How surfaces wear

Every wearing part tells the story of how it was worn. A loader bucket edge is scoured smooth and thin by rock sliding across it; a crusher hammer is battered and peened at its striking face; a pump impeller is eaten away in smooth, flowing hollows; a shaft journal is scored where its bearing ran dry; a chute is pitted where wet ore sat against it. Each kind of wear needs a different defence, and the hardfacing that resists one may fail quickly against another. Choosing a surfacing deposit starts with reading the wear, and a wrong reading leads to a deposit that is expensive, carefully applied and useless.

This chapter describes the main kinds of wear, introduces the Archard relationship that links wear to load, sliding distance and hardness, explains why hardness alone does not decide wear resistance, and shows how to measure wear so that a hardfacing job can be judged by its results. It introduces the workshop whose wearing parts the rest of the book follows.

Abrasion

Abrasion is the removal of metal by hard particles sliding or rolling across a surface. It is the commonest kind of wear in mining, quarrying, earthmoving and agriculture. In low-stress abrasion, particles slide across the surface without being crushed, as sand slides across a chute or soil across a tillage point, and they scratch out fine grooves. In high-stress abrasion, particles are crushed between two surfaces, as in a crusher or a grinding mill, and the fragments gouge the surface. In gouging abrasion, large, hard lumps tear out chunks of metal under heavy load, as rock does on a bucket’s teeth. Abraded surfaces show scratches or grooves running in the direction the material moved, and their depth hints at which kind of abrasion it was.

Impact

Impact is the repeated striking of a surface by heavy objects, such as rock hitting a crusher hammer or an excavator tooth striking hard ground. Impact deforms the surface, cracks brittle materials and can spall off pieces of a hard deposit. Parts subject to heavy impact need toughness as well as hardness, and the hardest, most brittle deposits fail under impact, often by cracking and losing large flakes. Many real parts suffer impact and abrasion together, and the balance between them decides the best deposit.

Erosion

Erosion is wear by particles carried in a moving fluid, such as sand in water through a pump or dust in air through a fan. Its effect depends strongly on the angle at which the particles strike: at low angles, they cut like abrasives, favouring hard surfaces; at high angles, near perpendicular, they hammer the surface, favouring tougher materials. Eroded surfaces have smooth, flowing shapes, often with wavy patterns that follow the flow.

Adhesion and galling

Adhesive wear happens when two metal surfaces slide against each other and bond together at their high spots, tearing metal from one surface to the other. It is the wear of dry or poorly lubricated bearings, of metal-to-metal sliding parts and of tools forming metal. Galling is its severe form, with torn and smeared surfaces. Hardfacing for adhesion must suit the mating surface as well as the load, because two surfaces of similar material gall more readily than dissimilar ones.

Corrosion and its partners

Corrosion removes metal chemically, and in many plants it combines with mechanical wear: wet ore abrades a chute and the abrasion removes the protective corrosion film, exposing fresh metal to corrode, so that the combined loss is far more than either would cause alone. Deposits for such service need corrosion resistance as well as hardness, which often points to chromium-rich or nickel-based alloys.

Figure 1Kinds of wear and what resists them

	Typical parts	Signs on the part	What resists it
Low-stress abrasion	chutes, tillage points, auger flights	fine scratches along the flow	hard carbides in a supporting matrix
High-stress and gouging abrasion	crushers, mills, bucket teeth	grooves, gouges, chunks torn out	hard carbides with enough toughness
Impact	crusher hammers, excavator teeth	battered, peened, flattened faces	tough, work-hardening deposits
Erosion	pump impellers, fan blades, pipe bends	smooth, flowing hollows	hard surfaces at low angles; tough at high angles
Adhesion and galling	dry bearings, sliding metal parts	torn, smeared metal	dissimilar, hard surfaces; lubrication
Corrosion with wear	wet chutes, slurry parts	pitting with scoring	chromium-rich or nickel-based deposits

The Archard relationship

A simple relationship, associated with J. F. Archard, describes how much material sliding wear removes: the volume worn away is proportional to the load pressing the surfaces together and the distance they slide, and inversely proportional to the hardness of the softer surface.

Figure 2The Archard wear relationship

V = K × F × s ÷ H

V

volume of material worn away

F

load pressing the surfaces together

s

sliding distance

H

hardness of the worn surface

1. doubling the load or the distance doubles the wear
2. doubling the hardness, other things equal, halves the wear
3. K changes by orders of magnitude between mechanisms, so it must come from experience

The relationship is approximate and works best for comparing similar situations. Its message for hardfacing is that, for the same wear mechanism, a harder surface wears more slowly in proportion to its hardness. A tillage point surfaced with a deposit three times as hard as its plain steel might be expected, on this basis, to last roughly three times as long in the same soil, though real results also depend on the deposit’s structure, toughness and how it is placed.

Why hardness is not the whole story

Hardness numbers measured with an indenter describe a deposit’s average resistance to indentation, but abrasion resistance depends on the size, hardness and distribution of the hard particles in the deposit as well. A deposit of hard chromium carbides in a softer iron matrix may measure moderately hard overall, yet resist abrasion far better than a uniformly hard martensitic steel, because the carbides themselves are much harder than the abrasive and stand proud as the softer matrix wears around them. Conversely, a very hard deposit that is brittle may crack and spall under impact long before it wears away. Chapters 4 to 6 explain how the families of deposits balance these properties.

