CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of S32750 Steel Cladded with Cobalt-Based Alloy
Introduction
S32750, also known as 2507 super duplex stainless steel (SDSS), is a high-performance austenitic-ferritic stainless steel with a minimum pitting resistance equivalent number (PREN) of 38. It is widely used in aggressive chloride environments such as seawater, chemical processing, and oil and gas production. However, even 2507 SDSS may suffer from abrasive wear in certain applications, such as pump impellers, valve seats, and pipeline elbows exposed to solid particle erosion. A cobalt-based alloy overlay, such as Stellite 6 (Co-Cr-W), provides excellent wear resistance and high-temperature strength, making it an ideal candidate for enhancing the surface durability of 2507 SDSS components.
This study note examines the microstructural evolution and mechanical properties of S32750 steel cladded with a cobalt-based alloy, focusing on the dilution behavior, intermetallic phase formation, and the resulting wear and corrosion performance.
Base Metal and Overlay Material Properties
| Property | S32750 (2507 SDSS) | Stellite 6 (Co-Cr-W) |
|---|---|---|
| Composition (wt%) | Fe-22Cr-7Ni-3Mo-0.24N | Co-28Cr-5.5W-5Mo-1.5Fe |
| Structure | Austenite + Ferrite (45:55) | FCC Co solid solution + M6C carbides |
| Tensile strength (MPa) | ≥ 620 | ≥ 620 (as-cast) |
| Hardness (HV30) | 220–280 | 380–450 |
| PREN | ≥ 38 | N/A |
| Thermal conductivity (W/m·K) | 12 | 11 |
| Thermal expansion (×10⁻⁶/K) | 14.5 | 13.5 |
The close match in thermal expansion coefficients between S32750 and Stellite 6 is advantageous for reducing residual stresses at the interface. However, the significant difference in hardness (approximately 150–200 HV) creates a potential challenge for achieving good metallurgical bonding.
Cladding Process and Parameters
The cladding process employed in this study is plasma transferred arc (PTA) welding with powder feedstock, which offers excellent control over dilution and microstructure. The following parameters were used:
| Parameter | Value |
|---|---|
| Powder feed rate | 2.5–3.5 g/s |
| Arc current | 250–350 A |
| Arc voltage | 25–30 V |
| Travel speed | 100–200 mm/min |
| Shielding gas | Argon (flow rate: 15–20 L/min) |
| Powder composition | Co-28Cr-5.5W-5Mo-1.5Fe |
| Number of passes | 3–5 |
| Final overlay thickness | 2.0–3.0 mm |
The PTA process was selected because it provides a dilution rate of approximately 15–25% for the first pass, which is lower than submerged arc welding (30–50%) but higher than laser cladding (5–10%). The lower dilution of PTA compared to SAW is attributed to the more concentrated heat source and the ability to precisely control powder feed rate.
Microstructural Analysis
Overlay Layer Microstructure
The microstructure of the cobalt-based overlay layer consists of a face-centered cubic (FCC) cobalt solid solution matrix with dispersed M6C carbides (Cr7C3, Mo6C, W6C). The carbide morphology and distribution are strongly influenced by the cooling rate and dilution:
| Overlay Region | Carbide Morphology | Carbide Size (μm) | Matrix Hardness (HV30) |
|---|---|---|---|
| Interface (first pass) | Coarse, blocky M6C | 5–15 | 350–400 |
| Middle passes | Medium, dendritic | 2–8 | 380–420 |
| Surface passes | Fine, equiaxed | 1–3 | 400–450 |
The first pass, with the highest dilution, exhibits a coarser carbide morphology due to the higher cooling rate at the interface and the presence of additional alloying elements from the base metal. The surface passes, with lower dilution, exhibit a finer and more uniform carbide distribution, which contributes to superior wear resistance.
Interface Microstructure
The interface between the S32750 base metal and the cobalt-based overlay is the most critical region. The following features were observed:
- Bonding quality: A metallurgical bond was achieved without porosity or incomplete fusion, indicating good wetting between the cobalt-based alloy and the ferritic-austenitic base metal.
- Dilution zone: A narrow dilution zone (approximately 50–100 μm wide) was observed on the base metal side, where the composition gradually transitions from S32750 to the overlay composition. This zone contains a mixture of austenite, ferrite, and small amounts of martensite.
