CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of SMAW Overlay Metal
Literature Overview
Shielded metal arc welding (SMAW), also known as manual arc welding or stick welding, remains one of the most widely used processes for overlay welding applications due to its simplicity, portability, and low equipment requirements. The literature under review examines the microstructure and mechanical properties of overlay deposits produced using SMAW with various electrode types, including low-hydrogen electrodes, stainless steel electrodes, and specialty overlay electrodes designed for wear-resistant applications.
The study provides a comprehensive analysis of how electrode composition, welding parameters, and deposition sequence influence the microstructure, hardness, and wear resistance of SMAW overlay deposits. The work is particularly relevant to field applications where SMAW is the only practical overlay process available.
Core Technical Viewpoints
Electrode Classification and Overlay Properties
| Electrode Type | Composition (wt%) | Overlay Hardness (HRC) | Dilution Rate (%) | Typical Application |
|---|---|---|---|---|
| E6010 (low-hydrogen) | C<0.20, Mn 1.0, Si 0.6 | 25–35 | 30–45 | Build-up, corrosion repair |
| E7018 (low-hydrogen) | C<0.10, Mn 1.2, Si 0.6 | 25–35 | 25–40 | General overlay |
| E309L (stainless) | Cr 23–26, Ni 12–14, C<0.03 | 25–30 | 25–35 | Corrosion resistance |
| E309Mo (stainless) | Cr 23–26, Ni 12–14, Mo 2–3, C<0.03 | 25–30 | 25–35 | Corrosion resistance |
| E316L (stainless) | Cr 16–18, Ni 10–14, Mo 2–3, C<0.03 | 25–30 | 25–35 | Chemical resistance |
| E347 (stainless) | Cr 19–22, Ni 24–30, Nb 0.65–1.10, C<0.08 | 25–30 | 25–35 | High-temperature service |
| E70T-8 (high-Mn) | Mn 12–14, C 0.9–1.2 | 30–40 (as-welded), 45–55 (work-hardened) | 20–30 | Impact-abrasive wear |
| E310L (stainless) | Cr 24–27, Ni 19–22, C<0.03 | 25–30 | 25–35 | High-temperature corrosion |
Microstructural Analysis of SMAW Overlays
The microstructure of SMAW overlay deposits is governed by several factors:
- Cooling rate: SMAW typically produces cooling rates of 5–50 °C/s, depending on section thickness, preheat, and interpass temperature. Higher cooling rates promote martensitic transformation, while lower rates allow pearlitic or ferritic structures to form.
- Electrode alloy content: The alloying elements in the electrode determine the phase composition of the overlay. High-carbon electrodes produce martensitic structures with carbide precipitation, while high-nickel electrodes promote austenitic structures.
- Dilution with base metal: The dilution rate in SMAW is typically 25–45%, which is higher than most gas-shielded processes. This dilution reduces overlay alloy content and can significantly affect properties.
- Deposition sequence: Multi-pass overlays develop a layered microstructure where each pass experiences a different thermal history. The first pass (root pass) experiences the highest dilution, while subsequent passes experience lower dilution as the previous pass material dilutes the base metal.
Effect of Welding Parameters on Overlay Properties
| Parameter | Range | Effect on Hardness | Effect on Dilution | Effect on Defects |
|---|---|---|---|---|
| Current | 80–250 A | Higher current → lower hardness | Higher current → higher dilution | Higher current → less cracking |
| Travel speed | 50–200 mm/min | Faster → higher hardness | Faster → lower dilution | Faster → more porosity |
| Electrode diameter | 2.5–5.0 mm | Larger → lower hardness | Larger → higher dilution | Larger → more porosity |
| Electrode angle | 10°–30° from vertical | Steeper → lower hardness | Steeper → higher dilution | Steeper → more undercut |
| Interpass temperature | 50–250 °C | Higher → lower hardness | Higher → higher dilution | Higher → less cracking |
The literature identifies an optimal parameter window for most SMAW overlay applications: current density of 10–15 A/mm² (electrode cross-section), travel speed of 100–150 mm/min, electrode angle of 15°–20° from vertical, and interpass temperature of 100–150 °C.
Process and Standards Analysis
Comparison of SMAW with Other Overlay Processes
| Property | SMAW | GMAW | SAW | FCAW |
|---|---|---|---|---|
| Equipment cost | Low | Medium | Medium-High | Medium |
| Portability | Excellent | Moderate | Poor | Good |
| Dilution rate | 25–45% | 15–30% | 10–20% | 20–35% |
| Deposition rate (kg/h) | 1–3 | 3–8 | 5–15 | 2–6 |
| Overlay hardness consistency | Moderate | Good | Excellent | Good |
| Field applicability | Excellent | Moderate | Limited | Good |
| Operator skill requirement | High | Medium | Low | Medium |
| Cost per kg of overlay | High | Medium | Low | Medium |
Defect Analysis and Prevention
| Defect | Root Cause | Detection Method | Prevention |
|---|---|---|---|
| Porosity | Hydrogen absorption, slag entrapment | RT, UT |