What is Hardfacing in Welding and How Can You Apply It?

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Welding is typically a process of joining two pieces of material using the heat generated by an electric arc and often a filler material to add and strengthen the resulting joint. However, the same basic technique can be used in other ways by adjusting a few parameters. A primary example is plasma cutting, which is largely the same as arc welding but with a controlled jet of gas used to melt and blow away material to cut rather than combine.

A similar adjustment of parameters leads to hardfacing. Some think of hardfacing as a complex and advanced technique that only the most experienced welding operators can pull off. Others just consider it an irritation and not worth the time and effort required to perform it. What is it, though, and how can you use the process? Let&#;s discuss.

What is Hardfacing and How Does it Work?

Hardfacing is a process where you use the general concept of welding &#; specifically the deposition of material &#; to coat the outside of a base metal with a tougher, harder metal. This can make the resulting workpiece more durable, more resistant to abrasion, wear, and damage, and expand its longevity. Hardfacing is performed using specialized electrodes or filler rods but largely uses the same kind of arc welding process to melt those fillers onto the surface of the base material.

This isn&#;t quite the same as something like adhering a shell to the outside of a workpiece. Since the welding process is used, the surface of the base material is melted enough to combine with the filler material, resulting in a merger of the two. This creates a thick, dense layer between 1 and 10 mm, made of a highly bonded, wear-resistant alloy combining the base and filler metals.

Hardfacing improves the surface strength, ductility, wear resistance, corrosion resistance, and erosion resistance of the original workpiece. It can be performed on cast iron, copper, and nickel alloys, stainless steel, carbon steel, and manganese steel.

One key to successful hardfacing is minimizing heat. While you need enough surface melt to the base material to adhere to the hardfacing filler, you want as little mixing as possible because the base material will soften the added material and reduce the utility of the hardfacing. Therefore, knowing what material you&#;re hardfacing is critical so you can identify ideal temperatures.

What Are the Benefits of Hardfacing?

Hardfacing is a way to strengthen and extend the working life of metal parts that see a lot of wear, particularly surface wear. By hardfacing a worn part, additional lifespan can be drawn from the base workpiece, extending longevity and reducing the need to replace it; this saves money over the long run.

Hardfacing is commonly used in agricultural and mining situations and is frequently also an option for getting more useful life out of a part while a replacement is on order. In remote locations or for specialized parts that need lead time to be made and shipped, when the difference is between halted operations and continued operations, hardfacing can make the difference.

Overall, hard facing can be used to minimize the downtime spent replacing worn or broken parts, allows you to store fewer replacements on-site if you typically do so, and increases the average lifespan of those parts. In many cases, the lifespan of a working piece can be extended by up to 300% with dedicated hard facing, which can mean overall savings for the company of 25% to 75% in the cost of replacements.

Where is Hardfacing Often Used?

Hardfacing can be used in a wide range of different applications. Since it&#;s coating the outside of a workpiece, it&#;s generally used on pieces that have to resist impacts or abrasions over time.

In construction, an excavator&#;s plow is one common piece that has to endure an immense amount of impact and erosion damage. Heavy use of an excavator can wear it down, reducing the strength of the plot and even the workable dimensions of the machine. Hardfacing the plow strengthens it against this kind of damage.

In agriculture, the manufacture of sugar involves crushing sugarcane in a roller. Despite being a plant &#; which you might not think of as being able to damage metal parts &#; the sugarcane plant is quite strong and can give as good as it gets. These rollers can be hard-faced to help resist the damage of continually crushing sugar cane and extend the operating life of the machine.

Another common example from mining is the crusher that breaks up larger chunks of ore-bearing material for processing. There are a wide range of different designs for these machines, but many of them end up using plates of metal and heavy motors to crush rock. Rock, too, resists being crushed and will damage and abrade the crusher over time. Hardfacing the jaws of the crusher will help them resist that force and can be used as a repair to restore functionality to damaged jaws before they need to be fully replaced.

Are There Different Techniques for Hardfacing?

Yes. Hardfacing can be performed in two ways: overlay and build-up. These are roughly equivalent to enhancing existing materials and repairing damaged materials.

