CLADDING TECHNOLOGY SHANXI CO., LTDWear Resistance of Cemented Carbide Overlay Layer After Heat Treatment
Literature Overview and Research Background
Cemented carbide (WC-Co based) overlay layers represent one of the highest-performance wear-resistant coatings available for industrial applications, particularly in mining, oil and gas drilling, and heavy-duty machinery. The literature under study investigates the effect of post-weld heat treatment on the microstructure, hardness, and wear resistance of cemented carbide overlay layers deposited by laser cladding onto a low-alloy steel substrate. The research is particularly relevant because improper heat treatment can either enhance or severely degrade the performance of these expensive overlay coatings, making process optimization essential for cost-effective deployment.
Core Technical Findings
The study examines three heat treatment conditions: as-deposited (AD), solution treatment at 1100°C for 2 hours followed by water quenching (ST), and solution treatment plus aging at 700°C for 4 hours (ST+AG). The results demonstrate that heat treatment significantly influences the WC-Co interface morphology, cobalt matrix microstructure, and overall wear performance.
| Heat Treatment Condition | Surface Hardness (HV₀.₅) | Wear Volume Loss (mm³) | WC Particle Integrity |
|---|---|---|---|
| As-Deposited | 1420 | 0.082 | Moderate cracking |
| Solution Treatment (1100°C/2h) | 1380 | 0.065 | Improved bonding |
| Solution + Aging (700°C/4h) | 1350 | 0.058 | Optimal interface |
The as-deposited overlay exhibits the highest hardness due to the rapid solidification microstructure produced by the laser cladding process, but it also shows microcracks around WC particles caused by residual thermal stresses. The solution treatment at 1100°C effectively relieves these residual stresses and promotes diffusion bonding at the WC-Co interfaces, reducing microcracking. However, excessive solution treatment temperatures above 1200°C risk partial dissolution of WC particles, which would reduce the hard phase volume fraction and degrade wear resistance.
The aging treatment at 700°C after solution treatment produces the best overall wear performance. This condition promotes the precipitation of fine γ-Co (FCC cobalt) phases within the cobalt matrix, creating a more homogeneous and tough binder phase. The resulting microstructure features well-bonded WC particles embedded in a fine-grained cobalt matrix, which effectively resists both abrasive and adhesive wear mechanisms.
Process Analysis and Defect Control
The laser cladding process parameters used in this study include: laser power of 3.5 kW, scanning speed of 200 mm/min, powder feed rate of 8 g/min, and shielding gas (Ar) flow rate of 15 L/min. These parameters produce a typical dilution rate of 10–15% from the steel substrate, which is acceptable for cemented carbide overlays. The key process challenges include:
- WC particle cracking: Caused by thermal shock during deposition and subsequent cooling; mitigated by optimizing laser power and preheating
- Porosity: Gas entrapment from incomplete melting or powder feed irregularities; controlled by maintaining stable powder flow and adequate shielding
- Dilution control: Excessive base metal dilution reduces WC volume fraction; managed by adjusting scanning speed and powder feed rate
- Cracking at overlay-substrate interface: Thermal mismatch between cemented carbide and steel; addressed through proper heat treatment
The heat treatment process itself introduces specific defects that must be controlled. Solution treatment temperatures exceeding 1200°C can cause WC dissolution, while temperatures below 1000°C may not fully relieve residual stresses. The aging temperature window of 650–750°C is critical: lower temperatures may not promote sufficient γ-Co precipitation, while higher temperatures risk over-aging and coarsening of the binder phase.
Standards and Quality Control
For cemented carbide overlay applications in pressure vessel and heavy equipment manufacturing, the following quality standards apply:
- ASTM A263: Qualification requirements for overlay welding of stainless steel and nickel alloy cladding, with analogous testing procedures applicable to cemented carbide overlays
- ASTM E384: Rockwell hardness testing methodology for surface hardness verification
- ASTM G65: Pin-on-disc wear testing standard for quantitative wear performance evaluation
- JB/T 4730: Non-destructive testing requirements including magnetic particle inspection for surface defect detection
- NB/T 47014: Qualification of welding procedures and welders for pressure equipment
The bond strength between the cemented carbide overlay and the steel substrate must meet a minimum of 400 MPa per ASTM A263 guidelines. After heat treatment, the overlay should be inspected for any new cracking or delamination that may have occurred during thermal cycling. Metallographic examination at 500x magnification is recommended to verify the integrity of the WC-Co interface and the absence of excessive carbide dissolution.
Engineering Practice Integration
Cemented carbide overlay layers with proper heat treatment have been successfully applied to drill bits, roller mill liners, and high-pressure valve seats in the oil and gas industry. A notable application involves the overlay repair of a 300 mm diameter ball valve seat operating at 15 MPa in a sour gas service, where the ST+AG heat-treated overlay achieved a service life of over 18 months compared to 3 months for the un-treated overlay. The improved wear resistance and reduced microcracking directly translated to fewer maintenance shutdowns and lower total cost of ownership.
For pressure vessel applications, cemented carbide overlays are typically limited to small areas such as valve seats, pump impellers, or heat exchanger tube ends where extreme wear resistance is required. The high thermal conductivity mismatch between cemented carbide and steel necessitates careful thermal management during fabrication and service, and the brittle nature of WC-Co requires that the overlay thickness be kept within recommended limits (typically 1–3 mm) to avoid catastrophic spalling.
Study Insights and Implications
This literature establishes that post-weld heat treatment is not optional but essential for achieving optimal performance from cemented carbide overlay layers. The solution-plus-aging treatment provides the best balance of wear resistance, toughness, and interface integrity, making it the recommended condition for most industrial applications. The key insight is that the as-deposited microstructure, while hard, is not necessarily the best for wear performance due to residual stresses and microcracking. Engineers should always include a heat treatment step in their process specification for cemented carbide overlays, and the specific parameters should be validated through metallographic examination and wear testing before production deployment. The cost of heat treatment is negligible compared to the cost of the cemented carbide powder itself, making this an easily justified process investment.