Frequently asked questions about hardfacing
Understanding what it is and how its used
|This robot is hardfacing an agricultural tool.|
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.
|Figure 1 |
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.
ASTM G65 Test Apparatus
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 42037, Cleveland, OH 44142, 216-265-9000, www.postle.com.
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