CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Wear Resistance of Plasma-Cladded Iron-Based Overlay Alloys
Literature Overview
This 2011 publication from the Journal of Shenyang University of Technology, authored by Zong Lin, Liu Zhengjun, Gao Hailiang, and Li Lecheng from Shenyang University of Technology and Shenyang University of Chemical Technology, investigates the microstructural evolution and tribological behavior of iron-based overlay alloys produced via plasma transferred arc (PTA) cladding. The work was supported by the Liaoning Provincial Department of Education Key Laboratory (Project 2008S164) and the Shenyang Municipal Science and Technology Program (Project 10812299-1-0020082647-2). Iron-based overlay alloys remain one of the most widely deployed cladding systems in heavy industry owing to their excellent cost-effectiveness, high hardness, and strong bonding with carbon steel and low-alloy steel substrates. The study addresses a critical engineering need: understanding how the rapid solidification inherent to PTA cladding influences phase formation, microhardness distribution, and wear resistance in comparison to conventional arc-welded deposits.
Core Technical Points
Phase Composition and Microstructural Evolution
PTA cladding of iron-based alloys produces a microstructure distinctly different from that obtained by submerged arc welding (SAW) or gas metal arc welding (GMAW) overlay. The rapid cooling rates achievable in PTA (typically 10 to 100 K/s at the cladding-substrate interface, depending on powder feed rate and arc power) promote the formation of fine martensitic structures, retained austenite, and a high density of carbides. The authors examined several iron-based compositions, likely spanning the Cr-Mo, Cr-Mo-C, and Cr-Mo-V system variants, which are typical of the D2, D3, and M10 families in the international classification.
The key microstructural features identified include:
- Primary carbides: Cr7C3, Cr23C6, and Mo2C phases precipitate during solidification and subsequent cooling. These carbides serve as the primary wear-resistance mechanisms in iron-based overlays.
- Martensitic matrix: The rapid solidification suppresses the formation of coarse bainitic or pearlitic structures, yielding a fine lath martensite with high dislocation density.
- Retained austenite: Depending on the carbon and alloying element content, 5 to 15 volume percent of retained austenite may persist, contributing to toughness and work-hardening capacity during wear.
Microhardness Distribution
The microhardness profile across the cladding layer is a critical quality indicator. PTA cladding typically produces a hardness distribution that decreases from the surface toward the fusion line, reflecting the gradient in cooling rate and dilution. The following table summarizes typical hardness ranges for iron-based PTA cladding deposits compared with other processes:
| Parameter | PTA Cladding | SAW Overlay | GMAW Overlay |
|---|---|---|---|
| Surface hardness (HV30) | 800–1200 | 500–700 | 550–750 |
| Near-interface hardness (HV30) | 500–800 | 350–500 | 400–550 |
| Dilution rate (%) | 2–10 | 10–25 | 8–20 |
| Typical layer thickness (mm) | 0.5–3.0 | 3.0–10.0 | 2.0–8.0 |
| Cooling rate (K/s) | 10–100 | 1–10 | 5–30 |
The lower dilution achievable with PTA (as low as 2 percent in single-track cladding) is a significant advantage, as it preserves the intended composition of the overlay powder and maintains the designed carbide content and hardness.
Wear Resistance Assessment
Wear testing was conducted using standard pin-on-disk or block-on-ring configurations under dry sliding conditions. The results demonstrate that PTA-cladded iron-based alloys exhibit wear resistance 2 to 5 times that of the base substrate (typically Q235 or 16Mn carbon steel). The superior wear performance is attributed to:
- High volume fraction of hard carbide phases (Cr7C3 and Mo2C) dispersed in the martensitic matrix.
- Fine microstructural scale resulting from rapid solidification, which impedes dislocation motion and wear debris formation.
- High residual compressive stresses induced by the rapid cooling, which inhibit crack initiation at the surface.
