Zhuojin Pipe Fitting Co., LtdHardness Distribution and Impact Toughness of Q345 Steel-Based Overlay Composite Plates
Literature Overview
This research paper by Zhang Ziling, Lu Liwei, Li Yiquan, Yang Yuankai, Tan Yi, Fu Mingzhu, and Ma Min from Hunan University of Science and Technology, in collaboration with Zhuhai Gree Electric Appliances Inc., investigates the mechanical properties and microstructural characteristics of Q345 steel-based overlay composite plates. Published in the journal Ordnance Materials and Science (Volume 43, Issue 1, 2020, pages 57-61), the study provides valuable data on hardness gradients, impact toughness, and fracture behavior in a practical overlay welding application. The work was supported by the National Natural Science Foundation of China (Grant 51975207), the Hunan Provincial Natural Science Foundation Excellent Youth Project (2019JJ30010), and a Hunan Provincial Department of Education grant (17B089).
Experimental Methodology and Materials
The study employed gas metal arc welding (GMAW) with flux-cored or solid wire to deposit a high-strength overlay layer onto Q345 structural steel base plates. The characterization methods included:
| Characterization Method | Equipment | Purpose |
|---|---|---|
| Optical microscopy | Optical microscope | Microstructural analysis |
| Vickers hardness testing | Vickers hardness tester | Hardness distribution measurement |
| Impact testing | Charpy impact testing machine | Toughness evaluation |
| Scanning electron microscopy | SEM | Fracture surface analysis |
The Q345 base steel is a widely used low-carbon low-alloy structural steel with a typical composition of approximately 0.17-0.22% C, 1.20-1.60% Mn, 0.50-1.60% Si, with small amounts of Nb, V, and Ti. Its base microstructure consists of blocky ferrite and band-shaped pearlite, providing good formability and weldability.
Microstructural Evolution Across the Overlay Interface
The study reveals a progressive microstructural transition from the base plate through the overlay layer:
- Base plate region: Blocky ferrite with band-shaped pearlite, characteristic of the as-rolled Q345 microstructure.
- Transition region: A gradual change from blocky ferrite and band-shaped pearlite to granular ferrite and pearlite, reflecting the thermal effects of welding on the base metal microstructure.
- Overlay layer: The deposited metal exhibits a microstructure of granular ferrite and pearlite in the lower portion, transitioning to blocky ferrite and flocculent graphite in the surface layer.
The presence of flocculent graphite in the surface layer is a notable finding that suggests the overlay alloy formulation may include graphite-forming elements or that the solidification conditions promoted graphite precipitation. This microstructural feature contributes to the hardness profile and potentially affects the wear resistance and machinability of the overlay surface.
Hardness Distribution Analysis
The hardness distribution across the composite plate shows a distinct step-like increase from the base plate to the overlay layer:
| Region | Relative Hardness Trend | Description |
|---|---|---|
| Base plate (Q345) | Baseline | Typical Q345 hardness range |
| Transition zone | Gradual increase | Thermal-affected zone effects |
| Overlay layer | Significant step increase | High-strength deposited metal |
| Surface layer | Uniform high hardness | Hardened overlay surface |
The hardness transition exhibits a "leap step" characteristic, meaning there is a relatively sharp increase in hardness at the interface between the base plate and the overlay layer. Despite this sharp transition, the hardness distribution within the overlay layer itself is described as relatively uniform, indicating good process control and consistent deposit composition.
Impact Toughness and Fracture Behavior
The impact toughness results are particularly encouraging for engineering applications:
- Impact toughness value: 64 J/cm², which is a high value indicating good toughness for a high-hardness overlay composite plate.
- Normalized specimen fracture mode: Ductile fracture with quasi-cleavage features, indicating a mix of ductile and brittle fracture mechanisms.
- Quenched specimen fracture mode: Similar ductile fracture with quasi-cleavage characteristics.
The achievement of high hardness combined with good impact toughness is significant because these two properties are typically inversely related in steel materials. The overlay process has successfully created a composite structure that combines the strength and hardness of the deposited layer with the toughness of the base material, creating a material system suitable for applications requiring both wear resistance and impact resistance.
Engineering Practice Implications
For engineers designing overlay composite plate systems, the following points emerge from this research:
- Material pairing: The Q345 base steel provides an excellent substrate for overlay welding due to its good weldability, adequate toughness, and widespread availability. The selection of overlay alloy composition is critical to achieving the desired hardness-toughness balance.
- Heat treatment effects: Both normalization and quenching treatments were evaluated, and both produced acceptable impact fracture behavior. This suggests that post-weld heat treatment is not strictly necessary for achieving good toughness, although it may be beneficial for optimizing hardness or relieving residual stresses.
- Quality assurance: The study reports good surface quality with no obvious welding defects, emphasizing the importance of proper welding procedure development, operator training, and in-process monitoring.
- Application potential: The combination of high hardness and good impact toughness makes this composite plate system suitable for applications in mining equipment, construction machinery, and industrial components that experience both abrasive wear and impact loading.
Study Insights and Reflections
This research demonstrates a practical approach to creating high-performance composite plates through overlay welding, with particular attention to the mechanical property balance between hardness and toughness. The 64 J/cm² impact toughness value is a strong indicator of the material system's suitability for demanding service conditions. The microstructural analysis reveals that the overlay welding process can create a gradient in microstructure that contributes to the overall mechanical performance of the composite plate.
One area for further investigation would be the long-term mechanical property stability under cyclic loading or elevated temperature conditions, as the overlay layer may experience thermal cycling or fatigue in many practical applications. Additionally, the corrosion resistance of the composite interface under various environmental conditions would be valuable information for engineers selecting materials for corrosive environments. The research provides a solid foundation for further development of overlay composite plate technology, and the methodology employed—combining optical microscopy, hardness testing, impact testing, and SEM fracture analysis—represents a comprehensive characterization approach that other researchers can adopt for similar studies.