Zhuojin Pipe Fitting Co., LtdDevelopment of K118 Nickel-Free Wear-Resistant Alloy Carbon Arc Surfacing Powder Block
Literature Overview
This paper, authored by Ge Changlu, Shan Liliyun, Kan Zhouping, and Liu Jiajun from China University of Mining and Technology and Xuzhou Mining Bureau Second Machinery Factory, published in "Coal Science and Technology" (Vol. 16, No. 9, 1988, pp. 15-16), describes the development of K118, a novel nickel-free wear-resistant alloy carbon arc surfacing powder block. The material is designed for application in coal mining equipment, particularly for the wear surfaces of scraper conveyors, where severe abrasive wear conditions prevail.
The development of nickel-free wear-resistant surfacing materials was particularly significant in the context of the 1980s, when nickel was an expensive and strategically important metal. The traditional wear-resistant surfacing alloys often relied on nickel to enhance toughness and wear resistance, but the high cost and limited availability of nickel prompted research into alternative alloying elements and microstructural designs that could achieve comparable performance without nickel.
Core Technical Analysis
Material Design Philosophy
The K118 powder block is designed as a carbon arc surfacing material, which means it is applied using carbon arc gouging or surfacing equipment. The carbon arc process involves using a carbon electrode to melt the base metal and the surfacing material simultaneously, with the carbon arc providing the heat source and the surfacing material being fed as a powder or wire.
The key design features of K118 include:
- Nickel-free composition: The absence of nickel is compensated by other alloying elements and a carefully designed microstructure that provides wear resistance through alternative mechanisms.
- Hypereutectic microstructure: The material is designed to produce a hypereutectic microstructure, which contains hard carbide phases (such as boron carbides) dispersed in a tougher matrix. The hypereutectic composition ensures a high volume fraction of hard phases, which is essential for wear resistance.
- Boron carbide formation: The inclusion of boron in the alloy composition promotes the formation of boron carbides (B4C or B2C), which are extremely hard phases (hardness >2000 HV) that provide excellent abrasion resistance.
Microstructural Analysis
The wear resistance of K118 is attributed to its hypereutectic microstructure, which consists of hard boron carbide particles dispersed in a matrix of austenite, martensite, and possibly retained austenite. The hardness of the individual phases and the volume fraction of hard phases are critical factors in determining the overall wear resistance.
| Microstructural Feature | Description | Contribution to Wear Resistance |
|---|---|---|
| Boron carbide particles | Hard, angular particles (Hv >2000) | Primary abrasion resistance mechanism |
| Austenite matrix | Ductile, shock-absorbing | Provides toughness and prevents cracking |
| Martensite | Hard, strong phase (Hv 500-800) | Secondary wear resistance contribution |
| Retained austenite | Stress-relieving, ductile | Accommodates residual stresses |
The hypereutectic composition is achieved by adjusting the carbon and boron content to exceed the eutectic composition, ensuring that primary carbide particles form during solidification. The size, shape, and distribution of these carbide particles are critical for wear resistance, as they determine the effectiveness of the abrasion resistance mechanism.
Welding Process Parameters
The carbon arc surfacing process for K118 requires careful control of the welding parameters to achieve the desired microstructure and wear resistance. Key parameters include:
- Carbon electrode diameter: Typically 6-12 mm, depending on the thickness of the surfacing layer.
- Welding current: 200-400 A, depending on the electrode diameter and surfacing layer thickness.
- Arc voltage: 20-30 V, controlled by the arc length.
- Travel speed: 50-150 mm/min, depending on the desired deposition rate and layer thickness.
- Preheating temperature: 200-300°C, to reduce the cooling rate and minimize the risk of cracking.
- Interpass temperature: Maintained above 200°C to prevent excessive cooling between passes.
The use of an AC welding machine is specified, which provides alternating current to the welding circuit. AC welding can help reduce arc blow and improve arc stability, particularly in the carbon arc process where the carbon electrode acts as a consumable electrode.
