Zhuojin Pipe Fitting Co., LtdMicrostructure and Properties of Overlay Deposits on Q345E Steel Substrate A Comparative Study of Two Consumable Types
Literature Overview
This 2016 paper by Li Fangzheng and co-authors from Dalian Jiaotong University, Dalian Huarui Heavy Industry, and Dalian Locomotive and Rolling Stock Co., Ltd., published in Hot Working Technology, presents a systematic metallurgical investigation of two different overlay consumables applied to Q345E low-alloy steel. The study compares RD-YD414(Q) and SHS9700U16 welding wires, examining their microstructure, hardness distribution, and wear resistance to provide guidance for material selection in overlay applications.
Substrate and Application Context
Q345E is a low-alloy structural steel widely used in heavy machinery, railway vehicles, and industrial equipment in China. The "E" designation indicates improved low-temperature impact properties, making it suitable for cold-climate applications. When this steel is exposed to abrasive wear conditions, overlay hardfacing becomes a practical solution for extending component life without complete replacement.
The substrate characteristics relevant to overlay welding include:
| Substrate Property | Q345E Specification |
|---|---|
| Yield strength | ≥345 MPa |
| Carbon equivalent | 0.40–0.55% |
| Hardness | 150–200 HB |
| Dilution potential | Moderate to high |
| Preheat requirement | 100–200°C |
| PWHT requirement | Recommended for thick sections |
The moderate carbon equivalent of Q345E means that cracking susceptibility in the heat-affected zone is a concern, particularly for thick sections or in cold conditions. This influences the welding procedure design and the choice of overlay consumables.
Consumable Comparison and Results
The two consumables investigated represent fundamentally different alloy systems:
| Property | RD-YD414(Q) | SHS9700U16 |
|---|---|---|
| Alloy system | Martensitic (Fe-Cr-C) | Boron carbide composite |
| Microstructure | Lath martensite + ferrite + bainite + carbides | Ledeburite matrix + lamellar cementite + boron carbides + intermetallics |
| Average hardness | 41.0 HRC | 63.5 HRC |
| Wear resistance | Moderate | Excellent |
| Toughness | Relatively good | Lower (brittle carbide-rich structure) |
| Crack resistance | Good | Limited |
| Application suitability | Moderate abrasion, impact loading | Severe abrasion, low impact |
The microstructural analysis reveals the fundamental differences in wear resistance mechanisms. RD-YD414(Q) achieves its moderate hardness through martensitic transformation, with carbide precipitation providing secondary hardening. The SHS9700U16 consumable relies on a matrix of extremely hard boron carbides and complex intermetallic compounds embedded in a ledeburite matrix, achieving very high hardness at the expense of toughness.
Microstructural Analysis
The paper provides detailed metallographic observations that are instructive for understanding overlay metallurgy:
For RD-YD414(Q), the overlay microstructure consists of:
- Lath martensite as the primary matrix phase, formed by rapid cooling from the austenite region.
- Retained ferrite at grain boundaries, resulting from incomplete transformation.
- Bainite in regions of slower cooling or higher dilution.
- Discrete carbide particles (primarily Cr7C3 and Fe3C) providing dispersion strengthening.
For SHS9700U16, the overlay microstructure consists of:
- A ledeburite-type matrix formed by the eutectic reaction of the high-carbon composition.
- Lamellar cementite (Fe3C) distributed throughout the matrix.
- Fine, complex boron carbide (B4C or B4C-Fe2B) particles providing extreme hardness.
- Iron-boron intermetallic compounds contributing to the overall hard phase fraction.
The hardness distribution within both overlay layers was found to be relatively uniform, indicating consistent welding parameters and good process control. However, the transition zone between the overlay and the substrate showed a hardness gradient, which is typical of arc welding overlay processes and represents the dilution zone.
Wear Test Results and Interpretation
The wear resistance comparison confirms the hardness-wear resistance correlation for these material systems. SHS9700U16 demonstrated significantly superior wear resistance compared to RD-YD414(Q), consistent with its much higher hardness (63.5 vs. 41.0 HRC). The wear mechanism for both materials was primarily abrasive, with material removal occurring through micro-ploughing and micro-cutting by the abrasive counterface.
The practical implication is clear: for severe abrasive wear applications where impact loading is minimal, SHS9700U16 is the superior choice. However, for applications involving combined abrasion and impact (such as crusher jaws, excavator bucket teeth, or railway components), RD-YD414(Q) may be more appropriate due to its better toughness and crack resistance.
Engineering Practice and Material Selection Guidance
This study provides valuable data for engineers making overlay material selections. The key decision factors include:
- Wear mechanism: Abrasive (high hardness preferred) vs. impact-abrasive (toughness required).
- Operating temperature: High temperatures may cause softening of martensitic overlays.
- Substrate constraints: Preheating and PWHT availability affect process selection.
- Overlay thickness requirements: Multi-pass welding may be needed for thick overlays.
- Service life expectations: Higher-cost materials may justify their premium through extended service intervals.
The paper demonstrates that overlay material selection is not simply a matter of choosing the hardest material available. The balance between hardness, toughness, and cost must be optimized for each specific application. Engineers should always perform wear testing representative of actual service conditions before committing to a material selection for critical components.