What Is Hardfacing?

Hardfacing refers to a process of metalworking where tougher, harder material is put onto to a base metal. Generally, hardfacing takes the form of TIG or filler rod welding or arc welding. PTA or powder metal alloys are used. These are referred to as thermal spray processes, powder plasma welding systems, fuse, spray, or plasma spray.

Link to JINHUA HARDFACING

The Application of Hardfacing To New Parts

The thermal spray process can be used to refurbish a surface that is worn-down on used parts. But to increase wear resistance of new parts, during their production, hardfacing is applied. Arc welding applied versions are used to extend an industrial component&#;s life and is a surfacing operation. It can be part of a maintenance program or used preemptively on new components. The result is significant savings in production costs and lessened machine downtime. This is the main reason that the process of hardfacing has become so popular in so many industries. Industries can include sugarcane and food, power, petrochemical, mining, cement, and steel, just to name a few.

Hardfacing Materials

Materials commonly applied in hardfacing include NOREM, chromium carbide alloys, nickel based alloys, and stellite or other cobalt based alloys. To add instructional information, add color, or refinishing, hot stamping is sometimes used after hardfacing. Films or foils can be used to create additional protection or for a metallic appearance.

The Various Welding Methods Used to Deposit Hardfacing

• Hardpaint
• Laser cladding
• Cold polymer compounds
• Thermal spraying
• Powder plasma welding system or PTAW (plasma transferred arc welding)
• ESW (electro slag welding)
• SAW (submerged arc welding)
• OFW (oxy-fuel welding)
• GMAW (gas metal arc welding) &#; this includes open arc welding and gas shielded welding
• SMAW (shielded metal arc welding)

Fuse and Spray Hardfacing&#;s Advantages

Hardfacing minimizes oxidation and distortion, resists chipping upon impact, and is extremely hard. It enables easy edge surfacing, is corrosion resistant, and provides a metallurgical bond of protective surfacing to substrates.

The feedback of the service performance and the component service conditions are used to determine the optimal an alloy selection. For every phenomenon involving wear, in each and every industry imaginable, there is an appropriate welding electrode that will offer maximum resistance to wear and erosion. The process used to protect parts and machinery today were unimaginable decades ago. One can only speculate as to the future of parts, machinery, and product protection.

Hardfacing Research

New welding processes and a wide range of alloys have been the result of development and extensive research on hardfacing. An experimental study was done using GMAW & SMAW hardfacing processes. It was determined that they were highly useful in wear reduction on the moldboard plowshare. In fact, using those processes of hard facing, the plowshare lifespan was increased no less than two times.

Rest assured, A&A C has been involved in much of that research and will be involved in the future of protective coatings as well. If your industry would like to improve their bottom line by protecting parts, machinery, and the finished product, as well as lighten the load on your maintenance crew, protective coating is something you&#;ll want to look into. We are here to help. Contact us at A&A C today.

A: The issue of whether hardfacing adversely affects the properties of duplex stainless-steel base materials depends on several factors.

Hardfacing (a standard term for a wear-resistant weld overlay) is commonly used for seat and guide surfaces on valve trim of all styles. The workhorse hardfacing alloy in the valve industry is generically known as &#;alloy 6,&#; although it is often called &#;Stellite 6&#; (Stellite is a registered trademark of Deloro Stellite Company, part of the Kennametal group). This alloy is commonly specified by customers around the world on valve specification sheets. Because it is the most common, we&#;ll focus our discussion initially on hardfacing with alloy 6.

This material is a cobalt-chromium-tungsten alloy with about 1% carbon. A carbon level this high results in a microstructure consisting of a network of carbide particles in a soft matrix. The hardness can range from 35-45 Rockwell Hardness (HRC) depending on the application method and the amount of dilution (the amount of mixing with base metal). Because of the presence of the carbide network, the overlay is somewhat prone to cracking, although resistance to cracking is very good for an alloy of this hardness level.

For small, simple parts, cracking is not a usually a problem. However, in larger and more complex parts, cracking can occur from the buildup of thermally-induced stresses caused by solidification shrinkage and thermal expansion and contraction.

