Roll Failure Costing You Downtime? Here‘s How to Choose the Right Rolling Mill Roll

Keywords: roll failure, rolling mill rolls, roll material selection, roll breakage causes, thermal breakage, fire cracks in rolls, roll spalling, roll hardness, residual stresses, iron based rolls, steel based rolls, tungsten carbide rolls, indefinite chill rolls, spheroidal graphite iron rolls, double pour rolls, roll wear resistance, extend roll life, rolling mill downtime prevention

Rolling mills need rolls that maintain their shape and profile for longer periods.
This helps extend rolling campaigns and improves productivity.

Normally, the life of a roll ends when it reaches its planned discard diameter.
However, failures and abnormalities still happen during service, even with careful handling from both the supplier and the mill operator.

A roll failure is a major catastrophe.

It can cause partial or total loss of the roll.

It also forces the mill to remove the resulting cobble, stops production, and may damage mill equipment.

All of these hurt mill performance.

Therefore, roll failures must be avoided.

Roll Components

Depending on the product profile, the roll body can be smooth or grooved.

• Smooth rolls are used for rolling flats, such as sheets, plates, or strips.
• Grooved rolls are used for rolling rounds and shaped materials like sections.

A roll has three main components (Fig 1):

1. Roll body – the part that contacts and deforms the metal
2. Roll necks – support the roll in the mill housings
3. Wobblers – drive or rotate the rolls

The barrel is the working portion where actual rolling takes place.

Required Properties of a Good Roll

A high-quality roll must have several key properties.

These include:

• High wear resistance – for longer life and better economy
• Fracture resistance – to withstand increasing rolling loads
• Fire cracking resistance – to handle steep temperature gradients between the rolling stock and the roll
• Spalling resistance – to prevent premature failure from thermal and pressure gradients
• Good surface finish – to produce high-quality product surfaces
• Good biting properties – to grip the material effectively

Parameters That Control Roll Properties

The main parameters influencing roll properties are:

• Mono or compound roll design.
• Chemical composition of the roll material.
• Casting conditions (mould design, temperatures, weights, inoculation, down cooling).
• Heat treatment.

Choosing the Right Roll Material

Selecting the right roll material is vital for good rolling practice.

Important properties include:

• Chemical composition.
• Microstructure.
• Hardness.
• Yield strength.
• Ultimate tensile strength.
• Young’s modulus.
• Poisson’s ratio.
• Coefficient of thermal expansion.
• Heat conductivity.
• Coefficient of heat transmission.

A general rule: harder rolls wear better but are less tough. The opposite is also true.

There is no universal rule that one roll technology is always better than another.
The final choice usually depends on the cost per ton of material rolled.
Mills value rolls by their cost of use and the surface quality they produce.
Low-priced rolls are not always better and can become counter-productive.

Historical Development of Rolls

In the 19th century, mills mostly used non-alloy grey iron and forged steel rolls.
Cast iron grades ranged from mild hard to clear chill.
These rolls had a white iron barrel with a grey iron core and necks.

Later, cast steel rolls were developed and are still produced today.

Around 1930, ICDP (Indefinite Chill Double Pour) rolls appeared for hot rolling.
In the late 1990s, carbide-enhanced ICDP rolls emerged.

Around 1950, nodular (spheroidal graphite) iron was introduced.
These rolls offered both good wear resistance and strength.

High chromium iron (2–3% C, 15–20% Cr) and later high chromium steel (1–2% C, 10–15% Cr) brought much higher wear resistance.

In early 1985, high-speed tool steel was introduced for rolls.
After solving initial problems, these rolls performed much better.

For wire rod mills, sintered tungsten carbide (HIP) rolls were developed.
Ceramic rolls are still under development.

For cold rolling mills, forged steel rolls improved with higher alloy content (chromium increased from 2% to 5%).
Chrome plating of work rolls was also introduced to improve surface roughness.

Classification of Rolls

Rolls can be classified in several ways.

