What is Hard Facing?

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

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 hard-facing products.

What base metals can be hard-faced?

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

The following base metals can be hard-faced:

  • Stainless steels
  • Manganese steels
  • Cast irons and steels
  • Nickel-base alloys
  • Copper-base alloys
Is there a convenient way to categorize the many alloys when determining which hard facing to use?

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.

  • 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 harnesses 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.
Many hard facing alloys crack, is this normal?

It depends on the hard-facing 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.

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. 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.

What is chromium carbide hard-facing?

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.

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.

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 micro-structure 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 micro-structure. You should consult with the manufacturer for recommendations.

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.

What type of gas is used in PTAW hard-facing?

Low penetration and dilution are the major objectives in hard-facing, so pure argon and mixtures of argon with hydrogen generally will produce the desired result.

Must parts be preheated before hard-facing?

As a rule, you should bring all parts at least to room temperature. You can select higher preheat and inter-pass temperatures based on the base metal chemistry and hard-facing product you’re using.

When is a cobalt or nickel hard-facing 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.

Why are some hard-facing 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 hard-facing deposit.

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 hard-facing deposit. If the hard-facing 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

Can cast iron be hard-faced?

Yes, but you must take preheat and inter-pass 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 hard-facing. Hard-facing product manufacturers and specialists can contribute to a greater in-depth understanding of hard-facing and help assist you in product and process selection for your application

What are the benefits of Hard-facing and Coatings ?
  • Reduces Cost: Restoring a worn part to “as new” condition generally costs between 20-70% of a brand new replacement part.
  • Prolongs Equipment Life: Service life increases of 3 to 10 times are common with properly coated parts.
  • Reduces Downtime: Parts last longer and fewer shutdowns are required.
  • Less Spare Parts Inventory: There is no need to keep numerous spare parts when worn parts can be rebuilt.
  • Environmentally conscious: the ability to reuse and recycle rather than waste and replace.
What base materials can be over-layed?

Almost any steel, or stainless based alloy can be over-layed, including manganese based or non-magnetic materials.

What causes porosity during welding?

In any welding process, porosity can be caused by the presence of contaminants or moisture in the welding zone, which includes the base metal, filler metal, shielding gas, and the surrounding atmosphere. Contaminants can include oil, dirt, grease, or cutting fluids. Concurrently, moisture can collect in the flux, shielding gas, or on the base metal, or come from the atmosphere.

Porosity occurring in a welding process that utilizes an external shielding gas can occur from using too much or too little gas flow, poor gas quality, or a defective welding torch, gun, or hose.

Operator technique can also cause porosity. Electrode, torch, or gun angle can lead to porosity, as can excessive arc length, electrode extension, or travel speeds.

Plasma transfer arc (PTA) welding is a process, in which the joining of materials is produced by the heat of a constricted arc between an electrode and a base metal.  In PTA welding, a shielded arc is struck between a non-consumable electrode (Tungsten) and the torch body, and this arc transforms an inert gas (Argon) into plasma by heating it to a high temperature. The PTA welding process uses this plasma to transfer an electric arc to a work piece. Metal powder is metered, under a positive pressure of Argon flow, from the bottom of the torch into a pool of molten metal on the workpiece surface.

The torch is then either moved by a side-beam carriage over the workpiece, or the workpiece is rotated or moved under the torch, or a combination of both to produce a weld overlay deposit. The plasma arc deposit is fully dense and metallurgically bonded to the workpiece.  One of the most important features of the PTA process is the control of dilution. PTA produces dilution as low as 5%, compared to 20-25% typically obtained when hard-facing by MIG and (TIG) processes.  So it is possible to maintain the noble properties of deposit even in one single pass.

What are the key advantages of PTA overlays?
  • Accuracy: automated controls provide highest levels of consistency controlling heat input
  • Quality: coating purity is extremely high with very little dilution into the base material
  • Productivity: much higher production rates than other processes of similar quality (laser)