CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Wear Resistance of High-Chromium Open-Arc Weld Overlay Alloy
Literature Overview
This research by Gong Jianxun and Xiao Yifeng from the School of Mechanical Engineering at Xiangtan University investigates the microstructure and wear resistance characteristics of high-chromium open-arc weld overlay alloys. Published in the Transactions of the Welding Institute of China in 2012, this study addresses the fundamental metallurgical and tribological properties of high-chromium overlay systems that are widely used in industrial wear-resistant applications.
Core Technical Content
High-chromium weld overlay alloys, typically containing 20–40% Cr, are widely employed for wear protection in applications involving abrasive, erosive, and adhesive wear. The primary wear-resistant mechanism in these alloys is the formation of hard chromium carbides (M7C3, M23C6, and Cr7C3) in a martensitic or austenitic matrix. The open-arc welding process (unshielded arc welding) introduces unique metallurgical challenges due to the absence of shielding gas, which affects the oxygen and nitrogen pickup in the weld metal.
Microstructural Characteristics
The microstructure of high-chromium overlay alloys deposited by open-arc welding typically consists of:
| Phase | Composition | Hardness (HV) | Volume Fraction | Distribution |
|---|---|---|---|---|
| M7C3 carbides | (Cr,Fe)7C3 | 1500–1800 | 30–50% | Dendritic interdendritic |
| M23C6 carbides | (Cr,Fe)23C6 | 1200–1500 | 10–20% | Grain boundaries |
| Martensitic matrix | BCC, high Cr | 500–700 | 30–50% | Continuous |
| Austenitic matrix | FCC, high Cr | 300–400 | 5–15% | Dendritic cores |
| Cr7C3 carbides | Cr7C3 | 1800–2000 | 5–15% | Fine dispersion |
Carbon Content Effect
Carbon content is the most critical factor governing the carbide volume fraction and morphology in high-chromium overlay alloys. The relationship between carbon content and microstructure can be summarized as:
| Carbon Content (%) | Primary Carbide Type | Carbide Morphology | Hardness (HV) | Wear Resistance |
|---|---|---|---|---|
| 1.0–1.5 | M7C3 | Coarse, blocky | 800–900 | Moderate |
| 1.5–2.5 | M7C3 | Semi-continuous | 900–1100 | Good |
| 2.5–3.5 | M7C3 + Cr7C3 | Fine, dispersed | 1100–1300 | Excellent |
| 3.5–4.5 | Cr7C3 | Very fine, continuous | 1300–1500 | Very good (brittle) |
| >4.5 | Cr7C3 dominant | Continuous network | 1500–1700 | Poor (excessive brittleness) |
Interpretation of Key Technical Points
Open-Arc Welding Metallurgical Challenges
Open-arc welding, also known as bare-arc welding or unshielded arc welding, involves welding without the use of external shielding gas or flux. While this process is simple and cost-effective, it introduces significant metallurgical challenges for high-chromium overlay alloys:
- Oxygen pickup: The molten weld pool is exposed to atmospheric oxygen, leading to significant oxidation of chromium and other alloying elements. This results in the formation of oxide inclusions that can degrade the mechanical properties and wear resistance of the overlay layer.
- Nitrogen pickup: Atmospheric nitrogen dissolves in the molten pool, forming nitride phases that can alter the carbide formation kinetics and morphology.
- Surface oxidation: The lack of shielding leads to a heavily oxidized surface on the deposited bead, which must be removed by grinding or machining to achieve the required surface finish and mechanical properties.
- Spatter: The unshielded arc produces significant spatter, which reduces the effective deposition efficiency and increases the material consumption.
Despite these challenges, open-arc welding remains in use for certain applications where process simplicity and cost are prioritized over maximum metallurgical quality. The study provides valuable insights into the actual microstructure and properties achieved under these conditions, which is essential for understanding the performance limits of open-arc deposited high-chromium overlays.