By Application (Fig 2)

• Longitudinal rolling (flats or sections).
• Transverse rolling.
• Thread rolling.
• Ring rolling.
• Tube piercing (seamless pipe).
• Skew rolling.

By Production Method

• Cast rolls (single pour, double pour, centrifugal cast).
• Forged rolls.
• Sintered rolls.
• Other methods.

Each method has its own advantages, disadvantages, and limits.
These limits come from roll dimensions, material composition, required hardness, wear resistance, and production costs.

By Microstructure

• Hypo-eutectoid steel rolls.
• Hyper-eutectoid steel (Adamite) rolls.
• Graphitic hyper-eutectoid steel rolls.
• High alloy steel rolls.
• Spheroidal graphite iron rolls.
• Indefinite chill cast iron rolls.
• Special materials (sintered carbide, ceramic).

By Surface Hardness (BHN)

By Material

• Iron-based rolls.
• Steel-based rolls.
• Special materials (tungsten carbide).

Iron-Based Rolls

Iron-based rolls are widely used because they provide good finish and wear properties.

Modern alloying elements, improved casting, and heat treatment have greatly enhanced their performance.

Types include:

• Grey iron rolls.
• Alloy iron rolls.
• Clear chill (definite chill) rolls.
• Double pour rolls.
• Indefinite chill rolls.
• Spheroidal graphite rolls.
• Centrifugally cast rolls.

Grey Iron Rolls

These are sand-cast and contain flake graphite.
The structure is uniform and very resistant to fire cracking.
Free graphite provides some self-lubrication.

Alloy Iron Rolls

Small amounts of nickel, chromium, and molybdenum increase hardness.
These rolls wear better than straight grey iron.
Higher alloy content gives higher hardness and higher cost.

Clear Chill Rolls

Used for sheets, plates, and small sections.
A chill mould creates a hard white iron surface layer.
The core remains grey iron.
Hardness drops quickly if the chill layer wears through.

Double Pour Rolls

Also called composite or duplex rolls.
The outer shell is hard, and the inner core is tough.
Two different metals are poured sequentially.
These are expensive but perform well.

Indefinite Chill Rolls

The hardness drop from surface to core is very gradual (about 1° Shore per 10 mm depth).
There is no sharp transition zone.
These rolls resist fire cracking and spalling better than clear chill rolls.
They are often used in intermediate and finishing stands.

Spheroidal Graphite Iron Rolls

Graphite forms as spheroids (nodules) instead of flakes.
This increases ductility and fracture resistance.
Strength is about twice that of flake graphite iron.
Toughness is about twelve times higher.
These rolls wear evenly but are more prone to fire cracking.

Centrifugally Cast Rolls

The mould spins at high speed during casting.
This creates a denser, more uniform shell.
These are superior to static double pour rolls.

Steel-Based Rolls

Steel rolls are stronger and tougher than iron rolls.
They allow heavier drafts and handle shock loading better.
However, carbon steel rolls wear faster because of lower hardness.

Cast Steel Rolls

Carbon content can be as high as 2.5%.
Plain carbon steel rolls are the cheapest.
Adding molybdenum, chromium, and nickel improves wear resistance and strength.

Adamite rolls are the most common in this group.
They are graded A (softest, toughest) to E (hardest, least tough).

Forged Steel Rolls

Forged rolls are denser and tougher than cast steel rolls.
However, they have lower carbon content, so they wear faster.
They are used where high loads are expected, such as blooming mills.

Forged rolls are also used as back-up rolls in 4-high mills.
Hardened forged rolls serve as work rolls in cold rolling (90–100° Shore).

Tungsten Carbide Rolls

Tungsten carbide (WC) is extremely hard.
These rolls are ideal for long rolling campaigns, such as high-speed wire rod mills.

Tungsten carbide is the main hard phase.
Cobalt or nickel serves as the binder.

Important notes:

• Cooling water must have a narrow pH range.
• Wrong pH leaches the binder and causes surface failure.
• Carbide sleeves can be mounted on steel shafts for intermediate mills.

