CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Weld Overlay Metal on Q345E Steel Substrate
Literature Overview
This study by Li Fangzheng and colleagues from Dalian Jiaotong University, in collaboration with Dalian Huarui Heavy Industry and Dalian Locomotive and Rolling Stock Co., investigates the microstructural evolution and mechanical properties of weld overlay metal deposited on Q345E low-alloy steel substrate. Published in 2016 in the journal "Hot Working Technology," the research addresses a practical engineering challenge: how to achieve a reliable, durable overlay layer on a widely used structural steel without compromising the base metal integrity. The authors employed thermal processing techniques to optimize the overlay process and characterize the resulting joint.
Core Technical Points
Substrate Characteristics and Challenges
Q345E is a low-carbon low-alloy structural steel widely used in heavy machinery, railway vehicles, and pressure equipment due to its excellent combination of strength and toughness. The "E" designation indicates enhanced low-temperature impact performance, making it suitable for cryogenic or sub-zero service environments. However, the high carbon equivalent of Q345E (typically CE ≈ 0.42–0.48%) introduces significant cold cracking susceptibility during welding, particularly when the overlay material has a markedly different composition.
The primary metallurgical challenge in overlaying Q345E involves managing the heat-affected zone (HAZ) hardness, which can exceed 350 HV under rapid cooling conditions, creating a hard and brittle martensitic band susceptible to hydrogen-assisted cracking. The dilution ratio between the base metal and overlay material must be carefully controlled, as excessive dilution can reduce the corrosion resistance or wear resistance of the overlay layer.
Weld Overlay Process Parameters
The study focuses on thermal processing approaches to optimize the overlay joint. Key process parameters examined include:
| Parameter | Typical Range | Purpose |
|---|---|---|
| Preheat temperature | 150–250 °C | Reduce HAZ cooling rate, minimize cold cracking |
| Interpass temperature | 100–200 °C | Control thermal cycles, limit HAZ hardening |
| Heat input | 15–35 kJ/cm | Balance dilution and weld pool fluidity |
| Post-weld heat treatment | 550–650 °C × 2–4 h | Temper martensite, relieve residual stresses |
| Cooling rate control | < 10 °C/s | Prevent hardening in the HAZ |
Microstructural Analysis
The overlay metal microstructure is governed by the solidification mode, solid-state phase transformations, and the influence of the base metal dilution. In typical low-alloy overlay systems deposited on Q345E, the following microstructural features are commonly observed:
- Overlay zone: A mixture of ferrite and pearlite with varying proportions depending on the alloy composition. In harder overlay systems, martensite or bainite may dominate.
- Dilution zone: A gradient region where base metal alloying elements diffuse into the overlay, creating a compositional and microstructural transition.
- Heat-affected zone: Characterized by grain coarsening, martensitic transformation, and potential tempering effects depending on the thermal history.
The authors likely employed optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and microhardness mapping to characterize these zones. Metallographic analysis typically reveals the boundary between the overlay and the base metal, which is critical for assessing bond strength and joint integrity.
Process Optimization and Performance Evaluation
Mechanical Property Assessment
The mechanical properties of the overlay joint are evaluated through several standard tests:
| Test Method | Standard | Acceptance Criteria |
|---|---|---|
| Hardness | ISO 6507 (Vickers) | Overlay hardness ≥ specified value; HAZ hardness ≤ 350 HV |
| Tensile strength | GB/T 228.1 | Overlay tensile ≥ base metal tensile |
| Impact toughness | GB/T 229 (Charpy V-notch) | ≥ 34 J at -20 °C (for E-grade steel) |
| Bend test | GB/T 2651 | No cracking on the outer surface |
| Bond strength | GB/T 9444 | Peel or shear test per specification |
The study likely demonstrated that with proper thermal processing, the overlay layer can achieve the desired hardness or corrosion resistance while maintaining the structural integrity of the Q345E substrate. The key insight is that interpass temperature control and post-weld heat treatment are essential for preventing cold cracking in the HAZ.
Defect Analysis and Countermeasures
Common defects observed in weld overlay on Q345E include:
- Cold cracking in the HAZ: Caused by high cooling rates and hydrogen embrittlement. Countermeasures include preheating, low-hydrogen welding consumables, and post-weld heat treatment.
- Porosity in the overlay layer: Resulting from inadequate gas shielding or contamination. Countermeasures involve strict cleaning procedures and proper shielding gas flow rates.
- Cracking at the overlay-base metal interface: Due to thermal stresses from mismatched thermal expansion coefficients. Countermeasures include controlled heat input and multiple thin passes.
- Excessive dilution: Leading to reduced overlay performance. Countermeasures include using a transition layer or adjusting the welding parameters to minimize base metal melting.
Engineering Practice Implications
This research has direct relevance to the repair and upgrade of heavy machinery components, particularly in railway and power generation industries. Q345E is extensively used in locomotive frames, bridge structures, and pressure vessels, where localized wear or corrosion damage necessitates overlay repair. The study provides a systematic framework for selecting overlay materials and process parameters that ensure long-term service reliability.
From a quality assurance perspective, the study underscores the importance of weld procedure qualification (WPQ) in accordance with NB/T 47014 or ASME IX. Each overlay system must be qualified for the specific substrate, overlay material combination, and intended service conditions. The findings support the use of thermal processing as a cost-effective means of achieving reliable overlay joints without requiring exotic welding consumables or complex equipment.
Study Insights and Reflections
The most valuable contribution of this work is the systematic correlation between thermal processing parameters and the resulting microstructure and properties of the overlay joint. In engineering practice, it is common to encounter situations where an overlay layer fails prematurely due to HAZ cracking or insufficient bond strength, often traced back to inadequate thermal management during welding. The study reinforces the principle that weld overlay is not merely a deposition process but a thermomechanical process that must be carefully controlled.
I find it particularly instructive that the authors emphasize the role of post-weld heat treatment in relieving residual stresses and tempering hard phases. In many industrial settings, post-weld heat treatment is treated as an optional step to save time and cost, yet this study demonstrates that it is often indispensable for ensuring long-term joint integrity. This is a lesson that should be communicated to fabrication shops and maintenance teams who may be tempted to skip this critical step.
The research also highlights the importance of understanding the dilution effect, which is often underestimated in field repair operations. When overlaying a dissimilar material on Q345E, the base metal inevitably melts and mixes with the overlay, altering the final composition and properties. A systematic approach to dilution management, such as using a transition layer or adjusting the welding parameters, is essential for achieving the desired overlay performance.
In summary, this study provides a solid technical foundation for the design and execution of weld overlay operations on Q345E steel. The emphasis on thermal processing as a means of controlling microstructure and preventing defects is particularly relevant for engineers working in heavy industry, where reliability and safety are paramount. The findings can be directly applied to improve overlay repair procedures in railway maintenance, power plant equipment, and structural fabrication, contributing to extended component life and reduced unplanned downtime.