Wear Mechanisms in High-Chromium Alloys
The wear behavior of high-chromium overlay alloys is governed by the interaction between the hard carbide phases and the ductile matrix. The primary wear mechanisms include:
| Wear Mechanism | Description | Mitigation Strategy |
|---|---|---|
| Abrasive wear | Hard particles plow through the surface | Increase carbide volume fraction and hardness |
| Adhesive wear | Material transfer between contacting surfaces | Increase surface hardness, reduce ductility |
| Fatigue wear | Surface cracking from cyclic loading | Optimize carbide distribution, improve toughness |
| Erosive wear | Material removal by fluid-borne particles | Optimize carbide morphology for impact resistance |
| Corrosive wear | Combined chemical and mechanical degradation | Increase chromium content, improve passivation |
The optimal balance between hardness and toughness is critical for wear-resistant applications. Excessive carbide volume fraction increases hardness but reduces toughness, leading to brittle fracture under impact loading. Conversely, too low a carbide fraction results in insufficient hardness and rapid wear.
Heat Treatment Effects
Post-weld heat treatment can significantly modify the microstructure and properties of high-chromium overlay alloys. Common heat treatment processes include:
| Heat Treatment | Temperature (°C) | Duration (h) | Effect |
|---|---|---|---|
| Annealing | 800–900 | 1–2 | Reduce residual stress, soften matrix |
| Solution + aging | 1000–1100 + 600–700 | 1–2 + 4–6 | Refine carbides, improve toughness |
| Subcritical annealing | 700–800 | 2–4 | Reduce hardness slightly, improve ductility |
| Normalizing | 850–950 | 1–2 | Homogenize microstructure |
Process and Standards Analysis
Welding Procedure Development
The development of a welding procedure for high-chromium overlay alloys requires careful consideration of several factors:
- Filler metal selection: The carbon content and chromium content of the filler metal must be selected to achieve the desired microstructure and properties. Common filler metals include A5, A6, and A7 (per AWS A5.15) or equivalent Chinese specifications.
- Preheat requirements: High-chromium alloys are susceptible to cracking during welding due to their high hardenability and residual stress sensitivity. Preheating to 150–250 °C is typically required to reduce the cooling rate and minimize cracking.
- Interpass temperature control: The interpass temperature must be maintained below 300 °C to prevent excessive grain growth and carbide coarsening.
- Post-weld heat treatment: PWHT at 700–800 °C for 2–4 hours is recommended to relieve residual stresses and improve the toughness of the overlay layer.
Non-Destructive Testing Requirements
The NDE requirements for high-chromium overlay welds are more stringent than for conventional structural welds due to the criticality of the wear-resistant function:
| NDE Method | Coverage | Acceptance Criteria | Purpose |
|---|---|---|---|
| UT | 100% | No indications > reference block | Bond strength verification |
| MT | 100% | No cracks, no continuous indications | Surface crack detection |
| PT | 100% | No cracks, no linear indications | Surface defect detection |
| RT | Spot check | No porosity clusters, no lack of fusion | Volumetric defect detection |
| Hardness test | 100% | Within specified range | Property verification |
| Wear test | Sampling | Meets specified wear rate | Performance verification |
Standards Compliance
The fabrication and inspection of high-chromium overlay welds must comply with relevant standards:
- AWS A5.15: Specification for stainless steel and cast iron electrodes for welding
- ASME IX / NB/T 47014: Welding procedure qualification
- ASME V / JB/T 4730: Non-destructive testing requirements
- ASTM G99: Standard test method for instrumented reciprocating abrasive wear testing
- ASTM G65: Standard test method for pin-on-disk abrasive wear testing
- ISO 9074: Wear tests — Abrasive wear by reciprocating contact
Integration with Engineering Practice
Application Areas
High-chromium open-arc weld overlay alloys find extensive application in:
- Mining and mineral processing: Crusher jaws, conveyor rollers, hopper linings, and pump impellers exposed to abrasive ore and slurry
- Cement industry: Kiln liners, mill balls, and grinding media subjected to severe abrasive wear
- Power generation: Coal handling equipment, ash handling systems, and boiler tubes exposed to fly ash erosion
- Agricultural machinery: Plowshares, disc blades, and tillage equipment subjected to soil abrasion
- Construction equipment: Excavator buckets, dozer blades, and scraper components
Practical Considerations for Open-Arc Welding
While open-arc welding is less commonly used for high-chromium overlay applications compared to shielded arc processes, it remains relevant in certain situations:
- Field repair: Where shielding gas equipment is not available, open-arc welding provides a practical alternative for emergency repairs
- Large-scale repair: For extensive surface repairs where the cost of shielding gas is prohibitive
- Rustic environments: In remote locations where process infrastructure is limited
The key to achieving acceptable results with open-arc welding of high-chromium alloys is careful control of the welding parameters and thorough post-weld cleanup. The heavily oxidized surface must be completely removed by grinding or machining to expose the underlying microstructure with acceptable properties.
