Zhuojin Pipe Fitting Co., LtdHardness Distribution and Impact Toughness of Q345 Steel-Based Overlay Composite Plates
Literature Overview
This paper by Zhang Ziling and colleagues from Hunan University of Science and Technology and Gree Electric Appliances, published in Arms Materials and Metallurgy (Vol. 43, No. 1, 2020), investigates the microstructure, hardness distribution, and impact toughness of Q345 steel-based overlay composite plates produced by gas metal arc welding (GMAW). The study employs optical microscopy (OM), Vickers hardness testing, Charpy impact testing, and scanning electron microscopy (SEM) to characterize the overlay composite plate from the base plate through the high-strength overlay layer to the surface.
The research is supported by the National Natural Science Foundation of China (Grant No. 51975207), the Hunan Provincial Natural Science Foundation Excellent Youth Project (2019JJ30010), and the Hunan Provincial Department of Education Funded Project (17B089). The collaboration between academia and industry (Gree Electric Appliances) suggests practical applications in the HVAC and refrigeration industry, where composite plates with combined strength and toughness properties are valuable.
Core Technical Findings
Microstructural Evolution
The study reveals a systematic microstructural evolution from the base plate through the overlay layer:
| Region | Microstructure | Formation Mechanism |
|---|---|---|
| Base plate (Q345) | Blocky ferrite + lamellar pearlite | Normalized microstructure of Q345 steel |
| Transition zone | Granular ferrite + pearlite | Partial recrystallization, modified cooling rate |
| Surface layer | Blocky ferrite + spheroidal graphite | High carbon enrichment, modified solidification |
The transition from lamellar pearlite in the base plate to granular ferrite and pearlite in the transition zone, and finally to blocky ferrite with spheroidal graphite at the surface, reflects the progressive influence of the welding thermal cycle on the microstructure. The spheroidal graphite at the surface is particularly notable, as it suggests significant carbon enrichment in the overlay layer, possibly due to the use of a high-carbon consumable or the interaction of the molten weld pool with the base material.
Hardness Distribution
The hardness profile exhibits a characteristic step-like distribution with a sharp increase (hardness jump) at the interface between the base plate and the overlay layer. The hardness distribution within the overlay layer is described as relatively uniform, indicating a consistent microstructure and composition throughout the overlay thickness.
| Region | Relative Hardness | Microstructural Basis |
|---|---|---|
| Base plate | Baseline (Q345 typical: 150–200 HV) | Blocky ferrite + lamellar pearlite |
| Transition zone | Intermediate | Granular ferrite + pearlite |
| Overlay layer | Elevated, uniform | Blocky ferrite + spheroidal graphite |
| Surface | Highest | Spheroidal graphite, possible carbide precipitation |
The hardness jump at the interface is a direct consequence of the microstructural transformation from the base plate microstructure to the overlay microstructure. The presence of spheroidal graphite at the surface contributes to the elevated hardness, as graphite particles act as hard phases within the ferrite matrix.
Impact Toughness
The impact toughness of the overlay composite plate reaches 64 J/cm², which is a significant value for a steel with elevated hardness. The fracture mode analysis reveals two distinct fracture behaviors:
- Normalized specimens: Ductile fracture with quasi-cleavage fracture features.
- Quenched specimens: Ductile fracture with quasi-cleavage fracture features.
The presence of quasi-cleavage fracture indicates that while the overall fracture behavior is ductile, there is some degree of brittle fracture contribution. This is consistent with the presence of hard phases (graphite, possible carbides) in the microstructure, which can act as crack initiation sites.
