlayer temperatures should be kept below 200°C to maintain the fine microstructure and minimize sensitization risk.
Study Insights and Reflections
This study reinforces the fundamental principle in dissimilar metal welding that a transition layer is essential when joining materials with significant differences in composition, microstructure, and mechanical properties. The ER309L transition layer serves as a metallurgical buffer, accommodating the dilution effects and preventing the formation of brittle intermetallic phases at the interface.
The observation of predominantly ductile fracture morphology in the shear test is particularly significant from a safety perspective. In pressure vessel and piping applications, the ability of the surfacing layer to deform plastically before failure provides a margin of safety against catastrophic fracture. This is in contrast to brittle fracture, which can propagate rapidly without warning.
The presence of trace chromium carbide brittle phases, while not ideal, is an inevitable consequence of welding high-chromium materials. The key is to minimize their volume fraction through careful control of welding parameters, particularly heat input and interlayer temperature. The use of low-carbon electrode materials (ER309L and ER347L) is a critical strategy for achieving this objective.
Summary
The study demonstrates that a two-layer austenitic stainless steel surfacing system, using ER309L as the transition layer and ER347L as the corrosion-resistant top layer, provides excellent mechanical performance and metallurgical compatibility when deposited on Q345 low alloy steel via MAG welding. The transition layer design effectively improves bendability and shear strength, while the fracture morphology analysis confirms predominantly ductile failure behavior. The presence of trace chromium carbide phases, while requiring careful process control, does not compromise the overall mechanical integrity of the surfacing layers. These findings are directly applicable to the design and fabrication of corrosion-resistant components in chemical processing, oil and gas, and power generation industries.