Cladding Materials for Steel Rolling Mill Guide Plates and Their Applications

Overview and Engineering Context

Rolling mill guide plates serve as critical wear components that direct and shape the strip as it passes through the rolling mill rolls. These components operate under extremely severe conditions characterized by high contact stress, abrasive wear from oxide scale, thermal cycling, and mechanical impact from strip flutter. The selection of appropriate cladding materials and the optimization of overlay welding parameters are therefore decisive factors in extending service life and reducing unplanned downtime. This study note examines the key findings regarding cladding material selection for rolling mill guide plates, drawing upon metallurgical principles, process parameters, and field performance data.

Core Material Systems and Their Metallurgical Basis

The primary material systems investigated for guide plate overlay include high-chromium cast irons, nickel-chromium alloys, cobalt-chromium alloys, and carbide-enhanced austenitic steels. Each system offers distinct advantages depending on the specific operating conditions encountered in different sections of the rolling mill.

High-chromium cast irons, particularly those in the Cr15-Cr26 range, form a dense network of M7C3 carbides that provide exceptional resistance to abrasive wear from iron oxide scale. However, these materials are inherently brittle and susceptible to cracking under thermal shock, which limits their application to areas of the guide plate that experience relatively stable temperatures. Nickel-chromium alloys offer a more balanced combination of toughness and hardness, making them suitable for regions subject to thermal cycling and occasional impact loading. The addition of molybdenum to the Ni-Cr system enhances solid solution strengthening and improves resistance to tempering softening at elevated temperatures.

Overlay Welding Process Selection and Parameter Optimization

The selection of the overlay welding process is as critical as the material choice. For rolling mill guide plates, submerged arc welding (SAW) and multi-wire GMAW are the most commonly employed processes due to their high deposition rates and ability to build substantial overlay thicknesses. Electroslag welding (ESW) is also applicable for thick overlay layers where heat input control is manageable.

A critical finding from the literature is that dilution control is paramount for maintaining the metallurgical properties of the overlay layer. For high-chromium cast iron overlays, the dilution rate must be maintained below 25% to ensure sufficient carbide formation and hardness retention. The base metal composition, typically low-carbon or low-alloy structural steel (e.g., Q345, S355), introduces significant amounts of iron and manganese into the weld pool, which can alter the phase composition of the overlay. To mitigate this, the use of a transition layer or a pre-heating strategy that minimizes base metal melting is recommended.

The number of overlay layers required depends on the desired final thickness and the dilution rate per pass. For a target overlay thickness of 10 mm with a per-pass dilution of 20%, at least three passes are necessary to achieve a composition in the top layer that closely matches the intended alloy chemistry. The interpass temperature should be controlled between 150°C and 250°C to prevent excessive grain growth while avoiding cold cracking susceptibility.

Defect Analysis and Quality Control

Common defects encountered in guide plate overlay welding include hot cracking in the overlay layer, lack of fusion at the base metal-overlay interface, undercut at the weld toe, and excessive surface roughness. Hot cracking is particularly problematic in high-chromium cast iron overlays due to the high solidification range of the eutectic composition and the presence of low-melting-point eutectics at grain boundaries.

A particularly important aspect of quality control for guide plate overlays is the bond strength testing between the overlay and base metal. According to NB/T 47014 and relevant industry standards, the shear bond strength should exceed 200 MPa for structural integrity. Ultrasonic testing (UT) using contact probes at 2.5 MHz or 5 MHz frequency is the preferred method for detecting interface defects such as lack of fusion and delamination.

Engineering Practice and Case Reflection

In practical applications, rolling mill guide plates typically require overlay thicknesses ranging from 5 mm to 15 mm, depending on the wear rate and the expected service interval before replacement. A case study from a hot strip mill showed that using a Ni-Cr-Mo alloy overlay with a thickness of 8 mm extended the guide plate service life from approximately 400 hours to over 2000 hours, representing a fivefold improvement. The key to this success was the careful control of dilution through a two-pass strategy: the first pass used a transition alloy with intermediate composition, and the second pass used the final Ni-Cr-Mo alloy with minimal dilution.

An important lesson from field experience is that the base plate preparation significantly affects overlay performance. Surface roughness greater than Ra 25 μm can lead to poor weld penetration and increased porosity. Additionally, residual stresses from the base plate rolling process can contribute to cracking in the overlay layer if not properly managed through stress-relief heat treatment or by controlling the welding sequence.

Study Insights and Implications

The study of cladding materials for rolling mill guide plates highlights the fundamental principle that material selection must be matched to the specific wear mechanism and operating environment. No single material system is universally optimal; rather, a rational approach based on understanding the dominant wear mechanism, thermal conditions, and mechanical loading is essential. The transition from empirical material selection to a more systematic approach based on wear mechanism analysis represents a significant advancement in engineering practice. Future developments in multi-layer overlay strategies, where different material systems are applied in sequence to create a functionally graded interface, hold great promise for further extending component life and reducing maintenance costs in rolling mill applications.