Overlay Repair and Heat Treatment of 60CrMnMo Steel Rolls

Literature Overview

The paper by Zhao Hui, Xu Ling, and Fu Chenggang (2009), published in Metal Heat Treatment (Vol. 34, Issue 4, pp. 82-84), documents the successful repair of a large-diameter (φ1150 mm) 60CrMnMo steel roll using overlay welding and post-weld heat treatment. The study combines academic research from Shenyang Ligong University with industrial practice at Benxi Steel's roll repair facility. The authors developed a localized repair welding material specification and applied submerged arc overlay welding (SAW) with Stellite multipass 224 flux-cored wire and HJ107 flux, supplemented by preheating and post-weld heat treatment (PWHT). The practical outcome—a 50% increase in roll service life—demonstrates the significant economic value of proper overlay repair technology.

Core Technical Findings

The repair methodology combines several critical elements: preheating to minimize thermal stresses, localized repair welding for surface defects, full-surface SAW overlay for wear protection, and controlled post-weld heat treatment for stress relief and microstructure optimization. The use of Stellite multipass 224 (a cobalt-based alloy) with HJ107 flux provides a proven combination for heavy-section overlay applications.

Interpretation of Technical Points

Preheating Strategy for Heavy-Section Rolls

The 60CrMnMo steel is a high-carbon, high-alloy steel with significant hardenability and susceptibility to cold cracking. At the large diameter of φ1150 mm, the thermal mass is substantial, and the cooling rate after welding can be relatively slow, which is generally beneficial for avoiding quench cracking. However, the preheating temperature must be carefully controlled—too low, and the risk of hydrogen-induced cracking persists; too high, and the base metal microstructure may soften excessively, reducing the effectiveness of the overlay bond. The authors' approach of preheating before localized repair welding is consistent with best practices for high-carbon steel welding, as codified in standards such as GB/T 985 and API 5L.

Stellite Multipass 224 Selection

Stellite multipass 224 is a cobalt-chromium-tungsten-based alloy specifically designed for multi-pass overlay applications. The "multipass" designation indicates that the alloy is formulated to maintain consistent composition and properties across multiple weld passes, which is essential for thick overlay builds. The cobalt matrix provides excellent hot hardness and oxidation resistance, while the chromium and tungsten carbides (primarily Cr₇C₃ and WC) provide wear resistance. For roll applications, where the surface is subjected to high contact stresses, elevated temperatures, and abrasive wear from scale and oxide layers, this alloy system is an appropriate choice.

HJ107 Flux Characteristics

HJ107 is a manganese-silicon type low-hydrogen flux that provides stable arc characteristics, good deoxidation, and low hydrogen pickup. The low hydrogen content is particularly important for high-carbon steel applications, where hydrogen-induced cracking is a primary concern. The flux also provides adequate alloying to the weld metal, contributing to the overlay's mechanical properties. The combination of Stellite multipass 224 with HJ107 flux is a well-established pairing in Chinese welding practice for cobalt-based overlay applications.

Post-Weld Heat Treatment

The PWHT is a critical step that cannot be omitted in the repair of 60CrMnMo rolls. The high carbon and alloy content of the base metal, combined with the thermal cycle of welding, creates significant residual stresses in the heat-affected zone (HAZ). Without PWHT, these stresses can lead to delayed cracking, distortion, and reduced service life. The tempering treatment also stabilizes the microstructure of the HAZ, converting any untempered martensite to tempered martensite and reducing brittleness. The specific PWHT parameters (temperature, holding time, cooling rate) are not detailed in the abstract but are undoubtedly optimized for the 60CrMnMo base metal.

Engineering Practice Integration

The following practical lessons emerge from this study for engineers involved in roll repair and heavy-section steel component refurbishment:

1. Preheating is non-negotiable for high-carbon, high-alloy steels. The preheat temperature should be determined based on the carbon equivalent (CE) of the base metal and the section thickness. For 60CrMnMo with a CE of approximately 0.55-0.60, preheating to 250-350°C is appropriate.
2. Multi-pass overlay with dilution control is essential. The first pass of Stellite multipass 224 will experience significant dilution from the 60CrMnMo base metal, which can reduce the cobalt content of the overlay. Subsequent passes experience less dilution as they are deposited on previously deposited cobalt-rich material. The "multipass" formulation of the alloy accounts for this dilution effect.
3. PWHT parameters must be tailored to the base metal. The tempering temperature for 60CrMnMo should be in the range of 550-650°C, with a holding time of 2-4 hours per 25 mm of thickness. The cooling rate should be controlled to avoid re-hardening.
4. Quality control should include hardness profiling, macrographic examination, and non-destructive testing (NDT). Hardness across the overlay thickness should show a smooth transition from base to overlay. Macrographic examination should reveal no cracks, porosity, or lack of fusion. RT or UT should be performed on the overlay for volumetric defect detection.

Key Questions and Reflections

The study reports a 50% improvement in service life but does not provide detailed wear test data or tribological analysis to explain the mechanism of life extension. Was the improvement due to superior wear resistance of the Stellite overlay, reduced thermal fatigue cracking, or a combination of both? Additionally, the study does not address the long-term oxidation resistance of the overlay at the operating temperatures of the rolling mill, which is a critical factor in hot rolling applications.

The paper also does not discuss the economic analysis of the repair approach compared to replacement. While a 50% life extension is clearly beneficial, the cost of Stellite multipass 224 wire, HJ107 flux, preheating, and PWHT must be weighed against the cost of a new roll. For a φ1150 mm roll, the economics likely favor repair, but this should be quantified for each application.

The combination of academic research and industrial practice in this study is commendable. The collaboration between Shenyang Ligong University and Benxi Steel's roll repair facility exemplifies the productive relationship between research institutions and industry, where fundamental materials science is translated into practical engineering solutions.

Study Insights and Implications

The successful repair of a φ1150 mm 60CrMnMo roll with a 50% service life improvement is a compelling demonstration of the economic and technical value of overlay welding technology in heavy industry. The study confirms that a systematic approach—combining appropriate material selection (Stellite multipass 224 with HJ107 flux), process control (preheating, multi-pass SAW), and post-weld treatment (PWHT)—can significantly extend the service life of expensive rolling mill components. For engineers, the key insight is that overlay repair is not merely a surface treatment but a comprehensive engineering solution that requires careful attention to metallurgical compatibility, process parameters, and quality control at every stage. The 50% life extension reported here is not an isolated achievement but a replicable engineering methodology that can be adapted to other heavy-section steel components in similar service conditions.