Surfacing Technology Summary for 750 Bloom Mill Rolls

Literature Overview

This 1990 paper published in "Shandong Metallurgy" (Volume 12, Issue 3) presents a technical summary of the surfacing technology applied to rolls in a 750 mm bloom mill. The classification codes TG333.17 and TG455 indicate the focus on rolling mill equipment and welding processes, respectively. Although the paper is over three decades old, the fundamental principles of roll surfacing technology remain relevant to modern rolling mill maintenance and refurbishment practices.

Technical Background

The 750 mm bloom mill is a primary rolling mill that reduces hot-rolled slabs (typically from a continuous caster or ingot) to blooms of a smaller cross-section. The rolls in such a mill are subjected to extreme conditions: high temperature, high contact stress, abrasive wear from scale and oxide layers, and thermal cycling. Surfacing (hardfacing) is a standard practice to extend roll life by depositing a wear-resistant layer on the roll surface.

The surfacing process for rolling mill rolls involves:

Roll preparation: The worn roll surface is ground to remove damaged material and provide a clean, slightly roughened surface for bonding.
Preheating: The roll is preheated to 200-400°C to reduce thermal stresses and minimize the risk of cracking.
Surfacing deposition: Multiple passes of surfacing material are deposited using SMAW, SAW, or oxy-fuel processes.
Post-weld treatment: The roll may be stress-relieved, quenched, and tempered to optimize the microstructure and hardness of the surfacing layer.
Machining and finishing: The roll surface is ground to the required profile and finish.

Surfacing Material Selection

The selection of surfacing material depends on the specific service conditions and the desired balance between wear resistance, toughness, and thermal stability.

Surfacing Material Type	Typical Composition	Hardness (HV)	Application
High-carbon martensitic	1.5-2.5% C, 0.5-1.0% Cr	500-650	General wear, moderate impact
Medium-carbon martensitic	0.8-1.5% C, 0.5-1.0% Cr	400-500	Impact-resistant applications
Austenitic	0.5-1.0% C, 10-20% Mn, 3-8% Cr	250-400	Impact-resistant, thermal shock
Ductile iron (nodular)	3.0-3.6% C, 1.5-2.5% Si, 0.1-0.3% C (matrix)	300-450	High impact, thermal cycling
Stellite-type	Co-Cr-W base	400-600	Severe abrasive wear
Chrome carbide	Cr7C3, Cr3C2 in iron matrix	800-1200	Severe abrasive wear, low impact

For a 750 mm bloom mill, the surfacing material must withstand:

Thermal shock: The roll surface is exposed to temperatures of 800-1200°C during rolling, with rapid cooling between passes.
Abrasive wear: Scale and oxide layers are removed from the slab surface and abrade the roll surface.
Contact stress: The contact pressure between the roll and the slab can exceed 1000 MPa.
Impact loading: Slab entry and exit, as well as roll change operations, introduce impact loads.

Surfacing Process Parameters

The surfacing process for rolling mill rolls requires careful control of welding parameters to ensure proper bonding, uniform hardness, and minimal distortion.

Parameter	Typical Value	Rationale
Preheat temperature	250-350°C	Reduces thermal stress, prevents cracking
Welding process	SMAW or SAW	SMAW for repair, SAW for full surfacing
Current	150-250 A (SMAW)	Adequate penetration without excessive heat
Voltage	20-30 V (SMAW)	Stable arc, consistent deposition
Travel speed	10-20 cm/min (SMAW)	Controls heat input and deposition rate
Interpass temperature	150-250°C	Prevents excessive softening, controls cooling rate
Number of passes	3-8 (depending on thickness)	Builds up required surfacing thickness
Post-weld cooling	Controlled (furnace or air)	Prevents thermal shock cracking
Post-weld heat treatment	Quench + temper (if martensitic)	Optimizes hardness and toughness

Quality Control and Defect Analysis

Quality control for roll surfacing operations includes:

Pre-Weld Inspection

Visual inspection of the roll surface for existing cracks, gouges, or other defects.
Magnetic particle testing (MT) of the roll body to detect subsurface cracks.
Measurement of roll diameter and profile to determine the required surfacing thickness.

In-Process Monitoring

Preheat temperature verification using thermocouples.
Interpass temperature monitoring.
Visual inspection of each pass for lack of fusion, undercut, or excessive spatter.

Post-Weld Inspection

Visual inspection of the complete surfacing layer.
MT of the surfacing layer and interface.
Hardness survey across the surfacing layer to verify uniformity.
Dimensional measurement of the roll profile.

Defect Type	Potential Cause	Detection Method	Countermeasure
Surface cracking	Excessive cooling rate, high carbon equivalent	Visual, MT	Increase preheat, reduce heat input
Undercut	Excessive arc length, wrong travel angle	Visual, profile gauge	Maintain arc length, correct angle
Lack of fusion	Insufficient current, poor surface prep	UT, MT	Increase current, improve surface prep
Hardness non-uniformity	Inconsistent deposition, parameter drift	Hardness survey	Process parameter control, operator training
Spalling	Poor interface bonding, thermal mismatch	UT, pull-off test	Proper surface prep, transition layer

Engineering Practice and Modern Relevance

Although this paper was published in 1990, the principles of roll surfacing technology have remained largely unchanged. Modern developments include:

Advanced surfacing materials: High-entropy alloys, nanostructured coatings, and functionally graded materials are now available for specialized applications.
Automated surfacing systems: Robotic welding systems provide consistent parameter control and reduce operator variability.
Thermal spray technologies: HVOF (High-Velocity Oxygen-Fuel) and plasma spraying offer alternative methods for depositing wear-resistant coatings with fine microstructures.
In-situ monitoring: Real-time monitoring of welding parameters and thermal profiles allows immediate corrective action.

However, the fundamental challenges—controlling residual stress, ensuring metallurgical bonding, achieving uniform hardness, and managing thermal distortion—remain the same. The systematic approach to process design, parameter optimization, and quality control described in this paper remains a valid and effective framework for roll surfacing operations.

Study Insights

This technical summary provides a comprehensive overview of the surfacing technology applied to a specific industrial application—the 750 mm bloom mill rolls. The paper's value lies in its practical orientation: it translates fundamental welding principles into specific process parameters, material selections, and quality control procedures that can be directly applied in a production environment. For engineers involved in rolling mill maintenance, this paper serves as a useful reference for process design and troubleshooting. The key insight is that successful surfacing of rolling mill rolls requires a holistic approach that integrates material science, process engineering, and quality management, with each element carefully optimized to address the unique demands of the application.