Microstructure and Properties of Weld Overlay Metal on 45 Steel

Literature Overview

This research investigates the microstructure and mechanical properties of weld overlay metal deposited on 45 steel, a medium-carbon structural steel widely used in pressure vessels, machinery components, and structural applications. The study is particularly relevant for engineers working with bimetallic components where a corrosion-resistant or wear-resistant overlay is applied to a cost-effective carbon steel substrate. The authors examine the effect of welding parameters on dilution, microstructure evolution, hardness distribution, and tensile strength of the overlay metal, providing data that can inform weld procedure qualification and material selection decisions.

Microstructural Analysis

The overlay is deposited using gas metal arc welding with a stainless steel consumable wire, typically ER308L or ER316L, depending on the intended service environment. The dilution rate, defined as the proportion of base metal alloying elements incorporated into the weld metal, is a critical parameter that directly affects the corrosion resistance and mechanical properties of the overlay. The study reports dilution rates ranging from 15 to 35 percent depending on the number of passes and the heat input per pass. Higher dilution rates result in lower chromium and nickel content in the weld metal, which can compromise the passive film stability and increase susceptibility to intergranular corrosion.

The microstructure of the overlay metal consists of austenite with varying amounts of ferrite, depending on the dilution level. At low dilution rates below 20 percent, the weld metal is predominantly austenitic with less than 5 percent delta ferrite. At higher dilution rates above 30 percent, the ferrite fraction increases to 15 to 20 percent, which can introduce cracking susceptibility in thick sections. The grain structure shows columnar grains growing from the fusion boundary toward the weld surface, with a transition to equiaxed grains in the upper portion of the weld where the cooling rate is lower.

Mechanical Property Assessment

The hardness of the overlay metal increases with dilution rate due to the higher carbon and manganese content inherited from the 45 steel base. The tensile strength follows a similar trend, with values ranging from 580 to 680 MPa across the dilution range studied. The elongation decreases with increasing dilution, dropping from 35 percent at 15 percent dilution to 22 percent at 35 percent dilution. This reduction in ductility is associated with the increased ferrite fraction and the coarsening of carbide precipitates in the weld metal. For pressure vessel applications governed by GB/T 150 or ASME VIII Div.1, the overlay metal must meet the minimum tensile strength requirements specified in the applicable code, and the dilution rate must be controlled to ensure adequate corrosion resistance without compromising mechanical integrity.

Engineering Considerations

For engineers specifying overlay cladding on 45 steel components, the key challenge is balancing corrosion resistance against mechanical strength. A dilution rate below 20 percent is generally recommended for applications requiring full stainless steel corrosion resistance, while higher dilution rates may be acceptable for mild corrosion environments where mechanical strength is the primary requirement. The study also highlights that the interpass temperature must be kept below 150 degrees Celsius to minimize grain growth and maintain fine microstructure in the overlay. Post-weld heat treatment at 1050 degrees Celsius followed by water quenching can homogenize the microstructure and reduce residual stresses, but must be carefully controlled to avoid sensitization of the stainless steel phase.

Conclusion

This study provides valuable data on the relationship between dilution, microstructure, and mechanical properties of weld overlay metal on 45 steel. The findings underscore the importance of controlling welding parameters to achieve the desired balance of corrosion resistance and mechanical performance. Engineers should use the dilution rate data presented here as a baseline for weld procedure qualification and adjust parameters based on the specific service requirements of their application.