A primer on hardfacing metal

Welders that help to maintain equipment used in construction and mining are probably familiar with hardfacing because the process helps to extend the life of roughly used components. Getty Images

Hardfacing is a cost-effective way to minimize wear and increase service life in industrial parts and equipment. At first glance, hardfacing can be confusing and troublesome, but it's really not once you know the facts.

The following answers to 22 frequently asked questions may help you understand the basics of the technology so you can select the hardfacing products that are most appropriate for your application.

Metal parts often fail their intended use not because they fracture, but because they wear by abrasion, impact, or metal-to-metal contact, which causes them to lose dimension and functionality. Hardfacing, also known as hard surfacing, is the application of a buildup or wear-resistant weld metal onto a part's surface by means of welding to extend the life of the part. The weld metal may be applied as a solid surface or in a pattern, such as a waffle, herringbone, or dot pattern.

Hardfacing is becoming increasingly important for many industries to protect equipment that is exposed to wear and abrasion. Extending the life of wear parts can save you thousands of dollars and improve productivity. Hardfacing can be used to recondition parts that have already been exposed to wear and have lost their useful life, or it can be used in the manufacture of new parts to improve their life before they are put into service. The three main types of hardfacing applications are:

Buildup or rebuilding.

Hardfacing or overlay.

Combination buildup and overlay.

Carbon and low-alloy steels with carbon contents of less than 1 percent can be hardfaced. Medium carbon and low-alloy steels are very common, since they provide higher strength than mild steels and better abrasion resistance. High-carbon alloys may require a special buffer layer. The following base metals can be hardfaced: stainless steel, manganese steel, carbon and alloy steels, cast iron, nickel-based alloys, and copper-based alloys.

Carbon and low-alloy steels are strongly magnetic and can easily be distinguished from austenitic manganese steel, which is nonmagnetic. Many low-alloy and higher-carbon steels are used for manufacturing equipment and spare parts, especially equipment that requires higher strength and abrasion resistance. They are not easily differentiated but should be identified to determine the proper preheat and post-heat temperature.

As the alloy content increases, the need for preheat and postheat becomes more critical. For example, steel made from 4130 generally requires a preheat of 400 degrees F. Steel used for rails is typically higher carbon and requires a minimum preheat of 600 to 700 degrees F. Manganese steel does not require preheat. In fact, steps should be taken to keep the base metal heat below 500 degrees F.

Figure 1 Many factors affect the economics of hardfacing, but a major factor is the deposition rate, estimated here for each process.

In order of popularity, the following welding processes are used to apply hardfacing:

Flux-cored arc welding (FCAW) with open-arc or gas-shielded hardfacing wire

Gas metal arc welding (GMAW) with gas-shielded wire

Shielded metal arc welding (SMAW)

Submerged arc welding (SAW)

Gas tungsten arc welding (GTAW)

Oxyfuel welding (OFW) or oxyacetylene welding

Plasma transferred arc welding, laser welding, thermal spray, and spray and fuse

The current trend is toward the use of semiautomatic and automatic welding processes using FCAW and GMAW, which are about the same in terms of popularity. GMAW using either a solid wire or metal-cored welding wire requires a gas shield, whereas FCAW employs welding wires that are used open-arc or gasless, as well as with a gas shield. SMAW with flux-coated electrodes is still very popular, especially for field on-site hardfacing applications, because the equipment is inexpensive and portable.

Consider the following factors when selecting a welding process:

Welding equipment availability, including size of power source

Use of stick electrode or semiautomatic wire

Availability of hardfacing consumables

Size of welding consumable

Use of gas-shielded wire, open-arc wire, or submerged-arc wire

Operator skill available

Welding location—indoors or outdoors

Component size and shape, and the area to be hardfaced

Thickness of deposit

Deposition rate

Welding position

Machining requirements

Desired finish

Component preparation for previously hardfaced parts

• Preheat and postweld treatments (temper/slow cool/air cool)

Following are the three most common types of wear:

Abrasive wear, which makes up 40 to 50 percent of all wear cases, occurs when materials such as grain, soil or sand, coal, or minerals slide across a metal surface. Abrasive wear can be divided into gouging abrasion, high-stress grinding abrasion, and low-stress scratching abrasion.

Impact wear (20 percent) occurs when an object is struck by another object, such as a crusher part or a railcar wheel rolling over a manganese crossing, resulting in material flaking off or spalling.

Adhesive wear (metal to metal) (15 percent) occurs when two metallic surfaces slide against each other under pressure, creating a micro-welding condition from frictional heat. It generally occurs in nonlubricated or dry conditions.

Heat and corrosion are two other types of wear, each of which make up about 5 percent of all cases.