Application to Scraper Conveyors
The primary application of K118 is for the wear surfaces of scraper conveyors used in coal mining. Scraper conveyors are subjected to severe abrasive wear from the coal and rock material being transported, as well as from the scraper chains and plates that move the material. The wear surfaces of scraper conveyors include the trough lining, scraper plates, and chain guides.
The wear resistance of K118 is evaluated by comparing the service life of scraper conveyor components surfaced with K118 against those surfaced with conventional wear-resistant materials or unprotected base materials. The paper reports that K118 provides significantly improved wear resistance compared to conventional materials, with a service life extension that justifies the additional cost of the surfacing operation.
Engineering Practice Integration
Selection Criteria for Wear-Resistant Surfacing Materials
The selection of a wear-resistant surfacing material for a specific application requires consideration of several factors:
- Type of wear: Abrasive wear (as in scraper conveyors), adhesive wear (as in sliding contacts), or erosive wear (as in fluid-solid particle interactions) each require different material properties.
- Service temperature: The material must retain its hardness and wear resistance at the operating temperature. For scraper conveyors, the service temperature is typically ambient, but for other applications, elevated temperatures may require heat-resistant alloys.
- Impact loading: The material must have sufficient toughness to resist cracking under impact loading. The hypereutectic microstructure of K118 provides a balance between hardness and toughness through the combination of hard carbide particles and a ductile matrix.
- Corrosion resistance: In some applications, the surfacing material must also resist corrosion. K118 is not designed for corrosion resistance, but for applications requiring both wear and corrosion resistance, a different material selection would be required.
- Cost: The cost of the surfacing material, the welding consumables, and the application labor must be considered in the overall cost-benefit analysis.
FMEA Analysis for Surfacing Application
A failure mode and effects analysis (FMEA) for the K118 surfacing application on scraper conveyors can identify potential failure modes and their countermeasures:
| Failure Mode | Potential Cause | Effect | Countermeasure |
|---|---|---|---|
| Cracking of surfacing layer | Excessive cooling rate; high residual stress | Loss of wear protection; reduced service life | Preheating; controlled interpass temperature; post-weld stress relief |
| Porosity in surfacing layer | Moisture contamination; improper arc length | Reduced density; reduced wear resistance | Dry electrode storage; proper arc length control |
| Incomplete fusion | Insufficient heat input; surface contamination | Delamination; premature failure | Surface cleaning; adequate heat input |
| Excessive dilution | High welding current; thin surfacing layer | Reduced hardness; reduced wear resistance | Optimized welding parameters; multiple thin layers |
| Spalling of surfacing layer | Thermal expansion mismatch; high residual stress | Loss of wear protection | Controlled cooling rate; stress relief treatment |
Key Questions and Reflections
The development of K118 raises several important questions that are relevant to the broader field of wear-resistant surfacing materials:
- How does the microstructure of K118 evolve during service, and what is the mechanism of wear? Detailed tribological analysis, including surface profilometry and scanning electron microscopy of worn surfaces, would provide insights into the wear mechanism and help optimize the material design.
- What is the effect of the carbon arc process on the microstructure of the surfacing layer compared to other surfacing processes such as arc welding or flame spraying? The welding process can significantly influence the microstructure and properties of the surfacing layer, and a comparative study would be valuable.
- How does the performance of K118 compare with more modern wear-resistant surfacing materials developed in subsequent decades? The evolution of wear-resistant materials has been rapid, and a historical perspective on the development of K118 and its successors would be informative.
Study Insights and Implications
This paper represents an important contribution to the development of nickel-free wear-resistant surfacing materials for coal mining applications. The use of boron carbides as the primary hard phase, combined with a hypereutectic microstructure, demonstrates a novel approach to achieving high wear resistance without relying on expensive nickel.
The broader implication of this research is that the strategic importance of nickel-free materials extends beyond cost considerations to include supply chain security and environmental concerns. The avoidance of nickel in wear-resistant alloys reduces dependence on a strategically important metal and eliminates the potential for nickel-related health and environmental issues.
For engineers involved in the selection and application of wear-resistant surfacing materials, this paper provides a useful reference for understanding the relationship between microstructure and wear resistance, and the role of boron carbides in achieving high abrasion resistance.