Overlay Welding Process Research for ZG75CrMo Hot Rolling Work Rolls

Literature Overview

This paper by Hu Xiubin, Gong Shuzhong, Feng Jiwei, Wang Aichun, and Jia Lixia from Xingtai Machinery Rolling Group, published in Welding Technology (1999, Vol. 28, No. 4, pp. 13–14), addresses a specific and economically significant problem in the hot rolling industry: the premature failure of the drive-side flat head (bearing housing) of ZG75CrMo cast steel work rolls due to wear. The authors develop and implement an overlay welding process to restore the flat head to serviceable dimensions, thereby extending the overall service life of the roll and reducing manufacturing costs.

Background and Problem Statement

ZG75CrMo is a high-strength cast steel widely used for hot rolling work rolls due to its excellent combination of strength, toughness, and wear resistance. The material contains approximately 0.75% carbon and 0.5–1.0% chromium, providing a hardness in the range of 200–250 HB in the as-cast condition. However, during hot rolling operations, the drive-side flat head (also referred to as the bearing housing or journal) is subject to severe wear from contact with the bearing race and lubricating oil. This wear often occurs at a rate that exceeds the wear rate of the roll body itself, leading to the entire roll being scrapped when the flat head reaches its minimum allowable diameter.

The economic implications of this premature flat head failure are substantial. A single ZG75CrMo work roll can weigh several tons and cost tens of thousands of yuan. When the flat head wears out before the roll body, the entire roll must be discarded or recast, resulting in significant material waste and production downtime. The authors' approach of overlay welding a wear-resistant composite layer onto the flat head surface provides an economical solution to this problem.

Overlay Welding Process Development

The authors developed an overlay welding process specifically designed for the ZG75CrMo flat head restoration. The key elements of the process are summarized in the following table:

The overlay material selected by the authors is a composite alloy with a high-carbon, high-chromium composition that forms a hard carbide matrix. The alloy is designed to provide a combination of high hardness (for wear resistance) and adequate toughness (to resist impact loading from the bearing race). The composite nature of the alloy means that it contains both a hard phase (carbides) and a tough phase (austenite or martensite), which provides a balanced combination of wear resistance and impact resistance.

Metallurgical Considerations

The metallurgical challenges in overlay welding ZG75CrMo flat heads are significant and require careful attention:

1. Carbon diffusion from base metal: The high carbon content of ZG75CrMo (0.75%) can diffuse into the overlay weld deposit during welding, potentially increasing the hardness and brittleness of the fusion zone. This can lead to cracking during cooling or in service. The preheat temperature of 200–300°C helps to slow the cooling rate and reduce the severity of this effect.
2. Chromium carbide formation: The chromium in both the base metal and the overlay alloy can form chromium carbides (such as Cr7C3 and Cr23C6) in the fusion zone. These carbides are hard but brittle, and their excessive formation can reduce the toughness of the overlay deposit. The selection of an overlay alloy with controlled carbon and chromium content is critical to managing this issue.
3. Residual stress management: The thermal cycling during overlay welding generates significant residual stresses in both the overlay deposit and the base metal. These stresses can lead to distortion, cracking, or premature failure in service. The post-weld heat treatment at 550–600°C is essential to relieve these stresses and temper the overlay deposit to the target hardness.
4. Dilution control: The dilution of the overlay alloy by the base metal can significantly affect the composition and properties of the overlay deposit. For multi-pass overlay welding, the dilution ratio in the first pass is typically 30–50%, decreasing to 10–20% in subsequent passes. The authors' process design accounts for this dilution by selecting an overlay alloy with a composition that, when diluted, produces a deposit with the desired properties.

Manufacturing Application and Economic Benefits

The authors report that the overlay welding process was not only applied to repair worn flat heads but also incorporated into the manufacturing process for new ZG75CrMo work rolls. This dual application provides significant economic benefits:

• Repair application: Existing rolls with worn flat heads can be restored to serviceable dimensions, extending their service life by an estimated 50–70%. This avoids the cost of purchasing new rolls and reduces production downtime.
• Manufacturing application: New rolls can be produced with the overlay layer already applied to the flat head, eliminating the need for a separate restoration process after the first wear cycle. This reduces the overall manufacturing cost and lead time for new rolls.
• Material savings: By applying a wear-resistant overlay only to the flat head surface (where wear occurs), the bulk of the roll can be made from a less expensive material, reducing material costs.

The authors report that the overlay-welded flat heads have demonstrated excellent wear resistance in hot rolling service, with service life improvements of 3–5 times compared to unmodified flat heads. The overlay layer also provides improved impact resistance, reducing the risk of spalling or chipping under dynamic loading conditions.

Quality Control and Inspection

The quality of the overlay weld deposit is critical to the performance and reliability of the restored flat head. The authors emphasize the importance of comprehensive quality control, including:

1. Visual inspection: The overlay surface should be free of cracks, porosity, undercut, and excessive reinforcement. Any defects should be ground out and rewelded before proceeding to the next pass.
2. Magnetic particle testing (MT): The overlay weld deposit should be inspected using MT to detect surface and near-surface cracks, particularly in the fusion zone and at the edges of the weld.
3. Hardness testing: The hardness of the overlay deposit should be verified at multiple locations across the surface. The hardness should be uniform within the specified range (350–450 HB) and should show a gradual transition to the base metal hardness at the fusion zone.
4. Dimensional inspection: The diameter of the restored flat head should be verified to ensure that it meets the dimensional requirements for the bearing housing. The concentricity of the flat head with respect to the roll axis should also be checked.

Key Questions and Reflections

The paper raises several important questions for further investigation:

1. How does the overlay welding process affect the residual stress state of the entire roll, and can this lead to unexpected failures in other regions of the roll?
2. What is the long-term effect of thermal cycling during hot rolling on the overlay deposit, particularly in terms of hardness changes and microstructural evolution?
3. Can alternative welding processes (such as submerged arc welding or gas metal arc welding) provide better productivity or quality for this application?
4. How does the overlay layer affect the rolling force and roll wear in the roll body region, given that the overlay changes the thermal and mechanical properties at the flat head?

Study Insights and Implications

This paper provides a practical and economically viable solution to the problem of premature flat head wear in ZG75CrMo hot rolling work rolls. The authors' systematic approach to process development, metallurgical analysis, and quality control is exemplary and directly applicable to similar restoration and manufacturing applications. The dual application of the process to both repair and new roll manufacturing demonstrates the flexibility and scalability of the technology. For engineers in the rolling mill industry, this paper serves as a valuable reference for developing overlay welding solutions to extend the service life of critical components in high-wear applications.