CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of 310 Stainless Steel CMT Overlay Welding on Q235 Steel Surface
Literature Overview and Technical Background
The combination of austenitic stainless steel overlay layers on carbon steel substrates is a common strategy for providing corrosion resistance to structural components while retaining the mechanical strength and cost-effectiveness of the base metal. However, the direct welding of austenitic stainless steels such as 310 (high-temperature grade) onto carbon steels such as Q235 presents significant metallurgical challenges due to the large difference in thermal conductivity, thermal expansion coefficient, and carbon activity between the two materials. The literature under study examines the microstructure and mechanical properties of 310 stainless steel deposited onto Q235 steel using the Cold Metal Transfer (CMT) welding process, which is characterized by low heat input and controlled wire feeding.
The CMT process was selected for this application because of its inherently low heat input (typically 0.5–1.5 kJ/mm), which minimizes dilution of the base metal into the overlay layer and reduces the thermal distortion of the component. This is particularly important for thin-walled or geometrically complex components where excessive heat input could cause warping or cracking. The literature investigates the effect of welding parameters on the microstructure, hardness, corrosion resistance, and bond strength of the 310 stainless steel overlay layer.
Welding Process Parameters and Microstructural Analysis
The CMT welding process was performed with the following parameters:
| Parameter | Value |
|---|---|
| Wire diameter | 1.0 mm (310 stainless steel) |
| Welding current | 60–100 A |
| Arc voltage | 14–18 V |
| Travel speed | 150–300 mm/min |
| Shielding gas | Ar (99.99%) |
| Preheat temperature | 100–150°C |
| Interpass temperature | ≤200°C |
| Wire oscillation amplitude | 0–2 mm |
| Wire oscillation frequency | 50–200 Hz |
The microstructural analysis revealed a typical three-zone structure: the overlay layer, the heat-affected zone (HAZ), and the base metal. The overlay layer exhibited a fully austenitic microstructure with a grain size of 20–40 μm, with occasional delta ferrite inclusions at grain boundaries. The HAZ in the Q235 base metal showed a transition from fine ferrite-pearlite near the weld to coarse ferrite-pearlite further away, with a maximum hardness of 250 HV in the HAZ compared to 120 HV in the base metal.
The dilution ratio was measured at 8–15%, depending on the welding parameters. Lower current and higher travel speed resulted in lower dilution (8–10%), while higher current and lower travel speed resulted in higher dilution (12–15%). The dilution ratio directly affects the hardness and corrosion resistance of the overlay layer: lower dilution preserves the austenitic microstructure and higher chromium content, resulting in better corrosion resistance.
Mechanical Properties and Corrosion Resistance
The mechanical properties of the overlay layer were evaluated as follows:
| Property | Value | Comparison with 310 SS Base |
|---|---|---|
| Hardness (overlay) | 180–220 HV | 150–180 HV |
| Hardness (HAZ) | 200–250 HV | N/A |
| Hardness (base) | 110–130 HV | N/A |
| Tensile strength (overlay) | 550–650 MPa | 520 MPa |
| Elongation (overlay) | 30–40% | 40% |
| Bond strength (tensile) | 350–450 MPa | — |
The corrosion resistance was evaluated by immersion testing in 3.5% NaCl solution at 60°C for 168 hours. The overlay layer showed no pitting or crevice corrosion, while the base metal exhibited severe general corrosion. The overlay layer also showed excellent resistance to intergranular corrosion when tested by the ASTM A262 Practice E method, with no intergranular attack observed after sensitization at 870°C for 4 hours.
The bond strength between the overlay and the base metal was measured by tensile testing of the overlay/base metal joint. The results showed a bond strength of 350–450 MPa, which exceeds the tensile strength of the Q235 base metal (375 MPa), indicating that the bond is stronger than the base metal itself. This is a critical requirement for the overlay to function as a protective layer under mechanical loading.
Defect Analysis and Process Optimization
The following defects were identified and analyzed:
| Defect | Frequency | Root Cause | Countermeasure |
|---|---|---|---|
| Porosity | Low (2–5%) | Inadequate shielding gas coverage | Increase gas flow rate; reduce travel speed |
| Cracking | Very low (<1%) | Low carbon content of 310 SS; low heat input | No countermeasure needed; CMT process inherently crack-resistant |
| Excessive dilution | Moderate (10–15%) | High current and low travel speed | Reduce current; increase travel speed |
| Incomplete bond | Low (3–5%) | Insufficient base metal melting | Increase current by 10–20%; add edge pass |
The FMEA analysis identified excessive dilution as the primary risk, with a criticality rating of 15. The countermeasure of reducing current and increasing travel speed reduced the dilution ratio from 15% to 8%, improving the corrosion resistance of the overlay layer without compromising the bond strength.
Engineering Practice Insights
The literature highlights the unique advantages of the CMT process for depositing austenitic stainless steel overlays on carbon steel substrates. The low heat input of CMT (0.5–1.5 kJ/mm) is significantly lower than that of conventional GMAW (2.0–4.0 kJ/mm) or SAW (3.0–6.0 kJ/mm), which results in lower dilution, reduced distortion, and a more controlled microstructure. This makes CMT particularly suitable for thin-walled components, geometrically complex parts, and applications where thermal distortion is a concern.
The literature also notes that the wire oscillation feature of CMT allows for the deposition of wider beads in a single pass, which increases the deposition efficiency without increasing the heat input per unit length. This is particularly useful for covering large surface areas with a uniform overlay thickness. The recommended oscillation amplitude is 1–2 mm for a 1.0 mm wire, with a frequency of 100–150 Hz.
From a metallurgical perspective, the fully austenitic microstructure of the 310 overlay provides excellent corrosion resistance and high-temperature stability, but the presence of delta ferrite inclusions at grain boundaries should be monitored, as excessive delta ferrite (above 5%) can reduce the corrosion resistance and ductility of the overlay. The literature recommends that the delta ferrite content be controlled by adjusting the welding parameters and, if necessary, by adding a small amount of niobium or titanium to the wire composition.
Summary and Conclusions
The CMT welding of 310 stainless steel onto Q235 carbon steel produces a high-quality overlay layer with a fully austenitic microstructure, low dilution (8–15%), excellent corrosion resistance, and a bond strength exceeding the tensile strength of the base metal. The low heat input of the CMT process is the key factor enabling the achievement of low dilution and a controlled microstructure, making it an ideal process for depositing austenitic stainless steel overlays on carbon steel substrates. The mechanical properties and corrosion resistance of the overlay layer meet or exceed the requirements for most industrial applications, and the process is suitable for both manufacturing and repair applications. This work demonstrates that the CMT process is a viable and advantageous alternative to conventional GMAW and SAW for austenitic stainless steel overlay welding, particularly in applications where low dilution and low distortion are critical.