Sintered Flux Weld Overlay Repair of 650mm Rolling Mill Rolls

Literature Overview and Research Background

This 1997 publication by Zhu Yan from Dalian Second Steel Mill, published in Mechanical Design and Manufacturing, addresses the application of sintered flux in the weld overlay repair of 650mm rolling mill rolls. Rolling mill rolls are critical components in steel production, subjected to extreme mechanical and thermal loads during the rolling process. Surface damage, wear, and defects in the roll surface must be repaired promptly to maintain production efficiency and product quality. The study investigates the use of sintered flux in submerged arc welding (SAW) for overlay repair, focusing on the process parameters, microstructure, and mechanical properties of the deposited layer.

Rolling Mill Roll Service Conditions and Repair Challenges

Rolling mill rolls operate under severe conditions that include high contact pressures, sliding friction, thermal cycling, and the presence of scale and lubricants. The following table summarizes the typical service conditions and their implications for repair:

Service Condition	Typical Value	Repair Challenge
Contact pressure	100-300 MPa	Overlay must withstand high stress
Sliding speed	1-5 m/s	Overlay must resist adhesive wear
Roll surface temperature	200-600°C	Overlay must resist thermal degradation
Thermal cycling	50-200 cycles/hour	Overlay must resist thermal fatigue
Roll diameter	650 mm	Large surface area requires efficient repair
Service life	10-50 hours	Repair must be rapid and cost-effective

The repair of rolling mill rolls must be performed quickly to minimize production downtime, while ensuring that the overlay layer provides adequate wear resistance and mechanical strength for the remaining service life.

Sintered Flux Characteristics and Advantages

Sintered flux is a type of welding flux that is manufactured by mixing raw materials, forming into granules, and sintering at elevated temperatures. Compared to fused flux, sintered flux offers several advantages for rolling mill roll overlay repair:

Characteristic	Sintered Flux	Fused Flux
Moisture content	Low (1-3%)	Very low (<0.5%)
Arc stability	Good	Excellent
Deposition rate	High	High
Mechanical properties	Good	Excellent
Cost	Lower	Higher
Reusability	Limited	Good
Slag fluidity	Moderate	Good

The study finds that sintered flux provides adequate arc stability and deposition rate for rolling mill roll overlay repair, while offering cost advantages over fused flux. The lower cost of sintered flux is particularly important for high-volume repair operations where cost efficiency is critical.

Process Parameters for Roll Overlay Repair

The SAW process parameters for rolling mill roll overlay repair are optimized to achieve high deposition rates while maintaining adequate mechanical properties. The following table summarizes the typical parameter ranges:

Parameter	Typical Value	Effect on Overlay Quality
Current (A)	500-700	Higher current increases deposition rate
Voltage (V)	32-40	Higher voltage increases arc width
Travel speed (mm/min)	400-600	Higher speed reduces heat input
Wire feed speed (mm/min)	700-1000	Must match travel speed
Wire diameter (mm)	3.2-4.0	Larger wire increases deposition rate
Flux coverage	10-15 mm	Must cover the arc completely
Preheat temperature (°C)	100-200	Reduces cracking susceptibility
Number of layers	2-4	Multi-layer improves properties

The study emphasizes that the high deposition rate of the SAW process with sintered flux is a significant advantage for rolling mill roll repair, where rapid repair is essential to minimize production downtime. The typical deposition rate can reach 5-10 kg/h, allowing for the repair of large surface areas in a short time.

Microstructure and Mechanical Properties

The microstructure of the overlay layer deposited using sintered flux is characterized by a martensitic structure with carbide precipitates, depending on the wire composition. The study investigates several wire compositions, including high-carbon martensitic steels and high-chromium alloys:

Wire Type	Composition	Hardness (HV30)	Wear Resistance
High-C martensitic	C 0.6-0.8%, Cr 3-5%	400-500	Good
High-Cr alloy	Cr 12-15%, C 2.5-3.0%	600-700	Excellent
Stellite-type	Co 60%, Cr 25%, W 10%	350-400	Excellent (high temp)

The mechanical properties of the overlay layer must be balanced between hardness and toughness to resist both abrasive wear and impact loading. The study finds that high-carbon martensitic overlays provide a good balance of properties for rolling mill roll applications, while high-chromium overlays offer superior wear resistance for more severe conditions.

Engineering Practice and Quality Control

The repair of rolling mill rolls requires careful quality control to ensure that the overlay layer meets the required performance standards. The following quality control measures are essential:

Control Measure	Purpose	Method
Surface preparation	Remove scale, rust, and contaminants	Grinding, wire brushing
Preheat control	Reduce cracking susceptibility	Infrared pyrometer
Flux management	Maintain low moisture content	Oven drying, storage control
Hardness testing	Verify overlay hardness	Vickers or Rockwell hardness tester
Wear testing	Assess wear resistance	Pin-on-disk or block-on-ring test
Visual inspection	Detect surface defects	Visual examination
Dimensional verification	Ensure roll geometry	Calipers, micrometers

The study also addresses the importance of post-repair machining to restore the roll surface to the required geometry and surface finish. The overlay layer must be machined to the correct diameter and surface roughness to ensure proper rolling performance and product quality.

Study Insights and Practical Recommendations

The study demonstrates that sintered flux SAW is a viable and cost-effective method for rolling mill roll overlay repair, offering high deposition rates and adequate mechanical properties. The key findings include the importance of process parameter optimization, the selection of appropriate wire composition based on service conditions, and the implementation of rigorous quality control measures. For engineers involved in rolling mill maintenance, this literature provides practical guidance on the application of sintered flux SAW for roll repair, including process parameters, material selection, and quality control requirements.

This literature provides a valuable technical foundation for implementing sintered flux SAW in rolling mill roll repair operations, and its principles remain relevant for modern maintenance practices in the steel industry, where rapid and cost-effective repair is essential for maintaining production efficiency.