CLADDED WEAR PLATES FIND MINING APPLICATIONS
This article appeared in the 1985 edition of "MINETEC 89"
Earth moving and mining has yielded an abundance of man's most basic necessities for thousands of years. The movement of the earth's crust requires tough tooling and ground engagement implements built to survive some of the most hostile environments. It has been a constant struggle to improve methods and materials to effectively reduce costly downtime and component replacements. Wear plates or abrasion resisting plates as they are often referred to, continue to fill a wide variety of applications and have enjoyed a great deal of success in applications involving abrasion and impact resistance. Ceramics are on the far end of the toughness spectrum. They provide excellent abrasion resistance, but are limited in applications involving moderate to heavy impact, and are not weldable. However, cladded wear plates provide the abrasion resistance that approaches ceramics, the weldability of mild steel, and moderate impact properties. These unique properties offer the user opportunities to significantly extend component life and reduce downtime.
The history of wear plate is rooted in the steels that were readily available in the field, namely construction steels. More recently, modern steel making processing and control has significantly improved steel wear properties and has provided the user with products aimed at this particular problem. The introduction of cladded wear plate has added yet another dimension to wear plates. A brief outline of wear plate history will help clarify where this unique product fits into the scenario.
Construction steels provided the first source of wear plate, but only afforded marginal wear resistance. These steels were usually low in carbon (the principal hardener in steel) with little alloying used. The absence of alloy produced steels with lower hardness beneath the surface. In the process of quenching from high temperatures only the very skin surface of the material resulted in hardnesses that could resist abrasion. The inner core of the material was much softer and unable to withstand the abrasive environment when the surface was lost. Abrasion resistance came only as a result of the hard skin on the surface of the plate. Once the surface of the plate was worn away the soft core quickly disappeared. Frequent replacement was necessary. The lack of proper alloying not only prevented thru-hardening, it also resulted in steels that were low in ductility in the hardened condition. This prevented any significant forming or shaping for applications. It became evident that these steels would be limited for use in thin, simple shapes and sections.
To overcome the inadequacies associated with construction steels, metallurgists judiciously added alloying elements to the steel that enhanced uniform hardness in thicknesses up to 3in (76mm) thick. Ductility and impact properties were improved through the use of special processing techniques to control non-metallic inclusion size, shape and volume. Additions of titanium were made to the melt to induce the formation of abrasion resistant carbonitrides, yielding steels of superior wear resistance. The development of these steels has evolved into what is generically called AR plate. Today they occupy a major portion of the wear plate market and continue to enjoy progressive growth.
Thru-hardened steels enjoyed a variety of applications involving moderate impact and abrasion. But when it came to severe abrasion with low impact, a new product was needed. Increasingly the alloy content of AR steels resulted in poor ductility, weldability, and formability. It was true that certain high chromium white iron castings provided greater wear resistance than AR steels, but they too were nonweldable and limited to predetermined cast shapes. It became evident that a product was needed that combined the wear resistance of a high chromium white iron casting, was weld- able and capable of being formed into commonly used mining components. Since hardfacing electrodes and wires could be produced with high chromium white iron chemistries, cladding the surface of a mild steel plate with these consumables was the answer. Thus was born the cladded wear plate. The mild steel backing plate could be welded to most existing structures, and the highly abrasive resistant hardfacing de- posit provided the wear protection for the most severe abrasion conditions. Formability resulted from the occurrence of check relief cracks in the weld deposit. The check relief cracks occurred as a result of cooling from the elevated welding temperatures and appeared about every 2"(51 mm) perpendicular to the weld bead direction. Studies have shown that these cracks did not penetrate the low carbon steel baseplate and did not significantly affect the overall integrity or the wear properties of the plate. As a result, forming could be successfully done on sections with the cladding to the inside radius.
Cladded wear plates opened the door for a number of applications involving severe abrasion and replaced many of the field welding and hardfacing operations. This meant that maintenance crews spent less time weld repairing and had more time for other maintenance programs.
Microstructure and hardness are the most important characteristics of a steel to combat wear by abrasion. Hardness is a direct result of the steels microstructure and is the easiest property to measure. Unfortunately it is often erroneously used as the sole criteria for measuring the relative wear resistance of a steel. The hardness of two steels may be exactly the same but their abrasion resistance may vastly differ. This is due to the differences in microstructure and their effect on hardness. Hardness may be achieved in two different ways:
1. By the formation of a hard metallurgical structure known as martensite. This is usually accomplished by alloying with controlled amounts of carbon, manganese, chromium and molybdenum, and quenching from high temperatures. High strength-low alloy steels are examples of this type. Hardnesses in the range of 600 BHN are obtainable.
2. By the formation of discrete hard
particles known as carbides. Alloying with high amounts of chromium and carbon
promotes the formation of chromium carbide, an extremely hard particle (1700 BHN). These
hard chromium carbides are supported by a softer, but much tougher matrix. It is the
combination of these two structures that yield the overall hardness of the deposit of
about 600 BHN. Cladded wear plates are examples of this group.
As indicated above it is possible to have two products of the same relative hardness (600 BHN) and be vastly different in wear resistance. The difference in wear resistance is attributed to the difference in the microstructures.
