CLADDING TECHNOLOGY SHANXI CO., LTDBimetal Wear-Resistant Overlay Plates for Mining and Cement Applications
Literature Overview and Technical Context
Bimetal wear-resistant overlay plates are a critical solution for extending the service life of equipment components in the mining and cement industries, where abrasive wear is the dominant failure mode. These plates consist of a structural carbon steel or low-alloy steel base plate with a high-chromium cast iron or high-chromium alloy overlay layer deposited by flux-cored arc welding (FCAW) or submerged arc welding (SAW). The overlay layer typically achieves a hardness of HRC 58–65, providing excellent resistance to abrasive wear while the base plate maintains the structural integrity and weldability of the assembly.
Core Technical Requirements and Material Selection
The selection of the overlay material is driven by the specific wear conditions encountered in the application. For mining applications involving quartzite, granite, or other hard rock abrasives, high-chromium cast irons with hardness above HRC 60 are typically specified. For cement applications involving limestone, clinker, and gypsum, slightly lower hardness levels of HRC 58–62 may be sufficient, depending on the specific abrasives present.
Typical Material Properties
| Property | Base Plate (Q345R) | Overlay Layer (High-Cr) |
|---|---|---|
| Hardness | HB 170–210 | HRC 58–65 |
| Carbon content | 0.12–0.20% | 2.5–3.5% |
| Chromium content | 0.5–1.5% | 12–28% |
| Tensile strength | ≥470 MPa | Not applicable (brittle) |
| Impact toughness | ≥34 J (20°C) | Not applicable (brittle) |
The overlay layer is inherently brittle due to the high carbon and chromium content, which is necessary to achieve the required hardness through the formation of hard carbide phases. This brittleness means that the overlay layer cannot be relied upon for structural loading, and the base plate must be designed to carry all structural loads. The transition zone between the base plate and the overlay layer is a region of significant metallurgical complexity, where dilution creates a gradient of composition and properties.
Welding Process Parameters
| Parameter | FCAW (Open Arc) | SAW (Submerged Arc) |
|---|---|---|
| Current | 250–400 A | 350–600 A |
| Voltage | 25–35 V | 30–40 V |
| Travel speed | 150–300 mm/min | 200–400 mm/min |
| Wire diameter | 1.6–2.4 mm | 1.6–2.4 mm |
| Flux coverage | None (open arc) | Complete flux coverage |
| Typical dilution | 15–30% | 20–35% |
The choice between FCAW and SAW depends on the application requirements. FCAW offers greater flexibility and is suitable for smaller components or field repair, while SAW provides higher deposition rates and is preferred for large plate production. The open-arc FCAW process produces a characteristic surface pattern with transverse cracks, which is considered a normal process feature rather than a defect, as these micro-cracks are confined to the overlay layer and do not affect the functional performance of the plate.
Surface Crack Characteristics and Acceptance Criteria
The transverse cracks observed on the surface of high-chromium overlay plates are a well-documented phenomenon resulting from the high cooling rates, the formation of hard brittle carbides, and the thermal stresses induced during solidification. These cracks are typically shallow, confined to the upper portion of the overlay layer, and do not extend into the base plate. The acceptance criteria for these cracks are typically defined in the product specification or standard, with limits on crack depth, spacing, and density.
Acceptance Criteria for Surface Cracks
| Parameter | Typical Acceptance Limit |
|---|---|
| Maximum crack depth | ≤ 1/3 of overlay thickness |
| Maximum crack spacing | ≤ 10 mm |
| Crack density | ≤ 3 cracks per 100 mm² |
| Crack extension into base | Not permitted |
It is important to distinguish between these normal process-induced cracks and true defects such as incomplete fusion or base metal cracking. The former are superficial and do not affect the wear resistance of the overlay layer, while the latter indicate a fundamental process failure that requires corrective action.
Common Defects and Countermeasures
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Excessive dilution | Too high heat input, too fast travel | Reduce current, increase travel speed, use appropriate wire diameter |
| Base metal cracking | Excessive preheat, high carbon content base | Reduce preheat, use low-carbon base plate, post-weld stress relief |
| Poor bond strength | Incomplete fusion, surface contamination | Proper surface preparation, adequate current, single-layer first pass |
| Excessive spatter | Too high current, incorrect gas shielding | Optimize current, ensure proper gas flow, use appropriate nozzle |
| Uneven overlay thickness | Travel speed variation, wire feed inconsistency | Stable travel speed, consistent wire feed, proper gun angle |
Engineering Practice and Application Cases
In the cement industry, bimetal wear-resistant overlay plates are widely used for溜槽 (chutes), 料斗 (hoppers), and 选粉机叶片 (classifier blades). The typical service life extension achieved by overlay plates compared to unclad carbon steel plates is 3–8 times, depending on the specific application and operating conditions. For example, a classifier blade made of Q345R steel with a 6 mm high-chromium overlay layer may achieve a service life of 6–8 months, compared to 1–2 months for an unclad plate of the same thickness.
A notable engineering consideration is the design of the plate geometry to minimize bending and impact loading on the overlay layer. Since the overlay layer is brittle, it cannot tolerate significant plastic deformation. The plate should be designed with adequate thickness of the base plate to prevent bending, and the overlay layer should be applied to surfaces that experience primarily compressive or shear loading rather than tensile loading.
Inspection and Quality Control
The quality of the overlay plate is verified through a combination of visual inspection, hardness testing, and thickness measurement. The hardness of the overlay layer is measured at multiple locations across the plate surface to ensure uniformity, with a typical acceptance range of HRC 58–65. The overlay thickness is measured by ultrasonic thickness gauging at regular intervals, with a typical minimum thickness of 3–6 mm depending on the application. The bond strength between the overlay layer and the base plate is verified by the absence of delamination or spalling during the hardness testing process.
Study Insights and Implications
The bimetal wear-resistant overlay plate technology represents a practical and cost-effective solution for extending the service life of equipment components in abrasive environments. The key to successful implementation lies in the careful selection of the overlay material, the precise control of welding parameters, and the proper design of the component geometry to minimize bending and impact loading on the brittle overlay layer. The characteristic surface cracks are a normal process feature that should be understood and accepted within the defined limits, rather than being treated as defects. Engineers working in this field must develop a thorough understanding of the metallurgical behavior of high-chromium alloys and the process parameters that influence the dilution rate, hardness, and bond strength of the overlay layer.