Technical Overview of the Developments in Rolling Mill Liners and Wear Parts 1

Technical Overview of the Developments in

Rolling Mill Liners and Wear Parts

Charles E. Churchill1

Abstract

Rolling Mill productivity and quality are significantly affected by the integrity of

various liners and wear part components of the mill. Today’s mill operators have a

wide choice of material grades and types to consider when specifying these critical

mill components. Liners need to be viewed as an overall technically integrated

system to ensure maximum value and performance of these “consumable” parts.

Technical details of these items can and should vary depending on the type of

rolling mill and the location of the part within the roll stack.

Understanding the nature of liner wear and the metallurgical aspects of liner design

will enable the mill operator to properly specify and choose among the options

commercially available. Use of special tool steels, corrosion resistant materials and

composites as well as hydraulically expandable liners should be considered as

proven methods to enhance the operations of hot and cold rolling mills.

1 Vice President of Technology ASKO, Inc., Homestead, Pennsylvania, USA

Technical Overview of the Developments in Rolling Mill Liners and Wear Parts 2

Introduction

The proper specification of mill liners and wear plates is important for a variety of

reasons. Principally, these parts minimize the surface wear on roll chocks and mill

housings thus protecting the more expensive mill components from wear and

damage. Also, when maintained properly, they improve the quality of rolled

products by assisting in proper roll alignment and gauge and shape control.

Additional benefits in lower total maintenance and operating costs can also be

realized. While there are a significant variety of mill stand designs, a typical Mae

West design is shown in the attached figure. During the course of this presentation,

we will discuss the various liner and wear components separately: Mill Housing and

Mae West liners, roll chock liners, and load bearing wear plates located at the

bottom of the roll stack (Figure 1).

Figure 1: Typical Mill Stand

Technical Overview of the Developments in Rolling Mill Liners and Wear Parts 3

Function of Liners and Types of Wear

Wear plates are designed to be consumable products within the mill system, but the

intent is to maintain the integrity of these parts as long as possible. Once significant

wear begins to occur, desired roll alignment is affected and resulting gauge and

shape problems may occur. Oftentimes to prevent quality problems and operating

problems, mill speeds are reduced or other corrective actions are implemented.

Regardless, the end result of premature and unpredictable wear of liners is reduced

productivity, higher maintenance costs, and deteriorated product quality.

It is important in evaluating and designing wear plates that we consider the various

causes for liner deterioration. The causes of wear can be characterized as follows:

impact, abrasion, corrosion and erosion/cavitation.

Impact related wear on liners can be caused by normal rolling forces, by the

thrusting of the roll stack into the mill housing particularly in reversing mills and

impact during roll changes or other rolling related mishaps (Figure 2). This type of

wear can cause catastrophic liner failure in the form of cracking or breakage.

Figure 2: Causes of Impact Wear/Failure

Technical Overview of the Developments in Rolling Mill Liners and Wear Parts 4

Abrasion-type wear is caused by normal frictional forces between components and

is accentuated by the presence of scale, lack of proper lubrication, roll changes, and

the continual minor movements of the rolls caused by automatic gauge and shape

control systems (Figure 3).

Figure 3: Causes of Abrasive Wear/Failure

Corrosion and erosion — which are particularly problematic in hot rolling mills — are

caused by water and humidity, elevated temperatures and high-pressure descaling

water impinging on the various mill components.

Engineering a Solution to Liner Wear

To address these various forms and modes of failure, it is important that the

reasons for wear are properly identified. This can be done by a periodic visual

inspection of the liners during roll changes or mill outages. Once this is

accomplished, wear plate metallurgy can be chosen to solve the specific problems

in the mill. Modern technology has made a wide variety of liner materials and

specifications available to meet the unique challenges of each mill wear plate

application. To solve specifically identified problems, liners can be designed with

varying degrees of toughness, wear resistance, and corrosion resistance. It is

likewise important that liners and wear plates be looked at in a holistic, mill-wide

manner. In today’s steel mills, it is not uncommon for different responsibilities,

such as maintenance, operations, or roll shops, to be responsible for purchasing

various wear plates that interact with one another. That being the case, it is critical

Technical Overview of the Developments in Rolling Mill Liners and Wear Parts 5

that these units coordinate their activities so that the most effective combination of

products are specified and purchased.