The formation of carbides is not the only criteria for good abrasive resistant cladded plate. Carbide volume and orientation is very critical. Chromium carbides are hexagonal, rod-like formations. Wear resistance is optimized when the long cylinder-like chromium carbides are perpendicular to the surface. They are anchored within the tough matrix and do not become dislodged or fractured easily as wearing progresses. Poor wear resistance occurs when the carbide formations lie horizontal to the surface. They are prone to come out or fracture as the softer matrix wears away. Chromium carbide volume and orientation can be controlled to some extent by carefully controlling the cooling process, employing the proper welding wire formulation, and controlling the welding parameters.
Residual stresses that are locked within the plate after welding are another critical factor in the wear resistance of cladded plate. If a cladded plate has a high amount of residual stress, distortion will develop in sections cut.from it. Straightening of the plate will be required. Severe cracking and spalling of the cladding from the base plate is possible as a result of high residual stresses and straightening. Applications involving high impact can be particularly critical to the amount of residual stress within the cladded plate. A cladded plate with the least amount of residual stress is the most desirable.
The method of cladding low carbon steel base plates directly affects the residual stress within the final cladded product. Base plates are usually cladded either on a horizontal table or on a pre-formed and welded cylinder. The former method results in very little distortion, lower residual stress, and requires little or no straightening after cladding. Subsequent forming of components is much easier and less prone to failure in plates produced in the flat condition. The latter method requires the cladded plate to be cut and flattened, thus producing a high amount of residual stress and associated problems.
A particularly attractive feature of cladded plate is its versatility. Plates can be field cut and positioned easily onto buckets, trucks and chutes and welded into place with all-position electrodes. Large areas can be covered quickly without excessive down-time. Fillet and plug welds are the most popular methods of attachment. Stud welding is becoming more popular for flat sections and for applications where dislodging by underplate build-up of material is unlikely.
Thin cladded plates are fillet welded or plug welded to support structures with Type 312 Stainless Steel electrodes. In the course of fillet or plug welding the cladding deposit is inadvertently melted and mixed with the weld bead. If mild or low alloy electrodes are used, a brittle deposit develops and cracking may result. A Type 312 electrode is recommended because of its high ferrite content and its ability to withstand dilutions of high carbon. It is an excellent choice for dissimilar metal joining and will provide high strength, and ductile weld joints. Thicker cladded plates can often be attached to support structures with mild or low alloy steel electrodes, such as E8018-C3, where the danger from brittle welds is less likely. Thicker base plate provides opportunities for reducing the dilution of the cladding into the weld deposit. Quite often the fillet or plug weld will require wear protection. In this case a high chromium iron hardfacing rod can be used to cap the fillet or plug welds. Low heat inputs are essential to reduce the tendency of heat affected zones cracking and spalling. Capping should be limited to two layers, as thicker layers often lead to spalling.
Cladded plate has a high chromium content which makes it difficult to flame cut. Cutting is usually carried out with a plasma arc torch. Experience has shown that it is a good practice to cut from the baseplate side in order to avoid any of the high carbon cladding material from washing over the base plate. The high carbon wash often contributes to weld cracking if not removed. Field cutting can be accomplished by arc gouging the mild steel base plate and then fracturing the cladding along predetermined lines. The edges may be rough, but in applications where edge welding and capping will be performed, this may be acceptable. Gouging can also be performed on the cladding side, allowing flame cutting methods to be performed on the low carbon steel base plate. This method leaves a U-shaped edge which is well suited for attachment to an item to be wear protected .
Forming material with numerous cracks appears to be quite difficult at first glance, but with proper procedures and guidelines forming of cladded plate can be done easily and effectively. Cladded plate forms as easily as low carbon steel of the same thickness. A forming radius equal to approximately 10 times the nominal thickness can be used as a guide line. It is recommended that all forming should be done with the cladding on the inside radius. Forming of cladded plates with the cladding on the outside radius is risky. A very generous radius is required to successfully accomplish this.
Cladded plates of 96" (2438mm) x 120" (3048mm) are typically manufactured in various nominal thicknesses ranging from 3/8" (9.5mm) to 1" (25.4mm). The actual thickness of cladding is either 1/4" (6.4mm) for one-layer or 3/8" (9.5mm) for two-layer deposits. Cladding thickness greater than 3/8in are generally too brittle for practical use. Competent fabrication shops can plasma cut and form sections to customer specifications.
Cladded plate finds applications wherever high abrasion under moderate impact loads are a problem, such as in truck bed liners, conveyor chute liners, target plates, screw auger flights, draggling bucket floor/side plates and dozer blade components. This product's wear resistant properties offer the user a chance to cut down the amount of material being used by the component. For example: a 2" (50.8mm) low alloy AR plate liner used in a bucket floor was replaced by 3/4" (1 9mm) cladded plate. The user not only increased the liner life by using cladded plate, he substantially reduced the weight the bucket had to carry by 62-1/2 %. Cladded plate does well where abrasion and moderate impact are involved. Users should be cautioned about applications involving heavy impact and where impact is at the plate edge. Direct blows are not as harmful as glancing angle blows.
The mining industry has enjoyed a progressive movement in the development of wear plates. Construction steels provided the first steps in reducing costly downtime due to wear. Technological advances in the steel making process now make it possible to select from high quality AR Plates, which represent a significant increase in component life. Cladded plates now offer the user a unique combination of wear protection and versatility. They are weldable. and formable, and can easily be applied over large areas, thus reducing precious downtime in production. Applications continue to grow as the mining industry continues to strive to reduce the costs associated with wear. Cladded plate has enjoyed a spiraling acceptance throughout the world and will continue to flourish, limited only by the lack of maintenance ingenuity and creativity.
Robert F. Miller, Clad Technologies Inc. 1-800-978-9780