Abrasion occurs when materials are rubbed across one another over a period of time. Some abrasion is intentional, such as sanding, grinding, and blasting. However, unintentional abrasion can lead to component failure so it is important to use the proper materials to ensure that surface wear does not lead to unanticipated breakdown of structures or parts. While steel in general has excellent resistance to abrasion, not all steels are equal. There are several types of steels that are made particularly to prevent wear, known as abrasion resistant steels.
How Is It Made?
Abrasion resistant steel is made out of iron ore, carbon, and other alloying elements, similar to other types of steel. The iron ore is melted in a blast furnace which removes undesirable substances that may be in the ore. Carbon and other alloying elements are added during this time. Abrasion resistant steels in particular have extra amounts of carbon and alloying elements such as chromium and manganese. The increased amounts help to make the steel less susceptible to wear. Substances that prevent oxidation are also added to the molten pool.
Following all of this, the molten abrasion resistant steel is shaped, heat treated and cut.
How Does It Work?
The chemical composition of abrasion resistant steel is one of the attributes that make it more immune to wear than other types of steel. There are several alloys that can be used increase the abrasion resistance. Carbon helps block dislocations, which increases the hardness and strength of a steel. The added carbon also allows the steel to form microstructures with increased hardness when heated and quenched. There are other elements that can be added to abrasion resistant steel to increase its hardness value too. Chromium and manganese are also added to abrasion resistant steels to help reduce the negative effects caused by wear.
Heat treatment is another factor that helps the steel resist abrasion. Abrasion resistant steel must have a microstructure that allows it to have a high hardness. This is accomplished, in part, by adding the proper alloying elements. However, this alone is not enough to ensure the proper microstructure is formed. The steel must also undergo a heating and a rapid quenching process to form microstructures such as martensite and bainite which gives the steel the required high hardness values. Care must be taken when welding or heating abrasion resistant steels. If they are heated to a high enough temperature, it may have an annealing effect on the steel, causing it to lose some of its hardness and, therefore, its abrasion resistance.
What Types Are Available?
There are several different abrasion resistant steel grades. Each grade is typically made to a specific Brinell hardness value, as opposed to other steels that are made with tensile strength and toughness in mind. This is because hardness is one of the most important factors when trying to increase abrasion resistance.
One common grade of abrasion resistant steel is AR400. The “400” in its name indicates the Brinell hardness value. AR450 and AR500 are similar to AR400, the difference being a higher Brinell hardness value. While the higher Although it has a lesser Brinell hardness value, AR400 will be more formable. All of these three abrasion resistant steels are used in applications such as mining equipment, cement pouring and forming equipment, excavation equipment, and conveyor systems.
There are also proprietary types of abrasion resistant steel. Tradenames such as Hardox 400 and MAS500 AR are similar to the grades already mentioned. There are also grades such as Hardox 450 CR. The “CR” in the name means that it has been cold rolled. These proprietary grades are used for similar applications previously mentioned.
Abrasion Resistant Steel Uses
Abrasion resistant steel is best suited for applications with high wear, and as such are commonly found where there is constant motion or a flow of material. Industries that benefit most from the use of abrasion resistant steel are those which require large scale materials handling such as the mining and construction industries.
- Wear parts on mining/digging equipment including buckets for diggers & excavators, liners for dump trucks and dozer attachments.
- Conveyor belts used to transport material.
- Liners for bins, chutes and hoppers that see material flow, such as in the mining or cement industry.
Abrasion resistant steels will typically be used as a liner to protect more permanent structures from wearing away. These liners are generally designed to be replaced at regular intervals.
While a high carbon content makes steel products both harder and tougher, they are also more brittle and are not suitable for all applications. Abrasion resistant steels should not be used as structural steel, for example, as the brittle nature may lead to premature failure.
How the Quenched and Tempered Process Creates AR Plate
AR material is created by quenching and tempering forged steel blocks, or ingots. At the time of this process, the grain structure is modified to increase toughness and promote formability (or, to be less brittle), which leads to through-hardening of the material.
Quenching and tempering (Q&T) is a two-part process:
Quenching occurs when steel is brought to a high temperature, normally between 1,500 and 1,650 °F and is quickly cooled with water. This process allows crystal structures to form within the steel, increasing hardness.
Tempering is the process of re-heating quenched steel to a below-critical temperature (approximately 300 — 700 °F), and then enabling the plate to cool in normal air temperatures.
The reheating of material disintegrates the crystal structures formed at the time of the quenching process, while the long cooling enables the crystal structures to reform — preserving most of the hardness and strength, but increasing the overall ductility.
What’s with the F?
Usually, abrasion-resistant steel is termed “AR450F” and “AR400F” (and at times “AR500F”). At present, the AR material with or without the “F” is interchangeable, but — traditionally — material with an “F” simply implied that it was “formable” and can possibly be bent to a certain degree without cracking.
When mills created both non-formable and formable steel plates, formable was somewhat costlier. Yet, more competitive pricing and reduced demand have resulted in the production of formable-only AR steel.
AR400 versus AR450 versus AR500+
Before this article dives into the difference between these common types of AR steel, it is significant to explain that AR steels are not controlled by a particular chemistry, but by a level of hardness.
A variety of mills may have different “recipes” for AR steel, but the produced material is subjected to a hardness test — called the Brinell Test — for determining the class in which it falls.
The Brinell Hardness Number (BHN) is the technical difference between AR400, AR450, and AR500. BHN signifies the hardness level of the material:
- AR400: 360-440 BHN Typically
- AR450: 430-480 BHN Typically
- AR500: 460-544 BHN Typically
- AR600: 570-625 BHN Typically (less common, but available)
Therefore, what does this mean in terms of usability? How do you know which level of hardness the project requires?
Projects that need AR materials are usually those that require a precise balance between brittleness and hardness. As hardness is increased, brittleness is also increased, making the material hard to form, weld, and shape. For some projects, hardness is crucial, and therefore brittleness is compromised; however, in other cases, the material should be formed and handled, so the level of hardness should be decreased.
As a general rule, AR450 and AR400 are the “sweet spots” in terms of good formability combined with good hardness. AR500 and AR600 will last longer and need to be replaced less often for projects that actually beat up the material.
Eventually, the customers and their team will be the best judge of which material is required for their project. Obviously, cost becomes an important factor, as sometimes it is worth having a formable material that has to be changed more frequently.