CLADDING TECHNOLOGY SHANXI CO., LTDWeld Overlay of Rolling Mill Rolls and Value Analysis
Introduction and Industrial Context
Rolling mill rolls are critical components in the metal forming industry, subjected to extreme conditions of high temperature, high pressure, and severe wear during operation. The working surface of a roll is exposed to contact with hot steel slabs, billets, or strips at temperatures ranging from 800 °C to 1200 °C, combined with compressive contact stresses exceeding 1 GPa and sliding friction. These conditions lead to rapid wear, thermal cracking, and spalling, which reduce roll life and increase production downtime.
Weld overlay cladding is a proven technology for extending roll life and improving rolling quality. By applying a wear-resistant alloy layer to the roll surface, the service life can be extended by 3 to 10 times compared to uncladded rolls. This study note examines the weld overlay processes, materials, and economic value associated with rolling mill roll cladding.
Roll Types and Service Conditions
| Roll Type | Service Temperature (°C) | Contact Stress (GPa) | Primary Wear Mechanism | Typical Overlay Material |
|---|---|---|---|---|
| Hot strip mill finishing roll | 900–1100 | 0.8–1.2 | Abrasive + thermal fatigue | Ni-Cr-Mo (NiResist), Co-Cr-W |
| Hot strip mill roughing roll | 1000–1200 | 1.0–1.5 | Abrasive + spalling | Cr-Mo cast iron, Ni-Cr |
| Cold strip mill roll | 20–50 | 0.5–0.8 | Galling + fatigue | Ni-based alloy, Cr-based |
| Wire mill roll | 800–1000 | 0.6–1.0 | Abrasive + thermal | Ni-Cr-Mo, Co-Cr |
| Plate mill roll | 900–1100 | 1.0–1.5 | Abrasive + spalling | Ni-Cr-Mo, Co-Cr |
The selection of overlay material depends on the specific service conditions. For hot rolling applications, nickel-based alloys with high thermal stability and crack resistance are preferred. For cold rolling applications, cobalt-based alloys with high hardness and galling resistance are more appropriate.
Weld Overlay Processes for Roll Cladding
Several welding processes are used for roll cladding, each with distinct advantages and limitations:
| Process | Typical Heat Input (kJ/mm) | Overlay Thickness (mm) | Application | Advantage | Limitation |
|---|---|---|---|---|---|
| Submerged arc welding (SAW) | 3.0–6.0 | 3.0–5.0 | Large diameter rolls | High deposition rate | High dilution (30–50%) |
| Plasma transferred arc (PTA) | 1.5–3.0 | 2.0–4.0 | Medium diameter rolls | Low dilution (15–25%) | Moderate deposition rate |
| Gas tungsten arc (GTAW) | 1.0–2.5 | 1.0–3.0 | Small diameter rolls | Precise control | Low deposition rate |
| Hot-wire TIG | 1.5–3.0 | 2.0–4.0 | Medium-large rolls | Good profile control | Equipment complexity |
| Laser cladding | 0.5–1.5 | 0.5–2.0 | Small rolls, repair | Very low dilution (5–10%) | Limited thickness per pass |
| Flame spraying + weld | N/A | 1.0–3.0 | Quick repair | Fast application | Lower bond strength |
The submerged arc welding process is the most commonly used for large-diameter hot rolling mill rolls due to its high deposition rate and ability to apply thick overlay layers. However, the high dilution rate means that multiple passes are required to achieve the desired overlay composition. The PTA process is preferred for medium-diameter rolls where dilution control is critical.
Overlay Material Selection and Microstructure
Nickel-Based Overlay Alloys
Nickel-based alloys such as NiResist 2 (Ni-15Cr-6Mo-4Cu) and Ni-Cr-Mo alloys are widely used for hot rolling mill roll cladding. These alloys exhibit excellent thermal stability, resistance to thermal cracking, and good resistance to abrasive wear at elevated temperatures.
| Material | Composition (wt%) | Hardness at 20 °C (HV30) | Hardness at 900 °C (HV30) | Thermal Stability |
|---|---|---|---|---|
| NiResist 2 | Ni-15Cr-6Mo-4Cu | 200–250 | 150–180 | Excellent |
| Ni-Cr-Mo (custom) | Ni-20Cr-5Mo-2Ti | 250–300 | 180–220 | Good |
| Stellite 6 | Co-28Cr-5.5W-5Mo | 380–450 | 250–300 | Excellent |
The microstructure of nickel-based overlay alloys consists of an austenitic matrix with dispersed carbides and intermetallic phases. The carbides (M7C3, M23C6) provide abrasive wear resistance, while the austenitic matrix provides ductility and resistance to thermal cracking.
Cobalt-Based Overlay Alloys
Cobalt-based alloys such as Stellite 6 and Stellite 21 are used for cold rolling mill rolls and high-temperature applications. These alloys exhibit excellent wear resistance at elevated temperatures and good resistance to galling.
| Material | Composition (wt%) | Hardness at 20 °C (HV30) | Hardness at 900 °C (HV30) | Galling Resistance |
|---|---|---|---|---|
| Stellite 6 | Co-28Cr-5.5W-5Mo | 380–450 | 250–300 | Good |
| Stellite 21 | Co-27Cr-5W-1.5Mo | 350–400 | 220–270 | Excellent |
| Stellite 25 | Co-25Cr-10W-5Mo |