CLADDING TECHNOLOGY SHANXI CO., LTDHeat Treatment Effects on D618 Cladding Layer Microstructure and Mechanical Properties
Literature Overview
Published in 2002 in the journal Metal Heat Treatment, this study by researchers from the Department of Materials Science and Engineering at Luoyang Institute of Technology examines the influence of post-weld heat treatment on the microstructure and mechanical properties of D618 cladding alloy layers. The work was supported by the Henan Provincial Department of Education Key Project Program. D618 is a well-known cobalt-based wear-resistant alloy widely used in severe wear applications, and understanding how heat treatment modifies its properties is essential for optimizing component performance.
Core Technical Content
The D618 alloy system is characterized by a high cobalt content (typically 50-60 percent), with chromium, tungsten, and molybdenum as principal alloying elements. The as-welded microstructure typically consists of a solid solution matrix with dispersed hard carbides and intermetallic compounds. The study systematically investigates the effects of various heat treatment conditions on this microstructure.
As-Welded Microstructure
In the as-deposited condition, the D618 overlay exhibits a complex microstructure comprising:
- A Co-Cr solid solution matrix with high lattice distortion
- M6C and M23C6-type carbides (where M represents Cr, W, and Mo)
- Sigma phase (Cr-rich intermetallic) in some regions
- Residual stresses from the welding thermal cycle
The as-welded hardness typically ranges from 55 to 62 HRC, with excellent wear resistance but potentially elevated residual stresses that may affect dimensional stability and fatigue performance.
Heat Treatment Conditions Investigated
The study examines multiple heat treatment regimes, including:
| Heat Treatment Type | Temperature (degrees C) | Duration (hours) | Cooling Method | Purpose |
|---|---|---|---|---|
| Solution treatment | 1000-1100 | 2-4 | Water quench | Homogenization |
| Aging treatment | 700-850 | 4-8 | Air cool | Precipitation hardening |
| Stress relief | 500-600 | 2-4 | Furnace cool | Residual stress reduction |
| Dual aging | 700/850 | 4/4 | Air cool | Balanced properties |
Effects on Microstructure
Solution treatment at 1050 degrees Celsius followed by water quenching dissolves most of the equilibrium carbides, creating a supersaturated solid solution. Subsequent aging at 750 degrees Celsius promotes the precipitation of fine, uniformly distributed carbides, which significantly enhances hardness and wear resistance. The aging response shows a classic precipitation hardening curve, with peak hardness achieved at approximately 750 to 800 degrees Celsius.
Stress relief treatment at 550 degrees Celsius effectively reduces residual stresses by more than 70 percent without significantly affecting hardness or microstructure. This is particularly important for large components where dimensional stability is critical.
Mechanical Property Evolution
The mechanical properties show distinct trends with heat treatment:
- Hardness: Increases from 58 HRC (as-welded) to 65-68 HRC after optimal aging treatment
- Wear resistance: Improves by 40-60 percent after aging due to finer, more uniformly distributed carbides
- Toughness: May decrease slightly with excessive aging due to carbide coarsening
- Compressive strength: Generally increases with solution treatment and aging
Engineering Practice Implications
For engineering applications involving D618 overlays, the heat treatment sequence is critical. The recommended practice for critical components is:
- Solution treatment at 1050-1100 degrees C for 2-4 hours with rapid quenching
- Double aging at 750 degrees C for 4 hours followed by 850 degrees C for 4 hours
- Stress relief at 550 degrees C for 2 hours if dimensional stability is required
This sequence maximizes hardness and wear resistance while ensuring adequate toughness and dimensional stability. However, the thermal distortion caused by heat treatment must be considered during component design. Allowances for distortion should be built into the fabrication sequence, and post-heat-treatment machining should be planned accordingly.
A practical consideration is the interaction between the cladding layer and the base material during heat treatment. The differential thermal expansion and conductivity between the cobalt-based overlay and the steel substrate can generate additional residual stresses at the interface. For thick overlays on large components, preheating and controlled cooling rates are essential to prevent cracking at the bond line.
Key Questions and Reflections
The study raises the question of whether the optimal heat treatment parameters identified for laboratory specimens remain valid for large-scale industrial components. Thermal gradients in thick sections may result in non-uniform microstructural evolution, particularly at the interface region where cooling rates differ significantly from the bulk overlay.
Another important consideration is the effect of multiple welding passes on the final heat-treated microstructure. Each subsequent pass re-heats the previous pass, effectively subjecting it to a tempering cycle. This multi-pass thermal history must be accounted for when designing the final heat treatment schedule.
Summary
This research provides a comprehensive understanding of how post-weld heat treatment modifies the microstructure and properties of D618 cladding alloys. The findings directly support the development of optimized heat treatment protocols for cobalt-based overlay systems in industrial applications. The systematic investigation of solution treatment, aging, and stress relief conditions offers engineers practical guidance for achieving the desired balance of hardness, wear resistance, toughness, and dimensional stability in D618-clad components.