Weld Overlay Repair and Strengthening of a 1550 mm Support Roll

Literature Overview

This study note examines the repair and strengthening of a φ1550 mm support roll, a critical component in hot rolling mills where severe thermal and mechanical loading leads to surface degradation over time. The support roll endures continuous contact with the work roll and the hot strip, resulting in plastic deformation, thermal fatigue, and material loss at the bearing surface. The core challenge addressed in this literature is achieving reliable weld overlay repair that restores dimensional accuracy while enhancing the surface hardness and wear resistance without introducing residual stresses that could compromise structural integrity.

Core Technical Content

The repair process typically involves grinding away the damaged surface to expose sound base metal, followed by multi-pass weld overlay using a hardfacing consumable. The base metal of the support roll is usually a medium-carbon low-alloy steel such as 42CrMo or a similar grade, while the overlay material is selected for its resistance to thermal fatigue and abrasive wear. Common overlay compositions include high-chromium alloys, martensitic stainless steels, or carbide-reinforced nickel-based alloys.

The following table summarizes typical process parameters observed in the literature:

Parameter	Typical Range	Notes
Base metal grade	42CrMo / 40Cr	Normalized condition
Overlay material	High-Cr martensitic or Ni-based	e.g., Stellite or equivalent
Preheat temperature	200–300 °C	To reduce cracking risk
Interpass temperature	< 250 °C	Maintain during multi-pass welding
Post-weld heat treatment	600–650 °C, 2–4 h	Stress relief and tempering
Post-weld hardness	HRC 45–55	Dependent on overlay alloy

Welding Process Selection

Submerged arc welding (SAW) is often favored for bulk deposition on large-diameter rolls due to its high deposition rate and deep penetration, which promotes good metallurgical bonding. However, for the final surface pass, gas tungsten arc welding (GTAW) or plasma transferred arc (PTA) may be employed to achieve a smoother surface finish and more uniform microstructure. The transition from SAW to GTAW requires careful management of the dilution rate to ensure the overlay composition meets the required hardness specification.

Strengthening Considerations

Beyond simple repair, the strengthening aspect of this study focuses on achieving a hardness profile that resists plastic deformation under the extreme contact pressures encountered during hot rolling. The overlay layer must maintain its hardness at elevated temperatures (up to 200–300 °C) while also possessing sufficient toughness to resist thermal fatigue cracking. Post-weld heat treatment is critical: tempering at 600–650 °C relieves residual stresses while stabilizing the martensitic structure in high-chromium overlays, preventing delayed cracking during service.

Common Defects and Countermeasures

The following defects are frequently encountered during repair of large support rolls:

Defect Type	Root Cause	Countermeasure
Cracking at weld/bond line	Excessive residual stress, high carbon equivalent	Preheat, low interpass temperature, post-weld stress relief
Incomplete bond	Surface contamination, insufficient penetration	Thorough grinding, proper joint design
Hardness non-uniformity	Dilution variation, cooling rate differences	Controlled multi-pass strategy, proper heat input
Surface porosity	Gas entrapment, flux contamination	Clean consumables, proper shielding

Engineering Practice Integration

In practice, the repair of a φ1550 mm support roll must be coordinated with the mill shutdown schedule, which is often limited to a few days. This places significant pressure on the welding team to complete grinding, preheating, multi-pass overlay, post-weld heat treatment, and final grinding within a compressed timeline. The large diameter of the roll also introduces challenges related to distortion control and dimensional accuracy. After repair, the roll must be machined back to precise concentricity and roundness tolerances (typically within 0.05 mm), which demands that the overlay layer have uniform hardness and minimal internal defects.

Non-destructive testing is essential: magnetic particle testing (MT) is used on the surface to detect cracks, while ultrasonic testing (UT) verifies the bond integrity between the overlay and base metal. A bond strength test, such as a bending test or a shear test, may be performed on a coupon welded under identical conditions to the production repair to validate the bond quality.

Study Insights and Reflections

The study of support roll repair reinforces the principle that weld overlay is not merely a surface restoration technique but a holistic engineering solution that must integrate material selection, process control, thermal management, and post-weld treatment. The experience of dealing with large-diameter components highlights the importance of thermal distortion control, which is often more challenging than in smaller components. Furthermore, the strengthening aspect reminds us that the overlay must be designed not just for wear resistance but also for thermal fatigue resistance, which requires a careful balance between hardness and toughness. A key insight is that the post-weld heat treatment is not an optional step but a mandatory one, as the residual stresses from welding alone can lead to premature failure under cyclic thermal loading.