CLADDING TECHNOLOGY SHANXI CO., LTDManual Arc Weld Overlay Manufacturing and Repair of Tools
Introduction and Scope
Manual shielded metal arc welding (SMAW) remains one of the most versatile and widely used methods for weld overlay manufacturing and tool repair in industrial settings. Despite the availability of more automated and advanced processes such as plasma transferred arc (PTA) welding, laser cladding, and hot-wire TIG welding, SMAW overlay retains significant practical advantages including equipment simplicity, field applicability, low capital investment, and the ability to weld in all positions. This literature study focuses on the technical aspects of SMAW overlay for manufacturing new tools and repairing worn or damaged ones, with emphasis on process selection, electrode choice, and quality assurance.
Process Characteristics and Advantages
SMAW overlay offers several distinct advantages for tool manufacturing and repair applications. The process requires minimal equipment—essentially a DC power source, welding cable, and the electrode holder. The welding operator has direct control over the arc length, travel speed, and electrode angle, allowing real-time adjustment of the deposit profile. The flux coating on the electrode provides both shielding and alloying, eliminating the need for external shielding gas in most applications.
Typical SMAW Overlay Process Parameters
| Parameter | Typical Range | Notes |
|---|---|---|
| Current type | DCEN preferred | Better penetration and arc stability |
| Current density | 15–40 A/mm² | Depends on electrode diameter |
| Travel speed | 50–200 mm/min | Slower for thicker deposits |
| Arc length | 2–5 mm | Short arc for better control |
| Electrode angle | 15–30° from vertical | Drag or push angle |
| Layer thickness per pass | 3–6 mm | Controlled by electrode manipulation |
| Interpass temperature | < 200 °C | Prevents excessive grain growth |
Electrode Selection for Overlay Applications
The selection of welding electrodes is the most critical factor in SMAW overlay performance. Electrodes are classified according to AWS A5.15 (envelope electrodes for hardfacing) and AWS A5.16 (stick electrodes for hardfacing). The classification system provides information on the alloy composition, current type, and coating type.
Common Hardfacing Electrode Classifications
| AWS Classification | Type | Typical Composition | Hardness (HRC) | Application |
|---|---|---|---|---|
| AWS A6-14 | Type I, high carbon | 3.0–3.5% C, 2.0–3.0% Cr | 55–65 | Abrasive wear, moderate impact |
| AWS A6-16 | Type I, medium carbon | 1.5–2.5% C, 2.0–3.0% Cr | 45–55 | General abrasion |
| AWS A6-20 | Type II, high Cr | 2.5–3.5% C, 20–28% Cr | 55–65 | Severe abrasion |
| AWS A6-23 | Type II, high Cr | 3.0–4.0% C, 26–34% Cr | 60–68 | Severe dry abrasion |
| AWS A6-26 | Type III, Ni-Cr | 1.0–2.0% C, 10–14% Cr, 4–6% Ni | 45–55 | Impact abrasion |
| AWS A6-31 | Type IV, Ni-Co | 1.0–2.0% C, 5–10% Cr, Ni-Co bal | 50–58 | High-temperature abrasion |
| AWS A6-41 | Type V, Ni | 0.5–1.0% C, Ni bal | 25–40 (as-welded) | Erosive wear, corrosion |
Manufacturing of New Tools by SMAW Overlay
The manufacturing of new tools by SMAW overlay involves depositing a hardfacing layer onto a structural steel substrate to create a tool with the desired surface properties. This approach is cost-effective because the bulk of the tool can be made from inexpensive carbon steel, while only the critical surface layer requires the expensive alloy.
The process typically involves the following steps:
- Prepare the substrate by machining to the required dimensions and cleaning the surface to be overlaid.
- Apply a transition layer if necessary to ensure good metallurgical compatibility between the base metal and the hardfacing material.
- Build up the overlay layer in multiple passes, maintaining a consistent bead profile and controlling the dilution.
- Machine the overlay surface to the final dimensions, removing the oxide scale and achieving the required surface finish.
