CLADDING TECHNOLOGY SHANXI CO., LTDWear-Resistant Alloy Overlay Welding on Large Hot Rolling Slabbing Mill Rolls
Overview and Technical Significance
This 1992 publication by Zhang Zhongtao from Ansteel Northern Machinery Works addresses the specific challenge of applying wear-resistant alloy overlays to large-diameter slabbing mill rolls. Slabbing mill rolls are among the largest and most heavily loaded components in a steel mill, typically having diameters of 600–900 mm and lengths exceeding 2000 mm. They operate at temperatures of 800–1200 °C with contact stresses exceeding 2000 MPa, creating an environment where conventional case hardening is insufficient for economic service life.
The work documents a systematic approach to overlay welding large rolls, addressing challenges unique to large-diameter components including thermal distortion, residual stress management, and weld sequence optimization.
Process Design for Large Diameter Rolls
Challenges Specific to Large Rolls
Large slabbing mill rolls present unique overlay welding challenges:
- Thermal distortion – The large mass provides thermal inertia, but uneven heating can cause barrel distortion or ovality exceeding tolerance.
- Residual stress – Thick sections trap heat, creating high residual stresses that can cause delayed cracking or dimensional instability.
- Accessibility – The large diameter makes automated welding challenging; manual or semi-automated methods may be required.
- Cooling rate – The massive section provides slow cooling, which can lead to coarse microstructure in the overlay if not managed.
Welding Sequence Optimization
The authors developed a welding sequence based on thermal analysis principles:
- Start at the center of the roll length and weld toward both ends to minimize longitudinal distortion.
- Use a skip pattern – weld short beads (200–300 mm) and skip forward 2–3 beads before returning to fill skipped sections.
- Maintain interpass temperature below 300 °C by allowing natural cooling or using controlled water cooling between passes.
- Rotate the roll between passes to distribute heat around the circumference.
| Parameter | Value |
|---|---|
| Roll diameter | 600–900 mm |
| Roll length | 2000–2500 mm |
| Overlay thickness | 8–15 mm |
| Number of layers | 4–6 passes |
| Preheat | 200–300 °C (entire roll) |
| Interpass temperature | ≤300 °C |
| Post-weld tempering | 580–620 °C × 4 h |
| Maximum allowable ovality | ≤0.1% of diameter |
Overlay Material Selection
For slabbing mill service, the authors selected high-speed steel (HSS) type hardfacing alloys with the following typical composition:
| Element | C | Cr | W | Mo | V | Co |
|---|---|---|---|---|---|---|
| Content (%) | 4.0–5.5 | 10–14 | 8–12 | 3–5 | 4–6 | 0–5 |
The as-welded hardness was HRC 60–65, reduced to HRC 55–58 after tempering. The microstructure consisted of M7C3 and M2C carbides in a martensitic matrix, providing excellent resistance to abrasive wear at elevated temperatures.
Quality Control and Inspection
Non-Destructive Testing Protocol
Given the criticality of slabbing mill rolls, a comprehensive NDT protocol was established:
- Visual testing (VT) – 100% of overlay surface for cracks, undercut, and porosity.
- Magnetic particle testing (MT) – 100% of overlay surface for surface and near-surface cracks.
- Ultrasonic testing (UT) – Spot check for subsurface cracks and incomplete fusion at the overlay-base interface.
- Hardness testing – Grid pattern across the overlay surface, minimum 10 points per roll.
- Dimensional inspection – Ovality, taper, and surface roughness measurement.
Acceptance Criteria
| Test | Acceptance Criteria |
|---|---|
| Surface hardness | HRC 55–58 (after tempering) |
| Overlay thickness | Uniform within ±1 mm |
| Surface roughness | Ra ≤ 3.2 μm (after grinding) |
| Ovality | ≤ 0.1% of diameter |
| Cracks | No cracks longer than 5 mm |
| Porosity | No porosity > 1 mm in size |
Engineering Performance and Lessons Learned
The overlay-welded slabbing mill rolls demonstrated service life extensions of 3–5 times compared to conventionally hardened rolls. The primary wear mechanism was abrasive wear from scale removal, and the HSS-type overlay effectively resisted this mechanism through the hard carbide phases. However, the authors noted that thermal fatigue cracking could initiate at the overlay surface under rapid cooling conditions (such as water quenching between passes in the rolling mill). This was mitigated by selecting a slightly lower carbon content (4.0–4.5% rather than 5.0–5.5%) to improve thermal shock resistance at the expense of maximum hardness.
A critical lesson from this work is that large roll overlay welding requires careful thermal management throughout the entire process, from preheating through post-weld treatment. The massive section acts as a heat sink during welding but also traps heat, creating complex thermal gradients that must be managed through careful sequencing and temperature monitoring.
Study Insights
This work represents an important contribution to the understanding of hardfacing technology application on large-diameter components. The systematic approach to welding sequence design, combined with rigorous quality control, established a methodology that has been widely adopted in Chinese steel mills. The emphasis on post-weld tempering as a critical process step—rather than an optional treatment—reflects a mature understanding of residual stress management in thick-section components. Modern applications may benefit from combining the principles in this work with advanced techniques such as hot-wire TIG or multi-wire SAW for improved productivity, but the fundamental thermal management and metallurgical principles remain directly applicable.