Most worn parts don't fail from a single type of wear, such as impact, but from a combination, such as abrasion and impact. For example, a mining bucket tooth usually is subjected to both abrasion and impact, although one type might be more dominant than the other. The type of wear dictates which hardface welding product is used. Determining the wear mode and understanding the environment to which the part is exposed are crucial to choosing the most suitable hardfacing alloy for the application.

Iron-based alloys can be divided into four main categories:

Martensitic. This group includes all hardenable steels with Rockwell hardness from 20 to 65 HRC. These alloys, similar to tool steel, harden upon cooling. They are good for metal-to-metal and abrasive wear. They also can withstand a great deal of impact. The alloys with HRC less than 45 generally are used for buildup before hardfacing or to restore dimension, and the part must be machined after welding. Higher-hardness martensitic alloys, greater than 50 HRC, are used for abrasion resistance.

Austenitic. Austenitic alloys include work-hardening manganese steels and stainless. These alloys generally are soft when welded and harden only after the weld metal is worked under repeated impact. They have good impact properties and moderate abrasion resistance. Austenitic manganese steel generally is found around crushers, railroad track frogs and crossings, and shovel teeth.

Metal carbide and soft austenitic matrix. These alloys contain large amounts of metal carbides in a softer matrix and are good for severe abrasion applications. The alloys that contain large amounts of chromium and carbon, the chromium carbide family, are closer to a cast iron or white iron. They sometimes contain additional amounts of niobium and vanadium. Their hardnesses are from 40 to 65 HRC. Alloys that contain large amounts of tungsten and carbon, the tungsten carbide family, sometimes contain small amounts of boron that form borides and are good for severe abrasion applications.

Metal carbide in hard martensitic matrix. The martensitic matrix is essentially a tool steel with a hardness from 45 to 60 HRC. These alloys contain additions of niobium, vanadium, molybdenum, or titanium. With proper procedures they can usually be applied without stress cracks.

Many chromium carbide alloys check-crack when cooled to moderate temperatures, and this is normal. Others, such as the austenitic and martensitic families, don't crack when applied with proper welding procedures.

When check-cracking, or checking, occurs, cracks form perpendicular to the bead length. Generally occurring from 3/8 to 2 in. apart, they result from high stresses induced by the contraction of weld metal as it cools. The cracks propagate through the thickness of the weld bead and stop at the parent metal, as long as it's not brittle. If the parent metal is hard or brittle, select a buffer layer of a softer, tougher weld metal, such as an austenitic alloy.

Generally, chromium carbides are iron-based alloys that contain high amounts of chromium (greater than 15 percent) and carbon (greater than 3 percent). These elements form hard carbides that resist abrasion. The deposits frequently check-crack about every ½ in., which helps relieve stress from welding. Their low friction coefficient also makes them desirable in applications that require material with good slip.

Generally speaking, the abrasion resistance increases as the amount of carbon and chromium increases, although carbon has the most influence. Hardness values are 40 to 65 HRC. They may also contain other elements that can form other carbides or borides that help increase wear resistance in high-temperature applications. These alloys are limited to two or three layers.

Complex carbides generally are associated with chromium carbide deposits that have additions of columbium (niobium), molybdenum, tungsten, or vanadium. These elements and carbon form their own carbides or combine with the present chromium carbides to increase the alloy's overall abrasion resistance. They can have all of these elements or just one or two. They are used for severe-abrasion and high-heat applications.

These tool steel alloys have numerous tightly packed carbides of titanium, niobium, or vanadium. Carbides in martensite are an excellent choice for applications requiring crack-free deposits with good wear characteristics. Weld deposits generally exhibit the same wear-resistant characteristics that are expected from chromium carbide hardfacing products. Since these alloys do not crack, they tend to be easier to reapply.

With this process, also known as tungsten carbide embedding, the tungsten carbide particles are fed from a hopper directly into the molten weld puddle of PS98. When the weld bead cools, the resulting weld deposit contains large volumes of tungsten carbide particles embedded in a 55- to 60-HRC tool steel matrix. These extremely hard and wear-resistant particles protect bulldozer and grader blades, dragline and loader buckets, and many different types of hammers from premature wear in many challenging, highly abrasive applications.

For working in rocky earth, ore, or slag, the goal is to protect the metal surface from abrasion caused by the movement of the rocks over it. This can be done by applying a series of ridges or weld beads parallel to the flow of material, like rails, which prevent the rocky soil from coming in contact with the surface.

When working in dirt or sand, apply hardface weld beads spaced ¼ to 11/2 in. apart and perpendicular or against the flow of an abrasive material. Forcing the material to compact between the weld beads works well for finely grained sands and soils. Apply a dot pattern to areas that do not undergo heavy abrasion but are subjected to wear, or where weld areas are hard to reach.

