RA 602 CA®: An Alloy for the Most Extreme Temperatures
RA 602 CA® – also known as 602 CA® and NiCroFer 6025, which are trademarks of VDM – is a relatively new heat-resistant alloy with excellent creep strength at very high temperatures and oxidation resistance up to 2250°F (1232°C).
As a result of its excellent properties and its availability in plate, sheet and round bar, RA 602 CA® is seeing increasing use in the most demanding applications, including radiant tubes, muffles for use up to 2200°F (1204°C), fixturing for tool-steel austenitizing heat treatments and other extreme high-temperature applications.
The chemistry is shown in Figure 1. This alloy utilizes the benefits of chromium, aluminum and carbon, along with microalloying using zirconium, titanium and yttrium.
The relatively high carbon content combined with the high chromium produces large Cr23C6 carbides. In addition, submicron carbides of titanium and zirconium form. Even solution annealing at 2250°F will not resolutionize these carbides, thereby imparting both resistance to grain growth and excellent creep strength. A nominal aluminum content of 2.0% allows the formation of a continuous, self-healing alumina subscale, which enhances oxidation resistance, carburization resistance and metal-dusting resistance. The reactive yttrium increases the adhesion and spalling resistance of the oxide, thus enhancing oxidation resistance.
RA 602 CA has some of the best creep and rupture properties of the nickel-based wrought alloys. RA 602 CA outperforms HR-120®, HR-160®, RA333®, 601 and other heat-resistant alloys at the highest temperatures (Fig. 2). The heat-treating industry has accepted both HR-120 and Alloy 230 as alloys for use in trays and fixtures because of their perceived high-temperature creep strength. The technical literature available online for both HR-120 and Alloy 230 only provides creep-strength data up to 1800°F (982°C) for 1% in 10,000 hours and indicates that the data is extrapolated. RA 602 CA has creep strength data for 1% in 10,000 hours up to 2100°F (1150°C).
RA 602 CA can be used in oxidizing environments up to 2250°F. The results of two recent oxidation studies are shown in Figure 3. Alloy 230 was not available for these studies.
Metal dusting (also known as catastrophic carburization or carbon rot) is a phenomenon in which a high carbon potential renders the metal into graphite and metal at the surface. This in turn leads the metal to thin and disintegrate. Metal dusting typically occurs from 800-1200°F (427-650°C). A decade-old study (Table 1) shows RA 602 CA to be an excellent alloy to resist this phenomenon. The test conditions are 1200°F, with an atmosphere of hydrogen, carbon monoxide and water vapor in equilibrium to achieve a carbon potential significantly higher than 1.0%. Note that this alloy was in the black condition. Alloy 601 was surface-prepared in three ways, and the black condition yields the worst results. The result of a second study, conducted in the metal-dusting zone of a carburizing furnace, is shown in Table 2. In this study, 1-inch schedule-40 oxygen probes of various alloys, and in some cases multiple surface conditions, were subjected to a 1.2% carbon potential at 1700°F so that the pipe is only at approximately 1100°F (593°C) where it exits the refractory.
Historically, many consider RA333 to be the best choice for carbon-dusting resistance. In recent years, the availability of RA333 has become more limited. It is only readily available in plate, so efforts have been made to identify a suitable alternative. In the last several years, a major effort to identify such an alloy was undertaken by a major manufacturer of capital equipment in their heat-treating facility. Only two suitable alternatives were found: alloy 625, which is limited to a maximum furnace temperature of 1800°F because of its oxidation limit; and RA 602 CA, which has a much higher limitation and is thus the alternative of choice.
In addition to metal dusting, alloys used for components in the carburizing heat-treat process, such as fixtures and grids, are chosen for their ability to resist carburization because their carbon contents are also lower than the carbon potential of the process. It is well known that nickel has a very low solubility for carbon.[2,3] Therefore, alloys higher in nickel should be beneficial for carburization resistance.
In addition, Cr, Si and Al are all oxide formers, and continuous oxides of these elements lower the diffusion of carbon into the base metal. When alternating between oxidizing and carburizing conditions, however, internal carbides are converted to oxides, liberating CO, widening the grain boundaries and loosening the oxide so that the carburization resistance can be diminished. RA 602 CA with a combination of high nickel and a thermodynamically stable alumina sub-layer makes this alloy more effective for carburization resistance (Table 3).
Euclid Heat Treating in Euclid, Ohio, agreed to a trial of a three-legged configured radiant tube using RA 602 CA, with a 0.120-inch wall thickness. This replaced their 0.375-inch wall cast HX tube, which was typically obtaining four years of maximum service. The carburizing furnace was in continuous operation at 1750°F (954°C) at carbon potentials of 1.1-1.6. With proper maintenance, including burner alignment and weak carbon burn-off, they were able to achieve a tube life of close to nine years.