There are numerous factors that differentiate liner quality and, therefore, affect the

cost of these products. Raw material grade and type, dimensional tolerances

required, heat treatment and hardness are all critical elements to be considered. In

considering material types used for wear plates, the alternatives in order of

declining wear resistance include tool steels, corrosion-resistant steels, alloy steels,

carbon steels, bronze and brass, and plastic. Many of these materials are also

available as composites with a hardened outer layer and a mild steel backing to

contact the roll chock or mill housing.

Dimensional tolerances are critical in the specification of high-quality wear plates.

Critical specifications to consider (in addition to the overall size of the part) are

surface finish, flatness and parallelism of the part. Fine ground surface finishes will

minimize wear and reduce friction between mating parts. Proper as-manufactured

flatness will facilitate installation and assure that liners are not subjected to unusual

stresses that can cause cracking.

Heat treatment is critical in determining the metallurgical characteristics of a wear

plate. Proper heat treatment will give uniform, predictable, and repeatable hardness

as well as dimensional stability to the liner. Perhaps most important, the hardness

of properly manufactured wear plates can be customized to suit the specific

application within the mill stand.

In evaluating the metallurgical design of liners, taking into account the location

within the mill stack, liner performance can be optimized by an ideal combination of

wear resistance, toughness, hardness, corrosion resistance, and the proper material

grade and configuration. Considering the fact that liners are manufactured as

discrete pieces, it is possible to have slight differences in the hardness of parts that

slide against each other, making certain parts comparatively “sacrificial”. Proper

consideration of this concept of “Engineered Wear” allows smaller, more assessable

Technical Overview of the Developments in Rolling Mill Liners and Wear Parts 6

parts to be the sacrificial component. This ensures that larger, more expensive mill

components such as housing liners last as long as possible, and smaller parts such

as work roll chock liners that are easily assessable can be designed to wear out

slightly more quickly. Also, having a differential hardness between mating parts

avoids the possibility of galling or frictional pickup from one part to another.

Regarding maintenance, it is important that liners be periodically inspected for

wear. Customized “Wear Indicators” can be precisely ground into liners to facilitate

this process. The intent of this preventive action is to ensure that the various wear

plates are changed before they cause a deterioration in mill performance or product

quality and obviously before they cause an unexpected mill outage.

Liner Metallurgy

In evaluating the different materials that are available as wear plates, the most

common material used in high-quality wear plates are alloy tool steels which have

an excellent combination of wear resistance and toughness and also a modest

degree of corrosion resistance. Different grades of tool steel should be considered

for varying types of mills. Liners for Cold Reduction and Temper Mills can be made

from materials with very high hardness (60+Rc) and wear resistance. Liners for

Plate Mills and Hot Strip Mills require high toughness and wear resistance

(Figure 4).

Figure 4: ASKO Tool Steel Liners

Technical Overview of the Developments in Rolling Mill Liners and Wear Parts 7

Material type and hardness should be specifically designed for each application to

provide maximum value to the user. In recent years, the use of martensitic

stainless steel has become relatively common in hot rolling applications where the

principal mode of liner failure has been determined to be corrosion or erosion.

These materials contain approximately .5% Carbon and 14% Chrome, providing

hardnesses of approximately 55 Rockwell C and exceptional resistance to corrosion

(Figure 5). Again, since these parts are individually heat treated, it is possible to

vary the hardness of these plates depending on roll stack locations.

Figure 5: ASKO Corrosion Resistant (CR) Liners

There are a number of products that are available in clad or composite design.

Composite liners are produced with the outer hardened surface being tool steel,

corrosion-resistant martensitic stainless steel, or lower-alloy tool steels (Figure 6).

Figure 6: ASKO Clad/Composite Liners

Technical Overview of the Developments in Rolling Mill Liners and Wear Parts 8

In addition, modern technology allows for special materials such as bronze or brass

to be bonded to low-carbon steels for specialized mill applications. The differential

hardness of composite materials allows for the outer material to provide the

required levels of wear resistance and/or corrosion resistance while the softer

material protects the adjoining mill housing or roll chock from impact. Most

commonly, the outer hardened material comprises approximately 25% of the

overall thickness while the remaining 75% of the thickness is low-carbon mild steel.