- Perform heat treatment if required to achieve the desired hardness and relieve residual stresses.
Key Considerations for Tool Manufacturing by Overlay
- The substrate material should have sufficient ductility to accommodate the thermal stresses induced during welding without cracking.
- The transition layer should have a composition that bridges the gap between the base metal and the hardfacing material, typically containing 10–20% of the hardfacing alloy.
- The overlay thickness should be sufficient to accommodate the final machining allowance, typically 1.5–3 mm above the final dimension.
- The welding sequence should be planned to minimize distortion, using techniques such as skip welding, back-step welding, and balanced welding.
Repair of Worn Tools by SMAW Overlay
Repair of worn tools by SMAW overlay is a common industrial practice that extends tool life and reduces replacement costs. The repair process requires careful assessment of the worn surface, preparation of the repair area, and selection of appropriate hardfacing material.
Repair Procedure and Quality Control
| Step | Activity | Quality Requirement |
|---|---|---|
| 1 | Assess wear pattern and remaining material | Minimum 2× overlay thickness remaining |
| 2 | Machine or grind worn surface | Smooth, clean surface with good bond |
| 3 | Preheat if required | 150–300 °C for high-carbon substrates |
| 4 | Apply overlay in controlled passes | Uniform bead profile, no defects |
| 5 | Post-weld heat treatment | Relieve stresses, achieve target hardness |
| 6 | Machine to final dimensions | Verify geometry and surface finish |
| 7 | Hardness testing | Verify hardness within specification |
| 8 | Visual and dimensional inspection | No cracks, porosity, or dimensional deviation |
Common Defects and Their Prevention
SMAW overlay is susceptible to several defect types that can compromise the performance of the overlay layer. Understanding these defects and their causes is essential for maintaining quality.
Defect Analysis and Countermeasures
| Defect | Cause | Prevention |
|---|---|---|
| Cracking in overlay | Excessive carbon, rapid cooling, hydrogen | Preheat, controlled cooling, low-hydrogen electrode |
| Cracking at overlay-base interface | High dilution, poor metallurgical compatibility | Transition layer, controlled welding sequence |
| Porosity | Contaminated surface, excessive arc length | Surface cleaning, short arc length |
| Excessive spatter | Long arc, high current | Short arc, proper current setting |
| Uneven hardness | Inconsistent dilution, improper electrode technique | Consistent travel speed, proper electrode angle |
| Overlap (lack of fusion) | Excessive travel speed, improper bead placement | Controlled travel speed, proper overlap (50% of bead width) |
Study Insights and Practical Recommendations
The literature on SMAW overlay for tool manufacturing and repair emphasizes several key principles that are essential for successful practice. First, the dilution effect must be carefully managed, as it directly affects the final hardness and wear resistance of the overlay. The dilution rate in SMAW is typically 15–35% for the first pass and decreases to 5–15% for subsequent passes. This means that the first pass may not achieve the required hardness, and additional passes are necessary to build up the overlay to the required thickness and composition.
Second, the heat input must be carefully controlled to balance penetration (for good bond strength) against dilution (for composition control). Lower heat input reduces dilution but may result in incomplete fusion. Higher heat input improves fusion but increases dilution. The optimal heat input is process-specific and should be determined through welding trials.
Third, the welding operator's skill is a critical factor in SMAW overlay quality. Unlike automated processes, SMAW requires manual control of arc length, travel speed, and electrode manipulation. The operator must be trained and certified in the specific overlay technique to ensure consistent quality. This human factor is both an advantage (flexibility) and a challenge (variability) of the SMAW process.
Finally, the literature underscores the importance of post-weld heat treatment in SMAW overlay applications. The high cooling rates associated with manual welding can produce hard, brittle microstructures in the overlay and HAZ. Tempering treatments at 500–650 °C relieve residual stresses, stabilize the microstructure, and achieve the desired hardness-toughness balance. For high-carbon, high-chromium overlays, the tempering treatment is essential to prevent cracking during subsequent machining or service.