CLADDING TECHNOLOGY SHANXI CO., LTDCarbon Arc Surfcacing on Scraper Conveyor Middle Trough Test Study
Literature Overview
This 1983 study from the Zhangjiakou Coal Mining Machinery Factory and the Harbin Welding Research Institute documents an early systematic trial of carbon arc surfacing (oxy-acetylene-based carbon arc overlay) applied to the wear surfaces of scraper conveyor middle troughs used in underground coal mining. The publication appeared in the journal Coal Science and Technology, and it represents one of the earliest Chinese engineering attempts to extend the service life of heavy-duty conveyor troughs through weld overlay rather than through full component replacement.
Background and Engineering Motivation
The scraper conveyor middle trough is a heavily loaded structural component in longwall mining systems. It is subjected to continuous abrasive wear from coal, rock fragments, and the scraper chain, often in the presence of moisture and fine particulate matter. In the early 1980s, the standard practice was to replace worn troughs entirely, which was expensive and time-consuming. The carbon arc surfacing process offered a lower-cost alternative because it required only a flame and a carbon rod, with consumable overlay wire being the primary material cost. The trial aimed to verify whether the process could deliver a wear-resistant surface layer with adequate bond strength to the carbon steel base metal under the harsh conditions of underground mining.
Process Description and Key Parameters
Carbon arc surfacing, sometimes referred to as carbon arc gouging overlay, uses an electric arc struck between a carbon electrode and the base metal to create a local melt pool. Overlay wire is then fed into the molten zone. Unlike gas-shielded processes, there is no external shielding gas; the carbon electrode itself serves as both the heat source and a flux-like modifier. The process is relatively simple to set up and requires no complex equipment, which made it attractive for field repair and in-situ application in mining environments.
| Parameter | Typical Range Observed |
|---|---|
| Carbon electrode diameter | 20–35 mm |
| Overlay wire | Cast iron or high-carbon steel wire |
| Arc voltage | 25–40 V |
| Travel speed | 50–150 mm/min |
| Layer thickness per pass | 2–5 mm |
| Base metal | Q235 or Q345 carbon steel |
Microstructure and Wear Performance
The study examined the microstructure of the overlay layer and the dilution zone at the interface with the base metal. Carbon arc surfacing typically produces a layer with a higher carbon content than the base metal, often resulting in a cast iron-like microstructure with graphite or carbide phases depending on the wire composition. The wear resistance improvement was evaluated through abrasion tests simulating coal and rock contact. The results indicated that overlay layers with controlled carbon content could achieve a 2 to 3 times improvement in wear life compared to bare carbon steel troughs.
However, the study also identified several challenges. The carbon arc process introduces a high carbon content at the dilution interface, which can create a brittle zone susceptible to cracking under impact loading. The lack of shielding gas leads to oxidation and nitridation of the overlay surface, which can reduce the effective wear resistance. The study recommended multiple thin passes rather than a single thick pass to minimize dilution and improve layer uniformity.
Defect Analysis and Countermeasures
The following defects were documented during the trial:
| Defect Type | Cause | Countermeasure |
|---|---|---|
| Cracking at overlay-base interface | High carbon dilution, rapid cooling | Preheat base metal to 150–250°C; use multiple thin passes |
| Surface porosity | Oxidation and gas entrapment | Clean base metal thoroughly; control wire feed rate |
| Uneven layer thickness | Manual travel speed variation | Use semi-automatic wire feeding; practice consistent travel |
| Excessive dilution | High heat input per pass | Reduce arc voltage; increase travel speed; use smaller electrode |
Engineering Practice Insights
This study is historically significant because it demonstrates the practical viability of a low-cost overlay process for heavy-duty mining equipment. The simplicity of carbon arc surfacing made it accessible to field maintenance crews without specialized welding training. However, the study also highlights the limitations of the process: the lack of shielding, the difficulty in controlling dilution, and the potential for interface brittleness. For modern applications, these same troughs might be repaired using GMAW or FCAW overlay with flux-cored wire, which provides better control over dilution and shielding. Nevertheless, in remote mining locations where equipment availability is limited, carbon arc surfacing remains a viable backup option.
Reflections and Implications
Reading this 1983 study in the context of current practice, I am struck by how much the overlay field has advanced, yet also how many fundamental principles remain unchanged. The challenge of controlling dilution at the overlay-base interface is just as critical today as it was forty years ago. The study's emphasis on multiple thin passes and preheating is still standard practice in modern overlay welding. The key lesson for today's engineers is that process simplicity does not necessarily equate to process inferiority; carbon arc surfacing, despite its limitations, delivers a functional solution for wear repair in demanding environments. The study also underscores the importance of systematic trial and evaluation before adopting a new process, a principle that remains central to engineering practice.