The abrasive matters as much as the surface

The hardness of the abrasive relative to the surface changes wear dramatically. When the abrasive is much harder than the surface, as quartz is harder than ordinary steel, it cuts freely and wear is fast. When the surface is harder than the abrasive, the particles tend to blunt or roll rather than cut, and wear falls steeply. That is why hard carbides, harder than quartz, resist sand and rock so well, while a hardened steel that is still softer than quartz improves only in proportion to its hardness. Knowing the abrasive, quartz sand, limestone, granite, clay, grain, matters as much as knowing the part: the same deposit can last many times longer in limestone than in granite, which is mostly quartz and feldspar.

Particle size, speed and angle

Wear also depends on the size, speed and angle of the particles. Larger particles carry more energy and gouge rather than scratch; faster particles, in erosion, remove material roughly in proportion to a power of their speed considerably greater than one, so a small increase in a pump’s speed or a fan’s airflow can raise wear sharply; and the angle at which particles strike decides whether hard or tough surfaces resist best. The workshop asks its customers about these conditions as well as about the material: what size of rock, how fast the conveyor runs, how steeply the chute is inclined, what speed the pump turns at. Changing the conditions, a gentler chute angle or a slower pump, can sometimes do more than any deposit.

Temperature

Some wearing parts work hot, such as sinter plant parts, kiln components and hot-forging tools, and wear resistance at temperature is a different property from wear resistance at room temperature. Many hard deposits soften or oxidise when hot, while cobalt- and nickel-based alloys and some tool steel compositions keep their hardness. The workshop’s parts all work at ordinary temperatures, but a part that runs hot needs a deposit chosen for its hot hardness, and its maker’s data are the starting point.

Wear and the whole machine

A wearing part is part of a machine, and its wear affects the rest. A worn crusher hammer changes the product size and unbalances the rotor; a worn bucket edge increases the force needed to load the bucket, and with it fuel use and wear on the pins and bushes; a worn pump impeller lowers the pump’s output and efficiency. The cost of wear therefore includes these effects, and Chapter 10 counts them when judging whether a deposit was worth applying.

Measuring wear

Choosing deposits on evidence needs measurements of wear. The simplest are dimensional: the thickness of a bucket edge or a hammer’s striking face measured at fixed points with callipers or an ultrasonic thickness gauge, or the profile of a worn part traced against a template. Weighing small parts before and after a period of service gives the mass lost. Recording the hours, tonnes or hectares of service between measurements turns the loss into a wear rate: millimetres per thousand tonnes, grams per hour, or millimetres per hundred hectares. Wear rates let deposits be compared fairly, and let the replacement of parts be planned before they fail.

Looking at a worn part

The workshop’s first step with any worn part is to look at it closely, with a magnifying glass and good light, before cleaning it. The direction of scratches shows how material moved; the shape of worn areas shows where the load was concentrated; cracks, chips and spalled areas show impact; pits and staining show corrosion; smeared and torn metal shows adhesion. Where the part has been hardfaced before, the condition of the old deposit is especially telling: a deposit worn smooth and even has done its job and needs only renewing, while one cracked, flaked or undercut by wear of the base metal beside it was the wrong choice or wrongly placed. The workshop photographs each worn part on arrival and keeps the photographs with the job record.

The running case

This is an illustration. The workshop’s main wearing parts are:

• crusher hammers from the quarry’s impact crusher, manganese steel castings battered by rock at high speed, suffering impact and gouging abrasion;
• loader bucket edges, plain high-strength steel, scoured by rock and gravel, suffering low- and high-stress abrasion;
• tillage points and auger flights from the farms, suffering low-stress abrasion in soil and grain;
• chute liners and pump impellers from the grain terminal, suffering low-stress abrasion and erosion, some wet.

The quarry’s manager has asked the workshop to cut the cost of its crusher hammers and bucket edges per tonne of rock handled, and the farms want their tillage points to last a season without replacement. The rest of the book follows how the workshop meets those requests.

Wear is usually the cheapest failure

Of all the ways a part can fail, wear is usually the most predictable and the cheapest to manage, because it is gradual. A part that wears at a steady rate can be measured, its remaining life estimated, and its rebuilding or replacement scheduled for a convenient shutdown rather than forced by a breakdown. Hardfacing changes the rate, and the measurements tell whether it was worth doing. The danger is wear that goes unmeasured until a part breaks: a bucket edge worn through, a hammer that has lost so much mass that it unbalances the crusher rotor.

A failure: the wrong hard surface

This is an illustration. A farm had a set of auger flights hardfaced elsewhere with a very hard, brittle deposit, applied thickly along their edges. The flights handled grain without trouble for a few weeks, then a load contaminated with stones went through, and large flakes of the deposit broke off the edges, jamming the auger and wearing the exposed steel faster than if it had never been hardfaced.

The deposit had been chosen for abrasion by fine grain and had no toughness for the impact of stones. The workshop rebuilt the flights with a tougher deposit of slightly lower hardness, applied in thinner layers, and the flights lasted the season, stones and all. The lesson was to read all the wear a part would see, not only its usual load, and to ask the customer what else goes through the machine on a bad day.

Exercise: comparing two deposits

A tillage point of plain steel with a hardness of about 200 on a common hardness scale wears about 12 mm over 100 hectares. Using the Archard relationship as a rough guide, estimate the wear over the same area for points surfaced with deposits of 400 and 600 on the same scale. What else would you check?

On the Archard proportion, wear falls in proportion to hardness for the same mechanism. A deposit of 400 might wear about 12 × 200 ÷ 400 = 6 mm, and one of 600 about 12 × 200 ÷ 600 = 4 mm, over the same 100 hectares.

These are only rough guides. The workshop would also check the deposits’ microstructure, since a carbide-rich deposit may outperform its measured hardness in abrasion; their toughness, if the soil is stony; and their thickness, since a deposit that wears through exposes the plain steel. The surest check is a field trial of a few points of each, measured at fixed intervals.