Cladding Technology Summary for 750 mm Bloom Mill Rolls

Literature Overview

This 1990 technical summary published in Shandong Metallurgy provides a comprehensive overview of the cladding technology applied to 750 mm bloom mill rolls at a major Chinese steel mill. The paper documents the evolution of roll surface protection technology from simple hardfacing welds to multi-layer composite cladding systems designed to extend roll life in the demanding conditions of hot rolling mill service. Bloom mill rolls operate under extreme conditions involving high temperatures (800 to 1100 degrees Celsius at the roll surface), high contact pressures (100 to 300 MPa), severe abrasive wear from scale and iron oxide, and cyclic thermal loading.

Service Conditions and Failure Mechanisms

The 750 mm bloom mill processes slabs at temperatures of 900 to 1200 degrees Celsius through multiple stands with reduction ratios of 2:1 to 4:1 per pass. The roll surface experiences:

The primary failure mechanisms for bloom mill rolls include: abrasive wear from scale particles (dominant mechanism, accounting for 60 to 80 percent of roll wear); oxidative wear from scale formation and removal; thermal fatigue cracking from cyclic temperature changes; and plastic deformation under high contact pressure in the roll neck region.

Cladding Technology Evolution

First Generation: Single-Layer Hardfacing

The initial approach used a single layer of high-carbon martensitic hardfacing alloy (typically 4 to 6 mm thick) applied by submerged arc welding (SAW) or electroslag welding (ESW). The overlay composition was high in carbon (1.5 to 3.0 percent), chromium (8 to 15 percent), and manganese (2 to 5 percent), producing a hardness of 55 to 65 HRC in the as-welded condition.

Limitations: Cracking due to high carbon content and thermal stress; spalling of the overlay layer from the base material; limited service life of 200 to 400 slabs.

Second Generation: Multi-Layer Composite Cladding

The improved approach employed a multi-layer composite structure:

Performance: Service life increased to 500 to 800 slabs, representing a 2 to 3 times improvement over single-layer hardfacing.

Third Generation: Optimized Multi-Layer with Process Controls

The final evolution incorporated improved process controls and material optimization:

• Preheating: 250 to 350 degrees Celsius to reduce thermal gradient and prevent base material cracking
• Interpass temperature: 200 to 300 degrees Celsius to control microstructure
• Post-weld treatment: Stress relief at 550 to 650 degrees Celsius for 2 to 4 hours
• Surface treatment: Shot peening or induction quenching to introduce compressive residual stresses
• Roll grinding: Precision grinding to achieve surface finish Ra 0.8 to 1.6 micrometers and geometric accuracy

Performance: Service life achieved 800 to 1200 slabs, with some rolls reaching 1500 slabs under optimal operating conditions.

Process Parameters and Quality Control

Welding Process Parameters

Quality Control Measures

The quality control program for roll cladding included:

1. Pre-weld inspection: Visual examination and magnetic particle testing of the roll surface to identify existing defects; hardness testing of the base material to verify proper material condition.
2. In-process monitoring: Visual inspection of weld beads for proper profile and absence of cracks; periodic hardness testing of the overlay layers to verify compliance with specifications.
3. Post-weld inspection: Ultrasonic testing (UT) for subsurface defects; magnetic particle testing (MT) for surface and near-surface cracks; dimensional inspection after stress relief and grinding.
4. Performance verification: Hardness mapping across the full overlay cross-section; microstructural examination of critical regions (bond line, surface layer).

Common Defects and Remediation

Engineering Practice Insights

The evolution of bloom mill roll cladding technology from single-layer to multi-layer composite systems represents a fundamental shift in engineering philosophy: from maximizing hardness to optimizing the balance of properties across the entire overlay cross-section. The multi-layer approach recognizes that different regions of the overlay experience different loading conditions and requires different material properties.

The bonding layer (Layer 1) serves as a stress accommodation zone, absorbing the thermal and mechanical stresses that arise from the mismatch between the overlay and base materials. Its low hardness and high toughness prevent crack initiation at the critical bond interface. The transition layer (Layer 2) provides a gradual property gradient, preventing abrupt changes that could serve as crack initiation sites. The wear-resistant layers (Layers 3 and 4) provide the functional surface protection.

The process control requirements for successful multi-layer cladding are significantly more demanding than for single-layer hardfacing. Precise control of preheating, interpass temperature, welding parameters, and post-weld treatment is essential to achieve the desired microstructure and property distribution. The investment in process control infrastructure (temperature monitoring systems, automated welding equipment, quality control facilities) is justified by the dramatic improvement in roll service life and the corresponding reduction in production downtime.

Study Reflection

This 1990 technical summary captures a critical period in the development of Chinese metallurgical equipment manufacturing, when systematic approaches to roll surface protection were being established. The documented evolution from simple hardfacing to sophisticated multi-layer composite cladding reflects the growing maturity of the industry and the increasing sophistication of materials engineering approaches. The emphasis on process control, quality assurance, and systematic defect analysis provides a model for modern overlay welding practice. The principles established in this work—gradual property transitions, stress accommodation, and comprehensive quality control—remain fundamental to the design and fabrication of high-performance cladded components in modern industrial applications.