Microstructural Characteristics and Sliding Wear Performance of High-Chromium Clad Wear-Resistant Plates for Scraper Conveyor Middle Troughs

Literature Overview and Background

Scraper conveyors are critical material handling equipment in coal mining, cement, and bulk material processing industries. The middle trough, which forms the main body of the conveyor, is subjected to severe abrasive wear from the transported material and the scraper chains. To extend service life, high-chromium cast iron or high-chromium alloy overlay cladding is applied to the wear surfaces. This study investigates the microstructural characteristics of high-chromium cladded wear-resistant plates and evaluates their sliding wear performance under simulated service conditions, providing valuable guidance for material selection and cladding process optimization.

Core Technical Points on Microstructure

Carbide Morphology and Distribution

The wear resistance of high-chromium cladding alloys is predominantly governed by the type, size, shape, and volume fraction of carbides formed during solidification. In high-chromium alloys containing 10 to 30 percent Cr, the primary carbides are M7C3 (chromium carbide) and, in higher chromium compositions, M23C6 and MC (chromium-rich carbides).

The cladding process, whether performed by submerged arc welding (SAW), gas metal arc welding (GMAW), or plasma transferred arc (PTA) welding, significantly influences the solidification rate and consequently the carbide morphology. Rapid solidification during single-pass cladding produces fine, uniformly distributed carbides, while multi-pass cladding with lower cooling rates can lead to carbide coarsening and network formation at the interdendritic regions.

Matrix Microstructure

The matrix surrounding the carbides consists of martensite in as-welded conditions for high-carbon high-chromium alloys. The martensitic transformation is driven by the high carbon equivalent and the rapid cooling inherent to the welding process. Tempering or post-weld heat treatment converts the retained austenite and reduces internal stresses, but excessive tempering above 600 degrees Celsius can lead to carbide coarsening and significant hardness loss.

Interface Between Cladding and Substrate

The transition zone between the high-chromium cladding and the carbon steel substrate is a critical region for both mechanical integrity and wear performance. Dilution from the base metal reduces the chromium and carbon content at the interface, resulting in softer, carbide-free zones that are susceptible to preferential wear. Proper cladding design requires a minimum of 2 to 3 mm of undiluted cladding material above the dilution zone to ensure effective wear protection.

Sliding Wear Performance Evaluation

Test Methodology and Results

Sliding wear tests were conducted per ASTM G99 using pin-on-disk methodology with alumina (Al2O3) and silicon carbide (SiC) counterfaces at normal loads of 10 to 50 N and sliding speeds of 0.1 to 0.5 m/s. The wear rate was calculated from mass loss measurements:

• The high-chromium cladding with 20 percent Cr and 4 percent C exhibited a specific wear rate of 0.08 to 0.15 mm3/N.m, representing a 4 to 6 times improvement over unclad Q235 carbon steel.
• Increasing the sliding speed from 0.1 to 0.5 m/s increased the wear rate by 30 to 50 percent due to thermal softening of the martensitic matrix at elevated interface temperatures.
• Higher normal loads (above 30 N) caused a transition from mild abrasive wear to severe abrasive wear, with evidence of matrix cracking and carbide pull-out observed in scanning electron microscopy (SEM) analysis.

Wear Mechanism Analysis

The dominant wear mechanism under dry sliding conditions is abrasive wear, with two-body abrasion from hard asperities on the counterface ploughing through the softer martensitic matrix. The hard carbides act as barriers to abrasive groove formation, but when the applied load exceeds the matrix support capacity, carbide pull-out occurs, exposing fresh material and accelerating wear. Under lubricated conditions, the wear mechanism shifts toward a combination of abrasive and adhesive wear, with oxide film formation providing partial protection.

Engineering Practice Integration

Cladding Process Selection for Scraper Conveyor Applications

For scraper conveyor middle troughs, the following cladding approaches are commonly employed:

1. SAW overlay with flux-cored wire: Suitable for large flat surfaces, providing cladding thickness of 3 to 8 mm in 2 to 3 passes. Wire composition typically includes 20 to 25 percent Cr, 3.5 to 4.5 percent C, and 1 to 2 percent Mo.
2. GMAW spray transfer overlay: Offers better control of dilution (typically 15 to 25 percent) and is suitable for complex geometries. Multiple thin passes (1.5 to 2 mm each) are recommended.
3. Flame spraying or HVOF: Provides minimal dilution and uniform coating thickness of 0.5 to 2 mm, but requires additional substrate preparation and has lower bond strength compared to fusion cladding.

Quality Control Considerations

• Dilution rate should be verified through optical emission spectrometry (OES) at the cladding-substrate interface, with maximum allowable dilution of 25 percent for high-chromium alloys.
• Hardness testing per ASTM E10 should confirm minimum 55 HRC in the undiluted cladding region.
• Visual and magnetic particle inspection (MT) per JB/T 4730.4 should be performed to detect surface cracks and lack of fusion.
• Pull-off adhesion testing per ASTM C1024 should demonstrate minimum bond strength of 25 MPa.

Key Questions and Reflections

The study prompts important considerations regarding the trade-off between hardness and toughness in high-chromium cladding alloys. While increasing chromium and carbon content improves hardness and abrasive wear resistance, it simultaneously reduces impact toughness and increases susceptibility to brittle fracture under impact loading. In scraper conveyor applications, where the cladding is subjected to both abrasive sliding and occasional impact from material drops, a balanced composition is essential. The optimal Cr content appears to be in the 18 to 22 percent range, providing sufficient carbide volume fraction while maintaining acceptable toughness.

Furthermore, the effect of multi-pass welding on carbide coarsening deserves attention. Each subsequent pass reheats the previously deposited layer, potentially causing carbide spheroidization and coarsening. Process parameters such as interpass temperature control (maintained below 200 degrees Celsius) and appropriate pass sequencing can mitigate this effect.

Study Insights and Implications

The most valuable engineering insight from this study is the direct correlation between carbide morphology control and service life extension in scraper conveyor applications. By optimizing the cladding alloy composition to 20 percent Cr and 4 percent C, combined with controlled GMAW or SAW deposition parameters, service life improvements of 3 to 5 times compared to unclad carbon steel can be achieved. The key to successful implementation lies in maintaining low dilution rates, controlling interpass temperatures, and ensuring adequate cladding thickness to compensate for the inevitable dilution zone. Engineers should prioritize carbide morphology optimization over simple hardness maximization, as the former provides more durable and predictable wear performance in actual service conditions.