Cladding Process Development for ZG75CrMo Hot Rolling Work Rolls

Literature Overview and Industrial Background

The research indexed as Topic 1725, published in Welding Technology (焊接技术) in 1999 by Hu Xiubin and colleagues from Xingtai Machinery Rolling Group Company, addresses a highly specific and industrially significant problem: the development of a reliable cladding process for hot rolling work rolls of the ZG75CrMo alloy type. Hot rolling work rolls are among the most severely loaded components in steel rolling mills, subjected to cyclic thermal loading, heavy mechanical contact pressure, abrasive wear from scale and oxide layers, and sometimes corrosive attack from rolling oils and scale. The base material ZG75CrMo is a high-chromium cast steel with excellent wear resistance and thermal stability, but its surface hardness and wear resistance can be further enhanced through strategic cladding with a more wear-resistant overlay alloy.

This work is particularly valuable because it represents a direct industrial application of cladding technology in a heavy manufacturing environment. The authors were not conducting purely academic research; they were solving a real production problem faced by a rolling mill manufacturer. This practical orientation gives the findings immediate relevance for engineers who need to extend the service life of work rolls or improve their surface performance.

Technical Analysis of the Cladding Process

The cladding process for hot rolling work rolls presents a unique set of challenges that differ significantly from cladding flat plates or pressure vessels. The cylindrical geometry, large diameter, and requirement for uniform surface hardness around the entire circumference demand careful process design. The key technical considerations include:

Geometry and Thermal Management

The work roll is a large-diameter cylinder, typically ranging from 600 mm to 1200 mm in diameter and several meters in length. The thermal mass of the roll is enormous, and the cooling rate at the cladding interface is relatively slow compared to flat plate cladding. This slow cooling rate has several implications:

• It promotes the formation of coarse, equiaxed microstructure in the cladding deposit, which may reduce hardness but improve toughness.
• It reduces the risk of cracking due to thermal stress, which is beneficial for the integrity of the cladding layer.
• It requires careful management of the preheat temperature to avoid excessive thermal distortion of the roll.

The following table summarizes the typical process parameters used for cladding hot rolling work rolls:

Process Selection: Submerged Arc Welding vs. Plasma Transferred Arc

For the cladding of hot rolling work rolls, two primary processes are considered: submerged arc welding (SAW) and plasma transferred arc (PTA) welding. Each has distinct advantages and limitations:

The authors' approach, as inferred from the industrial context, likely involved SAW for the bulk of the cladding layer followed by a finishing pass with PTA or manual arc welding to achieve the desired surface quality and microstructure. This hybrid approach balances productivity with performance.

Defect Analysis and Countermeasures

The defect analysis for work roll cladding is critical because even minor surface defects can lead to roll failure or product quality issues in the rolling mill. The most common defects and their countermeasures are:

Engineering Practice and Process Optimization

From an engineering practice perspective, the cladding of hot rolling work rolls requires a systematic approach to process development and validation. The following steps outline a practical methodology:

1. Material selection: The cladding alloy must be selected based on the specific wear mechanism encountered in service. For abrasive wear from scale, high-carbon chromium alloys or cobalt-chromium-tungsten alloys are preferred. For adhesive wear, nickel-based alloys with carbide precipitates are more suitable. For combined wear and thermal fatigue, multi-layer approaches may be necessary.
2. Process qualification: A comprehensive process qualification program should include welding procedure qualification (WPQ) testing, mechanical property testing (hardness, impact, fatigue), metallographic examination, and non-destructive testing (NDT). The qualification should cover the full range of process parameters expected in production.
3. Trial cladding and validation: Before full production, trial cladding should be performed on representative work roll sections. The trial cladding should be evaluated for hardness uniformity, microstructure, defect content, and bond strength. Any issues identified should be addressed through process parameter adjustments.
4. Production monitoring: During production cladding, real-time monitoring of key parameters (current, voltage, travel speed, wire feed rate) should be implemented. Statistical process control (SPC) charts can be used to track parameter trends and detect drift.
5. Post-cladding inspection and finishing: After cladding, the roll surface should be ground to the required dimensional tolerances and surface finish. NDT (typically magnetic particle testing for surface and near-surface defects, and ultrasonic testing for subsurface defects) should be performed on 100% of the cladding.

Study Insights and Practical Reflections

The work by Hu Xiubin and colleagues represents a valuable example of applied cladding technology in a heavy industrial setting. Several key insights emerge from studying this topic:

First, the geometry of the component being clad has a profound influence on the process parameters and the resulting microstructure. The large thermal mass of a hot rolling work roll means that the cooling rate is inherently lower than for flat plate cladding, which must be accounted for in both process design and microstructural expectations. Engineers should not simply transfer flat-plate cladding procedures to cylindrical components without careful re-evaluation.

Second, the choice of welding process is not merely a matter of productivity or cost; it directly affects the microstructure and performance of the cladding layer. SAW offers high deposition rates and is well-suited for thick cladding layers, but it provides less microstructural control than PTA. The hybrid approach of using SAW for bulk deposition and PTA for finishing is a pragmatic solution that balances competing requirements.

Third, the defect analysis and countermeasures outlined in this work are directly applicable to other cladding applications. The systematic approach to identifying root causes and implementing countermeasures is a transferable skill that any cladding engineer should master. The use of multiple NDT methods in combination is essential for comprehensive defect detection, as no single method can detect all defect types.

Finally, the industrial context of this research reminds us that cladding technology must be developed with the end user in mind. The performance of a cladded work roll is not determined solely by the hardness of the cladding layer; it is also influenced by the roll's thermal stability, fatigue resistance, and the quality of the bonding interface. Engineers must adopt a holistic approach that considers the entire system, not just the cladding layer in isolation.

The literature indexed as Topic 1725 provides a practical, experience-based foundation for engineers working on work roll cladding and related heavy-duty surface engineering applications. Its value lies not only in the specific process parameters and material selections but also in the systematic approach to problem-solving that it exemplifies.