Zhuojin Pipe Fitting Co., LtdMicrostructural Evolution and Mechanical Properties of 35CrMo Gear Hardfacing Weld Joint
Literature Overview
This paper by Ma Zhipeng and colleagues, published in Chemical Engineering Machinery in 2017 (Vol. 44, No. 3, pp. 271-275), investigates the microstructure and mechanical properties of a hardfacing weld joint produced on 35CrMo gear steel using CO2 gas-shielded arc welding (GMAW-CO2) with NER70S-6 welding wire. The research is supported by the Heilongjiang Provincial Applied Technology Research and Development Program (GA13A402) and the State Key Laboratory of Novel Brazing Materials and Technologies open research fund (SKLABFMT-2015-04). This study is particularly relevant to the repair and enhancement of heavy-duty gear components used in petrochemical and oil industry equipment, where 35CrMo is a widely used alloy steel grade.
Core Technical Findings
The authors demonstrate that using NER70S-6 welding wire as the filler metal produces a good quality hardfacing weld joint on 35CrMo gear steel. The microstructural analysis reveals several important features: columnar grains are formed at the weld interface and grow connected with the grains in the fusion zone. At locations further from the weld, side plate ferrite, acicular ferrite, and Widmanstätten structures are observed. In the normalized zone of the heat-affected zone (HAZ), fine pearlite and ferrite structures are produced through two-stage recrystallization.
Microstructural Zones and Characteristics
| Zone | Microstructure | Hardness (HV) | Key Features |
|---|---|---|---|
| Weld Metal (center) | Columnar dendrites | Up to HV220 | NER70S-6 composition, good toughness |
| Fusion Zone | Columnar grains connected to weld | HV180-220 | Grain growth at interface |
| Fine-grained HAZ (normalized) | Fine pearlite and ferrite | ~HV200 | Two-stage recrystallization |
| Coarse-grained HAZ | Side plate ferrite, acicular ferrite, Widmanstätten | ~HV200 | Prior austenite grain boundary effects |
| Base Metal 35CrMo | Pearlite and ferrite | ~HV180-200 | Normalized microstructure |
Process and Standards Analysis
The use of CO2 gas-shielded arc welding for hardfacing 35CrMo is a practical and economical choice. CO2 shielding gas provides good penetration and high deposition efficiency, making it suitable for building up material on thick gear components. The NER70S-6 wire is a low-carbon, low-alloy steel wire classified under ISO 14341-A and AWS A5.18 standards, with a yield strength of at least 690 MPa. This wire is designed for welding high-strength steels and provides good weld metal toughness even at elevated hardness levels.
35CrMo is a chromium-molybdenum alloy steel specified in GB/T 3077, with a typical composition of 0.32-0.40% C, 0.90-1.20% Cr, and 0.20-0.30% Mo. This steel is commonly used for gears, shafts, and high-strength structural components due to its excellent strength, toughness, and hardenability. The normalized microstructure of 35CrMo typically consists of fine pearlite and ferrite, with hardness in the range of 200-250 HV after normalizing.
Welding Process Parameters
- Welding process: CO2 gas-shielded arc welding (GMAW-CO2)
- Filler metal: NER70S-6 (low-carbon low-alloy steel wire)
- Base material: 35CrMo (Cr-Mo alloy steel, GB/T 3077)
- Shielding gas: CO2
- Key microstructural features: Columnar grains at interface, acicular ferrite in HAZ, Widmanstätten structures at distance from weld
Integration with Engineering Practice
In the context of gear repair and restoration, this study provides valuable guidance on the metallurgical compatibility between NER70S-6 weld metal and 35CrMo base steel. The observation that the weld metal achieves a hardness of up to HV220, which is slightly higher than the base metal, indicates that the hardfacing layer provides enhanced wear resistance without introducing excessive hardness mismatch that could lead to cracking.
The formation of acicular ferrite and side plate ferrite in the HAZ is particularly significant. Acicular ferrite is a fine-grained, interlocking microstructure that provides excellent toughness and resistance to hydrogen-induced cracking. Its presence in the HAZ suggests that the welding process parameters were well controlled to avoid excessive cooling rates that could lead to brittle martensite formation. However, the observation of Widmanstätten structures at locations further from the weld indicates that the cooling rate in those regions was slow enough to allow ferrite to grow along prior austenite grain boundaries, which can reduce toughness.
FMEA Analysis of Potential Defects
| Potential Defect | Cause | Detection Method | Countermeasure |
|---|---|---|---|
| Cracking in HAZ | Excessive cooling rate, hydrogen embrittlement | MT/PT, visual inspection | Preheating, controlled cooling, low-hydrogen wire |
| Hardness mismatch | Excessive dilution or insufficient dilution | Hardness traverse test | Optimize welding parameters, adjust wire composition |
| Porosity in weld | Gas entrapment, surface contamination | RT, UT | Clean surfaces, proper gas flow rate |
| Widmanstätten in HAZ | Slow cooling rate in thick sections | Metallographic examination | Increase preheat, reduce heat input |
Key Questions and Reflections
One important question arising from this study is the long-term fatigue performance of the hardfacing joint. Gears are subjected to cyclic loading, and the presence of a weld joint with a hardness gradient and microstructural transition zones could serve as a stress concentration site for fatigue crack initiation. The study does not address fatigue testing, which is a significant gap for a component like a gear where fatigue life is often the design-limiting factor.
Another consideration is the residual stress state of the hardfacing joint. The CO2 gas-shielded welding process typically produces higher residual stresses than processes with lower heat input, due to the higher arc force and penetration. For a gear component, these residual stresses could interact with the operating bending and contact stresses to accelerate fatigue failure. Post-weld stress relief treatment should be considered in engineering applications.
The observation of two-stage recrystallization in the normalized zone of the HAZ is an interesting metallurgical phenomenon. This suggests that the peak temperature in this zone was high enough to cause complete austenitization, followed by a cooling rate that allowed fine-grained recrystallization. The resulting fine pearlite and ferrite structure provides a good combination of strength and toughness, which is desirable for the HAZ of a gear component.
Study Insights and Implications
The most valuable contribution of this paper is the detailed microstructural mapping of the hardfacing joint, which provides engineers with a clear understanding of how different zones will behave under service loading. The combination of NER70S-6 wire with CO2 shielding on 35CrMo produces a joint with acceptable hardness matching and beneficial microstructural features such as acicular ferrite in the HAZ. For engineering practice, this means that CO2 gas-shielded hardfacing is a viable repair method for 35CrMo gears, provided that appropriate preheating and post-weld treatment are applied to control residual stresses and prevent cracking. The study reinforces the importance of microstructural analysis in evaluating the quality of hardfacing repairs, as visual inspection and hardness testing alone cannot reveal the full metallurgical picture of a weld joint.