Determining The Correct Hardfacing Product

Help your customers choose the right product for their application.

Hardfacing is the addition of a wear-resistant coating to a surface to resist abrasion, impact, erosion, galling or cavitation. Available forms of hardfacing are covered electrodes, cored wire, solid wire, cast rods and powders. These hardfacing materials can be applied by many welding processes. The information in this article will be centered on shielded metal arc welding (SMAW) and flux-cored arc welding (FCAW).

If using SMAW, consider the electrode diameter suitable for the application and the current capacities of the electrode power source. If using FCAW, consider the electrode diameter suitable for the application and the wire feed and current capacities of the wire power source.

In order to recommend the proper choice of hardfacing material, it is crucial that salespeople understand the environment to which the piece is exposed during operation. Observe the environment and ask questions such as: Is the operating environment mostly impact, abrasion or a combination of each? Will the part be in contact with an abrasive slurry or corroalsive liquid in addition to abrasion or impact?

Table 1 is a condensed version of the iron base alloys available in AWS A5.13 (SMAW) and AWS A5.21 (FCAW). The buildup alloys are used to restore dimension and provide a solid base for the hardfacing alloys. The martensitic air hardening alloys are best for metal-to-metal wear and provide decent abrasion resistance and excellent weldability. The Hadfield Mn is used for rebuilding the standard 14 percent Hadfield Mn castings. The CrMn alloys are used for rebuilding the Hadfield Mn castings and also for joining the Hadfield Mn to carbon steel and other low alloy steels. There are many versions and variations of the Cr carbides. The abrasion resistance increases with a higher carbon content and the impact resistance decreases.

Cored wires for hardfacing are available as self shielded and also gas shielded. The above-mentioned alloys in a cored wire are manufactured with a carbon steel sheath and the addition of the alloys in a powder form to the core. The powders in the core along with the sheath all melt together in the weld puddle to form the weld metal structure. Coated electrodes for hardfacing in the above compositions are made with a carbon steel core wire; the coating contains the alloys and fluxes for welding. For higher alloy versions of hardfacing alloys not shown in Table 1, an alloyed sheath for the flux cored wire is used. Also, an alloy core wire for the coated electrodes is used to increase the overall alloy content of the weld deposit.

Nominal Chemistries taken from AWS A5.21

Applications
The application could involve a fabrication or joining operation. There are weld overlayed sheets of chrome carbide for abrasion resistance that can be cut to form a fan, bucket, chute or other processing equipment. This would require a joining material for the carbon steel base, then an overlay of the joint with a weld of similar properties to the weld overlayed plate. It may involve attaching a Hadfield Mn impact casting to a carbon steel or alloy steel base material.

Always consider the base metal and know what the hardfacing weld metal will be applied to. For alloy steels such as 4130, 4340, preheat is often required to prevent heat-affected zone cracking. The amount of preheat is also dependant on the thickness of the base metal. Also consider if the hardfacing is compatible with the base metal. Will an undesirable alloy such as a weak or brittle alloy be formed? An intermediate layer may be needed to counteract this prior to applying the hardfacing alloy.

Wear Resistance
Wear resistance is often classified as impact or abrasion resistant. For impact, the high manganese alloys are used. In order for these alloys to function as designed, there must be enough impact to work-harden the weld metal. A 500°F maximum interpass temperature must be maintained when welding the high manganese alloys to prevent embrittlement of the base casting and the weld metal. If the impact is low, a single layer of an abrasion-resistant alloy such as FeCr-A3A should be applied to allow time for the manganese weld metal to work-harden and provide wear resistance.

Abrasion-resistant alloys contain large amounts of carbides as the primary wear resistant particle in the microstructure. The primary carbides include chrome, tungsten, molybdenum and niobium. Additions of boron and the formation of iron borides, chrome borides and boron carbides further increase the abrasion resistance of these alloys. One method of classifying the abrasion resistance is to measure the volume loss using ASTM G-65. It is used to measure the amount of volume loss in a weld deposit by feeding sand between the weld sample and a rotating rubber wheel in a controlled test. Figure 1 shows many of the common alloys and the associated volume loss with each. During welding, many of these alloys will cross-check about every half-inch to one-inch along the bead. The cross-check acts as a stress relieve to keep the weld from pulling away from the base and spalling off.

Volume Loss, Weld Deposit (ASTM G-65, method A)

Cost
Cost is always a consideration of the alloy used for hardfacing. How long does it really have to last? Many applications can be rewelded multiple times during the life of the part. It may be a once and done. Generally the greater the wear resistance, the higher priced the product is due to a greater amount of alloy in the hardfacing product.

As in all welding processes, consideration must be given for smoke and fume collection. Guidelines and limits of the welding fumes are available from the MSDS and the American Welding Society. Weld metals are now available that are chrome free eliminating the concerns of Hexavalent Chromium originating from the weld metal. The chromium free alloys work in many of the same applications of the standard chromium carbides but do not have the corrosion resistance of the chromium containing alloys.

Gases and Welding Distributors Association
Dave Kiilunen Dave Kiilunen is a metallurgist and vice president of Cor-Met Inc., located in Brighton, Michigan, and at www.cor-met.com.