Cobalt-based Stellite 6 has been the workhorse for providing improved wear resistance and service life in valve components installed in power generating facilities for over 75 years. However, documented failures at combined cycle and supercritical power stations suggest that new materials and new (or refined) hardfacing processes are needed as the industry continues to move toward the higher operating temperatures in ultrasupercritical (USC) and advanced ultra-supercritical applications (A-USC).
Editor’s note: Kennametal, Velan and Carleton University formed a research team to evaluate new hardfacing alloys and application processes for advanced ultra-supercritical applications. This article is based on what that research team has discovered.
This article provides an overview of work done to evaluate suitable alternatives to Stellite 6. In addition, it identifies improved cobalt-based alloys, particularly Tribaloy T-400C, and a newly developed alloy, CoCrMoSi, which, when applied using innovative hardfacing processes, could guard against premature failure and increase the service life of components in demanding USC and A-USC applications.
High-temperature valves isolate, relieve pressure and/or control flow for the systems in which they are installed, serving as pressure vessels for the flow of liquids, gases and slurries. Steam valves, for example, regulate the flow and pressure levels of steam and heated water vapor.
Cobalt-based hardfacing alloys such as Stellite 6 (Co-29.0%Cr-4.5%W-1.1%C) are often used on seating faces to minimize wear and prevent both seizing and galling. The Stellite hardfacing overlays provide excellent mechanical wear resistance and good corrosion resistance at temperatures up to 1200°F (649°C). At higher temperatures for USC and A-USC applications (which can exceed 1300°F or 700°C), the hardness of Stellite 6 begins to diminish to a point where it is not sufficient to maintain adequate wear resistance (Figure 1).
Because of this, cobalt-based Tribaloy alloys and newly developed cobalt-based alloys are becoming increasingly popular. These alloys consist of cobalt, chromium and molybdenum, and they tend to stand up better under high abrasion and high corrosion applications. They are typically used in valves on the high wear surfaces such as stems, balls, seats, gates, discs, plugs, some bushings and possibly some of the flapper valves.
Cobalt-based alloy Tribaloy T-800 (Co-17.5%Cr-28.5%Mo-3.4%Si) is strengthened by intermetallic Laves phases and is believed to be stable up to 2240°F (1230°C). One consideration with this material is that it is hard to modify the microstructure by subsequent heat treatment after casting or making a hardfacing deposit1,2. The melting point of Tribaloy T-800 is about 100°F (40°C) higher than that of Stellite 6.
Figure 2 demonstrates that Tribaloy T-800 maintains higher hardness at elevated temperatures. The hardness (about 300 Hv) of T-800 at about 1400°F (760°C) is about the same hardness level of Stellite 6 at 800–1000°F (427–538°C). This indicates that Tribaloy T-800 and similar alloys could be good hardfacing alloy alternatives for USC and A-USC applications. However, while the T-800 provides excellent wear and corrosion resistance, it is extremely brittle and the welding of T-800 hardfacing overlays is very difficult.
In high-temperature applications, excessive oxidation may result in binding of moving parts. Lack of ductility often results in cracking. Therefore, it is advantageous to have access to a Laves phase alloy with enhanced oxidation resistance and ductility (Figure 3)3,4. (We performed thermal gravitational analysis at 760°C for 200 minutes to evaluate the oxidation resistance3).
An improved cobalt-based alloy Tribaloy T-400C (Co-14.0%Cr-26.0%Mo-2.6%Si) is more oxidation resistant than Tribaloy T-8003. T-400C is also less brittle than T-800, so the weldability of T-400C is improved. Fourteen coatings were tested to evaluate the steam oxidation and solid particle erosion5,6. Tribaloy T-400C was the only coating to perform well in oxidation and erosion at 1400°F (760°C). Based on this data, T-400C is considered a better hardfacing alloy alternative versus T-800.
A-USC applications need a more ductile and oxidation-resistant Tribaloy-type of alloy than that provided by available Laves phase T-800 and T-400C alloys because insufficient oxidation resistance and ductility can cause defects in welding or casting. An alternative hardfacing alloy that has high microstructure stability, high melting point and high hot hardness while maintaining excellent oxidation resistance, wear resistance and a weldability similar to (or better than) Stellite 6 and Tribaloy T-400C would be highly desirable. Such a new alloy is currently being evaluated. By modifying the composition of Tribaloy T-800 and Tribaloy T-400C, the new alloy (CoCrMoSi) is designed based on an electron vacancy calculation scheme known as PHACOMP (an acronym for phase computation).
The average electron hole number, Nv, is given as follows:
Nv = ∑(xiniv), where xi is the atomic fraction of element i and niv is the electron hole number of element i. Elemental values of niv for the Co-Mo-Cr-Si alloy system have been documented in literature7.
