CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Overlay Layer Number on Microstructure and Properties of Submerged Arc Welding WC-Reinforced Coatings
Literature Overview
This 2015 study published in Heat Processing Technology by Zhou Yongmei, Shen Yanjin, and Chen Xueyong investigates how the number of overlay layers affects the microstructure and mechanical properties of tungsten carbide (WC)-reinforced composite coatings deposited by submerged arc welding (SAW). The research was supported by the Hunan Provincial Department of Education Scientific Research Project (No. 14C1137). The work addresses a fundamental question in multi-pass overlay welding: how does the thermal history of successive passes influence the final coating quality?
Core Technical Content
Submerged arc welding is one of the most widely used processes for depositing thick overlay coatings due to its high deposition rate, low spatter, and ability to produce deep, uniform welds with good penetration. When WC particles are added to the flux or as a separate powder feed, the resulting composite coating can achieve extremely high hardness values (1,000 to 1,500 HV or higher), making it suitable for severe abrasion applications such as mining equipment, cement industry components, and hydraulic machinery.
The critical issue addressed in this study is the effect of the number of overlay layers (passes) on the final coating properties. Each additional layer subjects the previous layers to repeated thermal cycling, which can cause:
- Carbide dissolution: WC particles in previously deposited layers may partially dissolve during reheating by subsequent passes, reducing the hard phase content and hardness.
- Phase transformation: The thermal cycle can cause martensite to temper or transform, changing the matrix microstructure and properties.
- Residual stress accumulation: Each pass adds residual stress, and the interaction between stresses from different passes can lead to cracking or delamination.
- Dilution variation: The dilution ratio from the substrate is highest in the first layer and decreases with each subsequent layer, creating a non-uniform composition profile through the coating thickness.
Microstructural Evolution with Layer Number
| Layer Number | Dilution Ratio | WC Retention | Matrix Structure | Hardness (HV) | Cracking Risk |
|---|---|---|---|---|---|
| 1st layer | Highest (15–25%) | Partial dissolution | High-carbon martensite | 800–1,200 | High (hot + cold) |
| 2nd layer | Moderate (8–15%) | Better retention | Mixed martensite/bainite | 900–1,400 | Moderate |
| 3rd layer | Lower (5–10%) | Good retention | Tempered martensite | 1,000–1,500 | Lower |
| 4th+ layer | Lowest (3–8%) | Excellent retention | Stable martensite | 1,100–1,600 | Lowest |
The data above represents typical trends observed in multi-pass SAW overlay with WC reinforcement. The exact values depend on the specific flux composition, welding parameters, and substrate material.
Process Parameters
| Parameter | Typical Range | Notes |
|---|---|---|
| Welding current | 400–800 A | Higher current increases dilution and heat input |
| Arc voltage | 25–35 V | Affects arc stability and flux melting |
| Travel speed | 200–500 mm/min | Higher speed reduces dilution |
| Flux type | WC-containing flux or separate powder | Flux composition determines carbide formation |
| Flux composition | 5–15% WC + iron powder + glass flux | WC content affects hardness but also cracking |
| Interpass temperature | < 200 °C | Must be controlled to prevent excessive tempering |
| Layer thickness | 3–8 mm per pass | Thicker layers increase residual stress |
Mechanical Properties
The hardness profile through the coating thickness is typically non-uniform. The surface layer (last deposited) usually exhibits the highest hardness due to the lowest dilution and best WC retention, while the layer adjacent to the substrate has lower hardness due to higher dilution and possible carbide dissolution during subsequent passes.
The impact toughness of the coating is generally low due to the high hardness and brittle carbide phases. This is acceptable for abrasion-wear applications where the primary failure mode is material removal rather than fracture, but it becomes a concern for impact-wear applications such as mining equipment.
Defect Analysis
| Defect | Root Cause | Detection Method | Prevention |
|---|---|---|---|
| Cracking in first layer | High dilution, high carbon, hydrogen | Visual, MT, PT | Reduce current, increase travel speed, preheat |
| Delamination between layers | Residual stress, poor wetting | UT, TOFD | Control interpass temperature, use compatible flux |
| WC particle agglomeration | Poor powder mixing, segregation | Metallography | Thorough powder blending, consistent feeding |
| Excessive porosity | Moist flux, gas entrapment | RT, UT | Dry flux storage, adequate flux coverage |
| Inclusion of flux particles | Poor flux melting, excessive thickness | Metallography, MT | Optimize flux composition and welding parameters |
Engineering Practice Integration
In industrial practice, the number of overlay layers is determined by the required coating thickness and the acceptable dilution level. For thick coatings (over 10 mm), it is common to use 3 to 5 layers, with the understanding that the inner layers will have lower hardness than the outer layers. Some engineers use a hybrid approach, depositing the first 1 to 2 layers with a lower-carbon, more ductile flux to improve weldability, and then depositing the remaining layers with a high-carbon, WC-rich flux to maximize surface hardness.
The selection of the appropriate number of layers also depends on the service conditions. For components subjected to severe abrasion but minimal impact loading, a thicker coating with more layers is acceptable. For components subject to impact-abrasion wear, a thinner coating with fewer layers and lower residual stress may be preferable, even if the surface hardness is slightly lower.
Key Reflections
This study highlights a fundamental principle in multi-pass overlay welding: the thermal history of each layer is not independent but is coupled to all subsequent layers. This coupling means that the final properties of the coating are determined not only by the parameters of the last pass but by the entire sequence of passes. Engineers must therefore consider the entire welding sequence as a system rather than optimizing each pass in isolation. The number of layers, interpass temperatures, and parameter variations between passes all interact to determine the final coating quality.