Q: Can duplex stainless steels be hardfaced without adversely affecting the base material properties?
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.
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) MR0175/ISO 15156 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.