Effect of Aging Treatment on Microstructure and Properties of Remelted Co-Based Alloy Cladding Layers

Literature Overview and Research Significance

This study investigates the influence of aging treatment on the microstructure and mechanical properties of cobalt-based alloy cladding layers that have undergone a remelting process. The significance of this work lies in the common industrial practice of applying a remelting pass to Co-based overlays to eliminate residual stresses, refine grain structure, and homogenize the deposit composition. However, the subsequent aging treatment is often neglected or applied without systematic optimization, leading to suboptimal performance of these expensive overlays.

Experimental Methodology and Process Parameters

The study examined Co-based alloy cladding layers deposited by gas tungsten arc welding (GTAW) with remelting using a laser or plasma torch, followed by aging treatment at various temperatures and durations. The base alloy was a Co-Cr-W type system, typical of Stellite-type alloys, deposited on 316 stainless steel substrates.

The Co-based alloy composition was approximately 62% Co, 28% Cr, 5% W, 3% Ni, with trace Mo and Fe balance. The GTAW parameters for initial deposit were 120-150 A, 12-14 V, with a travel speed of 60-80 mm/min using ERCoCrA wire.

Microstructural Evolution During Aging

The as-remelted microstructure consisted primarily of a face-centered cubic (FCC) γ-Co matrix with dispersed M₂₃C₆ and M₆C type carbides along grain boundaries. After aging treatment, significant microstructural changes were observed:

1. At 700-750°C: Precipitation of fine γ' (Ni₃Co) type intermetallics within the matrix, resulting in solid solution strengthening. Hardness increased from 350 HV (as-remelted) to approximately 420-450 HV.
2. At 800-850°C: Carbide coarsening began, with M₂₃C₆ particles growing from 0.2-0.5 μm to 1.0-2.0 μm. Hardness decreased slightly to 380-400 HV, but toughness improved significantly.
3. At 900°C: Excessive carbide coarsening and partial dissolution occurred, leading to hardness reduction to 320-340 HV. However, the microstructure became more homogeneous with improved corrosion resistance.

Mechanical Property Results

The aging treatment produced distinct improvements in specific properties:

The optimal aging condition was identified as 800°C for 4 hours with furnace cooling, which provided the best balance of hardness, strength, and toughness. The improvement in creep resistance was particularly notable, representing a 167% increase in rupture life at 600°C under a stress of 100 MPa.

Engineering Practice Integration

From a practical standpoint, this study provides critical guidance for the fabrication of Co-based clad components used in high-temperature applications such as:

• Hydrogenation reactor internals: Where Co-based overlays must resist hot hydrogen attack at 400-600°C
• Turbine components: Where thermal fatigue resistance is paramount
• Petrochemical process equipment: Where high-temperature hydrogen service requires both strength and corrosion resistance

The study demonstrates that the remelting-aging sequence is not merely a post-processing formality but a critical step that can substantially extend component service life. The recommended process window of 800°C for 4 hours with controlled furnace cooling should be incorporated into fabrication specifications for Co-based clad pressure vessels and heat exchangers.

Key Questions and Reflections

A critical observation from this literature is the apparent contradiction between aging treatment effects on hardness and toughness. While moderate aging improves both properties through precipitation strengthening, excessive aging leads to carbide coarsening that degrades toughness. This suggests that the optimal aging condition is highly alloy-specific and must be determined for each Co-based alloy grade through systematic testing.

The study also raises an important question about the interaction between remelting and aging: does the remelting pass fundamentally alter the aging response compared to a non-remelted deposit? The researchers noted that remelted layers showed a more uniform aging response, suggesting that the homogenization achieved during remelting facilitates more predictable precipitation behavior. This finding supports the practice of always applying a remelting pass before aging treatment of Co-based cladding layers, regardless of the initial deposition method.