Strip Cladding of Rolling Mills and Its Technical Implementation

Literature Overview and Industrial Context

This 1999 publication by Zhao Weiyan, Wang Hui, Zhang Huifen from Dalian Heavy Industry Group, and Liu Qinglin from Jinan No. 2 Machine Tool Group, published in "Welding," addresses the application of strip cladding technology to rolling mill components. Rolling mills are among the most demanding industrial environments for wear-resistant surfaces, experiencing extreme contact pressures, high temperatures, and abrasive sliding against steel slabs. The paper presents the technical challenges and solutions for applying strip cladding to restore or enhance the wear resistance of rolling mill rolls, housings, and other critical components. At the time of publication, strip cladding was a relatively mature technology in Europe and Japan but was still developing in China, making this paper a valuable resource for domestic engineers seeking to implement the technology.

Strip Cladding Process Fundamentals

Strip cladding, also known as flash butt welding or explosion welding in some contexts, involves the welding of a wear-resistant strip to a base component through a combination of mechanical pressure and localized heating. The process can be performed using several methods: electroslag welding (ESW) with strip electrodes, submerged arc welding (SAW) with strip electrodes, or explosive cladding. The paper focuses primarily on the ESW and SAW methods, which are the most practical for rolling mill applications due to their high deposition rates and ability to handle large surface areas.

The following table summarizes the process parameters for strip cladding of rolling mill components:

Parameter	ESW Strip Cladding	SAW Strip Cladding
Base metal	Carbon steel or low-alloy steel	Carbon steel or low-alloy steel
Strip material	High-chromium cast iron or alloy steel	High-chromium cast iron or alloy steel
Strip thickness	3–10 mm	3–8 mm
Current	800–1,500 A	500–1,000 A
Voltage	35–45 V	25–35 V
Travel speed	50–150 mm/min	100–300 mm/min
Flux type	Granular flux, alkaline	Granular flux, alkaline
Preheat temperature	200–300 °C	150–250 °C
Interpass temperature	≤ 250 °C	≤ 200 °C
Deposition rate	20–40 kg/h	15–30 kg/h
Dilution ratio	10–20%	15–25%

The paper emphasizes the importance of the dilution ratio in determining the final properties of the cladding layer. For high-chromium cast iron cladding, the dilution ratio should be kept below 20% to ensure the formation of a sufficient volume of hard carbides (M₇C₃ and M₂₃C₆) in the weld metal. A dilution ratio exceeding 25% leads to a significant reduction in hardness from the desired 58–62 HRC to below 45 HRC, rendering the cladding ineffective for abrasive wear applications.

Material Selection and Metallurgical Considerations

The selection of cladding strip material is critical for rolling mill applications. The paper recommends high-chromium cast iron strips (Cr15 to Cr28) for applications involving abrasive wear against steel slabs, and alloy steel strips (such as 40CrNiMo) for applications involving adhesive wear and galling. The metallurgical behavior of the cladding layer is influenced by the cooling rate, which is determined by the base metal thickness and the welding process parameters.

For high-chromium cast iron cladding, the microstructure consists of a matrix of martensite and retained austenite with dispersed carbides. The volume fraction of carbides is directly related to the chromium content; a Cr15 strip produces approximately 30% carbides, while a Cr28 strip produces approximately 50% carbides. The paper notes that the retained austenite content should be controlled below 20% to prevent the formation of brittle cementite during cooling, which can cause cracking. A post-weld tempering treatment at 200–300 °C is recommended to stabilize the retained austenite and reduce residual stresses.

The bond strength between the base metal and the cladding strip is another critical consideration. The paper reports bond strength values of 280–350 MPa for ESW cladding and 250–320 MPa for SAW cladding, which are well above the minimum requirements of 200 MPa specified in industry standards. The bond strength is influenced by the cleanliness of the base metal surface, the flux composition, and the welding parameters. Surface contamination, particularly oil and rust, must be removed by grinding or shot blasting before cladding to ensure metallurgical bonding.

Application Cases and Performance Results

The paper documents the application of strip cladding to several rolling mill components. One case involves the cladding of a 1,200 mm wide flat rolling mill housing, which experienced severe wear on the bearing seats after 6 months of operation. The housing was clad with a 6 mm thick Cr20 high-chromium cast iron strip using ESW, with a deposition rate of 25 kg/h. After cladding, the housing demonstrated a service life of over 36 months, an improvement of 600% over the original unclad condition. The cladding layer hardness was measured at 59 HRC, with a carbide volume fraction of 42%.

Another case involves the cladding of a rolling mill roll neck, which is subject to high contact pressure and sliding friction. The roll neck was clad with a 5 mm thick 40CrNiMo alloy steel strip using SAW, achieving a hardness of 42–45 HRC and a surface roughness of Ra 1.6 μm. The cladded roll demonstrated a 400% improvement in service life compared to the original case-hardened roll.

Defect Analysis and Countermeasures

The paper identifies several common defects in strip cladding of rolling mill components. Cracking is the most serious defect, typically occurring in the heat-affected zone (HAZ) of the base metal or at the interface between the base metal and the cladding layer. The primary cause is the high carbon equivalent of the base metal combined with insufficient preheating. The countermeasure is to increase the preheat temperature to 300 °C for base metals with carbon equivalent exceeding 0.5% and to apply a post-weld heat treatment at 550–600 °C for 2–4 hours.

Porosity in the cladding layer is caused by moisture in the flux or contamination of the strip surface. The paper recommends baking the flux at 300–350 °C for 2 hours before use and ensuring the strip surface is free of oil and moisture. Undercut at the strip edge can be minimized by using a cored wire as a backing strip and by optimizing the welding current and travel speed.

Study Insights and Engineering Implications

This paper provides a practical guide to the implementation of strip cladding technology for rolling mill applications. The detailed process parameters and material selection recommendations are directly applicable to engineering practice. The emphasis on dilution ratio control and post-weld heat treatment reflects a deep understanding of the metallurgical challenges involved. In my experience, the key to successful strip cladding is not just the welding process itself but the comprehensive approach that includes proper surface preparation, material selection, parameter optimization, and post-weld treatment. Engineers should adopt a systematic approach to strip cladding, treating each component as a unique challenge that requires careful analysis of the service conditions and appropriate selection of materials and processes.