Process Analysis and Technical Interpretation
GMAW Overlay Process Parameters
The study uses GMAW (gas metal arc welding) for the overlay welding process. Key process parameters that influence the overlay microstructure and properties include:
| Parameter | Typical Range | Effect on Overlay |
|---|---|---|
| Current | 150–350 A | Higher current → deeper penetration, more dilution |
| Voltage | 20–30 V | Higher voltage → wider bead, shallower penetration |
| Travel speed | 100–400 mm/min | Faster speed → lower heat input, finer microstructure |
| Wire feed speed | 2–8 m/min | Higher speed → higher deposition rate |
| Shielding gas | CO₂, Ar, Ar/CO₂ mixtures | CO₂ → deeper penetration, more spatter; Ar → shallower, smoother |
| Interpass temperature | < 200°C (typical) | Higher temperature → coarser microstructure, lower hardness |
The study does not explicitly report all process parameters, but the resulting microstructure and properties suggest a moderate heat input with controlled interpass temperature. The uniform hardness distribution within the overlay layer indicates consistent process parameters throughout the deposition.
Dilution and Its Effects
The microstructural evolution from the base plate through the overlay layer is directly related to the dilution rate. The transition zone, where the microstructure changes from blocky ferrite and lamellar pearlite to granular ferrite and pearlite, represents the region of partial dilution. The surface layer, with blocky ferrite and spheroidal graphite, represents the region of minimal dilution, where the overlay alloy composition dominates.
The hardness jump at the interface is a direct indicator of the dilution rate. A sharper, more pronounced hardness jump indicates a lower dilution rate and a more distinct boundary between the base material and the overlay. Conversely, a gradual hardness increase indicates higher dilution and a more gradual transition.
Heat Treatment Effects
The study compares normalized and quenched specimens, revealing differences in fracture behavior. The normalized treatment likely produced a more uniform microstructure with reduced residual stresses, while the quenched treatment produced a harder, more brittle microstructure with higher residual stresses.
| Treatment | Microstructure | Hardness | Impact Toughness | Fracture Mode |
|---|---|---|---|---|
| As-welded | Heterogeneous, residual stresses | Moderate | Moderate | Mixed |
| Normalized | Homogenized, reduced stresses | Moderate | Higher | Ductile + quasi-cleavage |
| Quenched | Hardened, high stresses | Higher | Lower | Ductile + quasi-cleavage |
The observation that both normalized and quenched specimens exhibit ductile fracture with quasi-cleavage features suggests that the overlay layer retains a degree of toughness even after heat treatment. This is attributed to the presence of the ferrite matrix, which provides ductility, combined with the hard phases (graphite, carbides) that provide strength.
Engineering Practice Integration
Application Scenarios
The Q345 steel-based overlay composite plate is suitable for applications requiring a combination of base plate strength and overlay layer wear resistance or corrosion resistance:
| Application | Base Plate Requirement | Overlay Requirement | Process Suitability |
|---|---|---|---|
| HVAC compressor components | Structural strength, weldability | Wear resistance | Good - Q345 base, hard overlay |
| Refrigeration equipment | Low-temperature toughness | Corrosion resistance | Good - composite plate properties |
| Mining equipment | Impact resistance | Abrasion resistance | Good - high hardness overlay |
| Construction machinery | Structural strength | Wear resistance | Good - composite plate properties |
Quality Control Considerations
The production of overlay composite plates requires careful quality control at multiple stages:
- Base plate preparation: Surface cleanliness, dimensional accuracy, and material certification are essential. The base plate should be free from surface defects, scale, and contamination.
- Welding process control: Process parameters must be maintained within specified ranges to ensure consistent overlay properties. Real-time monitoring of current, voltage, wire feed speed, and travel speed is recommended.
- Post-weld inspection:
- Visual inspection for surface defects (cracks, porosity, undercut).
- Hardness mapping to verify hardness profile and uniformity.
- Impact testing to verify toughness requirements.
- Microstructural examination to verify microstructural evolution.
- Heat treatment control: If post-weld heat treatment is required, the treatment parameters (temperature, time, cooling rate) must be carefully controlled to achieve the desired properties without introducing detrimental effects.
