CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Co-Based Alloy Weld Overlay after Laser Remelting and Aging Treatment
Literature Overview
This study note examines a technical paper investigating the microstructure and mechanical properties of cobalt-based alloy weld overlay layers subjected to laser remelting and subsequent aging heat treatment. Cobalt-based alloys such as Stellite 6 (Co-Cr-W), Stellite 21, and Haynes 25 are widely used in high-temperature and high-wear applications, including gas turbine components, extrusion dies, and chemical processing equipment. The primary challenge in Co-based alloy weld overlay is the formation of coarse carbide networks and uneven microstructure in the as-welded condition, which adversely affects hardness uniformity, wear resistance, and creep strength.
Core Technical Content
The paper describes a two-stage post-weld treatment process: laser remelting followed by aging heat treatment. The laser remelting step is intended to homogenize the microstructure by melting and rapidly resolidifying the overlay layer, while the aging treatment is designed to precipitate fine carbides and optimize the hardness distribution. The study compares the as-welded condition, the laser-remelted condition, and the laser-remelted plus aged condition to demonstrate the incremental benefits of each treatment.
| Treatment Condition | Hardness (HV) | Carbide Distribution | Retained Austenite (%) |
|---|---|---|---|
| As-welded | 320–380 | Coarse network, irregular | 8–15 |
| Laser remelted | 380–420 | Refined, more uniform | 5–10 |
| Laser remelted + aged | 420–480 | Fine precipitates, homogeneous | 2–5 |
The laser remelting parameters investigated include laser power, scanning speed, beam diameter, and overlap ratio. The optimal parameters were found to be a laser power of 2.0–3.0 kW, a scanning speed of 200–400 mm/min, a beam diameter of 0.8–1.2 mm, and an overlap ratio of 30–50%. These parameters produced a remelted layer with a depth of approximately 0.5–1.5 mm, which is sufficient to homogenize the typical single-layer overlay thickness.
Metallurgical Analysis
Metallographic examination of the as-welded overlay revealed a dendritic microstructure with M7C3 and M23C6 carbides segregated at the dendrite boundaries. The carbide network was continuous and irregular, which served as crack initiation sites and reduced the ductility of the overlay layer. After laser remelting, the dendritic structure was significantly refined, and the carbide morphology changed from a coarse network to discrete particles distributed more uniformly throughout the matrix. The rapid solidification rate achieved by laser remelting (estimated at 10³–10⁴ K/s) suppressed the formation of coarse interdendritic carbides and promoted the nucleation of fine precipitates.
The aging treatment was conducted at temperatures ranging from 850 to 950 °C for durations of 2 to 8 hours, followed by air cooling. The optimal aging condition was found to be 900 °C for 4 hours, which produced the highest hardness of approximately 480 HV while maintaining adequate toughness. The aging process promoted the precipitation of fine M7C3 carbides within the matrix, which provided effective dispersion strengthening without causing excessive embrittlement.
Process Optimization and Defect Analysis
The study also addressed potential defects introduced during laser remelting, including lack of fusion, cracking, and porosity. The authors identified that excessive laser power or too slow a scanning speed could lead to overheating and grain coarsening, while insufficient power could result in incomplete remelting and a mixed microstructure. The overlap ratio was found to be critical for ensuring complete coverage of the overlay surface; insufficient overlap left unremelted regions with the original as-welded microstructure.
Aging treatment can also introduce defects if the parameters are not carefully controlled. Over-aging leads to carbide coarsening and a significant drop in hardness, while under-aging results in incomplete precipitation and suboptimal mechanical properties. The authors recommend performing a dilatometry test on a coupon of the overlay material to determine the precise transformation temperatures and guide the selection of aging parameters.
Engineering Practice Implications
For engineers working with Co-based alloy overlay layers, this literature provides clear guidance on the benefits of combined laser remelting and aging treatment. The process is particularly valuable for critical components such as gas turbine blades, extrusion dies, and chemical pump impellers, where uniform hardness and high wear resistance are essential. The laser remelting step can be performed in situ or in a dedicated processing cell, and the aging treatment can be carried out in a conventional furnace.
The combined treatment process should be qualified according to relevant standards such as NB/T 47014 or ASME IX, with appropriate test coupons fabricated and tested to verify the hardness, tensile strength, and impact toughness of the treated overlay layer. Non-destructive testing should be performed before and after the treatment to detect any defects introduced during the process.
Key Reflections and Study Insights
The most striking finding of this study is the dramatic improvement in hardness uniformity achieved by the combined laser remelting and aging treatment. The hardness variation across the overlay layer was reduced from approximately 60 HV in the as-welded condition to less than 20 HV in the treated condition, which is a significant improvement for applications requiring consistent wear performance. Another important observation is that the laser remelting step is not merely a surface treatment but a true microstructural refinement process that fundamentally alters the carbide morphology and distribution.
The study also highlights the importance of process parameter control. The interaction between laser power, scanning speed, and overlap ratio must be carefully optimized for each specific overlay thickness and alloy composition. Engineers should avoid assuming that a single set of parameters is universally applicable; instead, they should conduct systematic parameter studies on test coupons before applying the process to production components.
Summary
The combination of laser remelting and aging heat treatment offers a powerful approach to optimizing the microstructure and mechanical properties of Co-based alloy weld overlay layers. The laser remelting step refines the dendritic structure and homogenizes the carbide distribution, while the aging treatment precipitates fine strengthening phases that enhance hardness and wear resistance without sacrificing toughness. This two-stage treatment process is particularly valuable for high-performance applications where the overlay layer must withstand extreme wear and thermal cycling. Engineers should carefully qualify the process parameters, perform thorough metallurgical examination, and implement rigorous quality control to ensure the successful application of this technology in production environments.