Microstructure and Properties of the Bonding Layer in Overlay Welding on 35CrMo Steel

Literature Overview and Context

This study by Xu Yali, Xue Lintao, and Guan Xin, published in Metal Functional Materials (Vol. 29, Issue 3, 2022, pp. 67-72), investigates the microstructure and mechanical properties of the bonding layer in a multi-layer overlay welding configuration on 35CrMo steel. The research was funded by an autonomous region vocational education research project (XJZJKT-2021Y34) and conducted in collaboration between Xinjiang Institute of Industrial Vocational Technology and Baosteel Xinjiang Bayi Iron and Steel Co., Ltd. The work addresses a common engineering challenge: achieving a metallurgically compatible and mechanically robust transition between a low-alloy steel substrate and a wear-resistant overlay layer.

Welding Configuration and Process Parameters

The study employs a "substrate + transition layer + protective layer" configuration, which is a well-established approach in overlay welding to ensure adequate dilution control and mechanical compatibility. The substrate is 35CrMo steel, a quenched and tempered low-alloy structural steel widely used in heavy machinery, power generation equipment, and oil and gas components. The transition layer is deposited using ER83-1 low-alloy steel wire, and the protective (wear-resistant) layer is deposited using HS112 wear-resistant steel wire. The welding process is gas metal arc welding (GMAW) with argon shielding gas.

The left-hand welding technique is noteworthy. In left-hand GMAW, the torch moves in a direction such that the arc is on the leading edge of the weld bead. This technique tends to produce a flatter, wider bead with more heat input on the trailing side, which can influence the cooling rate and solidification microstructure. The selection of ER83-1 as the transition layer wire is deliberate: its chemical composition is designed to provide a smooth dilution gradient between the 35CrMo substrate and the high-carbon, high-alloy HS112 wear-resistant layer, thereby minimizing the risk of cracking and ensuring adequate toughness at the critical bond interface.

Microstructural Analysis

Substrate-Transition Layer Interface

At the interface between the 35CrMo substrate and the ER83-1 transition layer, the authors observe that ferrite lamellae and pearlite are distributed in an alternating elongated block pattern. This dispersed distribution morphology is significant because it promotes improved ductility and toughness in the transition layer. The elongated block structure suggests a eutectoid-type solidification pattern where the carbon content at the interface is elevated due to dilution from the 35CrMo substrate, leading to the formation of pearlite alongside ferrite. The alternating arrangement of these phases creates a composite-like microstructure that resists crack initiation and propagation.

Transition Layer-Protective Layer Interface

At the interface between the ER83-1 transition layer and the HS112 protective layer, the microstructure consists of fine acicular ferrite, a small amount of blocky ferrite, and pearlite. The acicular ferrite morphology is particularly favorable for toughness, as the fine, needle-like structure provides numerous crack-arresting interfaces. The presence of blocky ferrite, while generally associated with lower toughness, is limited in quantity in this case, and its distribution within the acicular ferrite matrix does not significantly compromise the overall mechanical performance.

Mechanical Properties

The protective layer achieves a hardness range of 47-49 HRC, which is consistent with the expected properties of HS112-type wear-resistant steels and is suitable for applications involving abrasive wear. The tensile strength reaches 765.8 MPa, indicating that the overlay weld joint maintains adequate strength despite the high hardness of the protective layer. These values suggest that the multi-layer approach successfully balances wear resistance with structural integrity, avoiding the brittleness that would be associated with a direct deposit of HS112 on 35CrMo without an intermediate transition layer.

Engineering Practice Integration

In practice, overlay welding of wear-resistant layers on 35CrMo steel is commonly required in components such as ball mill liners, crusher hammers, excavator bucket teeth, and mining equipment wear parts. The "substrate + transition + protective" approach is mandated by many specifications, including ASTM A234 and various Chinese national standards, to ensure that the dilution effects are managed and that the final overlay layer achieves its intended properties. The process parameters reported in this study—300 A current, 25 V voltage, 7 mm/s speed—are within typical ranges for multi-wire or single-wire GMAW overlay welding, though the specific combination should be validated against the particular geometry and thickness requirements of the application.

The left-hand welding technique used in this study is worth noting from a metallurgical perspective. Left-hand welding produces a different heat input distribution compared to right-hand welding, with more heat concentrated on the trailing edge. This can result in slower cooling rates on one side of the bead, which may influence the acicular ferrite fraction and the overall toughness of the weld metal. In engineering practice, the choice between left-hand and right-hand techniques should be documented in the welding procedure specification (WPS) and validated through qualification testing.

Key Questions and Reflections

One important question is whether the reported mechanical properties are representative of the entire weld cross-section or only specific locations within the protective layer. The hardness variation across the overlay layer—particularly near the interface with the transition layer—can be significant due to dilution effects, and a single hardness range of 47-49 HRC may not capture the full gradient. Additionally, the study does not report impact toughness values, which are critical for assessing the fracture resistance of the transition layer, especially in applications subject to impact or thermal cycling.

Another reflection is the role of heat input in determining the microstructure. The welding parameters used in this study result in a moderate heat input, and the authors do not explore the sensitivity of the microstructure and properties to variations in heat input. In practice, heat input control is one of the most critical aspects of overlay welding, as excessive heat input can lead to grain coarsening, increased dilution, and reduced hardness in the protective layer, while insufficient heat input can result in incomplete fusion and lack of bonding.

Summary

This paper presents a well-executed investigation of the bonding layer microstructure and properties in a multi-layer overlay welding configuration on 35CrMo steel. The use of ER83-1 as a transition layer between the substrate and the HS112 wear-resistant protective layer is a sound engineering choice that produces a metallurgically compatible joint with adequate toughness and wear resistance. The reported mechanical properties—47-49 HRC hardness and 765.8 MPa tensile strength—are consistent with the intended performance of the HS112 overlay layer and demonstrate the effectiveness of the multi-layer approach. The microstructural observations, particularly the fine acicular ferrite in the transition-protective interface and the alternating ferrite-pearlite blocks at the substrate-transition interface, provide valuable insights into the solidification behavior and dilution effects in overlay welding. This work contributes to the practical knowledge base for engineers designing overlay welding procedures for wear-resistant applications on low-alloy steel substrates.