Zhuojin Pipe Fitting Co., LtdEffects of Preheating Temperature and Welding Current on Overlay Layer of 4Cr5Mo2V Mold Steel
Literature Overview and Research Context
The 2024 study by Zuo Pengpeng, Liu Junwei, Tang Jian, Zhang Luyao, Zhuo Xiaoru, and Jin Senlin, published in Xuebao Cailiao Rechuli (Journal of Thermal Processing of Materials), provides a comprehensive investigation of how preheating temperature and welding current influence the microstructure and mechanical properties of overlay weld layers on 4Cr5Mo2V mold steel. Funded by the Shenzhen Natural Science Foundation Key Project (JCYJ20220818103601003), this research combines experimental characterization with numerical thermal simulation to elucidate the mechanisms governing overlay layer evolution. The study is particularly valuable because 4Cr5Mo2V is a high-alloy hot work mold steel widely used in hot forging, extrusion, and casting applications, where overlay welding is employed for surface repair and hardfacing.
Core Technical Parameters and Experimental Design
The researchers systematically varied two key process parameters: preheating temperature (from unpreheated to 450°C) and welding current (from 70 A upward). The following table summarizes the key measured outcomes:
| Parameter Condition | Melt Pool Width | Melt Pool Depth | Grain Length | Grain Width | Average Hardness Trend |
|---|---|---|---|---|---|
| No preheat, 70 A | 7.0 mm | 1.5 mm | 10.9 μm | 4.0 μm | Baseline (highest) |
| No preheat, higher current | Increased | Increased | Increased | Increased | Decreased |
| Preheated to 450°C | — | — | — | — | Decreased by 4.9% |
The numerical simulation of the temperature field provided critical insight into the mechanisms underlying these observations. Increasing the welding current causes a substantial rise in the peak melt pool temperature, which in turn increases the melt pool dimensions, grain size, and heat-affected zone (HAZ) width. This thermal escalation degrades the overlay layer properties because larger grains and excessive dilution reduce the hardness and wear resistance of the overlay.
Mechanism Analysis: Thermal Field and Microstructural Evolution
The numerical simulation results reveal that the relationship between process parameters and overlay properties is mediated through the thermal history of the weld. When the welding current is increased beyond 70 A without preheating, the peak temperature rises sharply, leading to:
- Larger melt pool dimensions — Increased heat input expands the liquid zone, allowing greater substrate dilution and coarser solidification structures.
- Coarser grain growth — Higher temperatures and slower cooling rates (due to increased thermal mass) promote grain growth during solidification and subsequent cooling.
- Wider HAZ — The elevated thermal input extends the zone of thermal softening and microstructural transformation in the base metal, potentially compromising the mechanical integrity of the joint.
Conversely, preheating the substrate to 450°C before welding reduces the cooling rate of the overlay layer during solidification. This slower cooling rate has a direct consequence: the average hardness of the overlay layer decreases by 4.9% compared to the unpreheated condition. However, the base metal retains its original hardness level, which is a favorable outcome because it means the preheating does not cause unacceptable softening of the substrate.
Engineering Practice Implications
For engineers performing overlay welding on 4Cr5Mo2V mold steel components — such as hot work dies, forging punches, and extrusion mandrels — the findings provide actionable guidance:
- Welding current should be kept at or near 70 A for single-pass overlay to minimize thermal input and maintain fine grain structures. If higher deposition rates are required, multi-pass strategies with controlled interpass temperatures should be employed.
- Preheating to 450°C is beneficial for reducing residual stresses and cracking susceptibility in this high-carbon, high-alloy steel, but it comes at the cost of approximately 5% hardness reduction in the overlay layer. This trade-off must be evaluated based on the specific service requirements.
- The strong-toughness matching achieved at 450°C preheat represents a balanced condition where the overlay retains sufficient hardness for wear resistance while the reduced cooling rate improves toughness and reduces cracking risk.
From a quality control perspective, the grain dimensions (10.9 μm length and 4.0 μm width at 70 A without preheat) provide a reference baseline for acceptance criteria. Deviations from these dimensions — particularly grain coarsening — should be flagged as potential indicators of excessive thermal input.
Reflections and Study Insights
This study exemplifies the power of combining experimental characterization with numerical simulation to achieve a mechanistic understanding of welding processes. The temperature field simulation not only explains the observed microstructural trends but also provides a predictive tool for parameter optimization. The finding that preheating reduces overlay hardness by approximately 5% while preserving base metal hardness is a nuanced result that challenges the simplistic assumption that preheating is always detrimental to overlay performance.
For engineers working with hot work mold steels, the key insight is that overlay welding involves a fundamental trade-off between crack resistance (favored by preheating and slower cooling) and overlay hardness (favored by rapid cooling and low dilution). The optimal process window lies at the intersection of these competing requirements, and the 450°C preheat condition with 70 A welding current appears to represent a well-balanced compromise for 4Cr5Mo2V steel.