Load bearing members of the liner system, generally found at the bottom of the roll

stack, present a unique challenge to liner manufacturers and mill operators (Figure

7). The normal mode of failure of these parts is often breakage so it is critical that

these parts have not only good wear resistance but also exceptional toughness.

Figure 7: Load Bearing Wear Plates

Materials have been developed by liner manufacturers that result in a hard, wear-

resistant outer surface that resists deformation and wear. The metallurgical design

of these parts is such that the inner core of the liner is exceptionally tough, thus

resisting cracking and breakage. This combination of properties is due to the proper

specification of tool steel and unique thermal processing. In certain circumstances,

Technical Overview of the Developments in Rolling Mill Liners and Wear Parts 9

it has been found advantageous to use corrosion-resistant materials in load bearing

applications but hardness control is critical.

Finally, certain applications, particularly in Cold Reduction Mills and Temper and

Skin Pass Mills, can warrant the use of unique products that provide solutions to

wear and vibration problems. These products can include expandable hydraulic

liners, and a variety of non-ferrous liners such as bronze, brass and various types

of plastic. Many high-speed cold rolling mills experience episodes of severe

vibration that can cause marking of the strip or marking of the back up rolls. Many

investigations have been made into the causes and preventive measures for

vibration or chatter but despite preventive measures, many mill operators find

themselves limiting rolling speeds and thus limiting productivity to avoid the onset

of chatter. Clearly in today’s quality driven market, chatter on finished sheet

product is unacceptable. Research has shown that 3rd octave vibration (125 to 250

Hz) is common in Tandem Cold Reduction Mills and 5th octave vibration (500 to 700

Hz) is most common in Single or Two Stand Temper Mills.

ASKO-patented hydraulically expandable mill post liners (Hydraliners) have been

shown to effectively dampen both 3rd and 5th octave vibrations allowing mills to run

at designed speeds and levels of productivity without chatter related quality

problems. Vibration amplitude can be significantly reduced by the use of

Hydraliners permitting higher operating speeds without harmful mill chatter

(Figure 8).

Figure 8: Effect of the Hydraliner, Impulse Response

Technical Overview of the Developments in Rolling Mill Liners and Wear Parts 10

Most often these expandable liners are installed adjacent to the bottom back up roll

chocks, both operator and drive sides of the mill. It is possible to also use these

liners on the mill housings adjacent to the top back up roll chock without affecting

gauge and shape control systems. In Tandem Cold Reduction Mills, expandable

liners are usually installed first in the next to last stand where the combination of

speed and reduction are greatest (Figure 9).

Figure 9

Technical Overview of the Developments in Rolling Mill Liners and Wear Parts 11

While somewhat costly, replacing conventional housing liners with expandable

liners, has been shown to have a very fast return on investment (Figure 10).

Figure 10: Hoogovens (Corus) Five Stand Cold Mill Effect of ASKO Hydraliners

Summary and Conclusions

It is important that wear plates and liners within a rolling mill are viewed as being

an engineered product rather than merely a consumable wear part. Properly

designed liners provide high value to the end user by providing exceptionally long

service and very slow, predictable rates of deterioration.

Examples of high quality liners providing extended service are common (Figure 11).

Figure 11

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Similarly, changing to advanced materials such as Martensitic Stainless Steel has

produced dramatic improvements in liner life (Figure 12).

Figure 12

This engineering approach to mill wear plates is intended to keep the mill closer to

design tolerances for longer periods of time with resulting higher product quality,

improved productivity and lower maintenance costs. Reliable producers of these

wear plates can and should work with mill operators and engineers to evaluate the

mode of failure of wear plate components and assist in the recommendation and

specification of liner materials to optimize the performance of rolling mills.

References

1. R.Schrama, Housing and Chock Clearances and Liner Materials, Lubrication

Engineering Vol. 50 1994.

2. ASKO Inc, Liner and Wear Part Technical Brief, 2004

3. F. ter Lingen, The Reduction of Cold Mill Chatter, AISI Publication 2000.