Cladding Technology Summary for 750 Bloom Mill Rolls

Literature Overview

This technical summary, published in 1990 by Shandong Metallurgy, documents the engineering experience accumulated during the cladding (weld overlay) of rolls used in a 750 mm bloom mill. The 750 mm bloom mill is a critical piece of equipment in the hot rolling production line, where the roll surface endures extreme thermal cycling, mechanical wear, and chemical attack from scale and lubricants. The document represents one of the earlier systematic engineering reports on heavy-duty roll cladding in China's metallurgical industry, reflecting the technological maturity reached by the late 1980s in weld overlay applications for rolling mill components.

Core Technical Content

The study addresses the selection of cladding materials, welding process parameters, heat treatment procedures, and quality control methods for restoring and enhancing the surface properties of bloom mill rolls. The fundamental engineering challenge is to achieve a cladding layer that simultaneously resists abrasive wear from scale removal, thermal fatigue from repeated contact with hot slabs at temperatures exceeding 1000 °C, and mechanical impact from roll-slab interaction forces.

Cladding Material Selection

The selection of cladding materials for bloom mill rolls follows a systematic approach based on the operating conditions of each roll type (upper backup roll, lower backup roll, and work roll). The table below summarizes the typical material combinations considered in this era of engineering practice.

The cladding materials selected for this application typically contain carbon equivalent levels between 0.6% and 1.2%, with alloying elements such as Cr, Mo, and V added to promote the formation of hard carbide phases. The hardness of the as-welded cladding layer is designed to achieve HRC 50-60, providing a significant improvement over the base steel hardness of HRC 35-42.

Welding Process Parameters

The welding process employed for bloom mill roll cladding involves multi-pass submerged arc welding (SAW) or gas shielded metal arc welding (GMAW), depending on the available equipment and the specific geometry of the roll. Key process parameters include:

The dilution rate between the base metal and the cladding layer is a critical parameter that directly affects the final composition and hardness of the overlay. For bloom mill applications, a dilution rate of 25-40% is generally acceptable, as it provides sufficient hardening elements while maintaining adequate toughness at the bond line.

Quality Control and Inspection

The quality assurance methodology for roll cladding includes visual inspection, magnetic particle testing (MT) for surface and near-surface defects, and ultrasonic testing (UT) for subsurface indications. Hardness profiling across the cladding layer thickness is performed to verify the uniformity of the overlay and to detect any soft zones that could lead to premature wear. The bond strength between the cladding layer and the base material is evaluated through a bend test or a transverse tensile test, with acceptance criteria requiring that fracture occurs within the base metal rather than at the interface.

Engineering Practice Insights

The 1990 summary reflects the practical challenges encountered during the transition from manual to mechanized welding for heavy-duty roll cladding. One of the key lessons documented is the importance of maintaining consistent interpass temperature control, as excessive cooling between passes leads to hardening of the previous pass and increases the risk of cracking in subsequent passes. The document also highlights the economic advantages of cladding repair over complete roll replacement, with typical service life extensions of 2-3 times the original uncladded condition.

A significant engineering consideration documented in this summary is the management of thermal distortion during cladding. The cylindrical geometry of the roll, combined with the asymmetric heat input from circumferential welding, creates complex residual stress patterns that must be carefully managed through symmetric welding sequences and controlled cooling rates. The use of intermittent welding patterns and the application of thermal compensation through pre-bending of the roll are discussed as effective countermeasures.

Study Reflections

This document serves as a valuable historical reference for understanding the evolution of cladding technology in the Chinese metallurgical industry. The systematic approach to material selection, process parameter optimization, and quality control documented here reflects the engineering maturity achieved through years of field experience. The emphasis on practical solutions rather than purely academic analysis makes this summary particularly useful for engineers currently working on roll repair projects who need to understand the foundational principles that still govern modern cladding practice. The key insight from this literature is that successful roll cladding requires not only appropriate material selection but also meticulous attention to process control and heat treatment, as the service life of the cladding is often determined by the quality of the bond line rather than the bulk properties of the overlay material.