Increasing the preheat temperature (the minimum temperature of the adjacent metal prior to starting any new weld pass) reduces this tendency. For some base materials, this is an acceptable solution to the cracking problem. However, it is not a viable approach for hardfacing duplex stainless steels.

Because of the compositional makeup and the duplex microstructure (half austenite and half ferrite) of duplex stainless steels, these materials are highly prone to a number of phase transformations that can cause embrittlement or loss of corrosion resistance. The transformations happen at relatively low temperatures, which is why these materials as a group are limited to a maximum service temperature of 600°F (316°C) in the American Society of Mechanical Engineers (ASME) codes.

If you are looking for more details, kindly visit Hardfacing Machinery.

For this reason, some parameters for welding of duplex stainless steels are commonly recommended. These include limiting the maximum heat input (current times voltage, divided by travel speed) and interpass temperature (maximum temperature of the adjacent metal before starting any new weld pass). The use of these parameters is absolutely necessary when trying to meet the requirements specified in National Association of Corrosion Engineers (NACE) MR/ISO and Norsok M-630, as well as many end-user specifications. Even when such specifications are not imposed, these parameters should be used to avoid compromising the properties of the base material.

This is where issues arise. Small, simple duplex stainless-steel parts can be hardfaced with alloy 6 using proper duplex stainless-steel welding parameters without too much trouble. However, when parts become larger or more complex, alloy 6 will crack unless the preheat temperature is increased. If that temperature is increased to the point that cracking is alleviated, the interpass temperature required to avoid adverse effects in the duplex stainless-steel base metal will be exceeded. In other words, once the size or complexity of the part reaches a certain level, you can either have alloy 6 hardfacing without cracks, or you can have unaffected duplex stainless steel, but you cannot have both.

Some claim they can successfully hardface large, complex duplex stainless-steel parts with alloy 6 using proper welding parameters, but they are likely misapplying the interpass temperature rule by assuming that each layer produced on the inside or outside diameter of a part by a spiral weld process is a single &#;pass.&#; That is not the case&#;one revolution in such a process is one pass, and the weld progression must stop any time the temperature ahead of the pass exceeds the interpass temperature, even if the layer is not complete. If improper practices are followed, the &#;interpass temperature&#; isn&#;t measured until after the completion of the layer, which results in the part preheated excessively by the welding heat input. This excessive preheat prevents cracking, but it also adversely affects the properties of the duplex stainless-steel base material. Unfortunately, this degradation of the base material isn&#;t outwardly apparent, and it may not manifest itself until the component is in service.

The way to solve the cracking problem while maintaining the proper duplex base material properties is to use a hardfacing alloy that has enough ductility to be applied without cracking and to use the appropriate welding parameters. Two hardfacing alloys that exhibit such ductility are alloy 21, commonly called Stellite 21, and Ultimet (Ultimet is a registered trademark of Haynes International). These alloys have far less carbon than alloy 6 (nominally 0.30% and 0.06%, respectively). As such, they provide much better ductility and resistance to ­cracking than alloy 6.

Even though these alloys are both inherently softer than alloy 6, they still provide excellent resistance to sliding wear, galling, liquid flow erosion and cavitation damage because they are cobalt-based. Because of its inherently corrosion-resistant composition (Co-26Cr-9Ni-5Mo-2W-0.08N), Ultimet has the added benefit of providing corrosion resistance on par with the superduplex stainless steels and superaustenitic stainless steels in many environments, particularly seawater and other high-chloride environments.

Don Bush is a principal materials engineer at Emerson Process Management&#;Fisher Valve ­Division (www.emersonprocess.com). Reach him at .

RELATED CONTENT

• Cobalt-based Alloy 6 Materials and Boiler Feedwater Service

  Q: I&#;ve heard that cobalt-base Alloy 6 materials should not be used in boiler feedwater service.

• NACE MR Material Compliance

  Q: I have a material certified to be compliant with NACE MR.

• Valves in Space

  All of these valves need to be built to precise fits and finishes and to stringent performance requirements because &#;On a spacecraft, everything has got to work. If it breaks, you&#;re done.&#;

  The company is the world’s best china welding flux mixer factory supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.