Carbide rolls are made by pressing, shaping, and sintering.
Diamond grinding or very hard turning tools are required for final machining.

Issues in Roll Production

To be commercially successful and offer reasonable prices, roll producers need:

• Understanding of roll application (load, speed, cooling).
• Choice of optimum material.
• Production of sound rolls without defects.
• Adequate heat treatment (strength, hardness, residual stresses).
• Ability to machine rolls to specifications.
• Ability to follow changing user requirements.

Common Technical Issues

Some rolls fail prematurely before reaching expected life.
Roll maker and roll user must agree on failure analysis and cost responsibility.

Rolling conditions are never truly stable.
They can change from good to worse during a campaign.

Some mills face frequent roll problems while similar mills do not.
Roll makers must get involved in solving these problems to maintain good relationships.

Roll design must balance two opposing needs:

1. Maximum strength – to handle separating forces, torque, and high pressure
2. Maximum wear resistance – in the contact area with the rolling stock

It is difficult to achieve both from a single material.
Technologies like double pouring and centrifugal casting help solve this problem.

Factors Affecting Roll Quality

Roll quality depends heavily on material, composition, and microstructure.
Two factors are especially important.

Roll Hardness

Higher hardness improves wear resistance but increases failure risk.
Higher hardness also makes machining and grinding more difficult.
Not everything improves with higher hardness. In fact, the opposite is often true.

Residual Stresses

Residual stresses act as pre-stress and strongly affect roll strength.

• Compression stresses – increase fatigue strength, reduce crack propagation, reduce shear stress, and improve work hardening
• Tensile residual stresses – can cause roll breakage

Compression and tensile stresses balance each other across the roll cross section.
The correct level of residual stresses must be controlled.

Reasons for Roll Failures

Roll failures can happen for one or more of the following reasons.

Thermal Breakage

The barrel breaks with radial-oriented fracture lines.
The origin is at or near the axis.
Fracture is perpendicular to the roll axis, usually near the barrel center.
Causes include thermal gradient, residual stresses, and core material integrity.

Torsional Breakage of Driven Roll Necks

This usually happens during rolling accidents.
Fatigue torsional failure of roll necks is rare.

Fire Cracks

Fire cracks are thermal shock cracks.
They form under very sharp cooling rates on the roll surface.
A fire crack pattern is common on hot rolling rolls with water cooling.
Fire cracks can grow deeper and develop into spalls.

Local Overloads

This is one of the most common causes of premature roll failure.

Roll Fatigue

Fatigue damage can start at the surface or below it.
High mill loads cause fatigue problems.
Corrosion fatigue is especially dangerous because there is no safe operating limit.

Spalling

Two types of spalls exist:

• Surface-initiated spalls (from local overload or rolling abnormalities).
• Sub-surface initiated spalls.

Five types of spalls:

1. Saddle spalls.
2. Pressure cracks and ribbon fatigue spalls.
3. Shell / core interface-bond related spalls.
4. Spalls due to insufficient shell depth.
5. Barrel edge spalls.

Hydrogen Damage in Steel Rolls

Hydrogen causes two problems:

• Special fatigue showing one or more round cracks perpendicular to the longitudinal direction.
• Cracks grow conically in both directions and take a long time to become visible.

Wear

Wear occurs mainly in areas of the highest friction between the roll and the rolled material.
Wear is not uniform along the barrel.
Wear decreases with a higher content of hard carbides.

Summary

Roll failures are costly and can be avoided.

Key takeaways:

• Choose the right roll material for your application.
• Control residual stresses carefully.
• Avoid thermal overloads and sharp temperature gradients.
• Use proper roll cooling within recommended parameters.
• Monitor fire cracks and spalling early.
• Match roll hardness to the required balance of wear resistance and toughness.

By following these guidelines, rolling mills can extend roll life, reduce unplanned stoppages, and improve overall performance.

Contact： cykc@cykcgroup.com.cn