FMEA for Overlay Composite Plate Production
A Failure Modes and Effects Analysis (FMEA) for the overlay composite plate production process identifies the following critical failure modes:
| Failure Mode | Cause | Effect | Severity | Detection | Prevention |
|---|---|---|---|---|---|
| Overlay cracking | Excessive residual stress, low toughness | Component failure | High | Visual, MT | Reduce heat input, control interpass temperature |
| Excessive dilution | High heat input, slow travel speed | Reduced overlay properties | Medium | Hardness mapping | Optimize process parameters |
| Incomplete bonding | Poor surface preparation, low heat input | Delamination | High | UT, peel test | Surface cleaning, adequate heat input |
| Hardness non-uniformity | Parameter variation, consumable inconsistency | Property variation | Medium | Hardness mapping | Process control, consumable quality |
| Surface porosity | Gas entrapment, contamination | Reduced surface integrity | Low | Visual, PT | Shielding gas control, surface cleaning |
Key Questions and Reflections
The Role of Spheroidal Graphite
The presence of spheroidal graphite in the surface layer is an unusual finding for a steel overlay produced by GMAW. Spheroidal graphite is typically associated with ductile iron (nodular cast iron), where it is produced through controlled graphitization during casting. In a weld overlay, the formation of spheroidal graphite suggests either:
- The use of a high-carbon consumable that promotes graphite precipitation during solidification.
- A post-weld heat treatment that promotes graphitization (though the study does not explicitly mention such treatment for the as-welded specimens).
- The interaction of the molten weld pool with the base material, leading to local carbon enrichment and graphite precipitation.
The formation of spheroidal graphite in a weld overlay is of considerable interest, as it could provide a unique combination of hardness and toughness that is not achievable with conventional overlay alloys. However, the stability of the spheroidal graphite under service conditions (particularly thermal cycling and mechanical loading) requires further investigation.
Dilution Rate Optimization
The hardness jump at the base plate-overlay interface is a direct indicator of the dilution rate. For applications requiring maximum overlay properties, a low dilution rate is desirable, as it minimizes the influence of the base material on the overlay composition and microstructure. However, a very low dilution rate may result in poor metallurgical bonding between the base plate and the overlay, leading to delamination under service loading.
The optimal dilution rate is a balance between overlay property retention and bonding integrity. For the Q345 steel-based composite plate, a moderate dilution rate (20–40%) may be optimal, providing sufficient bonding while maintaining the desired overlay properties.
Impact of Heat Treatment on Service Performance
The comparison of normalized and quenched specimens reveals that heat treatment can significantly influence the impact toughness and fracture behavior of the overlay composite plate. For applications requiring high impact toughness, normalization may be the preferred heat treatment, as it reduces residual stresses and promotes a more uniform microstructure. For applications requiring maximum hardness, quenching may be appropriate, but the reduced toughness must be considered.
The choice of heat treatment should be based on the specific service requirements of the component. For example, a compressor component in an HVAC system may require high impact toughness to resist impact loading, while a mining equipment component may require maximum hardness to resist abrasion.
Study Insights and Implications
The paper provides a comprehensive characterization of the microstructure, hardness distribution, and impact toughness of a Q345 steel-based overlay composite plate produced by GMAW. The systematic evolution of microstructure from the base plate through the overlay layer, the characteristic hardness jump at the interface, and the high impact toughness (64 J/cm²) are all significant findings with practical implications for the design and application of overlay composite plates.
The presence of spheroidal graphite in the surface layer is a particularly interesting finding that warrants further investigation. If the formation of spheroidal graphite can be reliably controlled, it could provide a unique combination of hardness and toughness that is not achievable with conventional overlay alloys. This could open new application areas for overlay composite plates, particularly in applications requiring both wear resistance and impact toughness.
For engineers involved in the design and fabrication of overlay composite plates, the study provides valuable guidance on the relationship between process parameters, microstructure, and mechanical properties. The emphasis on the hardness jump at the interface and the impact of heat treatment on fracture behavior are particularly relevant for quality control and process optimization.
The research methodology, combining microstructural analysis, hardness mapping, impact testing, and fracture surface analysis, provides a comprehensive characterization approach that can be applied to other overlay welding applications. The findings contribute to the broader understanding of overlay welding metallurgy and provide a foundation for developing new overlay processes and materials for demanding industrial applications.
This study represents a meaningful contribution to the field of overlay welding technology and provides practical insights for the production of high-performance overlay composite plates.