Imagine a workpiece with many gouges, scratches, abrasions, dings, scrapes, and other forms of damage from long use. The workpiece is worn and nearing the end of its lifespan. Such a piece can be restored with hard facing using the build-up technique. Build-up hardfacing entails using the welding process to deposit hardfacing material and spending additional time and care on the deeper gouges and abrasions to rebuild the working surface of the workpiece. A layer to smooth and even out the working surface is applied, and any finishing is performed.

Overlay hardfacing can be done on pieces that have been built up already using the previous technique or on brand-new or barely-worn pieces that need additional reinforcement to expand their lifespan. This technique simply uses repeated and iterative passes of hard facing to deposit a layer of material across the entire surface of the workpiece; no special care is necessary for repairs because no repairs are necessary. It&#;s simply a method of reinforcement.

How is Hardfacing Performed?

The step-by-step process of hard facing is relatively simple and similar to many other welding processes. The primary difference is that it takes place over the whole surface of a typically large workpiece rather than along a single seam or crack in the workpiece.

Step one is to clean the part. Since hard facing is typically used on working machines, these have likely built up grime, dirt, oils, greases, rust, and chemicals, all of which can inhibit the performance of a weld and cause inclusions, weaknesses, and faults in the resulting surface. A thorough cleaning is required to avoid cracking, warping, and damage to the workpiece. This process may even need to be performed on brand-new pieces, particularly if they&#;ve been painted or coated in rust-resistant chemicals.

Step two is to perform build-up hard facing on any deep gouges, abrasions, cracks, or other damage to the surface of the workpiece. Hardfacing requires that the surface be more or less the shape and form you want the finished piece to be, so any repairs you need to make should be made at this step in the process. This isn&#;t necessary for new pieces you&#;re reinforcing with hard facing, as there&#;s likely no damage to repair.

Step three is to &#;butter&#; the part. Buttering is the act of forming a thin buffer layer between the base material and the hardfacing material. It&#;s most commonly used in cases where the two materials are dissimilar and will have a hard time bonding with one another. By using an intermediary as a buffer layer, you can create more comprehensive bonds and reduce the chances of the final coating cracking or shrinking and causing problems.

The fourth and final step is to perform the actual hard facing. This is where you apply your coats of hardfacing material. This may be spotty and sporadic, or it may be comprehensive and can be anywhere from one to three coats for most situations.

One critical component of the actual hardfacing process is deciding on a hardfacing pattern. While a complete coat of cladding can be used, this is often unnecessary, depending on the purpose of the workpiece. Other patterns include:

• Dots. A dot pattern involves using a series of regularly spaced dot-shaped welds to deposit the hardfacing material across the surface. This is typically used on machines that deal with larger rocks and aggregate; smaller material can fill the gaps between the raised dots, while the raised dots become the source of impact and are strengthened against it. Other types of impact are cushioned by the &#;dead bed&#; between the dots.

• Stringers. Stringers are long, straight beads of welding, typically spaced some distance apart, between a quarter of an inch all the way up to 1.5 inches. This is typically chosen with beads that run parallel to the direction the material flows in use, so it doesn&#;t &#;catch&#; and cause more damage to the hard-faced material.

• Waffles. A waffle, criss-cross, or herringbone process is perpendicular beads that leave small square pockets behind. Again, this is usually used for dealing with larger aggregate materials, where smaller materials and sand form a cushioning bed in between.

All of these patterns are ways to save time, energy, and material when hardfacing, so you don&#;t have to coat the entire working surface of a piece completely.

Hardfacing Frequently Asked Questions

What does successful hardfacing look like? Ironically, hard facing often looks visually like bad welds because it&#;s an uneven and rough build-up of material on the surface of the workpiece. In a way, it&#;s almost like more of a &#;brute force&#; approach to welding rather than more elegant seams and connections.

Are you interested in learning more about Hardfacing Machinery? Contact us today to secure an expert consultation!

What materials can and can&#;t be hard-faced? Hardfacing is generally done on harder steels, like stainless, manganese, carbon, and alloy steels. It can also be performed on cast iron, nickel-base alloys, and copper-base alloys. Other base materials are often too soft to take hardfacing well or are not used in the kinds of applications where hardfacing is beneficial.

One of the greatest details of hardfacing is that it&#;s only applicable to high-wear pieces in heavy industry. It&#;s typically not used in cases where direct wear isn&#;t responsible for damage. That is, cases where the cause of damage is flexing stress or sheer stress are not going to benefit from hard facing.

