CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Heat Treatment on D618 Weld Overlay Microstructure and Properties
Literature Overview
Published in Metal Heat Treatment (金属热处理) in 2002, this paper by Peng Jixiang, Wang Shunxing, and Liu Yong from the Department of Materials Science and Engineering, Luoyang Institute of Technology, investigates the influence of post-weld heat treatment on the microstructure and mechanical properties of D618 weld overlay deposits. D618 is a high-alloy nickel-based overlay material designed for extreme wear and corrosion resistance applications.
Material Background
D618 is a proprietary high-alloy weld overlay material containing significant amounts of Ni, Cr, Mo, and W. It is designed to produce a martensitic or semi-austenitic microstructure with excellent resistance to abrasive and adhesive wear, as well as oxidation at elevated temperatures. The as-welded condition typically exhibits very high hardness (HV 900–1100) but limited toughness, making heat treatment essential for optimizing the performance-toughness balance.
Heat Treatment Parameters and Microstructural Evolution
Treatment Conditions Investigated
| Heat Treatment | Temperature (°C) | Holding Time | Cooling Method | Resulting Hardness (HV) |
|---|---|---|---|---|
| As-welded | — | — | Air cool | 950–1050 |
| Tempering 1 | 400 | 2h | Air cool | 850–900 |
| Tempering 2 | 500 | 2h | Air cool | 750–800 |
| Tempering 3 | 600 | 2h | Air cool | 650–700 |
| Solution + Quench | 1100 | 1h | Water quench | 950–1000 |
| Solution + Temper | 1100 + 500 | 1h + 2h | Water + Air | 750–800 |
Microstructural Changes
The heat treatment induces the following microstructural transformations:
- As-welded condition: Lath martensite with retained austenite (15–25%), fine carbides (M₇C₃, M₃C) dispersed throughout the matrix. High dislocation density and internal stresses.
- Tempering at 400°C: Partial precipitation of fine carbides from supersaturated martensite. Slight reduction in dislocation density. Retained austenite begins to decompose.
- Tempering at 500°C: Significant carbide coarsening and spheroidization. Retained austenite decomposes to martensite and carbides. Internal stresses substantially relieved.
- Tempering at 600°C: Carbide coarsening becomes pronounced. Tempered martensite structure fully developed. Maximum toughness achieved at the expense of hardness.
- Solution treatment + quench: Complete dissolution of carbides and homogenization of the solid solution. Rapid quenching produces fresh martensite with higher retained austenite content.
- Solution + temper: Combines the benefits of homogenization with stress relief, producing a refined, uniformly tempered martensitic structure.
Mechanical Property Evolution
| Property | As-Welded | 500°C Temper | Solution + 500°C Temper |
|---|---|---|---|
| Hardness (HV) | 1000 | 780 | 790 |
| Impact energy (J) | 5–8 | 15–22 | 18–25 |
| Wear resistance (mm³/Nm) | 0.8 | 1.2 | 1.1 |
| Residual stress (MPa) | 400–550 | 100–150 | 80–120 |
Engineering Practice Integration
Application Selection Based on Heat Treatment
| Application | Recommended Treatment | Rationale |
|---|---|---|
| Abrasive wear (dry) | As-welded or 400°C temper | Maximum hardness for abrasion resistance |
| Abrasive + impact | 500°C temper | Balance hardness and toughness |
| High-temperature service | Solution + 500°C temper | Homogeneous structure, stable properties |
| Corrosive + wear | 500°C temper | Reduced residual stress, improved corrosion resistance |
Inspection and Quality Control
For D618 overlay deposits in critical applications:
- Hardness mapping: Traverse testing at 1 mm intervals across the overlay surface to verify uniformity (±50 HV tolerance)
- Microstructural examination: Metallographic analysis of cross-sections to confirm phase distribution and absence of untempered martensite
- Residual stress measurement: X-ray diffraction or hole-drilling method to verify stress levels below 200 MPa
- Bond strength testing: Peel test or shear test per API 934 or equivalent standards
Key Reflections
The fundamental insight from this study is that heat treatment is not merely a stress-relief operation for D618 overlays — it is a critical process variable that determines the final performance characteristics. The choice of heat treatment must be driven by the specific service requirements: maximum hardness for pure abrasion, or a balanced property set for combined wear and impact loading.
A practical consideration is that large-scale overlays on heavy components (such as mill rolls or large excavator buckets) may not be amenable to full solution treatment due to distortion risks. In such cases, tempering alone provides adequate improvement while minimizing dimensional changes. The 500°C temper treatment emerges as the most versatile option, providing a good compromise between hardness, toughness, and dimensional stability.