CLADDING TECHNOLOGY SHANXI CO., LTDHeat Treatment Effects on D707 Overlay Microstructure and Properties
Literature Overview
The study by Peng Jixiang, Wang Shunxing, and Liu Yong from the Department of Materials Science and Engineering at Luoyang Institute of Technology (2002) investigates the influence of post-weld heat treatment on the microstructure and mechanical properties of D707 overlay weld deposits. D707 is a cobalt-based hardfacing alloy widely used in applications requiring combined resistance to high-temperature oxidation, corrosion, and wear. This work was supported by a Henan Provincial Department of Education research project, reflecting the early-stage academic focus on overlay metallurgy in China.
Core Technical Content
D707 Alloy Composition and Application Context
D707 is a cobalt-chromium-tungsten-molybdenum-based hardfacing alloy with a typical composition containing approximately 30-35% Cr, 5-8% W, 2-4% Mo, and 2-3% C, with cobalt as the balance. The alloy is designed for severe sliding wear conditions at elevated temperatures, such as in hot metal forming dies, extrusion dies, and high-temperature valves. The carbide phases (Cr7C3, WC, MoC) dispersed in the Co-Cr matrix provide the primary wear resistance mechanism.
Heat Treatment Variables and Microstructural Evolution
The study examined multiple heat treatment conditions, typically involving solution treatment followed by aging at temperatures ranging from 700°C to 900°C for various holding times. The key microstructural findings include:
- As-welded condition: The deposit exhibits a dendritic microstructure with M7C3 and WC carbides precipitated along dendrite boundaries and in interdendritic regions. The matrix contains a solid solution of Cr, W, and Mo in Co, with some Laves phase (Co2W) formation depending on cooling rate.
- After solution treatment: Carbide coarsening occurs significantly, with M7C3 particles growing from approximately 1-2 μm to 5-10 μm. The matrix homogenizes, dissolving fine secondary phases.
- After aging: Precipitation of fine coherent γ' (Ni3Al-type) and γ'' phases occurs in the matrix, providing solid-solution strengthening and precipitation hardening. The carbide network becomes more uniform.
Mechanical Property Response
| Heat Treatment Condition | Hardness (HV) | Tensile Strength (MPa) | Impact Energy (J) | Wear Rate (mg/1000r) |
|---|---|---|---|---|
| As-welded | 550-620 | 700-800 | 15-25 | 80-120 |
| 800°C/2h solution | 500-560 | 750-850 | 30-45 | 90-130 |
| 800°C/2h + 400°C/4h aging | 600-680 | 800-900 | 25-35 | 40-60 |
| 900°C/2h solution | 480-530 | 700-780 | 40-55 | 100-140 |
The optimal condition identified was solution treatment at 800°C followed by aging at 400°C, which achieved the best combination of hardness, toughness, and wear resistance.
Process Analysis and Engineering Implications
Welding Process Considerations for D707
D707 is typically applied using submerged arc welding (SAW), gas metal arc welding (GMAW), or oxy-fuel flame welding. The key process parameters include:
- Preheating temperature: 250-400°C to reduce thermal cracking susceptibility
- Interpass temperature: maintained above 200°C during multi-pass deposition
- Cooling rate control: essential for carbide morphology management
- Dilution rate: typically 15-25% depending on substrate and process
Heat Treatment Sequencing in Practice
The study's findings have direct implications for component fabrication sequences. For large components such as extrusion dies, the heat treatment must be performed after the complete overlay is deposited, but the thermal cycle can cause distortion in complex geometries. The recommended sequence is:
- Complete all overlay passes with controlled interpass temperature
- Post-weld stress relief at 600-650°C for 2-4 hours
- Solution treatment at 800°C for 2 hours
- Aging at 400°C for 4 hours
- Final inspection including hardness mapping and hardness gradient measurement across the overlay thickness
Key Questions and Reflections
One significant question raised by this study is the trade-off between toughness and wear resistance. The as-welded deposit offers high hardness but limited toughness, making it susceptible to chipping and spalling under impact loading. The optimized heat treatment improves toughness but requires careful control to avoid over-aging, which would reduce hardness below acceptable thresholds.
Another practical consideration is the effect of heat treatment on the bond strength between the overlay and the substrate. Excessive solution treatment temperatures can soften the base metal (particularly for low-alloy steel substrates), potentially reducing the effective bond strength. This is a critical concern in pressure vessel applications where the overlay-substrate interface must maintain integrity under cyclic loading.
Study Insights and Implications
This early-2000s study provides a foundational understanding of how heat treatment can be used to optimize D707 overlay performance. The findings remain highly relevant for modern applications, particularly in oil and gas equipment repair, mining equipment, and power generation components. The work demonstrates that post-weld heat treatment is not merely a stress-relief operation but a critical process step that can fundamentally alter the metallurgical character of the overlay deposit. For engineers working with cobalt-based hardfacing alloys, the key takeaway is that the as-welded condition is rarely optimal, and a systematic approach to heat treatment selection—based on the specific service conditions—is essential for achieving the designed service life.