Surfacing for Wear Resistance: Part One

The deterioration of surfaces is a very real problem in many industries. Wear is the result of impact, erosion, metal-to-metal contact, abrasion, oxidation, and corrosion, or a combination of these. The effects of wear, which are extremely expensive, can be repaired by means of welding.
Surfacing with specialized welding filler metals using the normal welding processes is used to replace worn metal with metal that can provide more satisfactory wear than the original. Hardfacing applies a coating for the purpose of reducing wear or loss of material by abrasion, impact, erosion, oxidation, cavitations, etc.

Wear

The deterioration of surfaces is a very real problem in many industries. Wear is the result of impact, erosion, metal-to-metal contact, abrasion, oxidation, and corrosion, or a combination of these. The effects of wear, which are extremely expensive, can be repaired by means of welding. Surfacing with specialized welding filler metals using the normal welding processes is used to replace worn metal with metal that can provide more satisfactory wear than the original. Hardfacing applies a coating for the purpose of reducing wear or loss of material by abrasion, impact, erosion, oxidation, cavitations, etc.

In order to properly select a hard facing alloy for a specific requirement it is necessary to understand the wear that has occurred and what caused the metal deterioration. The various types of wear can be categorized and defined as follows:

Impact wear is the striking of one object against another. It is a battering, pounding type of wear that breaks, splits, or deforms metal surfaces. It is a slamming contact of metal surfaces with other hard surfaces or objects. A good example is the impact encountered by a shovel dipper lip or tamper.

Abrasion is the wearing away of surfaces by rubbing, grinding, or other types of friction. It usually occurs when a hard material is used on a softer material. It is a scraping or grinding wear that rubs away metal surfaces. It is usually 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 or slurries that 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 its dimensional accuracy. This can be damaging when parts must maintain close dimensional tolerances.

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

Metal-to-metal wear is a seizing and 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 helps create this type of wear.

Corrosion wear is the gradual eating away or deterioration of unprotected metal surfaces 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 special type of wear indicated by the flaking off or crumbling of metal surfaces, which takes place when unprotected metal is exposed to a combination of heat, air, moisture. Rust is an example of oxidation.

Corrosion (erosion) wear takes place at the same time. This can happen when corrosive liquids flow over unprotected surfaces.

Thermal shock is a problem indicated by cracking or splintering, which is caused by rapid heating and cooling cycles. While not exactly a wear problem it is a deterioration problem and is thus considered here.

Many of the above types of wear occur in combination with one another. It is wise to consider not only one factor, but to look for a combination of factors that create the wear problem in order to best determine the type of hard facing material to apply. This is done by studying the worn part, the job it does, how it works with other parts of the equipment and the environment in which it works. With these factors in mind it is then possible to make a hardfacing alloy selection.

 

Hardfacing Alloy Selection

Unfortunately, there is no standardized method of classifying and specifying the different surfacing weld rods and electrodes. Many of the hard facing electrodes commercially available are not covered by any of most used specifications. Various filler metal suppliers provide data setting forth classes of service and have categorized their own products within these classes. Many suppliers also provide complete information for using their specific products for various applications and for different industries such as quarrying, steel mills, foundries, etc.

The best system of classification has been established by the American Society for Metals Committee on Hardfacing. In this system, there are five major groups classed according to total alloy content other than iron, with subdivisions based on the major alloying elements. Most of these alloys are available as solid bare filler rod in straightened lengths or in coils or covered electrodes. Some of the materials are available as powder for special applications.

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

Group 1 is the low-alloy steels that, with few exceptions, contain chromium as the principal alloying element. The subgroup 1A has alloy content 2-6% including carbon. These alloys are often used as buildup materials under higher-alloy hard facing materials. The Group 1B is similar except that they have a higher alloy content, ranging from 6-12%. Several alloys in the group have higher carbon content exceeding 2%, and include several alloy cast irons.

The alloys of Group 1 have the greatest impact resistance of all hardfacing 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 extremely popular. They are machinable and have a moderate improvement over the wear properties of the base metal to which they are welded. They have a high compressive strength and fair resistance to erosion and scratch abrasion.

Group 2 contains higher alloyed steels. Group 2A has chromium (Cr) as the chief alloying element with total alloy content of 12-25%. Many of these alloys also contain molybdenum. Those with over 1.75% carbon are medium-alloy cast irons. Group 2B has molybdenum (Mo) as the principal alloying element but many of these also contain appreciable amounts of chromium. The hardfacing alloys of Groups 2A and 2B are more wear resistant, less shock resistant, and more expensive 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 facing materials. They provide high impact resistance and good abrasion resistance at normal temperatures.

Group 2C contains tungsten and modified high-speed tool steels. They are excellent choices at service temperatures up to 590°C (1100°F) and when good resistance coupled with toughness is required. They are not considered as good high abrasion-resistant types but are resistant to hot abrasion up to 590°C (1100°F) and exhibit good metal-to-metal wear at elevated temperatures.

Group 2D are the austenitic manganese steels, which contain either nickel or molybdenum as stabilizers. The alloys in Group 2D are highly shock resistant but have limited wear resistance unless subjected to work hardening. The total alloy content ranges from 12-25%. This group is excellent for metal-to-metal wear and impact when the deposit is work hardened in use. The as-welded deposit hardness is low, from 70 to 230 BHN, but will work harden to 450-550 BHN. The deposit may deform under battering but it will not crack. The deposit should not be heated to above 260°C (500°F), which would cause embrittlement.

Group 3 contains higher-alloyed compositions ranging from 25-50% total alloy. They are all high-chromium alloys and some contain nickel, molybdenum, or both. 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. The 3B grouping will withstand elevated temperatures of up to 540°C (1000°F). The 3C group is high in cobalt which improves high-temperature properties. The Group 3 alloys are more expensive than Groups 1 and 2.

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

The Group 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 hard facing materials. The alloys with higher carbon are used for applications requiring high hardness and abrasion resistance but when impact is not as important. These alloys are excellent when service temperatures are above 650°C (1200°F). They resist oxidation temperatures of up to 980°C (1800°F).

The Group 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 scratch abrasion resistance, and corrosion resistance. They will retain hardness to 540°C (1000°F). The alloys with higher carbon content provide higher hardnesses but are more difficult to machine and provide for less toughness. These alloys show good oxidation resistance up to 950°C (1750°F).

The Group 4C alloys are the chrome-nickel cobalt alloys and all are recommended for elevated temperatures. The high-nickel alloy has excellent resistance to hot impact, abrasion, and corrosion and moderate resistance to wear and deformation at elevated temperatures. The medium-nickel alloy has high-temperature wear resistance and impact resistance. It also provides resistance to erosion, corrosion, and oxidation. The low-nickel alloy is used for moderate high temperatures and provides good edge strength, corrosion resistance, and moderate strength.

The Group 5 alloys provide a tungsten carbide weld deposit. This deposit consists of tungsten carbide particles distributed in a metal matrix. The matrix metals include iron, carbon steel, nickel-based alloys, cobalt-based alloys, and copper-based alloys. The tungsten carbide particles are crushed to mesh sizes varying from 8 to 10 down to 100 and have excellent resistance to abrasion and corrosion, and moderate resistance to impact. The matrix material determines the resistance to corrosion and high-temperature resistance. The finish of the deposit depends on the tungsten carbide particle size: the finer the particles the smoother the finish. The deposits are not machinable and are very difficult to grind.

 

October, 2004
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