Hardfaced or overlaid plate became commercially viable in 1965 when Roman F Arnoldy, founder of the Red Dog™ Corporation, invented and patented the bulk welding process. This invention enabled the economic deposition of a wear resistant chromium carbide alloy containing 4.5% - 5% carbon onto a ductile carbon steel base. Considerable refinement and development has taken place since then and today the range of overlaid plate available includes chromium and tungsten carbide grades with outstanding resistance to abrasion, erosion and impact at both ambient and elevated temperatures. In the majority of severe abrasion environments the chromium carbide alloys are the most economic solution.
The most commonly used alloy facing is a high chromium iron containing approximately one-third chromium and in excess of 4 percent combined carbon.
This corresponds to Red Dog T200X with a chemistry of:
C 5.4%, Mn 3.5%, Cr 34.0%, Others 1.3%, Balance Fe.
This standard alloy may be modified in a number of ways, either to increase abrasion resistance whilst reducing toughness, or vice versa. Conversely, the matrix may be hardened by reducing the manganese to 1 percent, with some loss in toughness. Further refinement can be achieved by the introduction of other alloying elements. (see Red Dog Plate Range Summary for further details).
The material that gives high chromium iron alloys their ability to resist abrasion is the formation of primary carbides from a chemical compound of chromium, iron, and carbon, or chromium iron carbide, also called simply chrome carbide. Pure chromium carbide can be produced, but it is prohibitively expensive for large-area protection so Red Dog uses a mixed carbide of both chromium and iron, which exists as a primary carbide with the formula M7C3, where M indicates the mixture of iron and chromium in the compound.
A typical (T200X) overlay alloy comprises a composite of chromium iron carbides in a matrix of a chromium iron carbon alloy. The hardness of primary chromium iron carbides is the equivalent of 1700HV compared with, for example, a typical workshop steel file, which has a hardness of 600HV. Generally the hardness of these alloys is measured using a Rockwell hardness tester, which although neither measure the carbide or the matrix, provides an acceptable general indication of the alloys hardness. A typical value being 54-60 HRc.
In addition to chemistry, the most important characteristic of the alloy overlay is it's microstructure. When viewed under a microscope the carbides will appear as white material against a dark background, the matrix. An ideal microstructure, for maximum abrasion resistance, will contain a dense arrangement of needle like carbides, which in cross section appear as slender hexagons with a small hole in the centre.
The presence of irregularly shaped spots or avenues of white, as "fish bone patterns," or as "ladders" having central poles with rungs on either side, is an indication that the carbon content is below optimum for maximum abrasion resistance but has increased impact resistant properties. See Red Dog Plate Range Summary for details of individual alloy formulations.
Micrographs typical of facings with different chemistry are illustrated below.
|Manganese||2.03%||Molybdenum||less than 0.5%|
Primary carbides can be identified by their crystalline structure. When cut by a perpendicular plane, they appear hexagonal. Regardless of the angle of the cut, the primary carbides usually appear with a black cavity near the centre, have sharp corners, are are crisply defined whit structures against the dark matrix background.
This micrograph shows an alloy of less than optimum composition in which there are no visible primary carbide structures. Much of the chromium content is visible as white specks in the matrix. Lesser carbides appear as round or fish bone structures. However, the type of alloy is suitable for high impact applications.
|Manganese||2.55%||Molybdenum||less than 0.05%|
|Silicon||0.51%||Boron||less than 0.5%|
Increasing the carbon and chromium content results in the formation of primary carbide structures, which appear as lighter hexagonal forms in a darker matrix.
Note the easily visible cavity inside each crystalline-shaped primary carbide.
|Manganese||3.20%||Molybdenum||less than 0.05%|
|Silicon||0.43%||Boron||less than 0.5%|
This sample, the first that falls within the desirable 4-5 percent carbon range, shows better primary carbide development. Structures are easily distinguished from the dark matrix, with an excellent view of the appearance of primary carbides sliced crosswise (smaller hexagonal shapes), lengthwise (the longer needle-like shape, centre), and transversely (upper right, with the hollow centre shown as an elongated cavity). Density of the carbides within the matrix material is at the low end of the acceptable 35-35 percent range.
|Manganese||3.34%||Molybdenum||less than 0.05%|
|Silicon||0.47%||Boron||less than 0.5%|
This micrograph shows the primary carbides as crisply defined and easily visible within the dark matrix material. Note the elongated appearance of the centre cavity of each primary carbide crystalline structure when it is sliced at an angle. Density is well within the desirable 35-to-45 percent range, with even distribution, which provides excellent wear characteristics.
|Manganese||3.09%||Molybdenum||less than 0.5%|
|Silicon||0.46%||Boron||less than 0.5%|
In this final sample the primary carbide structures are at their maximum density, within the 35-45 percent range, creating an alloy with maximum wear resistance without excessive brittleness. There is still ample matrix material to hold the primary carbides in place. The patterns of crystalline growth at right angles, are easily seen, along with the distinctive hexagonal crosscut shapes and the needle-like lengthwise shapes, each with a central cavity.
See Red Dog Plate Range Summary, which provides details of the properties of each alloy formulation and it's suitability for particular service conditions.
The appearance of an overlaid plate does not necessarily reflect it's resistance to wear and it is often the less cosmetically attractive materials which have the better mechanical properties.
When the carbon content of the overlay goes beyond the 4 percent range, the facing becomes increasingly viscous and tends to be somewhat uneven. It may even contain some small holes that extend down to the base plate. For most uses this lack of smoothness is unimportant.
If a smooth surface finish is important, for example when material flow is critical, smoother surfaces can be produced using the submerged arc welding process to deposit the overlay.
Another characteristic of good quality plate is the presence of stress relief cracks which, contrary to first impressions, are actually beneficial to this material. The presence of cracks in the hardfacing, at the correct frequency and spacing, allows the plate to be rolled, formed and bent without damage. This characteristic is one of the basic features of Red Dog's hardfaced plate patents.
Red Dog T200X general purpose chromium carbide overlay plate typically can deliver up to 20 times the life of carbon steel. Combined with it's versatility, this is the reason why T200X plate has become a great cost-reducer in the manufacture of aluminum, asphalt, cement, glass, petrochemicals, power, pulp and paper, steel and synfuels, as well as in dredging, mining, oil refining, food and refuse processing, and numerous material-handling applications, such as scrape and steel recycling.
The steel manufacturing industry, for example, requires wear protection for hoppers, conveyor liners, and grizzly bars that quickly erode without hardfacing protection.
In mining, without hardfacing protection, critical components such as chute liners, front-end loader liners, and wear pads prematurely lose their usefulness due to the impact of silicious matter.
Refuse processing plants use overlay products in their hydra- pulper liners and beaters, screw conveyors and troughs, conical section wear liners, baler liners, tub liners, transfer points, flop gates, fan blades and liners, and ash removal systems.
Hardness Conversion Tables For Steels
Reprinted with permission 1971 Society for Automotive Engineers Inc.
(values in parentheses beyond normal range - for information only)