What&#;s the best process for hard facing? Many forms of hard facing are performed using submerged arc welding. Another popular process is flux-core arc welding. However, any welding process can be used for hardfacing, up to and including plasma arc welding, laser welding, and even brazing.

What matters most is the heat control and deposition rates of the process you choose. Some processes have better deposition rates and can hardface a workpiece faster and more effectively. Others need more careful control to avoid overheating the area. At the end of the day, though, what matters most is that you pick a process you&#;re familiar with and can control well.

What is &#;wear&#; in the industry? Most often, wear is caused by abrasion with a material like rock or aggregate or by impact. Sometimes, wear can be defined as metal-to-metal abrasion, heat, or corrosion, but these are less common in the cases where hard facing is most beneficial. Additionally, many different kinds of wear can apply at once; using an excavator to plow can cause both abrasion and impact, for example.

How can you pick the best machine for hardfacing? The best machine for hardfacing is one that can handle long duty cycles with enough power to deposit material quickly and limit the heat-affected zone of the base material. As such, nearly any good welder can handle the process, but it can benefit more from a 220v machine over a 110v machine. Either way, there are many options you can explore to find the welder that best suits your needs. Our welding equipment rentals help you test and try out machines and, if you don&#;t like them, return and rent different machines.

Red-D-Arc, an Airgas company, rents and leases welders, welding positioners, welding-related equipment, and electric power generators &#; anywhere in the world. Our rental welders, positioners and specialty products have been engineered and built to provide Extreme-Duty&#; performance and reliability in even the harshest environments, and are available through over 70 Red-D-Arc Service Centers, strategically located throughout the United States, Canada, the United Kingdom, France, and the Netherlands, as well as through strategic alliances in the Middle East, Spain, Italy, Croatia, and the Caribbean. From our rental fleet of over 60,000 welders, 3,700 weld positioners, and 3,700 electric-power generators, we can supply you with the equipment you need &#; where you need it &#; when you need it.

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At first glance, hardfacing can be confusing and troublesome; in reality, it isn't. Understanding some of the basics about hardfacing can go a long way toward instilling confidence in your hardfacing product selection.

The following 19 answers to frequently asked questions may help you select hardfacing products that are most appropriate for your application.

1. What is hardfacing?

Metal parts often fail their intended use not because they fracture, but because they wear, which causes them to lose dimension and functionality. Hardfacing, also known as hardsurfacing, is the application of buildup or wear-resistant weld metals to a part's surface by means of welding or joining.

2. What base metals can be hardfaced?

Carbon and low-alloy steels with carbon contents of less than 1 percent can be hardfaced. High-carbon alloys may require a special buffer layer.

The following base metals can be hardfaced:

• Stainless steels
• Manganese steels
• Cast irons and steels
• Nickel-base alloys
• Copper-base alloys

3. What is the most popular procedure used to apply hardfacing?

In order of popularity, the following procedures can be used:

• Flux cored arc welding (FCAW)
• Gas metal arc welding (GMAW)
• 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 brazing

FCAW and GMAW may be interchangeable or the same in terms of popularity. However, the trend is toward use of semiautomatic and automatic welding procedures.

4. With so many welding processes available, which ones are the most economical?

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

5. Wear is such an all-encompassing term. Can it be broken down into more manageable categories?

Yes. Many different categories of wear exist&#;too many to cover in one article&#;but the most typical modes of wear are as follows (percentages are estimates of total wear):

• Abrasion&#;40 percent
• Impact&#;25 percent
• Metallic (metal to metal)&#;10 percent
• Heat&#;5 percent
• Corrosion&#;5 percent
• Other&#;5 percent

Most worn parts don't fail from a single mode of wear, such as impact, but from a combination of modes, such as abrasion and impact. For example, a mining bucket tooth usually is subjected to abrasion and impact, and depending on what type of material is mined (soft or hard rock), one mode may be more dominant than another. This will dictate the welding product used.

Determining the wear mode can be challenging and may require trial and error when you select hardfacing products.

