CLADDING TECHNOLOGY SHANXI CO., LTDOverlay Welding Repair of Hot Shear Blades
Literature Overview
This 1992 study by Wan Weiguo from the Research Institute of Maanshan Iron and Steel Company addresses the practical challenge of overlay welding repair for hot shear blades used in steel mill operations. Hot shear blades are critical components in the continuous casting and rolling mills, where they must withstand extreme temperatures, cyclic thermal stress, and abrasive contact with hot steel slabs. The overlay welding repair methodology described in this study represents a cost-effective alternative to blade replacement, significantly reducing maintenance costs and downtime.
Core Technical Analysis
Hot shear blades experience a complex combination of wear mechanisms during service: abrasive wear from contact with hot steel, adhesive wear from material transfer, thermal fatigue from cyclic heating and cooling, and oxidative wear from exposure to hot scale and atmosphere. The overlay welding repair process must address all of these wear mechanisms simultaneously.
Material Selection for Overlay Layers
| Overlay Material | Hardness (HV) | Temperature Resistance (°C) | Wear Mechanism Resistance | Application Suitability |
|---|---|---|---|---|
| Martensitic stainless steel (440C) | 550–650 | 400 | Abrasive, adhesive | General purpose |
| High-speed steel (M2) | 700–850 | 500 | Abrasive | High abrasion |
| Cobalt-based (Stellite 6) | 400–500 | 900 | Abrasive, adhesive, thermal | High temperature |
| Hardfacing alloy (Fe-Cr-C) | 800–1000 | 300 | Abrasive | Low temperature abrasion |
| Nickel-based (Inconel 625) | 250–350 | 1000 | Thermal, corrosion | Thermal fatigue |
For hot shear blades operating at temperatures above 500°C, cobalt-based alloys such as Stellite 6 are the preferred choice due to their exceptional temperature resistance and wear performance. For blades operating at lower temperatures (below 400°C), martensitic stainless steels or high-speed steel overlays provide adequate performance at lower cost.
Process Parameters for Hot Shear Blade Repair
| Parameter | Value | Notes |
|---|---|---|
| Preheat temperature | 200–300°C | Prevents cracking in the base material |
| Welding method | GTAW or GMAW | GTAW for precision; GMAW for larger areas |
| Shielding gas | Argon (100%) or Ar-5%CO₂ | Argon for cobalt-based; Ar-CO₂ for iron-based |
| Current (GTAW) | 100–180 A | Depends on wire diameter and deposit thickness |
| Travel speed | 200–400 mm/min | Slower for better fusion; faster for lower dilution |
| Interpass temperature | <250°C | Prevents excessive grain growth |
| Number of passes | 2–4 | Depends on required overlay thickness |
| Post-weld treatment | Stress relief at 500–600°C/2h | Reduces residual stresses |
Defect Analysis and Repair Protocol
The overlay welding repair of hot shear blades is susceptible to several types of defects that must be identified and addressed through a systematic quality control protocol:
| Defect Type | Root Cause | Detection Method | Acceptance Criteria |
|---|---|---|---|
| Hot cracking | Thermal stress during solidification | Visual + MT | No cracks > 0.5 mm |
| Cold cracking | Hydrogen embrittlement in HAZ | UT after 24h delay | No cracks > 0.3 mm |
| Porosity | Gas entrapment or incomplete fusion | RT or UT | Porosity area < 5% |
| Lack of fusion | Poor surface preparation or insufficient heat | UT | No unfused areas |
| Excessive dilution | High heat input | Microstructural analysis | Dilution < 25% |
FMEA-Based Risk Assessment
| Failure Mode | Occurrence | Detection | Severity | RPN |
|---|---|---|---|---|
| Hot crack in overlay | 6/10 | 5/10 | 9/10 | 270 |
| Cold crack in HAZ | 4/10 | 4/10 | 8/10 | 128 |
| Interface delamination | 5/10 | 5/10 | 7/10 | 175 |
| Excessive dilution | 7/10 | 4/10 | 6/10 | 168 |
| Surface roughness | 8/10 | 8/10 | 4/10 | 256 |
The high risk priority number for hot cracking emphasizes the need for careful process control, including adequate preheating, controlled heat input, and post-weld stress relief. The interpass temperature control is particularly critical for preventing cold cracking in the heat-affected zone of the base material.
Engineering Practice and Maintenance Strategy
The overlay welding repair of hot shear blades is typically performed as part of a planned maintenance schedule, with the following recommended protocol:
- Blade inspection: Measure blade thickness, identify wear patterns, and determine the extent of material loss.
- Surface preparation: Grind the worn surface to remove scale, oxide, and damaged material, exposing fresh base metal.
- Preheating: Apply uniform preheat at 200–300°C using induction heating or torch preheating.
- Overlay welding: Apply 2–4 passes of overlay material, maintaining interpass temperature below 250°C.
- Post-weld treatment: Stress relieve at 500–600°C for 2 hours to reduce residual stresses.
- Quality inspection: Perform MT or PT inspection of the overlay surface, and UT inspection of the interface if required.
- Grinding and finishing: Grind the overlay surface to the required profile and finish quality.
- Final inspection: Verify dimensional accuracy and surface finish before returning the blade to service.
Study Insights and Engineering Implications
The research provides practical guidance for the overlay welding repair of hot shear blades, emphasizing the importance of material selection, process parameter control, and quality assurance. The choice of overlay material is critical and must be based on the specific service conditions, including operating temperature, wear mechanism, and expected service life.
A key insight from the study is that the overlay thickness must be carefully controlled to balance wear resistance with blade strength. Excessive overlay thickness can lead to reduced blade stiffness and increased risk of deformation during shearing operations. The recommended overlay thickness is typically 2–5 mm, depending on the blade design and service conditions.
The study also highlights the importance of surface preparation in achieving reliable overlay bonding. Incomplete removal of scale, oxide, and damaged material can lead to poor fusion and interface defects, which are difficult to detect through non-destructive testing. A systematic surface preparation protocol, including grinding to a clean metal surface and verification through visual inspection, is essential for ensuring overlay quality.
From an economic perspective, overlay welding repair extends the service life of hot shear blades by 3–5 times compared to replacement, with a cost reduction of 60–80%. This makes it a highly attractive maintenance strategy for steel mills and other industries where hot shear blades are used extensively.
The systematic approach to overlay welding repair presented in this study provides a valuable framework for engineers seeking to implement cost-effective maintenance solutions for wear-critical components, emphasizing the importance of material selection, process optimization, and quality control.