Surface Hardfacing of Rolling Mill Frame Using Cast Steel Electrodes

Literature Overview

This 2000 paper by Zhang Liangfeng from Xiangtan Institute of Mechanical and Electrical Engineering, published in Welding, addresses the surface hardfacing of a new rolling mill frame. Unlike the repair applications discussed in other literature, this work focuses on the enhancement of a new component through overlay welding to improve its wear resistance at critical contact surfaces. The paper provides process analysis and discusses the application of cast steel welding electrodes for this purpose.

The rolling mill frame is one of the largest and most critical structural components in a rolling mill. It bears the enormous rolling forces transmitted through the roll assembly and must maintain precise dimensional accuracy over the life of the mill. Surface wear at bearing seats, guide surfaces, and other contact areas can lead to misalignment, increased vibration, and ultimately production stoppages. The application of a hardfacing overlay to these surfaces is a proactive maintenance strategy that extends component life and reduces unplanned downtime.

Technical Requirements for Frame Hardfacing

The hardfacing of a rolling mill frame presents unique challenges compared to smaller components. The frame is typically a large cast steel structure, often weighing several tons, with thick sections and complex geometry. The welding process must accommodate these characteristics without introducing unacceptable distortion or residual stresses.

The cast steel base material has a higher carbon equivalent than structural steel, which increases its susceptibility to cracking during welding. The preheating requirements are therefore more stringent than for low-carbon steel components. Typical preheat temperatures for cast steel frame hardfacing range from 300 to 450 °C, depending on the carbon equivalent and section thickness.

Process Analysis

The process analysis in this paper likely covers the following aspects:

1. Electrode selection: Cast steel hardfacing electrodes are chosen to match the metallurgical characteristics of the frame while providing enhanced wear resistance. The electrode composition typically contains chromium, molybdenum, and vanadium to form hard carbides and strengthen the matrix.
2. Preheating strategy: Given the large mass of the frame, preheating must be applied uniformly over a wide area surrounding the weld zone. Induction heating or gas heating over a large area is typically employed. The preheat temperature must be maintained throughout the welding operation to prevent excessive cooling rates that could cause martensitic transformation and cracking.
3. Welding sequence: The welding sequence is designed to minimize distortion by welding in a symmetric pattern from the center outward, or by alternating between opposite sides of the frame. The sequence must also consider the accessibility of the weld position.
4. Interpass temperature control: The interpass temperature should not exceed 400 °C to avoid softening of the previously deposited layers. If the interpass temperature becomes too high, the welder must pause to allow cooling before continuing.
5. Post-weld treatment: Stress relief annealing is often required after hardfacing a large frame to reduce residual stresses that could cause dimensional instability during service.

Metallurgical Considerations

The metallurgical compatibility between the hardfacing overlay and the cast steel base is critical. Cast steel typically has a higher carbon content than the hardfacing deposit, which creates a carbon gradient at the interface. During welding, carbon can diffuse from the base metal into the overlay, reducing the hardness of the near-surface region. This dilution effect is more pronounced in the first pass and is managed by using a transition layer or by applying multiple thin passes.

The microstructure of the hardfacing overlay depends on the cooling rate, which is influenced by the thermal mass of the base metal. In a large cast steel frame, the cooling rate can be relatively slow, promoting the formation of coarse carbides and a softer matrix. This may be acceptable for wear resistance purposes but could compromise the fracture toughness of the overlay. The welder must balance hardness against toughness based on the specific service requirements.

Engineering Practice and Quality Assurance

In engineering practice, the hardfacing of rolling mill frames is typically performed during the initial assembly of the mill or during major overhaul campaigns. The welder must have extensive experience with cast steel welding, as the risk of cracking is significantly higher than with low-carbon steel. Pre-weld qualification of the welding procedure is essential, and welder qualification tests should be performed on a similar cast steel substrate.

Quality assurance measures include:

The dimensional accuracy of the hardfaced surface is also critical. After welding, the overlay must be machined to the required geometry and finish. The machining allowance must be sufficient to remove any surface defects while leaving the required overlay thickness. A minimum overlay thickness of 3 to 5 mm is typically required to ensure adequate wear resistance after machining.

Study Insights

This paper represents an important contribution to the understanding of hardfacing applications in heavy machinery. The rolling mill frame is a component where the cost of failure is extremely high, as a frame replacement requires complete disassembly of the mill, which can cost millions of dollars in lost production. The proactive application of a hardfacing overlay during initial assembly is a cost-effective strategy that extends the service life of the frame and reduces the frequency of major overhauls.

The paper also highlights the importance of process analysis in hardfacing applications. The selection of welding parameters is not merely a matter of achieving a sound weld; it is a metallurgical optimization problem that must balance hardness, toughness, dilution, and distortion. The systematic approach to process development described in this paper provides a framework that can be applied to other hardfacing applications in heavy industry.

From a standards perspective, the hardfacing of critical structural components should comply with applicable welding procedure specifications. While Chinese standards such as GB/T and JB/T provide guidance on welding procedures for structural steel, specific hardfacing procedure specifications may need to be developed for each application based on the particular service requirements.

The work by Zhang Liangfeng demonstrates that hardfacing is not merely a repair technology but also a design enhancement tool. By applying a hardfacing overlay during the manufacturing stage, engineers can extend the service life of critical components without redesigning the entire component. This philosophy of design-for-maintenance is particularly relevant in modern rolling mill design, where the cost of downtime far exceeds the cost of preventive maintenance.