We determined that for cobalt-based alloys, the critical electron hole number is 2.74, above which the alloy composition would develop several intermetallic phases. Cr, Mo and Si have an adverse effect by increasing the electron hole number. For Laves-phase-strengthened cobalt alloys, the Cr level must be high for corrosion resistance; high Mo level is provided in the alloy to impart wear. Si is provided to enhance wear resistance in combination with Mo. However, higher Cr, Mo and Si contents result in a higher electron hole number, facilitating the formation of intermetallic phases producing a more brittle alloy. Therefore, the new alloy is based on the average electron hole number calculation required to balance wear, corrosion and oxidation resistance, ductility and weldability (Figure 4).
The wear resistance of the new alloy and Stellite 6 was studied by using a pin-on-disc apparatus under dry conditions. The wear loss of the new alloy is less than the wear loss of Stellite 6 hardfacing alloy (see Figure 5, page 37). The wear resistance of the new alloy was also evaluated by conducting the block-on-ring test according to ASTM G77 (Figure 6). The new alloy performs much better than Stellite 6, and also outperforms Tribaloy T-800 at higher load levels (150 and 210 pounds). The results lead to a conclusion that Tribaloy T-400C and the new alloy can be used as hardfacing alloys for USC and A-USC applications.
Table 1 summarizes the characteristics of the cobalt-based hardfacing alloys discussed in the work featured for this column.
Gas tungsten arc welding (TIG) and plasma-transferred arc welding (PTA) are the most common hardfacing processes for depositing cobalt-based alloys. ASME Section IX provides rules for the qualification of hardfacing procedures, and for the performance qualification of hardfacing welders and welding operators. Hardfacing overlay is considered a special process as defined in QW-251.4, with separate essential and non-essential variables in the applicable process tables of QW-250. Keys to success in welding hardfacing overlays include:
- Effective maintenance and soak of adequate preheat
- Not exceeding qualified interpass temperature limitations
- Adherence to qualified heat input parameters
- Slow cooling of completed components
All of the Stellite and Tribaloy alloys are a challenge to weld. The reason is that the large volume fraction of alloy carbides and Laves phases that make them so wear-resistant also make them highly intolerant of thermal stresses.
Meanwhile, over the past 10-15 years, the power generation industry has seen renewed concern about hardfacing applications because of a rise in in-service failures, including extensive cracking, disbonding, and even liberation of cobalt-based hardfacing in high-temperature valves8-11.
Evaluations of service history and failed components have led to an understanding that metallurgical changes within the microstructure during welding and high-temperature service exposure contribute to failure of the cobalt-based alloy hardfacing deposits. Cracking has been shown to be related to the formation of the brittle intermetallic Co-Fe sigma (σ) phase caused by high iron dilution. Multiple hardfacing layers, combined with a buffer layer, are often used to address the high dilution problems.
Various materials such as Stellite 21 and Inconel 625 are being used as buffer layers with inconsistent results in terms of Stellite 6 alloy hardfacing failures. Multiple hardfacing layers can achieve low dilution, which is typically contractually required for nuclear and defense applications. Table 2 presents the dilution for a typical setup of the TIG and PTA hardfacing processes. In third layers, the dilution can be controlled at a 5% level. Recently, a new process called Ultraflex has been introduced as a hardfacing alternative to TIG and PTA processes. One layer of Ultraflex can control the dilution to below the desired 5% level12.
Ultraflex is based on a powder metallurgy process where coating or hardfacing is applied to substrates in a “green” state and then heated to fuse the coating into a dense, uniform, metallurgically bonded layer with almost no porosity. In the Ultraflex process, carbide and metallic powders are mixed with a liquid medium to form a slurry or paste. This slurry can be applied to parts either through mechanical means such as dipping, brushing and troweling or by flow-coat methods. Once the slurry is applied, it is then dried to remove the excess moisture. Multiple coats can be applied to achieve the desired thickness of the final coating, typically in the 0.005–0.030 inch (0.12–0.75 mm) range. Once the coating has dried, the entire part is then heated in a vacuum furnace to fuse the coating. Figure 7 (see page 39) shows a typical Ultraflex cobalt-based alloy coating buffer layer on which Stellite 6 and other cobalt-based hardfacing alloys may be deposited by common hardfacing processes such as TIG and PTA. Figure 8 shows a wedge valve hardfaced with the cobalt-based alloy. In Figure 9, the new alloy is deposited by PTA process and the sound microstructure is shown.
Wear resistance and service life can be significantly improved in USC and A-USC applications for valve components hardfaced using the T-400C or the new alloy (CoCrMoSi) and applied with the Ultraflex, TIG and/or PTA processes.
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- A. Halstead, R.D. Rawlings, Structure and Hardness of Co-Mo-Cr-Si Wear Resistant Alloys (Tribaloys), Metals Science 18 (1984) 491-500
- J.B.C. Wu, M.X. Yao, Wear-resistant, corrosion-resistant cobalt-based alloys, U.S. Patent No. 6,852,176, Feb 8, 2005
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- M. Yao, R. Collier and D. DeWet, Dilution Control in Hardfacing Severe Service Components, U.S. Patent No. 8,828,312, Sep. 9, 2014