CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Weld Overlay Metal on 45 Steel
Research Background and Motivation
This study, conducted by Xie Jun, Zeng Zhichao, Zhong Yu, Qu Jinshan, and Zeng Qingbao from Xihua University and Sichuan Atlantic Welding Materials Co., Ltd., was published in the Journal of Xihua University (Natural Science Edition) in 2006. The investigation focuses on the microstructure and mechanical properties of weld overlay metal deposited on 45 steel, a widely used medium-carbon structural steel with approximately 0.42–0.50 wt% carbon content. Understanding the metallurgical behavior of overlay metal on this base is essential for applications where 45 steel components require surface enhancement for wear resistance, corrosion resistance, or functional improvement.
Metallurgical Analysis of the Overlay Zone
The overlay welding of dissimilar materials onto 45 steel creates a complex metallurgical structure at the interface. The base metal, with its medium carbon content and ferrite-pearlite microstructure, interacts with the deposited overlay metal through a series of solidification and transformation processes. The cooling rate at the weld interface, typically in the range of 10–100 °C/s for conventional arc welding processes, governs the microstructure that forms in the dilution zone.
| Zone | Microstructure | Typical Hardness (HV) |
|---|---|---|
| Base metal (45 steel) | Ferrite + pearlite | 180–220 |
| Dilution zone (interface) | Mixed ferrite, pearlite, martensite | 250–350 |
| Overlay weld metal (single layer) | Martensite + retained austenite | 350–450 |
| Overlay weld metal (multi-layer) | Fine-grained martensite, tempered carbides | 300–400 |
The dilution zone is of particular interest because it represents the transition region where the composition and microstructure change from the base metal to the overlay metal. In this zone, the carbon content may locally increase due to dilution from the base metal, promoting the formation of harder phases such as martensite or cementite. If not properly managed, this can lead to excessive hardness and brittleness at the interface, creating a potential site for crack initiation.
Mechanical Properties and Performance Evaluation
The mechanical properties of the overlay metal are strongly dependent on the welding process parameters, filler metal composition, and the number of overlay layers. Key properties evaluated in this type of study include hardness distribution across the overlay thickness, tensile strength, impact toughness, and wear resistance.
The hardness profile typically shows a gradient from the base metal to the overlay surface. The highest hardness values are found in the dilution zone and the first overlay layer, where the cooling rate is highest and the dilution from the base metal is most significant. Subsequent overlay layers, deposited on already-welded material with lower carbon content, exhibit more uniform and controlled microstructures.
Effect of Process Parameters on Overlay Properties
| Parameter | Low Value Effect | High Value Effect |
|---|---|---|
| Heat input | Coarse grain, high dilution | Fine grain, low dilution |
| Welding speed | High dilution, wide weld | Low dilution, narrow weld |
| Number of layers | High dilution, uneven properties | Low dilution, uniform properties |
| Interpass temperature | Rapid cooling, hard microstructure | Slow cooling, softer microstructure |
| Filler metal carbon content | Low hardness, high toughness | High hardness, low toughness |
Engineering Implications and Practice
For engineers designing overlay welding processes on 45 steel components, this research provides critical guidance on several fronts. First, the dilution effect must be explicitly accounted for in process design. A single-pass overlay will have significantly different properties than a multi-pass overlay, and the choice between them depends on the functional requirements of the application.
Second, the hardness-toughness balance is a central design challenge. High hardness improves wear resistance but reduces impact toughness, which can be detrimental in applications involving shock loading. The selection of filler metal composition and the control of cooling rate are the primary levers for achieving the desired balance.
Third, the study underscores the importance of microstructural characterization in validating overlay welding processes. Metallographic examination, hardness mapping, and mechanical testing are essential for confirming that the overlay meets specification requirements. Without such verification, there is no assurance that the overlay will perform reliably in service.
Summary and Key Takeaways
The study on 45 steel overlay welding provides a solid foundation for understanding the metallurgical behavior at the interface between medium-carbon steel and deposited overlay metals. The key insight is that the dilution zone is the critical region governing both the bond integrity and the functional performance of the overlay. Engineers must design processes that control dilution through multi-layer strategies, appropriate filler selection, and heat input management. The findings are directly applicable to overlay welding in pressure vessel fabrication, where 45 steel or similar medium-carbon steels are commonly used as base materials for stainless steel or alloy overlay applications.