1. Abrasive Wear
Foreign materials rubbing against a metal part cause abrasive wear. It accounts for 55-60 % of all wear on industrial metal components. Abrasive wear is really a group of wear problems. It can be broken down into three main categories:

• 1a. Low-stress scratching abrasion.
• 1b. High-stress grinding abrasion.
• 1c. Gouging abrasion.

1a. Low-Stress Scratching Abrasion
This is normally the least severe type of abrasion. Metal parts are worn away through the repeated scouring action of hard, sharp particles moving across a metal surface at varying velocities. The velocity, hardness, edge sharpness, angle of introduction and size of the abrasive particles all combine to affect the amount of abrasion. Alloys containing carbide (particularly chrome-carbide) are used successfully to resist low-stress abrasive wear. Due to the absence of impact the relatively brittle high carbon-chromium steel alloys are well suited for low-stress abrasive applications.

Typical components subject to low-stress scratching abrasion include agricultural implements, classifiers, screens, slurry pumps nozzles, sand slingers and chutes.

1b. High-Stress Grinding Abrasion
This is more intense than simple scratching. It happens when small, hard, abrasive particles are forced against a metal surface with enough force to crush the particle in a grinding mode. Most often the compressive force is supplied by two metal components with the abrasive sandwiched between them. Sometimes this is referred to as three-body abrasion. The surface becomes scored and surface cracking can occur. There are examples of softer, tough alloys outperforming harder alloys in grinding abrasion applications. The successful range of alloys includes austenitic manganese, martensitic irons and some carbide containing alloys (usually smaller carbides, like titanium carbide) in a tough matrix.

Typical components subject to high-stress grinding abrasion include augers, scraper blades, pulverizes, ball and rod mills, mullet tires, brake drums, roll crushers, rollers, sprockets and mixing paddles.

1c. Gouging Abrasion
The resulting wear can be extreme when high-stress or low-stress abrasions are accompanied by some degree of impact and weight. The metal surface receives prominent gouges and grooves when massive objects (often rock) are forced with pressure against them. A low velocity example of this is when a dragline bucket digs into the earth and a high velocity example would be rock crushing. In both instances the action of the material on metal is similar to that of a cutting tool. Gouging abrasion also places a premium on toughness. Sometimes this is at the expense of harder and more abrasion resistant alloys. Carbide containing alloys are used successfully, when supported by a tough alloy, preferably austenitic manganese.

Typical components subject to gouging abrasion include dragline buckets, power shovel buckets, clam shell buckets, gyratory rock crushers, roll crushers and jaw crushers.

2. Impact Wear
When the stress on a metal component exceeds the elastic limits of the metal it deforms including beneath the point and laterally across the surface away from the impact point.

Very brittle metal cannot withstand much deformation so it may crack from either a severe blow or repeated lighter blows.

Even if the metal is ductile enough to avoid cracking, repeated impact often compresses the surface, sometimes causing the metal to 'mushroom' at the edges and eventually chip off.

3. Adhesive Wear (Metal to Metal)
This accounts for as much as 15 % of all wear resulting from the non-lubricated friction of metal parts. Metal surfaces, regardless of their finish, are composed of microscopic high and low areas. As metal surfaces slide against each other, the high areas are broken and tiny fragments of metal are torn away. The continual removal of metal, roughens the working surface, and contributes to even more rapid wear.

The martensitic hard surfacing alloys are a good choice to resist metal-to-metal wear. Other alloys used successfully include austenitic manganese and cobalt based alloys.

4. High Temperature Wear
Steel surfaces exposed to high temperatures for long periods of time can steadily deteriorate. Heat affects the metal's microstructure and generally reduces its durability. The wear resistance of most alloys diminishes when exposed to high heat in service due to softening of inadvertent tempering.

A major cause of metal failure from high temperature service is thermal fatigue or fire cracking. This results from repetitive intense heating followed by quick cooling. The repeated expansion and contraction caused by this thermal cycling eventually exceeds the ability of the metal to recover and causes deep cracking.

A martensitic steel containing 5-12 % chromium is used extensively to combat thermal fatigue. Many chromium-carbide alloys retain their wear resistance up to temperatures of 1200 °F (630 °C). Service conditions over that temperature generally require a non-ferrous alloy. Typical components subject to high temperature wear include continuous caster rolls, steel mill work rolls, hot forging dies, tongs and sinter crushing equipment.

5. Corrosive Wear
Ferrous metals are subject to many forms of corrosion and each one can cause wear damage. The most common type of corrosion is rust. Rust transforms the surface of a metal into oxide that eventually flakes off reducing the original thickness of the metal.

Corrosion related to surfacing is usually a secondary wear factor. Although many hard surfacing alloys offer a certain amount of protection against corrosion the selection of a surfacing alloy should be handled as a separate issue for a specific corrosive service.

Surface Finish
Surface finish is very important when considering overall wear life in situations such as fan blades, spill faces, and chutes. Surface smoothness, uniform surface hardness and even carbide distribution are all critical to overall wear plate performance.

For instance: wear around bolt holes show up as comet trails; welded wear plate shows up as corrugations; and; casting imperfections show up as cavitation erosion. Generally a rough surface finish will accelerate wear anywhere from 200-600 % in comparison to a smooth surface of the same alloy structure and hardness.

Iron-Base and Non-Ferrous Hard Surfacing
Alloy Steel International manufactures products for hard surfacing in both the iron-base and non-ferrous categories. The iron-base alloys represent by far the largest usage of the hard surfacing alloys.