A photograph of the tube assembly after seven years in the furnace is shown in Figure 4 (intro). The inital cost per tube is significantly more than the cast tube it replaced – in this case somewhat more than 50%. However, the lifetime costs are significantly less using RA 602 CA. For a 50% higher cost, there is a 100% minimum increase in usable life. The real cost justification is the entire replacement cycle, which for a typical heat treater can be anywhere from three to five days to cool down the furnace, evacuate all the atmospheric gases and deem it safe for maintenance workers to enter, swap out the tubes and reheat the furnace. The revenue lost due to this downtime is the most significant cost for most heat treaters.
RA 602 CA is proving to be a suitable alternative for muffles that need to operate at the most extreme temperatures (2150-2200°F). One heat treater of tool-steel parts replaced a European high-nickel alloy muffle with RA 602 CA. At the time of writing, this heat treater has already achieved a two-year muffle life, with no sign of roof or side sag. The temperature of the process is 2150°F operating with a 100% nitrogen atmosphere. The prior muffles were made of 2.4879 or G-NiCr28 W. These muffles were failing between 1 and 1.5 years either from weld cracking or excessive creep. Based on the fact that there is no sag yet, the anticipation is that this muffle should last at least another year.
As a result of the high creep strength of this alloy, RA 602 CA is quite suitable for fixturing in many applications, including vacuum heat treating. In one application at a commercial heat treater, a serpentine grid constructed from RA 602 CA reported use exceeding 2,000 cycles. This grid was designed to support up to 2,000 pounds at temperatures of 2175-2260°F (1191-1238°C) and be subjected to quenching in a 6-bar nitrogen quench in a vacuum furnace. This particular tray was fabricated from 3/16-inch-thick plate to a grid dimension of 36 inches x 54 inches. A photograph of this particular grid is shown in Figure 5.
Similarly designed serpentine grids were fabricated for a heat treater that performs copper brazing of stainless steel heat exchangers. For this application both 36-inch x 48-inch and 36-inch x 72-inch grids have exceeded three years of service supporting 600-650 pounds at operating temperatures of 2045°F (1118°C). This alloy has replaced both 600 and molybdenum. As a result of its creep strength, such fixturing allows a 7-20% weight reduction compared to 600 and molybdenum, allowing either energy reduction or increased efficiency.
RA 602 CA was used to fabricate a wire bar basket for the heat treatment of high-speed tool-steel bits up to 2150°F. A typical hardening cycle is 20-45 minutes, followed by a 2-bar nitrogen quench, finishing with a 1050°F temper. A basket is used up to six cycles per day. The basket used 3/8-inch-diameter round bar. Some straightening is required with time, but the frequency and severity has been significantly reduced compared to the previous baskets using alloy 600. A photograph of these baskets after 500 cycles in 90 days is shown in Figure 6.
RA 602 CA was used to replace a high-temperature retort originally made of alloy 600 for a CVD aluminizing process with a process temperature of at least 2100°F and retort temperature approaching 2200°F. This new retort gives a six-fold increase in usable life of up to one year compared to alloy 600.
In the original trial, the customer agreed to construct the retort of some 600 channels and some RA 602 CA channels on the top and run at a process temperature of 2000°F (1093°C). This retort is partially shown in Fig. 7. After several months, the 600 channels warped and thinned from oxidation, and the RA 602 CA channels appeared almost new. This processer has switched alloys to RA 602 CA for all the retorts.
Additional Areas of Interest
As stated earlier, RA 602 CA exhibits excellent resistance to carburization, which would make it seem suitable for use in baskets to be used in carburization processes. With the higher carbon, however, it is unclear as to how a welded basket will respond to cyclical quenching. A trial is being conducted to study this under real conditions, with a resistance-welded sample welded to a new basket.
- D. C. Agarwall & U. Brill, “Performance of Alloy 602CA (UNS N06025) in High Temperature Environments up to 1200°C,” Corrosion 2000, Paper number 00521, NACE International, Houston, Texas, 2000
- D. C. Agarwall, U. Brill & J. Kloewer, “Recent Results on Metal Dusting of Nickel Base Alloys and Some Applications,” Corrosion 2001, Paper number 01382, NACE International, Houston, Texas, 2001
J. P. Kelly, Heat Resistant Alloys, Art Bookbindery, Canada, 2013
www.rolledalloys.com RA 602 CA case histories