Repair of Phi 650 Rolling Mill Rolls by Cladding with Sintered Flux

Overview of the Technical Context

Rolling mill rolls are among the most critical consumable components in steel and non-ferrous metal rolling operations. The φ650 roll diameter represents a medium-sized work roll used extensively in finishing mills for strip and sheet production. During service, the roll surface is subjected to extreme cyclic stresses, thermal gradients, and abrasive contact with hot metal, leading to wear patterns that include galling, chipping, and dimensional loss. When the roll diameter deviates from specification by more than 0.5 mm per side, the roll must be either reground or replaced. Regrinding consumes base material and shortens the service life cycle, whereas replacement incurs high procurement and logistics costs. Cladding repair with sintered flux offers a cost-effective and technically robust solution that restores both the geometric dimension and the surface tribological properties of the roll.

The literature reviewed in this study focuses on the application of sintered flux-based cladding processes—primarily submerged arc welding (SAW) and electroslag welding (ESW)—to restore φ650 rolls. The sintered flux serves a dual role: it acts as a shielding medium during welding and simultaneously delivers alloying elements to the cladding layer. The key insight from the literature is that proper flux selection, combined with optimized welding parameters, can produce a cladding layer with hardness exceeding 45 HRC and excellent bonding with the roll base steel without inducing excessive dilution.

Core Technical Analysis

Sintered Flux Composition and Selection

The performance of the cladding layer is fundamentally governed by the chemical composition of the sintered flux. The literature identifies three principal categories of sintered flux used for roll repair:

The manganese-silicon type flux is the most widely used due to its balance of weldability, hardness, and cost. The chromium-manganese type provides superior wear resistance through the formation of chromium carbides (Cr7C3 and Cr23C6) in the microstructure. The boron-carbide reinforced type achieves the highest hardness through the dispersion of hard boride phases but requires careful control of welding parameters to avoid cracking.

A critical finding from the literature is that the flux particle size distribution significantly affects the arc stability and metal transfer characteristics. Particles in the 0.25-0.85 mm range provide the optimal balance between arc stability and coating uniformity. Oversized particles (>1.0 mm) lead to arc instability and porosity, while undersized particles (<0.2 mm) cause excessive flux consumption and poor arc coverage.

Welding Process Parameters

The φ650 roll presents a cylindrical geometry that requires orbital welding or manual circumferential welding. The literature describes both automated and semi-automated approaches. For automated repair, the roll is mounted on a rotating fixture, and the welding head traverses the circumference with precise feed control.

The following table summarizes the recommended welding parameters for sintered flux cladding on φ650 rolls:

The interpass temperature control is particularly important for roll repair because the roll body is typically made of medium-carbon steel (such as 40CrNiMo or equivalent) with a base hardness of 22-28 HRC. Excessive interpass temperatures above 350 °C can cause softening of the base metal heat-affected zone (HAZ), reducing the fatigue resistance of the roll.

Multi-Pass Cladding Strategy

A single-pass cladding is rarely sufficient for restoring a worn roll to its original diameter. The literature describes a multi-pass strategy that typically involves three to five passes depending on the amount of metal to be restored:

1. First pass (build-up pass): A thicker wire (2.4 mm) is used at higher current to rapidly deposit metal and restore the bulk dimension. This pass may have slightly lower hardness but establishes the geometric foundation.
2. Intermediate passes: Medium-diameter wire (2.0 mm) with optimized flux composition builds up the remaining dimension while gradually increasing the hardness profile.
3. Final pass (surface hardening pass): A smaller wire (1.6 mm) with a high-alloy flux delivers the surface layer with maximum hardness and wear resistance. This pass is critical for achieving the desired surface properties.

The dilution ratio between the base metal and the cladding alloy is a key performance indicator. For the first pass, dilution can be as high as 30-40%, but for the final surface pass, dilution should be controlled below 15% to ensure the surface hardness meets specification. The literature recommends using a flux with higher alloy content in the final pass to compensate for dilution effects.

Defect Analysis and Countermeasures

Common Defects in Sintered Flux Cladding of Rolls

Based on the literature review and engineering experience, the following defects are most commonly encountered:

Case Study: Crater Cracking Prevention

The literature presents a notable case study involving crater cracking in the repair of a φ650 finishing roll made of 40CrNiMo steel. The roll had been ground down by 0.8 mm on each side due to galling wear. During the first repair attempt, crater cracks were observed at the end of each circumferential pass. The root cause analysis revealed that the sulfur content in the base metal was 0.032% (above the recommended 0.025% threshold), and the cooling rate at the crater was excessively high.

The corrective measures included:

• Increasing preheat temperature from 150 °C to 250 °C
• Reducing travel speed by 15% to increase heat input at the crater
• Applying a low-sulfur sintered flux with Mn ≥ 20% and S ≤ 0.01%
• Introducing a backfill pass after each circumferential circuit

After implementing these measures, the subsequent repair produced a crack-free cladding layer with a hardness of 46 HRC at the surface, declining to 32 HRC at the fusion line. The repaired roll completed a full production cycle of 180 hours before the next regrind, demonstrating excellent service performance.

Engineering Practice Integration

Quality Control Procedures

The literature emphasizes a comprehensive quality control procedure that follows the PDCA cycle:

• Plan: Define acceptance criteria based on roll specification (hardness, dimension, metallurgical bond). Select flux and wire according to the wear condition and service environment.
• Do: Execute the multi-pass cladding with strict parameter control. Monitor arc voltage, current, and travel speed in real time.
• Check: Perform hardness survey at 10 locations around the circumference at three depths (surface, 2 mm, 5 mm). Conduct magnetic particle inspection for surface cracks. Perform ultrasonic testing for subsurface defects.
• Act: If hardness or dimensional specifications are not met, adjust flux composition or welding parameters for the next repair cycle.

Performance Comparison with Conventional Methods

The data clearly demonstrates that sintered flux cladding provides the best balance of cost, service life, and surface performance. The hardness advantage of 5-10 HRC over the base metal translates directly into extended service life under abrasive conditions.

Key Reflections and Study Insights

The most valuable insight from this literature is the recognition that sintered flux cladding is not merely a welding operation but a materials engineering challenge. The flux composition must be carefully matched to the base metal chemistry, the service conditions, and the desired surface properties. A generic flux will not deliver optimal performance for every roll application.

Another important observation is the role of thermal management. The φ650 roll has a relatively thick cross-section, and the cumulative heat input from multiple passes can raise the core temperature significantly. The literature recommends intermittent welding—completing one circumferential pass and allowing cooling before starting the next—to manage the thermal profile. This approach may seem to reduce productivity, but it prevents HAZ softening and reduces residual stresses that could lead to roll failure in service.

The study also highlights the importance of operator skill in semi-automated applications. Even with automated orbital fixtures, the manual adjustment of parameters during the process—particularly the final pass—requires experienced welders who understand the relationship between arc characteristics and deposition quality. Investment in operator training and certification is therefore not optional but essential for consistent results.

In conclusion, the application of sintered flux cladding to φ650 roll repair represents a mature and well-documented technology that delivers significant economic and technical benefits when properly implemented. The key to success lies in the systematic selection of flux composition, the disciplined control of welding parameters, and the rigorous execution of quality control procedures. Engineers working in this field should treat each repair as a materials design exercise rather than a routine welding task, and should continuously refine their parameter databases based on post-service performance data.