6. Is there a convenient way to categorize the many alloys when determining which hardfacing to use?

Yes. Iron-base alloys can be divided into three main categories:

• Martensitic. This includes all hardenable steels with Rockwell hardness from 20 to 65. This group, similar to tool steel, hardens upon cooling. They are good for metal-to-metal and abrasive wear. They also can withstand a great deal of impact.
• Austenitic. Austenitic alloys include work-hardening steels, such as manganese and stainless. This group generally is soft when it's welded and hardens only after the weld metal is worked. They have good impact properties and moderate abrasion resistance. The stainless steel family is good for corrosion resistance.
• Metal carbide. These alloys contain large amounts of metal carbides in a soft, tough matrix and are good for severe-abrasion applications. The alloys that contain large amounts of chromium and carbon are known as the chromium carbide family and are closer to a cast iron or white iron. Their hardnesses are from 40 HRC to 65 HRC. Alloys that contain large amounts of tungsten and carbon belong to the tungsten carbide family. Some contain small amounts of chromium and boron that form borides and are good for severe-abrasion applications.

7. Many hardfacing alloys crack. Is this normal?

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

8. What is check-cracking?

Check-cracking, or checking as it's sometimes called, occurs in the metal carbide families and can be seen as cracks that are perpendicular to the bead length (see Figure 2). They generally occur from 3/8 to 2 inches apart and are the result of 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. In cases in which the parent metal is hard or brittle, you should select a buffer layer of a softer, tougher weld metal. The austenitic family is a good choice for a buffer deposit.

9. What is chromium carbide hardfacing?

Generally, these are iron-base alloys that contain high amounts of chromium (greater than 18 percent) and carbon (greater than 3 percent). These elements form hard carbides (chromium carbides) that resist abrasion. The deposits frequently check-crack about every 1/2 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 range from 40 HRC to 65 HRC. They also can 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.

10. What are complex carbides?

Complex carbides generally are associated with the chromium carbide deposits that have additions of columbium, molybdenum, tungsten, or vanadium. The addition of these elements and carbon form their own carbides and/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 or high-heat applications.

11. Can hardness values be used to predict abrasion resistance?

No, this isn't a good idea. A martensitic alloy and a chromium carbide alloy can have the same hardness, let's say 58 HRC, and perform vastly different under the same abrasive conditions. 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. You should consult with the manufacturer for recommendations.

12. If hardness is unreliable, then how is wear measured?

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. This essentially is a test in which 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 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 (seeFigure 3).

13. What type of gas is used in GMAW hardfacing?

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 carbon dioxide, but you'll get more spatter than you would with an argon mixture.

14. What is a ball, or globular, transfer, and why is it important?

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 and can be characterized as a smooth-sounding transfer. These wires are desirable in joining applications in which you 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 stickout, gas (if any), 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.

15. Must parts be preheated before hardfacing?

As a rule, you should bring all parts at least to room temperature. You can select higher preheat and interpass temperatures based on the base metal chemistry and hardfacing product you're using.

Manganese and some stainless steels and similar hardfacing products require no preheating, and welding temperatures should be kept as low as possible. Other steels usually require proper preheat and interpass temperatures. You should consult the manufacturer for the best combination to prevent cracking and spalling.

16. When is a cobalt or nickel hardfacing alloy used?

Cobalt alloys contain many types of carbides and are good for severe abrasion at high temperatures. They also have good corrosion resistance for some applications. Deposit hardness ranges from 25 HRC to 55 HRC. Work-hardening alloys also are available.
Nickel-base alloys can contain chromium borides that resist abrasion. They can be good particularly in corrosive atmospheres and high temperatures when abrasion is a problem.

17. Why are some hardfacing products limited to two or three layers?

Limited-layer products usually are in the metal carbide families, such as chromium carbide and tungsten carbide. You can apply martensitic and austenitic products in unlimited layers unless the manufacturer specifies otherwise.

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.

18. What is meant by a buildup or buffer alloy?

These alloys often resemble the parent metal alloy and are applied to severely worn parts to bring them back to dimension or act as a buffer for subsequent layers of a more wear-resistant hardfacing deposit. If the hardfacing produces check cracks, then it's wise to use a tough manganese product as the buffer to blunt and stop the check cracks from penetrating into the base metal.

19. Can cast iron be hardfaced?

Yes, but you must take preheat and interpass temperatures into account. Nickel and nickel-iron products usually are suitable for rebuilding cast iron. These products 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 assist you in product and process selection for your application

Bob Miller is a materials and applications engineer at Postle Industries Inc., P.O. Box , Cleveland, OH , 216-265-, www.postle.com.

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