Iron-Base
Iron-base hard surfacing alloys can be subdivided according to their metallurgical phase or microstructure. Each type of alloy resists certain forms of wear better and/or more economically than others. For simplification Alloy Steel International groups the different classifications into three main hard surfacing alloy families:

• 1. Austenitic Alloys
• 2. Martensitic Alloys
• 3. Carbide Alloy

Included in each family are products that combine properties of the main alloy family with properties common to other alloy families. These products have been developed by Alloy Steel International either to resist two kinds of wear simultaneously or incorporate certain desirable characteristics.

These three main hard surfacing alloys will be discussed in detail:

1. Austenitic Alloys
Austenitic alloys with up to about 0.7 % carbon and 20-30 % alloy (usually about equal parts of manganese and chrome with some nickel) provide stable austenite; even on carbon in high dilution situations, and low alloy steels. This makes them a much better choice than the austenitic manganese alloys for overlay on carbon and low alloy steels.

Well-designed austenitic surfacing alloys are extremely tough, ductile and work-hardenable. They offer excellent impact resistance but very low abrasion resistance (with little improvement as work-hardens) and have no relief checks. These alloys will normally work-harden to a nominal surface hardness up to 50Rc and although this improves their abrasion resistance they still retain their good impact resistance. The austenitic surfacing deposits like the austenitic manganese base metals (see Base Materials) should not be exposed for extended periods to temperatures over 500°F (260 °C). This is to minimize embrittlement.

2. Martensitic Alloys
Martensite is a hard micro structural phase that is formed in steels by rapid cooling. Since martensitic alloys are air-hardenable the cooling rate plays an important part in the final hardness. Faster cooling usually results in harder surfacing deposits. Preheating of 250 °F (121 °C) to 600 °F (316 °C) is generally required when working with martensitic alloys. This is to avoid cracking in the alloy deposit. Low carbon with low alloy (less than 5 %) martensitic alloys is used primarily for build-up on carbon and low alloy steels. Their relatively high compressive strength, toughness, and good metal-to-metal sliding are wear resistant.

Slightly higher carbon and higher alloy (6-2 %) martensitic alloys exhibit significantly higher regarding deposited hardness'. This hardness gives them better metal-to-metal and abrasive wear resistance.

Martensitic hard surfacing alloys provide a good balance of impact and abrasion resistance. By choosing the proper carbon-chromium content it is possible to choose the best compromise of abrasion, adhesion and impact resistance. Martensitic alloys have the ability to respond to heat treatment thus making it possible to change their hardness/toughness.

Summary:

• Good impact resist.
• Fair abrasion resist.
• Good metal-to-metal wear resist.
• Used both for build-up and overlay.

3. Carbide Alloy
By alloying several percent of carbon with a minimum, 12 % alloy (primarily chromium), hard carbides are formed and dispersed throughout the surfacing deposit.

These dispersed carbides are much harder than the surrounding matrix and provide excellent abrasion resistance. They are used when the primary wear factor is abrasion. At the lower end of the carbon range (less than 3 %) the quantity of carbides is small compared to the matrix where they are dispersed.

These alloys exhibit good abrasive wear resistance while retaining good toughness. These carbide-surfacing alloys are used to resist a combination of abrasion and impact. As the carbon content increases (to as much as 7 %), in the carbide containing alloys, the abrasion resistance increases and the toughness decreases. This is due to the higher percentage of carbides.

As carbides are undermined and knocked out by moving abrasive particles, additional carbides are uncovered to further resist abrasives and delay wear as shown in the diagram to the right.

Summary:

• Excellent abrasion resist.
• Good heat resist.
• Fair corrosion resist.
• Fair to low impact resist.

Non-Ferrous Hard Surfacing Alloys
For hard surfacing other non-ferrous alloys are used to a much lesser extent than iron-base alloys. This happens generally where service temperatures exceed those that the carbide alloys will tolerate. These Cobalt-base and Nickel-base alloys offer wear resistant properties to combat most of the major types of wear. However, due to their higher cost, they are used primarily in specific applications where their unique properties are economically justified.

These two hard surfacing alloys will be discussed in more detail.

Cobalt-Base
Cobalt-base alloys are most often used in high temperature applications but also have a combination of overall resistance to low stress abrasive wear. They also have the necessary toughness to resist some degree of impact. Depending on the alloy, they are capable of resisting metal-to-metal wear, corrosion, and oxidation.

Nickel-Base
Nickel-base hard surfacing alloys were developed as a substitute for certain cobalt-base alloys; primarily to reduce alloy cost. Nickel provides better matrix strength at high temperatures than iron. It has similar applications to cobalt alloys. Nickel-base alloys are basically a lower cost substitute in high temperature wear applications for cobalt-base alloys.

Hard Surfacing Misconception
Greater hardness does not always mean greater abrasion resistance or longer wear life. Several alloys may have the same hardness rating but vary greatly in their ability to withstand abrasive wear.

For example, many of the best Arcoplate surfacing alloys derive their high abrasion resistance from very hard carbides dispersed throughout a softer, tougher matrix. Bulk hardness tests (Rockwell or Brinell) that measure the average hardness of both the carbide and matrix together, over a relatively large area, often register the same hardness as other conventional metals. However, in actual performance, a carbide-containing surfacing alloy has a substantially better abrasive wear resistance.

When equally comparing several surfacing alloys with each other high bulk hardness ratings are not the only factor assuring resistance to wear. Resistance (especially to low and high-stress abrasion) depends on a combination of both hardness and the metallurgical microstructure of the alloy. The microstructures of alloys vary according to the ratio of carbides to matrix and the type of carbides in the alloy. The alloy with the hardest and most evenly dispersed carbides, along with the highest percentage of carbides, will have the best resistance to low-stress and high-stress abrasion.