CLADDING TECHNOLOGY SHANXI CO., LTDDevelopment of K118 Wear-Resistant Alloy Carbon Arc Overlay Powder Block
Literature Overview
This 1988 study published in "Coal Science and Technology" (煤炭科学技术) by Ge Changlu, Shan Liyun from China University of Mining and Technology, and Kan Zhouping, Liu Jiajun from Xuzhou Mining Bureau Second Machinery Factory, reports on the development of a K118 wear-resistant alloy powder block for carbon arc overlay welding. This research is historically significant as it represents an early effort to develop specialized consumables for the coal mining industry, where wear-resistant overlay welding is critical for extending the service life of mining equipment components.
Technical Background and Application Requirements
The coal mining industry faces severe wear challenges due to the abrasive nature of coal, rock, and debris encountered during mining operations. Equipment components such as crusher jaws, conveyor rollers, bucket teeth, and excavator buckets are subjected to intense abrasive wear, often in wet and corrosive environments. Traditional replacement strategies result in high downtime costs and material waste.
The K118 alloy powder block was developed to address the following application requirements:
| Requirement | Specification | Rationale |
|---|---|---|
| Abrasive wear resistance | Hardness above 55 HRC | Resists abrasive wear from coal and rock |
| Impact resistance | Charpy impact energy above 27 J at -20°C | Withstands impact loading during mining operations |
| Weldability | Low hydrogen content, good arc stability | Enables reliable field welding in mining environments |
| Cost effectiveness | Lower cost than imported alternatives | Economic viability for widespread adoption |
| Durability | Minimum overlay thickness of 3 mm | Provides sufficient wear life before re-overlay is required |
Alloy Design and Microstructural Analysis
The K118 alloy was designed based on the principles of carbide strengthening and martensitic transformation. The alloy composition is characterized by:
- High carbon content: 3.0–4.5 wt% C, which promotes the formation of hard carbide phases.
- High chromium content: 12–18 wt% Cr, which stabilizes austenite and promotes chromium carbide formation.
- Molybdenum addition: 1.0–2.0 wt% Mo, which increases hardenability and improves high-temperature wear resistance.
- Vanadium addition: 0.5–1.5 wt% V, which forms hard vanadium carbides that enhance wear resistance.
- Nickel addition: 3.0–5.0 wt% Ni, which stabilizes austenite and improves toughness.
The resulting microstructure consists of:
- Martensitic matrix: A high-carbon martensite with hardness in the range of 50–58 HRC, providing the base wear resistance.
- Chromium carbides (Cr7C3 and Cr23C6): Hard, angular carbide particles distributed throughout the matrix, providing primary abrasive wear resistance.
- Vanadium carbides (VC): Finer, more uniformly distributed carbide particles that enhance wear resistance at the microstructural level.
- Retained austenite: A small fraction of retained austenite (5–15%) that provides some strain-hardening capacity and improves impact toughness.
Carbon Arc Overlay Welding Process
The K118 powder block is applied using carbon arc overlay welding (also known as carbon arc gouging or carbon arc surfacing). This process is characterized by:
- Carbon electrode: A carbon rod is used as the electrode, with the alloy powder block fed into the arc.
- Arc temperature: The carbon arc operates at approximately 4000–5000°C, providing intense heat input.
- Dilution: The carbon arc process has relatively high dilution (15–25%) because the carbon electrode melts and mixes with the alloy powder.
- Deposition rate: High deposition rate (5–10 kg/h) makes the process economical for thick overlay layers.
- Shielding: Flux or shielding gas is used to protect the weld metal from atmospheric contamination.
Process Parameters
| Parameter | Typical Value |
|---|---|
| Carbon electrode diameter | 8–12 mm |
| Current | 150–250 A (DCEN) |
| Arc voltage | 20–30 V |
| Travel speed | 100–200 mm/min |
| Powder feed rate | 200–400 g/min |
| Interpass temperature | Below 150°C |
| Number of passes | 2–4 |
Performance Testing and Results
The K118 alloy was evaluated through a series of laboratory and field tests:
| Test Method | Result | Comparison with Baseline |
|---|---|---|
| Hardness (HV) | 850–950 HV | 2–3× higher than base steel |
| Abrasive wear (ASTM G65) | 0.5–0.8 mm³ loss | 5–8× better than uncoated steel |
| Impact toughness | 27–40 J at -20°C | Acceptable for mining applications |
| Fatigue life | 10⁵–10⁶ cycles | Comparable to cast alloy components |
| Field service life | 3–5× longer than replacement parts | Significant economic benefit |
Engineering Practice Insights
The development of the K118 alloy powder block represents a practical approach to solving a real industrial problem. The carbon arc overlay welding process is particularly well-suited for field applications in mining environments because it is portable, requires minimal equipment, and can be performed by trained welders without specialized infrastructure.
However, the study also highlights some limitations of the carbon arc process:
- High dilution: The high dilution rate can reduce the hardness and wear resistance of the overlay layer, particularly in the first pass.
- Carbon pickup: The carbon electrode introduces additional carbon into the weld metal, which can promote carbide coarsening and reduce toughness.
- Surface quality: The carbon arc process produces a rough surface finish that may require post-weld machining for some applications.
- Hydrogen sensitivity: The high heat input and carbon arc process can introduce hydrogen into the weld metal, increasing the risk of hydrogen-induced cracking.
The K118 alloy was designed to compensate for these limitations by using a high-alloy composition that maintains adequate hardness and toughness even with significant dilution. The addition of nickel and molybdenum improves the toughness of the martensitic matrix, while the controlled carbon content ensures that the carbide morphology remains favorable for wear resistance.
For engineers working in the mining industry, the key takeaway is that consumable selection must be matched to the specific application requirements. The K118 alloy was optimized for abrasive wear in mining environments, but it may not be the optimal choice for applications requiring high impact resistance or corrosion resistance. Engineers must carefully evaluate the service conditions and select consumables accordingly.