Case Study: Crusher Jaw Repair
A typical application of high-chromium open-arc overlay welding is the repair of worn crusher jaws in a mining operation. The crusher jaws, originally made of high-chromium cast iron, are worn to a critical thickness after 6–12 months of service. Instead of replacing the entire jaw, the worn surface is ground back to a uniform profile and rebuilt with 15–25 mm of high-chromium overlay weld metal using open-arc welding.
The repair procedure involves:
- Grinding the worn surface to remove all existing material and establish a flat, clean surface
- Preheating the jaw to 200 °C to minimize cracking risk
- Depositing the overlay in multiple passes, maintaining interpass temperatures below 300 °C
- Post-weld heat treatment at 750 °C for 3 hours to relieve residual stresses
- Grinding the surface to remove the oxidized layer and achieve the required surface finish
- NDE inspection including UT for bond strength and MT for surface cracks
The repaired jaw typically achieves 70–80% of the wear life of a new jaw, representing a significant cost saving compared to complete replacement.
Key Questions and Reflections
The Brittleness Challenge
The primary challenge with high-chromium overlay alloys is their inherent brittleness, particularly at high carbon contents where the carbide volume fraction exceeds 50%. This brittleness limits the application of these alloys to situations where the wear mechanism is primarily abrasive rather than impact or fatigue. Engineers must carefully evaluate the service conditions to determine whether a high-chromium overlay is appropriate or whether a more ductile overlay system would be more suitable.
The solution to the brittleness problem lies in optimizing the carbide morphology rather than simply reducing the carbide volume fraction. Fine, dispersed carbides provide wear resistance without creating continuous networks that act as crack propagation paths. Achieving this optimal morphology requires precise control of the welding parameters and post-weld heat treatment.
Oxygen Sensitivity
The sensitivity of high-chromium alloys to oxygen pickup during welding is a significant concern for open-arc welding. Chromium has a strong affinity for oxygen, and even small amounts of dissolved oxygen can form chromium oxide inclusions that degrade the mechanical properties and corrosion resistance of the overlay layer.
The study's findings on the microstructure and properties of open-arc deposited high-chromium alloys provide a baseline for understanding the performance limits of this process. The comparison with shielded arc deposited alloys would reveal the extent to which oxygen pickup degrades the wear resistance and mechanical properties.
Economic Considerations
The economic evaluation of high-chromium overlay welding must consider not only the initial welding cost but also the total cost of ownership, including:
- Material cost (filler metal consumption)
- Welding labor cost
- Post-weld machining cost
- Inspection and testing cost
- Downtime cost during repair
- Extended service life benefit
For many applications, the extended service life provided by high-chromium overlay welding more than justifies the additional cost compared to unprotected or conventionally protected components.
Study Insights and Implications
This research provides valuable insights into the microstructure-property relationships in high-chromium open-arc weld overlay alloys. The identification of the optimal carbon content range for achieving the best balance between hardness and toughness offers practical guidance for filler metal selection and welding procedure development.
The study's emphasis on open-arc welding, while addressing a less common process, provides important baseline data that can be used to evaluate the performance improvements achieved by shielded arc processes. This comparative perspective is valuable for engineers making process selection decisions based on cost and performance requirements.
For engineering practice, the key takeaway is that the wear resistance of high-chromium overlay alloys is primarily governed by the carbide volume fraction, morphology, and distribution rather than the matrix properties alone. Achieving optimal wear performance requires careful control of the welding parameters and post-weld heat treatment to produce a fine, dispersed carbide structure within a ductile matrix.
The research also highlights the importance of considering the specific wear mechanism when selecting overlay materials. Abrasive wear applications benefit from high carbide volume fractions, while impact and fatigue wear applications require more ductile overlay systems with lower carbide fractions. Engineers must carefully evaluate the service conditions to make appropriate material and process selections.