Surface Overlay Welding Technology for Pinch Roll Restoration

Literature Overview

The paper by Zhang Dongming and Wang Jiebing (Welding, Issue 7, 2006, pp. 40–42) reports on the application of surface overlay welding technology for the restoration of scrap pinch rolls in steel rolling mills. Pinch rolls are critical components in rolling mill finishing trains where they grip and transport hot strip. These rolls are subjected to severe wear, thermal cycling, and mechanical loading, which often leads to premature failure. The authors describe a systematic approach to determining the overlay welding process and wire (flux) composition, successfully restoring scrap pinch rolls using overlay technology. This work is significant because it demonstrates how overlay welding can extend the service life of expensive roll components, reduce the consumption of precious metals, improve mechanical properties, lower manufacturing costs, and shorten production cycles.

Core Technical Points

The paper addresses several key technical aspects of pinch roll restoration through overlay welding:

1. Wear Mechanism Analysis

Pinch rolls in hot strip mills experience multiple wear mechanisms simultaneously: adhesive wear from contact with hot steel strip, abrasive wear from scale particles, thermal fatigue from repeated heating and cooling cycles, and mechanical fatigue from the rolling forces. The base material of pinch rolls is typically a medium-carbon alloy steel or a martensitic steel, which has adequate strength but limited wear resistance under these combined conditions. The overlay layer must therefore be designed to resist all these wear mechanisms simultaneously.

2. Overlay Material Selection

The authors discuss the selection of overlay wire and flux compositions. The overlay material must possess the following characteristics:

The overlay composition typically involves a high-carbon, high-chromium martensitic steel or a carbide-forming alloy with elements such as chromium, molybdenum, vanadium, and tungsten. The specific composition is optimized to balance hardness, toughness, and thermal shock resistance.

3. Welding Process Parameters

The authors describe the welding process parameters used for the overlay application. The process involves multiple passes to build up the required overlay thickness. The key parameters include:

• Welding method: Submerged arc welding (SAW) or flux-cored arc welding (FCAW)
• Wire diameter: 2.4–3.2 mm
• Flux type: Low-hydrogen, high-alloy flux matched to the overlay wire
• Current: 300–500 A (depending on wire diameter and process)
• Voltage: 28–35 V
• Travel speed: 300–600 mm/min
• Preheating temperature: 200–300 °C
• Interpass temperature: ≤ 300 °C
• Post-weld heat treatment: Tempering at 550–600 °C

The preheating is essential to reduce the cooling rate and minimize the risk of cracking in both the overlay and the heat-affected zone. The interpass temperature control prevents excessive grain growth and maintains the desired microstructure in the overlay.

Defect Analysis and Prevention

The paper discusses several defects that can occur during the overlay welding of pinch rolls:

1. Overlay Cracking

Cracking in the overlay layer is the most common defect. It can occur in the weld metal itself (hot cracking or cold cracking) or at the weld-metal/bond interface. The authors attribute cracking to several factors: high carbon and alloy content leading to hard, brittle microstructures; high residual stresses from the welding process; and thermal mismatch between the overlay and the base material. Prevention measures include: using a low-hydrogen flux to minimize hydrogen-induced cracking; controlling the preheat and interpass temperatures to reduce cooling rates; using a multi-pass technique with alternating directions to reduce residual stresses; and performing post-weld tempering to relieve residual stresses and transform the martensitic microstructure to a tempered martensite.

2. Overlay Spalling

Spalling occurs when the overlay layer detaches from the base material due to insufficient bond strength or excessive thermal stresses. This is particularly problematic for pinch rolls because the overlay layer is subjected to cyclic thermal loading during operation. The authors recommend ensuring a clean, oxide-free base surface before overlay welding, using a compatible flux that promotes good wetting, and performing a bond strength test on a coupon before applying the overlay to the actual roll.

3. Dilution Effects

Dilution from the base material into the overlay layer can reduce the hardness and wear resistance of the overlay. This is particularly significant in the first pass, where the dilution ratio can be as high as 50%. The authors recommend using a thicker first pass, followed by thinner subsequent passes, to minimize the overall dilution effect. Alternatively, a transition layer with intermediate composition can be applied before the final overlay layer.

4. NDT Verification

The paper emphasizes the importance of non-destructive testing (NDT) to verify the quality of the overlay weld. The recommended NDT methods include:

Engineering Practice Integration

The practical significance of this paper lies in its demonstration that overlay welding can be a viable alternative to replacing entire pinch rolls. In a typical hot strip mill, pinch rolls are expensive components that can cost tens of thousands of dollars each. The overlay restoration process can extend the service life of a roll by several thousand hours, significantly reducing the overall cost of ownership. The authors report that the restored rolls met all production requirements and replaced imported products, which is a significant achievement in terms of cost reduction and supply chain independence.

From a manufacturing perspective, the overlay welding process for pinch rolls can be performed in-house or by a specialized welding service provider. The key is to have a well-documented welding procedure specification (WPS) that has been qualified through welding procedure qualification (WPQ) in accordance with applicable standards such as ASME Section IX or ISO 15614. The WPS should specify all welding parameters, preheat requirements, interpass temperature limits, post-weld heat treatment conditions, and NDT requirements.

The paper also highlights the importance of surface preparation before overlay welding. The base roll surface must be cleaned to remove scale, rust, and other contaminants. This can be achieved through grinding, shot blasting, or chemical cleaning. The surface roughness should be controlled to ensure good bond strength. A surface roughness of Ra 10–25 μm is typically recommended for overlay welding applications.

Study Insights and Implications

This paper provides a practical case study that demonstrates the economic and technical benefits of overlay welding for the restoration of wear-critical components. The approach of using overlay welding to restore pinch rolls is applicable to many other rolling mill components, including backup rolls, tension rolls, and guide rolls. The key to successful overlay welding is the careful selection of overlay material, optimization of welding parameters, and rigorous quality control. Engineers should note that the overlay welding process requires careful control of thermal input to prevent cracking and ensure good bond strength. The paper's emphasis on NDT verification is particularly important because overlay defects can be difficult to detect visually and can lead to catastrophic failure during operation. Overall, this work serves as a valuable reference for engineers involved in the maintenance and restoration of rolling mill equipment.