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Engineering Wear To Minimize Mill Maintenance Costs

Liner systems can be engineered to maximize liner life and minimize mill maintenance costs. Maintenance costs are reduced when the liner system is engineered so that the less expensive and easier-to-replace liners wear out, but the surfaces of the more expensive and difficult-to-replace components do not incur wear. Ensuring the compatibility of liners with the other wear surfaces maximizes wear resistance and keeps the mill closer to design specifications longer.

It is important that two surfaces in contact with each other have different hardness and good surface finish. Varying the hardness prevents galling and determines which parts wear more quickly.

Minimizing the wear rate of two liner surfaces reduces the maintenance costs associated with the costs of the parts and the downtime to replace the parts.

Liner Impact Resistance and Wear Resistance

Impact resistance is the ability of a liner to absorb impact without fracturing, and is sometimes referred to as "fracture toughness". To increase impact resistance, various alloys are added to the steel. Proper heat treatment also ensures that the toughness of any material selected is maximized. Liners must have good impact resistance since the mill rolls are moved by the strip or bar momentum upon entry and exit, forcing the chocks to impact against one side of the housing and its liners.

The impact a liner experiences varies depending on its application. For example, in reversing plate mills, with heavier reduction and changing directions, liners are subject to great impact. In cold rolling mills where the forces are less, impact on the liners is less. Abrasive wear on a liner varies depending on factors such as the presence or absence of scale and lubrication or automatic gauge and shape control. The chocks are in relative motion with the housing, causing a sliding mechanism between the housing and chock liner. At roll changes, sliding friction is also present.

The wear resistance of steel depends on the following interrelated factors:

• Steel composition
• Heat treatment
• Depth of hardness
• Surface finish

Corrosion Resistance
Corrosion of a liner varies depending on the presence of the corrosive media of water, roll coolant, humidity, and high temperatures. For example, corrosion is a problem in hot mills at or below the pass line where the parts are exposed to water and high temperatures. Corrosion can cause pitting and flaking, leading to liner wear. Water and scale at high pressures and temperatures causes the erosion of liner surfaces.

How Do You Measure Wear?
The easiest way to monitor the wear of a liner without removing the liner and other parts, is with wear grooves. This feature on a liner allows you to visually detect and easily measure the amount and uniformity of wear. Wear grooves are two shallow grooves precisely ground into the wear surface of the liner and running the length of the liner, on opposite sides. As the liner surface wears, its groove becomes visibly narrower, providing a quick and easy check on liner wear when its surface is exposed during maintenance or roll change. The precise amount of wear can be determined with the liner in position by measuring the groove with depth micrometers.

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