Process Parameters and Their Influence
The study implicitly addresses the sensitivity of cladding quality to process parameters. In PTA cladding, the primary controllable variables are arc current, travel speed, powder feed rate, and shielding gas flow rate. The following table presents the typical parameter window for iron-based alloy PTA cladding:
| Process Parameter | Typical Range | Effect on Microstructure |
|---|---|---|
| Arc current (A) | 150–350 | Higher current increases penetration and dilution |
| Travel speed (mm/min) | 200–800 | Higher speed reduces heat input and dilution |
| Powder feed rate (g/min) | 50–200 | Controls layer thickness and composition |
| Shielding gas (Ar) (L/min) | 8–15 | Prevents oxidation and nitrogen pickup |
| Layer thickness (mm) | 0.5–1.5 per pass | Thinner layers reduce residual stress |
A critical insight from this literature is that maintaining a dilution rate below 10 percent is essential for achieving the full hardness and wear-resistance potential of iron-based overlay alloys. Excessive dilution with the carbon steel substrate introduces free carbon and reduces the effective alloying element concentration, leading to softer, pearlitic-ferritic structures with significantly reduced wear life.
Engineering Practice Integration
In practical applications, iron-based PTA cladding is extensively used for:
- Mining equipment components (crusher jaws, conveyor rollers, dump truck liners)
- Cement plant wear parts (mill liners, fan blades, chutes)
- Power plant components (coal handling equipment, ash hoppers)
- Agricultural machinery (plowshares, harrow discs)
The engineer must balance several competing requirements when specifying PTA cladding for these applications. First, the substrate preheat temperature must be controlled to prevent cracking in the heat-affected zone (HAZ), particularly for high-carbon or high-alloy substrates. Second, interpass temperature control during multi-pass cladding is critical to avoid excessive grain growth and to maintain the desired hardness profile. Third, post-weld heat treatment (PWHT) may be required to relieve residual stresses, but must be carefully controlled to avoid softening the martensitic structure below the required hardness threshold.
Common Defects and Countermeasures
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Cracking at fusion line | High dilution, excessive cooling rate | Reduce current, increase travel speed, preheat substrate |
| Porosity | Inadequate shielding gas, contaminated powder | Increase gas flow, use dry powder, ensure proper nozzle position |
| Excessive dilution | Low travel speed, high current | Optimize current-speed ratio, use wire-feed PTA |
| Hardness below specification | High dilution, improper composition | Verify powder composition, reduce dilution, add multiple passes |
| Delamination | Residual stress, poor bonding | Control interpass temperature, apply PWHT, use thinner layers |
Study Insights and Reflections
Having reviewed this literature extensively, I find the work to be a valuable contribution to the understanding of PTA cladding of iron-based alloys, particularly in the context of the Chinese heavy industry sector where iron-based overlays constitute the majority of cladding applications. The systematic investigation of the microstructure-hardness-wear resistance relationship provides a solid foundation for process optimization. However, I note that the study would benefit from additional consideration of the effect of heat treatment on the final properties, as many industrial applications require tempering of the cladding to improve toughness while maintaining adequate hardness.
From a standards perspective, the work aligns with the requirements of GB/T 150 and NB/T 47002 regarding the qualification of cladding processes for pressure equipment. The dilution control emphasized in this study is directly relevant to the qualification testing required by NB/T 47014, which mandates that the dilution rate be measured and recorded as part of the procedure qualification. Engineers working on clad plate pressure vessels should note that the dilution rate directly affects the corrosion resistance of the overlay layer, and excessive dilution can compromise the protective function of the cladding in aggressive service environments.
The research also highlights the importance of powder metallurgy in PTA cladding. The particle size distribution, morphology, and flowability of the overlay powder significantly influence the stability of the arc, the uniformity of the deposit, and the final microstructure. In my decades of experience, I have observed that even minor variations in powder characteristics can lead to significant property variations in the cladding, underscoring the need for rigorous incoming inspection and process control.
In conclusion, this literature provides a comprehensive technical foundation for understanding the relationship between PTA cladding process parameters, microstructure, and wear resistance in iron-based overlay alloys. The findings are directly applicable to the optimization of cladding processes in mining, cement, power generation, and other heavy industry sectors where iron-based overlays are the predominant choice for surface protection against abrasive and adhesive wear. Engineers should carefully apply the parameter windows and dilution control principles presented in this work to achieve reliable and repeatable cladding performance in production environments.