Wear

The deterioration of surfaces is a huge problem in many industries. Wear is the result of erosion, abrasion, impact, metal-to-metal contact, oxidation, and corrosion, or a combination of these. The effects of wear, can be repaired and is generally very expensive.
Prevention and wear protection is the most economical way to offset the high costs associated with component repair or replacement. Keeping your equipment up and running is paramount in achieving profit and customer satisfaction.
Hard-face coatings are applied to problematic wear surfaces for the purpose of reducing wear and/or the loss of material by abrasion, impact, erosion, corrosion, oxidation, cavitations, etc.

In order to properly select a coating alloy for a specific requirement it is necessary to understand the type of wear that has occurred to cause surface deterioration. The various types of wear can be categorized and defined as follows:

Impact wear is the striking or slamming contact of one object against another. a battering, pounding type of wear that breaks, splits, and deforms metal surfaces.

Abrasion is the wearing of surfaces by rubbing, grinding, or other types of friction. It usually occurs due to metal-to metal contact. It is a scraping, grinding wear that rubs away metal surfaces and can be caused by the scouring action of sand, gravel, slag, earth, and other gritty material.

Erosion is the wearing away or destruction of metals and other materials by the abrasive action of water, steam, slurries which carry abrasive materials. Pump parts are subject to this type of wear.

Compression is a deformation type of wear caused by heavy static loads or by slowly increasing pressure on metal surfaces. Compression wear causes metal to move and lose dimensional accuracy.

Cavitation wear results from turbulent flow of liquids, that carry small suspended abrasive particles.

Metal-to-metal wear is a seizing and/or galling type of wear that rips and tears out portions of metal surfaces. It is often caused by metal parts seizing together because of lack of lubrication. It usually occurs when the metals moving together are of the same hardness. Frictional heat promotes this type of wear.

Corrosion wear is the gradual deterioration of unprotected metal surfaces, caused by the effects of the atmosphere, acids, gases, alkalies, etc. This type of wear creates pits and perforations and may eventually dissolve metal parts.

Oxidation is a type of wear causing flaking or crumbling layers of metal surfaces when unprotected metal is exposed to a combination of heat, air and moisture. Rust is an example of oxidation.

The above types of wear can occur in combination with one another and because of this, we must consider all of the factors to determine the type of coating material to apply. This is done by studying the worn part, it’s service requirement, it’s interaction with other parts and equipment and it’s environment.

Presently, there is not a governmental standardized method to classify or specifying different surfacing coatings. Experienced material engineers that have worked with coating suppliers can provide data to classify their industries products. Many suppliers also provide complete information for using their specific products for various applications and for different industries such as aerospace, quarrying, steel mills, foundries, etc.

Precision Coat of Florida’s Engineering Staff continually works toward the development of wear resistant material classifications. To date in our system, there are five major groups classified with subgroups to address alloying issues.

The following is a brief description of the five major groups, what they contain as alloys, and where they are recommended.

Group 1 is low-alloy steels that contain chromium as the principal alloying element.

Subgroup 1A has an alloy content of 2-6% including carbon. These alloys are often used as buildup materials under higher-alloy coatings.

Subgroup 1B is similar but have a higher alloy content, ranging from 6-12%. alloys in the group have higher carbon contents exceeding 2%.

Group 1 alloys have the greatest impact resistance of all coating alloys except the austenitic manganese steels (Group 2D) and have better wear resistance than low or medium carbon steels. They are the least expensive of the alloy surfacing materials and are machinable. This material also has a moderate improvement over the wear properties of the base metal with high compressive strength and fair resistance to erosion and abrasion.

Group 2 contains higher alloyed steels.

Subgroup 2A has chromium (Cr) as the chief alloying element with total alloy content of 12-25%. Many of these alloys also contain molybdenum.

Subgroup 2B has molybdenum (Mo) as the principal alloying element but many of these also contain appreciable amounts of chromium. The coating alloys of Groups 2A and 2B are primarily more wear resistant, less shock resistant, than those in Group 1.

Groups 2A and 2B are quite strong and have relatively high compressive strengths. They are effective for rebuilding severely worn parts and are used for buildup prior to using higher alloy coating materials. They provide high impact resistance and are good protection of abrasion wear. They are also excellent choices at service temperatures up to 1500°F and when good resistant to hot abrasion is required.

Subgroup 2C contain Tungsten, Nickel and Cobalt blends. They are excellent choices at service temperatures below 900°F and when extreme toughness is required. They are considered as very good high abrasion-resistant and exhibit the best protection for metal-to-metal wear.

Group 3 contains higher-alloyed compositions ranging from 25-50% total alloy. They are high-chromium alloys and some contain nickel, molybdenum or cobalt blends. The carbon can range from slightly under 2% to over 4%. The alloys in this group exhibit better impact, erosion resistance, metal-to-metal wear, and shock resistance than the previous groups.

Group 4 are nonferrous alloys either cobalt base or nickel base with total content of nonferrous metals from 50 to 99%.

Subgroup 4A alloys are the high-cobalt-based alloys with high percentage of chromium. These alloys are used exclusively for applications subjected to a combination of heat, corrosion, erosion, and oxidation. They are considered the most versatile of the wear resistant coating materials. The alloys with higher carbon are used for applications requiring high hardness and abrasion resistance when impact is not a concern. These alloys are excellent when service temperatures are above 1200°F. and resist oxidation

Subgroup 4B alloys are the nickel-based alloys which contain relatively high percentages of chromium. This group of alloys is excellent for metal-to-metal resistance, exhibits good abrasion resistance and corrosion resistance. They will retain hardness very well at higher temperatures and provide good oxidation resistance, but are more difficult to machine.

Subgroup 4C alloys are the chrome-nickel cobalt alloys and are recommended for elevated temperatures. The high-nickel alloy has excellent resistance to hot impact, abrasion, and corrosion and extreme resistance to wear and deformation at elevated temperatures and also provides resistance to erosion, corrosion, and oxidation with good edge strength and corrosion resistance.

Group 5 alloys provide a tungsten carbide nickel cobalt blend with tungsten carbide particles distributed in a cobalt matrix. The tungsten carbide particles are crushed to varying mesh sizes and have excellent resistance to abrasion, corrosion, and moderate resistance to impact. The matrix materials determine the coatings resistance to high-temperature corrosion and wear resistance. The coating is thought to be the toughest of all wear resistant coatings and are very difficult to grind.