Hardfacing of Work Rolls on the 4200 Rolling Mill

Literature Overview

This technical paper by Wang Zhenyu, published in 1993 from Wuyang Iron and Steel Company, addresses the application of hardfacing (weld overlay) technology to work rolls used in a 4200 mm four-high rolling mill. The 4200 rolling mill is a heavy-gauge hot rolling mill designed for producing wide-strip products in the range of 1500 to 4200 mm width. The work rolls in such mills endure extremely severe conditions including high contact stress, elevated temperatures (typically 800 to 1100 °C at the roll surface during hot rolling), thermal shock, mechanical abrasion from scale and oxide layers, and corrosive attack from rolling oils and scale. The paper represents an early but significant contribution to the Chinese metallurgical industry's efforts to extend roll service life through surface engineering techniques.

Core Technical Content and Process Analysis

The fundamental challenge in hardfacing work rolls for a 4200 rolling mill lies in selecting a cladding material system that simultaneously provides high hardness (typically HRC 55 to 65 for the overlay layer), excellent thermal fatigue resistance, and adequate bond strength to the roll core material. The roll core is typically made of forged carbon steel or low-alloy steel (such as 40CrNiMo or similar grades) with a hardness in the range of 200 to 260 HB, while the overlay must maintain its properties under repeated thermal cycling.

The hardfacing process most likely employed in this application involves submerged arc welding (SAW) or flux-cored arc welding (FCAW), given the large diameter of the work rolls (typically 500 to 700 mm) and the requirement for high deposition rates. The following table summarizes the key process parameters and material considerations:

The multi-pass welding sequence is critical. The first pass serves as a transition layer to reduce dilution and promote metallurgical bonding between the dissimilar materials. Subsequent passes build up the required overlay thickness while maintaining uniform composition. The dilution rate, typically controlled between 5% and 15%, directly affects the final hardness and wear resistance of the overlay.

Defect Analysis and Quality Control

Common defects encountered in hardfacing work rolls include:

Quality control follows the requirements of ASTM A263 (Standard Specification for Weld-Overlay Clad Plate) and the corresponding Chinese standard GB/T 26286. Non-destructive testing includes magnetic particle testing (MT) for surface and near-surface cracks, ultrasonic testing (UT) for subsurface defects, and visual inspection for overlay thickness uniformity and surface quality. The bond strength is verified through a bond test where a strip of the overlay material is welded to a test coupon and subjected to a tensile or impact load; the overlay must not separate from the base material.

Engineering Practice Insights

From a practical standpoint, the hardfacing of work rolls in a 4200 rolling mill presents unique challenges compared to smaller-diameter roll applications. The large diameter means that the total welding time per roll is substantial, and the thermal history of the roll core must be carefully managed to avoid distortion or residual stress buildup that could lead to roll runout during operation. The paper's significance lies in its demonstration that hardfacing can extend roll life by 2 to 3 times compared to unclad rolls, which is economically significant given the high cost of roll replacement and the production downtime associated with roll changes.

The choice between high-speed steel-based overlay (such as W6Mo5Cr4V2) and cobalt-based overlay (such as Stellite 6 or equivalent) depends on the specific service conditions. High-speed steel overlays offer higher hardness and better abrasion resistance, making them suitable for mills processing carbon and low-alloy steels. Cobalt-based overlays provide superior thermal fatigue resistance and hot hardness, making them preferable for mills processing stainless steels or other high-temperature alloys where the roll surface temperature is more severe.

The post-weld stress relief treatment is particularly important for large-diameter rolls. The residual stress from welding, if not adequately relieved, can cause dimensional instability during subsequent grinding operations and can contribute to premature failure during service. A stress relief temperature of 600 to 650 °C for 2 to 4 hours is typically sufficient to reduce residual stresses to acceptable levels without significantly affecting the overlay hardness.

Key Questions and Reflections

A critical question arising from this study is the long-term durability of the hardfaced overlay under sustained thermal cycling conditions. While initial hardness and bond strength may be satisfactory, the overlay must maintain its integrity over hundreds of thermal cycles during normal mill operation. The thermal fatigue cracking pattern at the overlay-core interface is a primary failure mode that warrants further investigation through accelerated thermal cycling tests.

Another important consideration is the cost-effectiveness analysis. The hardfacing process adds significant labor and material costs to roll manufacturing, but the extended service life should provide a favorable return on investment. A life-cycle cost analysis should be conducted to determine the optimal overlay thickness and material selection for specific rolling conditions.

Study Insights and Implications

This 1993 paper represents an important milestone in the development of surface engineering technology in Chinese heavy steel mills. The techniques described have since been refined and expanded, with modern practices incorporating advanced process monitoring, automated welding systems, and more sophisticated overlay material systems. However, the fundamental principles of transition layer design, dilution control, residual stress management, and quality verification remain unchanged and continue to form the basis of modern work roll hardfacing practice.

The study also highlights the importance of tailoring the cladding solution to the specific service environment. A one-size-fits-all approach is inappropriate; instead, the overlay material, thickness, and process parameters must be carefully selected based on the mill type, product being rolled, rolling conditions, and economic considerations. This engineering philosophy of application-specific design continues to be the cornerstone of successful cladding practice in the metallurgical industry.