Hot Rolling Mill Roll Cladding Materials and Process Research

Literature Overview

This 1998 study from Xi'an Jiaotong University addresses the cladding of hot rolling mill rolls, a critical application in the steel industry where roll surface integrity directly affects product quality, production efficiency, and equipment availability. Hot rolling mill rolls operate under extreme conditions characterized by high temperatures (up to 1200 °C at the roll surface), severe thermal cycling, mechanical loading (contact pressures exceeding 2 GPa), and chemical interaction with the steel being rolled. The study examines both the selection of cladding materials and the optimization of welding processes to produce durable roll surface coatings.

Core Technical Content

Hot rolling mill rolls typically consist of a cast iron or steel body with a hardened and wear-resistant surface layer. Conventional approaches include through-hardening of the entire roll, induction hardening of the surface, or the use of high-carbon cast iron compositions. However, these methods have limitations in terms of hardness uniformity, thermal fatigue resistance, and service life. Weld cladding offers a more flexible approach by allowing the selection of a specialized surface alloy that can be optimized for specific rolling conditions.

Cladding Material Selection

The study evaluates several cladding material systems for hot rolling mill roll applications:

Material System	Hardness (HV)	Thermal Fatigue Resistance	Wear Resistance	Oxidation Resistance	Cost
High-carbon martensitic steel (1.2–1.5% C)	600–700	Moderate	Good	Moderate	Low
Co-based alloy (Stellite type)	400–500	Excellent	Good	Excellent	High
Ni-based alloy (Inconel type)	350–450	Excellent	Moderate	Excellent	High
High-speed steel (W6Mo5Cr4V2)	800–900	Poor	Excellent	Poor	High
Fe-Cr-Al alloy (15Cr15Al)	500–600	Good	Moderate	Excellent	Moderate
Cu-based alloy (Cu-Cr-Zr)	200–300	Excellent	Moderate	Good	Moderate

For hot rolling applications, the optimal material depends on the specific rolling condition:

Hot slabbing mills: High thermal fatigue resistance is critical. Co-based or Ni-based alloys are preferred, with Cu-based alloys as a cost-effective alternative.
Hot strip mills (roughing stands): A balance of thermal fatigue resistance and wear resistance is needed. High-carbon martensitic steels or Fe-Cr-Al alloys are suitable.
Hot strip mills (finishing stands): Surface quality is paramount. Ni-based alloys with controlled microstructure provide the best surface finish.
Hot wide-band mills: Wear resistance is the dominant requirement. High-carbon martensitic steels or high-speed steel compositions are appropriate.

Welding Process Selection

The welding process must be selected based on the roll geometry, the required coating thickness, and the production volume. The following processes are considered:

Process	Coating Thickness	Deposition Rate	Dilution	Surface Quality	Equipment Cost
Submerged arc welding (SAW)	3–10 mm	High	Moderate	Poor (requires grinding)	Low
Gas metal arc welding (GMAW)	1–5 mm	Moderate	Moderate	Fair	Low
Plasma transferred arc (PTA)	0.5–3 mm	Moderate	Low	Good	Moderate
Powder flame spraying	0.5–2 mm	High	None	Good	Moderate
Cold spray	0.5–2 mm	Moderate	None	Excellent	High
Electroslag welding (ESW)	5–20 mm	High	High	Poor	Moderate

For hot rolling mill roll cladding, PTA and SAW are the most commonly used processes. PTA offers better dilution control and surface quality, while SAW provides higher deposition rates for thicker coatings. A hybrid approach using SAW for the bulk of the coating and PTA for the final surface pass is often employed in industrial practice.

Process Parameter Optimization

The following process parameters were optimized for PTA cladding of hot rolling mill rolls:

Parameter	Optimal Value	Acceptable Range	Effect of Deviation
Plasma current	250 A	200–300 A	Too low: incomplete melting; Too high: excessive dilution
Arc voltage	32 V	28–38 V	Too low: poor wetting; Too high: excessive spatter
Travel speed	300 mm/min	250–350 mm/min	Too slow: excessive dilution; Too fast: incomplete melting
Powder feed rate	0.5 g/s	0.3–0.8 g/s	Too low: thin coating; Too high: poor flowability
Shielding gas flow	20 L/min	15–25 L/min	Too low: oxidation; Too high: arc instability
Preheat temperature	200–300 °C	150–400 °C	Too low: cracking; Too high: coarse microstructure

Defect Analysis and Quality Control

Hot rolling mill roll cladding is subject to several characteristic defects:

Defect	Cause	Consequence	Prevention
Cracking	Thermal stress; high carbon content	Roll failure; production stoppage	Control carbon content; use ductile transition layer
Delamination	Poor bonding; thermal expansion mismatch	Loss of coating; roll damage	Proper substrate preparation; control dilution
Hardness variation	Non-uniform cooling rate	Uneven wear; surface quality defects	Optimize travel speed; use multi-pass strategy
Inclusion contamination	Flux contamination; powder contamination	Reduced coating life; surface defects	Use high-purity consumables; clean environment
Excessive dilution	High heat input; thin coating	Reduced coating hardness; loss of properties	Reduce current; increase travel speed; use finer wire

Engineering Practice and Implementation

The implementation of roll cladding technology in a steel mill requires careful planning and execution:

Pre-welding preparation: The roll body must be thoroughly cleaned to remove scale, oil, and contamination. Any existing cracks or defects in the roll body must be repaired before cladding. The roll should be preheated uniformly to 200–300 °C to reduce thermal stress.
Cladding execution: The cladding should be performed in a controlled environment with minimal vibration and temperature variation. Multiple passes should be applied with interpass temperature monitoring. The final pass should be performed with the finest consumable to achieve the best surface quality.
Post-welding treatment: After cladding, the roll surface should be ground to the required dimensional accuracy (typically Ra < 0.8 μm for hot rolling applications). A low-temperature stress relief treatment (300–400 °C) may be applied to reduce residual stresses without affecting the coating hardness.
Quality verification: Hardness testing should be performed at multiple locations across the roll surface. Ultrasonic testing should be used to verify bonding quality. Dimensional inspection should confirm that the roll geometry meets specification.
Service monitoring: During hot rolling operations, the roll surface should be monitored for signs of cracking, spalling, or excessive wear. Regular inspection intervals should be established based on the roll application and the expected service life of the coating.

Study Insights and Implications

This research from 1998 provides a foundational understanding of the materials and processes relevant to hot rolling mill roll cladding. While the specific alloys and processes have evolved since publication, the fundamental principles remain valid: the selection of cladding material must be matched to the specific rolling conditions, and the welding process must be carefully controlled to achieve the desired microstructure and properties.

A key insight is that the cladding material is not the sole determinant of roll performance. The interaction between the coating and the roll body, including thermal expansion compatibility and bonding strength, is equally important. Engineers should adopt a systems-based approach to roll cladding, considering the entire roll assembly (body, transition layer, coating) as an integrated component rather than treating the coating as an isolated surface modification.

The work also highlights the economic benefits of roll cladding technology. By extending roll life by 2–5 times compared to conventional hardening methods, the total cost of ownership is significantly reduced, even when accounting for the additional cost of the cladding process. For steel mills operating at high production volumes, the payback period for roll cladding technology is typically less than 12 months.