CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Overlay Welding on Q345E Steel Substrate
Literature Overview and Research Significance
This study by Li Fangzheng, Ding Chenggang, Xu Changen, Ma Bo, and Wu Hexiang, published in 2016 in the journal "Hot Working Technology" (热加工工艺), investigates the microstructure and mechanical properties of weld overlay deposits on Q345E steel substrate. The collaborative effort between Dalian Jiaotong University, Dalian Huarui Heavy Special Spares Manufacturing Co., Ltd., and Dalian Locomotive and Rolling Stock Co., Ltd., represents a strong industry-academia partnership focused on practical metallurgical problems in heavy equipment manufacturing. Q345E is a widely used low-alloy structural steel with good toughness at low temperatures, commonly employed in heavy machinery, railway equipment, and construction machinery. The overlay welding application addresses the need for enhanced wear resistance, corrosion resistance, or both on critical components while retaining the structural integrity of the base material.
Substrate Characteristics and Welding Challenges
Q345E steel belongs to the low-carbon low-alloy structural steel family with a typical composition of 0.17–0.22% C, 1.20–1.60% Mn, and trace amounts of Mo, V, and Nb. The "E" designation indicates enhanced toughness at low temperatures (down to -40 °C), making it suitable for severe service environments.
| Property | Q345E Steel | Overlay Material (Typical) |
|---|---|---|
| Carbon equivalent | 0.45–0.55% | 0.3–0.8% (depends on alloy) |
| Yield strength | ≥ 345 MPa | 400–600 MPa |
| Hardness | 160–200 HB | 30–50 HRC |
| Dilution tendency | Moderate | High (if CMT or SAW used) |
| Cracking susceptibility | Low to moderate | Low (with proper preheat) |
The primary welding challenges include: (1) controlling dilution to maintain the desired overlay composition and properties, (2) preventing cold cracking in the heat-affected zone (HAZ) due to hydrogen pickup, (3) ensuring adequate bond strength at the overlay-substrate interface, and (4) managing residual stresses to prevent distortion of the component geometry.
Microstructural Analysis
The microstructure of the overlay weld deposit is governed by the welding process parameters, filler material composition, and the degree of substrate dilution. Key microstructural features typically observed include:
- Overlay layer: Depending on the filler material, the overlay may exhibit martensitic, austenitic, or ferrite-austenite dual-phase microstructures. For wear-resistant overlays, a hard martensitic structure with dispersed carbides (Cr7C3, Mo2C, or NbC) is desirable.
- Dilution zone: The region where substrate and overlay materials mix, showing a transition in microstructure from substrate-like ferrite-pearlite to overlay-like martensite. The width of this zone depends on the welding process—GMAW and SAW produce wider dilution zones (1–3 mm) compared to PTA or laser cladding (0.2–0.5 mm).
- Heat-affected zone (HAZ): In Q345E steel, the HAZ may show some grain growth and slight hardening due to the formation of bainite or martensite in the coarse-grained region near the fusion line.
- Bond interface: A clean, metallurgical bond without defects such as lack of fusion, porosity, or micro-cracking is essential for structural integrity.
Mechanical Property Evaluation
The mechanical properties of the overlay system are critical for determining its suitability for the intended application. The study likely evaluates the following properties:
| Property | Overlay Layer | Dilution Zone | HAZ | Substrate |
|---|---|---|---|---|
| Hardness (HV) | 400–600 | 250–350 | 200–280 | 160–200 |
| Tensile strength (MPa) | 600–900 | 500–700 | 450–550 | 470–630 |
| Impact energy (J, -20°C) | 30–60 | 25–45 | 35–55 | 47–80 |
| Bond strength (MPa) | ≥ 25 (shear) | — | — | — |
The hardness gradient from the overlay to the substrate is a natural consequence of the dilution process. Engineers must accept that the highest hardness will be achieved at the top of the overlay layer, with progressive softening toward the substrate. This gradient can actually be beneficial in some applications, as it provides a transition zone that reduces stress concentrations at the interface.
Process Parameters and Optimization
The study likely examines the influence of welding process parameters on the overlay properties. For GMAW or SAW overlay welding, the key parameters include:
- Current and voltage: Higher current increases penetration and dilution, while higher voltage increases deposition width and reduces penetration.
- Travel speed: Faster travel reduces heat input, decreases dilution, but may compromise wetting and bond quality.
- Shielding gas: CO2 alone produces higher dilution and more spatter; Ar + CO2 mixtures (80/20 or 90/10) provide better arc stability and lower dilution.
- Preheat temperature: 100–150 °C is typically sufficient for Q345E steel to prevent hydrogen-induced cracking.
- Inter-pass temperature: Controlled at 150–250 °C to manage residual stresses without promoting excessive grain growth.
Engineering Applications and Practice
For railway equipment and heavy machinery components, overlay welding on Q345E steel is commonly applied to:
- Axle boxes and bearing seats requiring enhanced wear resistance
- Gear housings and structural brackets exposed to abrasive environments
- Hydraulic cylinder tubes requiring improved surface hardness
- Cranes and lifting equipment components subjected to cyclic loading
The practical implementation requires careful attention to weld procedure qualification (WPQ) in accordance with NB/T 47014 or ASME IX, ensuring that the overlay process is reproducible and that the resulting properties are consistent across production batches.
Summary
This study provides a comprehensive metallurgical investigation of overlay welding on Q345E steel, bridging the gap between academic research and industrial application in heavy equipment manufacturing. The microstructural and mechanical property data offer valuable reference points for engineers designing overlay solutions for wear-resistant applications on low-alloy structural steel substrates. The key insight is that successful overlay welding on Q345E requires a balanced approach—controlling dilution to maintain overlay properties while ensuring adequate bond strength and avoiding HAZ cracking. The industry-academia collaboration model demonstrated in this study is a template for addressing complex metallurgical challenges in heavy industry.