A dot pattern also is used on thin base metals when distortion and warpage may be an issue from overheating of the base metal. For working in soil with some clay content, the goal is to use a hardfacing pattern that traps the soil on the surface, forming a layer of trapped soil that will protect the surface underneath. This is best done with a crosshatch or waffle pattern. This pattern also works well when there is a combination of fine and coarse soil.

No. A martensitic alloy and a chromium carbide alloy can have the same hardness, let's say 58 HRC, and perform very differently under the same abrasive conditions. A chromium carbide alloy will provide better abrasion resistance than a martensitic alloy. The metallurgical microstructure is a better measuring stick, but that isn't always available.

The only time hardness can be used to predict wear is when the alloys being evaluated are within the same family. For example, in the martensitic family, a 55-HRC alloy will have better abrasion resistance than a 35-HRC alloy. This may or may not be the case in either the austenitic or metal carbide families. Again, you have to consider the microstructure.

It depends on the type of wear involved, but in the case of abrasive wear—by far the most predominant wear mechanism—the ASTM Intl. G65 dry sand rubber wheel test is used extensively. The sample is weighed before and after the test, and the result usually is expressed in grams of weight loss or volume loss. A sample is held against a spinning rubber wheel with a known force for a set number of revolutions. A specific type of sand, which is sized carefully, is trickled down between the sample and rubber wheel. This simulates pure abrasion, and the numbers are used as guidelines in material selection.

Low penetration and dilution are the major objectives in hardfacing, so pure argon and mixtures of argon with oxygen or carbon dioxide generally will produce the desired result. You also can use pure CO2, but you may get more spatter than you would with an argon mixture.

Welding wires produce either a spray transfer or a globular (ball) transfer of molten metal across the welding arc.

Spray transfer is a dispersion of fine molten metal drops, characterized by a smooth-sounding transfer. These wires are desirable in joining applications that require good penetration.

Ball transfer wires disperse larger molten metal drops, or balls. This type of transfer promotes low penetration and dilution, suitable for hardfacing. It has a noisier arc that produces an audible crackling sound and generally has a higher spatter level than spray transfer wires. Welding parameters such as electrical stick-out, gas, amperage, and voltage can affect the size of the ball and its transfer. Gasless or open-arc wires all have a globular or ball transfer.

Heat-affected zone cracking is always a concern when welding low-alloy and high-carbon steels, highly stressed parts, and parts with complex shapes. In general, all parts should be welded at least at room temperature; you might need higher preheat and interpass temperatures depending on the base metal chemistry and hardfacing product you’re using.

High-carbon steels require preheating. For example, steel made from 4130 generally requires a preheat of 400 degrees F. Steel for rails is typically high-carbon and requires a minimum preheat of 600 to 700 degrees F.

Manganese steel and some stainless steels require no preheating, and welding temperatures should be kept as low as possible. In fact, you should try to keep the manganese base metal below 500 degrees F. Consult the manufacturer for the best combination to prevent cracking and spalling.

Chromium carbide and complex carbides generally are limited in the number of layers that can be applied. The brittle nature of the metal carbides leads to check-cracking, and as multiple layers are applied, stress continues to build, concentrating at the root of the check cracks, until separation or spalling occurs between the parent metal or buffer and the hardfacing deposit.

Unless otherwise specified by the manufacturer, and with the correct procedures, martensitic hardfacing alloys can be applied in multiple layers. Austenitic manganese hardfacing products can be applied in unlimited layers unless the manufacturer specifies otherwise. If more layers are required, use a buildup or buffer alloy.

21. What Are Buildup and Buffer Alloys?

Buildup and buffer alloys are similar to the parent metal alloy in hardness and strength. They are applied to severely worn parts to bring them back to dimension when machining must be used after welding. With hardness of 30 to 45 HRC, they are applied as a buffer for subsequent layers of a more wear-resistant hardfacing deposit. If the hardfacing alloy produces check cracks, then use a tough manganese product as the buffer to blunt and stop the check cracks from penetrating into the base metal.

A mild steel electrode or wire should never be used for buildup or as a buffer layer. While mild steel welding products are great for joining and fabricating, they do not have the strength and hardness to support hardfacing. A soft mild steel buffer layer will collapse under the hardface layer, causing the hardface layer to spall off and fail.

22. Can Cast Iron Be Hardfaced?

Yes, but be sure to consider preheat and interpass temperatures. Nickel and nickel-iron products usually are suitable for rebuilding cast iron. They aren't affected by the carbon content of the parent metal and remain ductile. Multiple layers are possible. If further wear protection is required, metal carbide products can work well on top of the nickel or nickel-iron buildup.

These frequently asked questions only begin to address hardfacing. Hardfacing product manufacturers and specialists can contribute to a greater in-depth understanding of hardfacing and help you select products and processes for your application.

Bob Miller was a materials and applications engineer at Postle Industries Inc., 5500 W. 164th St., Cleveland, OH 44142, 216-265-9000, postle.com.