Hot Rolling Mill Roll Cladding Materials and Process Investigation

Literature Overview

This study, published in Mechanical Engineering Materials in 1998 by Shen Fenggang, Lu Xuegang, Xu Lengqian, and Xue Jin from the School of Mechanical Engineering at Xi'an Jiaotong University, addresses the critical challenge of extending the service life of hot rolling mill rolls through cladding technology. Hot rolling mill rolls are subjected to extreme conditions including high temperatures (up to 1200 °C), severe mechanical loading, thermal cycling, and chemical interaction with the hot steel being rolled. The development of effective cladding materials and processes is therefore of paramount importance to the steel industry.

Core Technical Content

The study systematically investigated various cladding materials and processes suitable for hot rolling mill roll applications. The primary cladding processes considered included:

• Electroslag welding (ESW) cladding: Suitable for thick cladding layers on roll bodies
• Submerged arc welding (SAW) cladding: Versatile process for various cladding geometries
• Flame cladding: For specific alloy compositions and thin coatings
• Plasma arc cladding: For high-quality surface layers with controlled dilution

The cladding materials evaluated were primarily chromium-based alloys with varying compositions designed to provide combinations of hardness, thermal fatigue resistance, and spalling resistance.

Material Selection and Design Principles

The selection of cladding material for hot rolling mill rolls depends on several factors related to the specific rolling application:

1. Roll type: Roughing rolls, intermediate rolls, and finishing rolls experience different service conditions and require different material properties. Roughing rolls require high thermal fatigue resistance and spalling resistance, while finishing rolls require high hardness and wear resistance for surface quality.
2. Steel being rolled: The composition and temperature of the steel being rolled affect the chemical interaction between the roll surface and the steel. Sulfide inclusion formation in the steel can lead to roll surface damage, requiring specific alloy additions in the cladding material.
3. Rolling conditions: Rolling speed,压下 (reduction), and temperature affect the thermal and mechanical loads on the roll surface. Higher temperatures and speeds increase the severity of thermal fatigue.
4. Service life requirements: The expected number of heats or tonnage rolled before regrinding or replacement determines the required coating thickness and material properties.

Process Development and Key Findings

The researchers investigated the process parameters for each cladding method and evaluated the resulting coating quality through metallographic examination, hardness testing, thermal fatigue testing, and field trials.

ESW Cladding: ESW produces thick, uniform cladding layers with good metallurgical bonding. The high heat input promotes complete melting of the cladding material, resulting in homogeneous composition. However, the high dilution from the substrate (typically 15-25%) reduces the effectiveness of the cladding alloy. Process parameters include:

• Welding current: 800-1200 A
• Voltage: 40-50 V
• Travel speed: 200-400 mm/min
• Electrode feed rate: 10-15 m/h
• Flux composition: Rutilic or basic fluxes

SAW Cladding: SAW offers better dilution control than ESW, with typical dilution rates of 10-15%. The process is well-suited for building up cladding thickness in multiple passes. Key parameters include:

• Current: 400-600 A
• Voltage: 28-35 V
• Travel speed: 300-600 mm/min
• Wire feed speed: 5-10 m/min
• Flux coverage: Adequate to prevent oxidation

Flame Cladding: Flame cladding using oxy-acetylene or oxy-propane torches with wire or powder feed is suitable for thin coatings and repair applications. The dilution rate is typically low (5-10%), preserving the cladding alloy properties. However, the process is less productive and requires skilled operators.

Microstructural Analysis

The microstructure of the cladding layers was examined through metallographic techniques. Key observations included:

• Cr20-Cr40 alloys: These hypereutectic chromium cast irons exhibit a microstructure consisting of ledeburite-like carbides in a martensitic or pearlitic matrix. The carbide morphology and distribution significantly affect wear and thermal fatigue resistance.
• FeCrB alloys: These alloys contain boron carbide particles in a martensitic matrix. The boron carbide particles provide excellent wear resistance but can be brittle and susceptible to cracking.
• Dilution zone: The region near the fusion boundary shows a gradient in composition and microstructure, transitioning from the cladding alloy to the substrate. This zone is critical for bonding strength and crack resistance.

Engineering Practice Integration

The study provided practical recommendations for implementing cladding on hot rolling mill rolls in production environments:

• Surface preparation: Roll surfaces must be machined to a precise roundness and surface finish (Ra 3.2-6.3 μm) before cladding. Any surface defects must be repaired before cladding begins.
• Preheating: Preheating the roll to 200-300 °C reduces thermal stress during cladding and minimizes the risk of cracking. The preheat temperature depends on the roll material and cladding alloy.
• Multi-pass strategy: For thick cladding layers, multiple passes are required. Each pass should overlap the previous pass by 30-50% of the bead width to ensure uniform coverage and bonding.
• Post-weld treatment: Stress relief annealing at 500-600 °C for 2-4 hours can reduce residual stresses in the cladding layer and improve service life. However, this must be performed carefully to avoid softening the cladding alloy.
• Quality control: Each cladding operation should be inspected for defects including porosity, cracking, incomplete fusion, and excessive dilution. Non-destructive testing methods such as magnetic particle inspection (MT) and ultrasonic testing (UT) are recommended.

Defect Analysis

Study Insights and Implications

This 1998 study represents a significant contribution to the understanding of hot rolling mill roll cladding technology in China. The systematic approach to material selection and process development provides a comprehensive framework for engineers working in this field. Several key insights emerge from the research:

1. Material-process synergy: The optimal cladding material depends not only on the service conditions but also on the cladding process used. For example, high-chromium alloys that are prone to cracking may be more successfully applied using ESW with its high heat input, while more dilution-sensitive alloys may be better applied using flame cladding.
2. Thermal fatigue as the primary failure mode: For hot rolling mill rolls, thermal fatigue rather than wear is often the primary failure mechanism. This insight guides material selection toward alloys with good thermal fatigue resistance rather than simply maximizing hardness.
3. Importance of dilution control: The dilution rate from the substrate significantly affects coating performance. Processes with lower dilution rates generally produce better coatings, but may require more passes and longer production times.
4. Field validation: The study emphasized the importance of field trials to validate laboratory results. Laboratory testing, while valuable, cannot fully replicate the complex service conditions experienced in actual rolling mills.

The work by these researchers at Xi'an Jiaotong University contributed to the development of domestic cladding technology for the Chinese steel industry, reducing dependence on imported rolls and coatings. The methodology and findings remain relevant today, providing a foundation for more advanced cladding technologies that have since been developed.