CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Performance of Weld Overlay on Tension-Piercing Mill Rolls
Literature Overview
The 2014 study by Chen Hua, Wang Liyan, and Liu Xiaochun, published in Thermal Processing Technology, investigates the microstructure and mechanical properties of weld overlay layers on tension-piercing mill rolls used in the steel wire rod production industry. This research was supported by the Jilin Provincial Natural Science Foundation (Project 201115143) and involved collaboration between Changchun University of Technology and the National Automotive Parts Quality Supervision and Inspection Center. Tension-piercing rolls are critical components in wire rod mills, operating under extreme conditions of high temperature, mechanical contact stress, and thermal cycling. The overlay layer must simultaneously provide wear resistance, thermal stability, and sufficient toughness to withstand the dynamic loading from the wire rod being pulled through the roll surface.
Microstructural Analysis of the Overlay Layer
The overlay layer studied in this research was deposited using submerged arc welding (SAW) with a high-alloy consumable designed for hot work applications. The microstructural examination revealed a complex multi-phase structure that is characteristic of high-alloy overlay deposits:
| Phase | Distribution | Morphology | Hardness (HV) |
|---|---|---|---|
| Martensite | Matrix phase | Lath structure | 550–650 |
| Ferrite | Isolated islands | Equiaxed | 200–300 |
| Chromium carbides | Dispersed particles | Cubic, 2–8 μm | 1200–1500 |
| Manganese sulfides | Linear chains | Elongated | 600–800 |
| Retained austenite | Grain boundaries | Interdendritic | 250–350 |
The authors conducted detailed scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analysis to map the phase distribution across the overlay depth. A notable finding was the presence of a gradient in carbide size and distribution from the surface to the fusion zone. Near the surface, carbides were finer (2–4 μm) and more uniformly distributed, while deeper in the overlay, carbides grew larger (5–8 μm) with a tendency to form networks along prior austenite grain boundaries. This gradient is attributed to the varying cooling rates experienced during multi-pass deposition.
Heat Treatment Effects
The study also examined the effect of post-weld heat treatment on the overlay microstructure and properties. Normalizing at 850°C followed by oil quenching produced a predominantly martensitic structure with fine carbide dispersion, achieving a hardness of 62 HRC with improved toughness compared to the as-welded condition. The as-welded condition contained approximately 15–20% retained austenite, which while providing some toughness, reduced the effective hardness. The heat treatment reduced retained austenite to below 5%, maximizing hardness but potentially increasing susceptibility to quench cracking if improperly controlled.
| Condition | Hardness (HRC) | Impact Energy (J) | Carbide Size (μm) |
|---|---|---|---|
| As-welded | 58–62 | 25–35 | 4–8 |
| Normalized + quenched | 62–65 | 15–22 | 2–5 |
| Tempered (550°C, 2h) | 55–58 | 35–45 | 3–6 |
Engineering Performance Assessment
The overlay layer was evaluated against several key performance metrics relevant to tension-piercing roll service:
- Wear resistance: Measured by pin-on-disk abrasion testing, the overlay demonstrated 3–5 times the wear life of uncoated roll steel, with wear depth of 0.08–0.12 mm per 1000 cycles under standardized conditions.
- Thermal stability: After 50 thermal cycles between room temperature and 800°C, the overlay retained approximately 90% of its original hardness, indicating good thermal stability.
- Bond strength: Transverse tensile testing revealed bond strength exceeding 550 MPa, well above the base metal yield strength, confirming that failure would occur in the base metal rather than at the overlay interface.
- Cracking resistance: The overlay layer showed no surface cracks after 100 thermal cycles, though microcracks were observed at the fusion zone interface after 200 cycles.
Process Optimization Insights
The research identified several critical process parameters that significantly influence overlay quality:
- Deposition rate: Higher deposition rates (>2.0 kg/h) led to increased porosity and incomplete fusion between passes, while rates below 0.8 kg/h produced excessive dilution with the base metal, reducing overlay hardness.
- Interpass temperature control: Maintaining interpass temperature between 100–200°C was found to be optimal. Lower temperatures increased cracking susceptibility due to high thermal stresses, while higher temperatures promoted carbide coarsening and reduced hardness.
- Wire feeding speed consistency: Variations in wire feeding speed greater than ±5% resulted in irregular bead geometry and inconsistent dilution, leading to non-uniform hardness across the overlay surface.
FMEA Analysis of Common Defects
| Defect | Root Cause | Likelihood | Severity | Countermeasure |
|---|---|---|---|---|
| Surface cracks | High carbon equivalent, low interpass temp | High | High | Preheat 200°C, control interpass <200°C |
| Undercut | Excessive travel speed, improper gun angle | Medium | Medium | Reduce travel speed 10–15%, adjust angle |
| Porosity | Moist flux, inadequate shielding | Medium | High | Dry flux at 300°C for 2h, ensure gas flow |
| Excessive dilution | Low current, fast travel | Medium | Medium | Increase current 10%, reduce travel speed |
| Hardness non-uniformity | Inconsistent wire feed, varying dilution | High | Medium | Calibrate wire feeder, standardize parameters |
Key Reflections and Engineering Recommendations
The most valuable finding from this research is the demonstration that the overlay microstructure can be effectively controlled through systematic optimization of welding parameters and post-weld heat treatment. The gradient in carbide distribution from surface to fusion zone, while initially concerning, actually provides a beneficial property gradient: the harder surface layer resists abrasion while the tougher subsurface layer absorbs impact energy and prevents crack propagation. This natural gradient is a testament to the inherent benefits of multi-pass overlay welding.
From a practical engineering perspective, several recommendations emerge:
- The overlay thickness should be maintained at 3–5 mm to ensure adequate wear life while avoiding excessive thermal distortion of the roll body.
- A two-pass approach is recommended: a build-up pass with lower alloy content to ensure good fusion with the base metal, followed by the final overlay pass with the full alloy composition.
- Regular hardness mapping across the overlay surface during production should be conducted to detect parameter drift early. A hardness variation exceeding 5 HRC across the surface indicates process instability.
- The transition zone at the roll end should receive particular attention, as stress concentrations at geometric discontinuities can initiate overlay spalling.
Summary
This research provides valuable insights into the microstructure-property relationships in weld overlay layers for tension-piercing mill rolls. The systematic approach to microstructural characterization, combined with practical performance evaluation, offers a comprehensive understanding of how welding parameters and heat treatment influence overlay quality. The key engineering takeaway is that successful overlay application requires a holistic approach that considers not only the overlay composition but also the interaction between the overlay, base metal, and service conditions. The identified process windows and defect countermeasures provide practical guidance for improving overlay quality in industrial applications, ultimately extending roll life and reducing production downtime in wire rod mills.