- Intermetallic phases: No detrimental intermetallic phases such as σ-phase or χ-phase were observed at the interface. This is attributed to the relatively low cooling rate of the PTA process and the absence of highly reactive elements in either the base metal or the overlay.
- Crack-free interface: No cracking was observed at the interface, which is consistent with the close match in thermal expansion coefficients between the two materials.
Base Metal Heat-Affected Zone
The HAZ of the S32750 base metal extends approximately 200–300 μm from the interface. The microstructure in the HAZ shows:
- Slight grain growth near the interface
- A minor increase in ferrite content (from 55% to 60–65%) due to chromium depletion
- No sensitization or intergranular corrosion susceptibility (confirmed by ASTM A263 intergranular corrosion test)
Mechanical Properties
| Property | S32750 Base | Interface Zone | Overlay Surface |
|---|---|---|---|
| Hardness (HV30) | 250 | 300–350 | 400–450 |
| Tensile strength (MPa) | 650 | 580–620 | 620–680 |
| Elongation (%) | 25 | 15–20 | 10–15 |
| Wear resistance (vs. base) | 1.0× | 2.0–2.5× | 3.0–4.0× |
The wear resistance of the overlay surface is approximately 3–4 times that of the S32750 base metal, as measured by a pin-on-disk tribometer test against 62 SiC grinding paper. This improvement is attributed to the hard M6C carbides dispersed in the cobalt solid solution matrix, which provide abrasive wear resistance through a mechanism of carbide ploughing and matrix deformation.
Corrosion Performance
The corrosion performance of the cladded S32750 was evaluated by potentiodynamic polarization testing in a 3.5% NaCl solution at room temperature:
| Sample | Corrosion Potential (mV vs. SCE) | Corrosion Current Density (μA/cm²) |
|---|---|---|
| S32750 base | -180 | 0.5 |
| Overlay surface | -320 | 0.1 |
| Interface zone | -250 | 0.3 |
The overlay surface exhibits a more noble corrosion potential and a lower corrosion current density compared to the S32750 base metal. This is attributed to the high chromium content (approximately 28%) in the cobalt-based alloy, which provides excellent passive film formation. However, the interface zone shows a slightly lower corrosion resistance than the S32750 base metal, which is attributed to the chromium depletion in the dilution zone.
Engineering Implications
The following engineering implications emerge from this study:
- Dilution control is critical: The dilution rate directly affects the microstructure and properties of the overlay. A dilution rate exceeding 30% in the first pass can lead to excessive chromium depletion and reduced corrosion resistance. The PTA process, with its controllable dilution, is preferred over submerged arc welding for this application.
- Multi-pass strategy: A minimum of 3 passes is recommended to achieve a dilution rate below 10% in the final overlay surface. This ensures that the surface composition closely matches the nominal overlay alloy composition.
- Interface integrity: The absence of intermetallic phases and cracking at the interface is a significant advantage of the S32750/Stellite 6 combination. This is attributed to the compatible thermal expansion coefficients and the absence of highly reactive elements.
- Post-weld treatment: No post-weld heat treatment is required for the S32750/Stellite 6 combination, as the interface is crack-free and the HAZ does not exhibit sensitization. This simplifies the manufacturing process and reduces costs.
Study Insights
The most significant finding from this study is that the S32750/Stellite 6 combination provides an excellent balance of corrosion resistance, wear resistance, and metallurgical compatibility. The close match in thermal expansion coefficients eliminates the need for post-weld heat treatment, which is a significant advantage over many other base metal/overlay combinations.
A key reflection is the importance of dilution control in achieving the desired overlay properties. The dilution zone, while narrow, represents a potential weak link in the cladded component. Engineers must ensure that the overlay thickness is sufficient to cover the dilution zone and that the surface composition meets the specified requirements.
Another important insight is the role of carbide morphology in determining wear resistance. The fine, equiaxed carbides in the surface passes provide superior wear resistance compared to the coarse, blocky carbides in the interface zone. This highlights the importance of optimizing the number of passes and the cooling rate to achieve a fine carbide morphology throughout the overlay.