Microstructure and Wear Resistance of Dissimilar Material Weld Overlay Layers

Overview and Motivation

The selection of dissimilar material weld overlay layers is a critical engineering decision in the fabrication of components subjected to corrosive, abrasive, or erosive service environments. This study note summarizes key insights from a technical paper examining the microstructural evolution and wear performance of weld overlay layers deposited on dissimilar substrate materials. The research addresses a fundamental challenge in overlay engineering: achieving both metallurgical compatibility and functional performance when the base material and overlay alloy have significantly different thermal expansion coefficients, melting points, and solidification behaviors.

In practice, engineers frequently encounter scenarios where a carbon steel or low-alloy steel pressure vessel or structural component requires a corrosion-resistant or wear-resistant overlay, yet the substrate cannot be replaced entirely due to cost, design constraints, or in-service repair conditions. The study examined several representative material combinations, including carbon steel substrates with austenitic stainless steel overlay, nickel-based alloy overlay, and high-chromium cast iron overlay, and analyzed how the resulting microstructures influence wear resistance under different tribological conditions.

Core Findings on Microstructural Evolution

The microstructure of a weld overlay layer is governed by the interaction between the base metal and the deposited alloy during solidification, as well as by the thermal history experienced during multi-pass deposition. The study identified several recurring microstructural features that are critical to understanding wear behavior.

The most significant finding was that the dilution zone at the base metal/overlay interface consistently represented the weakest link in terms of both mechanical integrity and wear performance. When dilution exceeded 10%, the overlay layer incorporated substantial amounts of base metal constituents, leading to the formation of mixed carbide networks that were neither as hard as the intended overlay carbides nor as tough as the base metal. This intermediate zone was particularly susceptible to crack initiation under cyclic loading or thermal cycling conditions.

The study also highlighted the role of retained austenite in austenitic overlay layers. In stainless steel overlay systems deposited on carbon steel substrates, the cooling rate at the interface was often high enough to stabilize a portion of the austenite phase. This retained austenite contributed to the toughness of the overlay layer but could transform into martensite during subsequent welding operations or service heating, leading to dimensional instability and potential cracking.

Wear Mechanism Analysis

The wear resistance of the overlay layer was evaluated through pin-on-disk abrasion testing and dry sliding wear experiments. The study identified three dominant wear mechanisms depending on the overlay composition and substrate combination:

1. Adhesive wear dominated in low-carbon overlay systems where the surface hardness was insufficient to resist material transfer under high contact pressure.
2. Abrasive wear was the primary mechanism for high-carbon and high-chromium overlay layers, where hard carbide particles (such as Cr7C3, Cr23C6, and M7C3) provided ploughing resistance but could be dislodged if the matrix was too soft.
3. Oxidative wear became significant at elevated temperatures, where surface oxide layers formed and were repeatedly fractured and removed during sliding contact.

The optimal wear performance was achieved in overlay layers where a balanced distribution of hard carbide particles was embedded in a tough matrix. For example, a 316 stainless steel overlay deposited on a Q345R low-alloy steel substrate achieved a hardness of approximately 280 HV with a dilution ratio of 6%, demonstrating a good compromise between corrosion resistance and moderate wear resistance. In contrast, a high-chromium white iron overlay deposited on the same substrate reached hardness values above 600 HV but exhibited poor impact toughness, making it suitable only for applications where abrasion was the sole concern and impact loading was absent.

Engineering Practice Implications

The study's findings have direct implications for overlay process design in pressure vessel fabrication and repair. The following engineering recommendations emerge from the analysis:

• Multi-pass deposition is essential: A minimum of three passes is recommended to reduce base metal dilution below 5%, particularly when the substrate carbon equivalent exceeds 0.45%.
• Interpass temperature control: Maintaining interpass temperatures between 100°C and 200°C helps manage the cooling rate at the interface and reduces the risk of martensitic transformation in the dilution zone.
• Preheat considerations: For substrates with high carbon equivalent or high restraint, preheating to 150–250°C is advisable to reduce residual stresses and minimize cracking susceptibility at the interface.
• Post-weld heat treatment: A stress-relief anneal at 600–650°C for austenitic stainless steel overlay layers can stabilize the microstructure and reduce residual stresses, but must be carefully controlled to avoid sensitization.

A practical case from the literature involved the repair of a hydrogenation reactor shell where a 316L overlay layer was deposited on a 16MnDR carbon steel substrate. The initial two-pass overlay exhibited 12% dilution and a hardness of only 200 HV, well below the required specification. After implementing a six-pass deposition strategy with controlled interpass temperatures, the dilution was reduced to 4% and the overlay hardness reached 260 HV, meeting the acceptance criteria for the design service environment.

Key Questions and Reflections

The study raises several important questions that warrant further investigation in engineering practice. First, the long-term stability of retained austenite in overlay layers under cyclic thermal loading remains uncertain. In hydrogenation reactors, where temperature fluctuations of ±50°C are common during startup and shutdown cycles, the transformation behavior of retained austenite could affect dimensional stability and fatigue life. Second, the interaction between overlay microstructure and the underlying heat-affected zone (HAZ) of the base metal is often overlooked in standard qualification procedures. The HAZ of the base metal may undergo microstructural changes that compromise the bond strength at the interface over time.

The study also underscores the importance of dilution control as a quality parameter. While many fabrication specifications focus on overlay layer thickness and hardness, the dilution ratio at the interface is arguably the most critical factor governing long-term performance. Future qualification procedures should consider incorporating dilution measurement as a routine acceptance test, particularly for critical components in high-pressure or high-temperature service.

Summary and Outlook

The study provides valuable insights into the relationship between microstructure, dilution, and wear resistance in dissimilar material weld overlay layers. The central conclusion is that achieving optimal wear performance requires not only selecting an appropriate overlay alloy but also carefully controlling the deposition process to manage dilution, cooling rate, and residual stresses at the interface. For engineers involved in pressure vessel fabrication and repair, these findings reinforce the need for process qualification that goes beyond conventional mechanical property testing to include microstructural evaluation of the dilution zone. The practical guidance on multi-pass deposition, interpass temperature control, and post-weld heat treatment offers actionable recommendations that can be directly applied to improve overlay quality in industrial settings. Further research into the long-term behavior of overlay interfaces under combined mechanical and thermal loading would be particularly beneficial for extending the service life of clad and overlay pressure vessels in demanding industrial applications.