Wear-Resistant Alloy Overlay Welding for Large Hot Rolling Slab Mills

Literature Overview and Technical Background

Large hot rolling slab mills are among the most demanding applications for wear-resistant overlay welding due to the extreme combination of thermal, mechanical, and chemical loads imposed on the rolls. The slab mill rolls are exposed to red-hot steel slabs at temperatures exceeding 1000 °C, subjected to rolling forces in the range of 200 to 400 kN per meter of roll length, and continuously exposed to scaling and oxidation. The literature reviewed here addresses the application of wear-resistant alloy overlay welding technology to extend the service life of these large rolls, which typically have diameters ranging from 800 to 1200 mm and lengths up to 3000 mm.

The overlay alloys discussed in the literature include high-chromium white cast iron, high-manganese austenitic steel, and nickel-based alloys with carbide-forming elements such as chromium, molybdenum, and tungsten. Each of these materials offers different wear resistance characteristics, and the selection depends on the specific operating conditions of the slab mill. The literature also covers various welding processes including submerged arc welding (SAW), gas metal arc welding (GMAW), and plasma transferred arc (PTA) welding, with emphasis on their respective advantages and limitations for large roll overlay applications.

Core Technical Points and Process Selection

The selection of the overlay welding process is critical to achieving the desired surface properties and service life, particularly for large rolls where the total overlay volume can be substantial. The literature provides a comparative analysis of the available processes, highlighting their suitability for different roll sizes and operating conditions.

The following table compares the key characteristics of the main overlay welding processes used for large hot rolling slab mill rolls:

For large slab mill rolls, SAW is often the preferred method for the base layer due to its high productivity and ability to deposit thick layers quickly. However, SAW typically produces a rough surface that requires machining, and the dilution from the base steel can be significant. PTA welding is preferred for the final surface layer because it offers excellent control over dilution and produces a smooth, uniform surface. The literature recommends a multi-layer approach where SAW is used for the initial layers and PTA is used for the final surface layer to achieve both economy and quality.

The preheating requirements for large roll overlay are critical to prevent cracking and ensure proper bonding. The literature recommends preheating the roll surface to 250 to 400 °C before welding, with the exact temperature depending on the roll material and the overlay alloy being used. For high-carbon steel rolls, preheating to 350 to 400 °C is recommended to reduce the risk of cold cracking. The interpass temperature should be maintained below 250 °C to prevent excessive softening of the base material.

Metallurgical Considerations and Wear Mechanisms

Understanding the metallurgical behavior of the overlay layer under operating conditions is essential for predicting service life and optimizing the overlay design. The literature provides detailed analysis of the microstructure development in different overlay alloys and their response to thermal cycling.

High-chromium white cast iron overlays develop a microstructure consisting of a martensitic matrix with primary and secondary carbides. The primary carbides, which are typically M7C3 type, provide excellent wear resistance but can be prone to cracking under impact loading. The literature recommends using alloys with a controlled carbide morphology, such as those containing boron to promote the formation of finer, more uniformly distributed carbides.

High-manganese austenitic steel overlays rely on work hardening to provide wear resistance. Under impact loading, the austenite transforms to martensite, increasing the hardness significantly. However, this transformation can also lead to residual stresses and potential cracking. The literature suggests using alloys with a slightly higher carbon content to promote more complete martensite formation and improve wear resistance.

Nickel-based alloy overlays with carbide-forming elements offer excellent wear resistance at elevated temperatures due to their thermal stability. These alloys maintain their hardness even at temperatures above 600 °C, making them suitable for the hottest zones of the roll surface. The literature highlights that these alloys are particularly effective in preventing adhesive wear, which is a major concern in hot rolling applications.

Engineering Practice and Quality Control

The quality of the overlay weld on large rolls is verified through a combination of non-destructive testing and performance testing. The literature emphasizes the importance of measuring the hardness profile across the overlay interface and verifying the absence of defects that could lead to premature failure.

The recommended quality control sequence includes visual inspection of the entire overlay surface, magnetic particle testing (MT) to detect surface-breaking cracks, and ultrasonic testing (UT) to detect subsurface defects and measure overlay thickness. For critical applications, eddy current testing (ET) may also be performed to detect surface and near-surface defects without requiring surface preparation.

Destructive testing includes microhardness profiling, metallographic examination of the weld cross-section, and wear testing using standardized methods such as the ASTM G99 pin-on-disk test. The literature recommends performing wear testing on coupons taken from representative areas of the overlay to predict the service life under actual operating conditions.

The literature also highlights the importance of monitoring the residual stress distribution in the overlay layer. Residual stresses can affect the wear resistance and fatigue life of the overlay, and excessive tensile stresses can lead to cracking. The literature recommends performing X-ray diffraction (XRD) residual stress measurements on representative areas to verify that the stresses are within acceptable limits.

Key Questions and Reflections

Several important questions arise from studying this literature that merit further investigation. First, the long-term behavior of overlay layers under cyclic thermal loading remains an area of uncertainty. While the initial wear resistance may be excellent, the accumulation of thermal stresses over thousands of cycles could potentially lead to delamination or cracking at the interface. Second, the effect of post-weld machining on the residual stress state and wear performance of the overlay layer is not fully understood. Machining can introduce additional stresses and potentially affect the microstructure of the surface layer.

From a practical standpoint, the literature provides valuable guidance on welding sequence planning for large rolls. The recommended approach is to weld in a circumferential pattern, with each pass overlapping the previous one by at least 50 percent of the electrode diameter. This approach minimizes the accumulation of residual stresses and reduces the risk of distortion. The literature also highlights the importance of maintaining a consistent interpass temperature throughout the welding operation, as fluctuations can lead to variations in dilution and weld quality.

Study Insights and Implications

The study of wear-resistant alloy overlay welding for large hot rolling slab mill rolls provides valuable insights into the complex interplay between metallurgy, process parameters, and service performance. The key takeaway is that achieving a reliable overlay requires a holistic approach that addresses material selection, process optimization, and thorough quality verification. The literature underscores that the selection of the overlay process should be based on a careful analysis of the operating conditions, the required surface properties, and the economic constraints of the application.

The engineering implications are significant: proper process selection and parameter control are essential for achieving the desired wear resistance and service life. The recommended multi-layer approach using SAW for the base layer and PTA for the surface layer provides an effective strategy for balancing economy and quality. Furthermore, the emphasis on thorough NDT and performance testing highlights the importance of quality assurance in ensuring the long-term reliability of these critical components.

In conclusion, the literature on wear-resistant alloy overlay welding for large hot rolling slab mill rolls offers a comprehensive guide to addressing one of the most demanding applications in the field of wear-resistant overlay welding. The combination of metallurgical understanding, process control, and rigorous quality verification provides a solid foundation for successful engineering implementation.