Weld Overlay Repair of Hot-Rolled Vertical Strip Mill Upright Rolls

Literature Overview

This study note focuses on the weld overlay repair technology applied to hot-rolled vertical strip mill (VSB) upright rolls at Meishan Steel Company, authored by Wang Yinjun, Li Yungang, Dong Hanjun, and Wang Kaisheng (2006). The research addresses a critical industrial problem: the severe wear and surface damage experienced by upright rolls during the vertical strip rolling process, where rolls operate under extreme thermal cycling, mechanical loading, and abrasive contact with hot steel slabs. The collaboration between Meishan Steel's Technical Center and Nanjing University of Aeronautics and Astronautics reflects the practical engineering orientation of this work, combining plant-level operational experience with academic welding expertise.

Core Technical Challenges

Vertical strip mill rolls are subjected to a combination of wear mechanisms that make repair particularly challenging. The primary failure modes include abrasive wear from scale and oxide scale removal, thermal fatigue cracking due to repeated heating and cooling cycles during rolling, and plastic deformation under high contact stress. Unlike horizontal mill rolls, upright rolls experience asymmetric loading and often require repair at the plant floor with limited access to conventional machining equipment.

The key technical parameters governing the repair process include:

Parameter	Typical Value	Notes
Roll material	4Cr5MoSiV or similar high-speed steel	High hardness, good hot hardness
Base metal hardness	52-58 HRC	Requires proper preheating
Overlay thickness	3-8 mm	Depends on wear allowance
Preheat temperature	250-350°C	Prevents cracking
Interpass temperature	200-300°C	Controls cooling rate
Post-weld cooling	Controlled to <50°C/hr	Prevents thermal cracking

Weld Overlay Process Analysis

The repair process typically employs a multi-pass approach using either submerged arc welding (SAW) or gas metal arc welding (GMAW) with appropriate consumables. For high-speed steel rolls, the selection of welding consumables is critical. Common approaches include:

Matched consumables — Using wire with similar composition to the base roll material to maintain hardness and hot hardness properties.
Soft weld overlay — Applying a lower-hardness transition layer followed by a hard surfacing layer to accommodate thermal expansion differences.
Nickel-based or cobalt-based alloys — For cases where corrosion resistance or extreme wear resistance is required.

The welding sequence must be carefully planned to minimize residual stresses. A typical approach involves:

Step 1: Grind the damaged area to remove all cracks and severely worn material.
Step 2: Apply a transition layer (if composition mismatch exists) with a compatible filler metal.
Step 3: Build up the overlay layer in multiple passes, maintaining strict interpass temperature control.
Step 4: Machine the surface to the required dimensional tolerance and surface finish.
Step 5: Perform post-weld heat treatment (PWHT) to relieve residual stresses and restore tempering balance.

Defect Analysis and Countermeasures

The most common defects encountered during roll repair include:

Defect Type	Cause	Countermeasure
Cracking	High carbon content, rapid cooling	Increase preheat, reduce interpass temp, use low-hydrogen consumables
Porosity	Contaminated surface, improper flux	Thorough surface preparation, flux baking
Lack of fusion	Insufficient heat input	Increase current, improve travel speed control
Hardness variation	Inconsistent dilution	Multi-pass with consistent parameters
Delamination	High residual stress	Controlled cooling, PWHT

The challenge of repairing high-carbon, high-alloy steel rolls lies in the high hardenability and susceptibility to cold cracking. The carbon equivalent (CE) of 4Cr5MoSiV is approximately 0.6-0.7%, which demands careful thermal management throughout the welding process.

Engineering Practice Insights

From the perspective of practical implementation, several lessons emerge from this type of repair work. First, the economic viability of roll repair versus replacement must be evaluated based on the extent of damage, remaining life, and availability of replacement rolls. Second, the dimensional accuracy after machining is critical — the roll surface must meet tight tolerance requirements (typically ±0.1 mm) to ensure proper strip gauge control. Third, the repair procedure must be qualified according to relevant standards such as NB/T 47014 or equivalent welding procedure qualification standards.

A key insight from this research is that the welding parameters must be optimized for each specific roll geometry and damage pattern. Standardized procedures alone are insufficient; experienced welders who understand the material behavior under thermal cycling are essential for achieving reliable repairs. The integration of preheating, interpass temperature monitoring, and post-weld inspection (particularly magnetic particle testing for surface cracks) forms the backbone of quality assurance in this application.

Study Reflections

This study highlights the importance of understanding the service environment when designing weld repair procedures. The VSB upright roll operates in a fundamentally different thermal and mechanical regime compared to standard pressure vessel or structural applications. The rapid thermal cycling (from ambient to 1200°C+ during rolling, then rapid cooling) creates a unique fatigue environment that weld repair must accommodate. The research underscores that successful roll repair is not merely about depositing material — it is about restoring the functional integrity of a component that must withstand extreme operating conditions for extended service intervals.