Overlay Repair of 2800 Rolling Mill Bloom Roll - Literature Study Note

Literature Overview

This 2004 study by researchers from Wuhan Iron and Steel Company documents the overlay welding repair of bloom rolls for a 2800 mm rolling mill. Bloom rolls are among the most heavily loaded components in a hot strip mill, experiencing extreme thermal cycling, mechanical wear, and thermal fatigue during continuous operation. The overlay repair program represents a significant engineering challenge requiring careful selection of overlay materials, welding processes, and post-weld treatment strategies.

Technical Background and Challenges

Bloom rolls in a 2800 mm mill operate under conditions of:

Operating temperature: 900–1200 °C (contact with hot steel slabs)
Cooling method: Water spray cooling, causing thermal shock
Service life requirement: Minimum 6–12 months between repairs
Wear mechanism: Abrasive wear, thermal fatigue cracking, and erosion
Roll diameter: Typically 900–1200 mm
Roll length: Up to 2800 mm

The challenge of overlay repair on such large-diameter rolls involves managing residual stresses, ensuring uniform overlay thickness over the entire working surface, and preventing cracking due to the high carbon equivalent of the roll base metal (typically medium-carbon alloy steel such as 50Mn or 50MnV).

Overlay Material Selection

Material Option	Composition Type	Hardness (HV)	Application Suitability
Cr-Mo alloy steel	Medium alloy	280–350 HV	General wear resistance
High-speed steel	High alloy	500–650 HV	Severe abrasive wear
Maraging steel	Ni-based	450–600 HV	Thermal fatigue resistance
Ductile iron overlay	Fe-Cr-Ni-B	350–450 HV	Cost-effective repair

The research team evaluated multiple overlay material systems, ultimately selecting a Cr-Mo-V alloy steel overlay with a hardness range of 320–380 HV, which offered an optimal balance between wear resistance, thermal fatigue crack resistance, and weldability.

Process Design and Execution

Welding Procedure

The overlay repair process employed a combination of:

GMAW (gas metal arc welding) for打底 (root pass) and intermediate layers
SAW (submerged arc welding) for build-up layers to achieve required thickness
Post-weld heat treatment (PWHT) to relieve residual stresses

Process Parameter	Value	Rationale
Preheat temperature	250–300 °C	Prevent cold cracking in high-carbon base
Interpass temperature	≤ 300 °C	Control HAZ hardness
Overlay thickness	8–15 mm	Restore dimensions plus wear allowance
SAW current	800–1000 A	High deposition rate for build-up
SAW voltage	30–35 V	Stable arc for wide bead
Travel speed	300–500 mm/min	Optimize cooling rate
PWHT temperature	600–650 °C	Stress relief
PWHT holding time	2 h per 25 mm thickness	Adequate diffusion

Defect Analysis and Countermeasures

Defect Type	Cause	Countermeasure
Hot cracking	High sulfur in base metal	Preheat + low-sulfur consumables
Cold cracking	Hydrogen embrittlement	Preheat + low-hydrogen flux/wire
Thermal fatigue cracks	Residual stress + thermal cycling	PWHT + multi-pass with controlled interpass temp
Poor bond	Incomplete fusion at interface	Proper edge preparation + adequate heat input
Undercut	Excessive travel speed or arc length	Parameter optimization + backing bar

Quality Assurance and Performance Verification

The overlay repair quality was verified through:

Hardness survey: Cross-sectional hardness profile from overlay to base to confirm HAZ hardness below 350 HV (critical for preventing cracking)
Metallographic examination: Confirmation of sound bonding and absence of microcracks
Service performance monitoring: Tracking of roll life extension compared to new rolls
Dimensional verification: Post-grinding dimensional accuracy within ±0.5 mm

Engineering Practice Reflections

This study highlights a critical principle in overlay repair engineering: the overlay material selection must consider not only wear resistance but also thermal fatigue crack resistance for hot rolling applications. A hard overlay that cracks under thermal cycling provides no benefit. The successful repair program demonstrated that with proper procedure qualification, preheat control, and PWHT, bloom roll overlay repair can achieve 70–80% of the service life of new rolls at significantly lower cost.

The work also underscores the importance of considering the base metal's carbon equivalent when designing overlay repair procedures. High-carbon base metals require more aggressive preheating and slower cooling rates to prevent cracking, which in turn affects productivity. The engineering team's systematic approach to process development, including parameter optimization through trial welds and metallographic evaluation, serves as an excellent model for overlay repair program development.