RA330© was developed over 60 years ago by Rolled Alloys and continues to be used for a multitude of heat-treating applications.
Many factors go into the selection of this alloy, including availability, versatility to function and resistance to a range of oxidizing and reducing atmospheres common in heat treating. RA330 possesses reasonable creep strength at temperatures up to 1800°F (982°C), good oxidation resistance up to 2100°F (1150°C) and is economically feasible. Since it is a wrought alloy, it is weldable even after extended use at temperatures. As a result of all of these features, RA330 is a great choice for a wide variety of operations, particularly when versatility and ability to function in a variety of environments and processes is important. RA330 is often the alloy of choice for retorts, muffles, fixturing and baskets. Due to controlled chemistry and proprietary heat treatment, RA330 outperforms the generic 330 versions.
RA330 is a 35% Ni-19% Cr alloy with an addition of Si and the balance iron, as shown in Table 1. The nominal 1.25% Si addition enhances all of the previously mentioned properties.[1,2]
Physical Properties and Attributes
Table 2 shows the physical properties typical for RA330.
Table 3 compares creep strengths of various heat-resistant alloys. RA330 has moderate creep strength up to 1800°F. In general, it outperforms all stainless steels except RA 253 MA® and the higher-carbon variants of 304 and 316. In general, nickel alloys except alloy 600 outperform RA330 in creep strength. The stainless steels with lower creep strength and an inability to resist surface heat treatments over long periods of time preclude their use in many heat-treating applications. Higher-nickel alloys will also carburize over time, and the cost differential drives the selection of RA330 for many heat-treating applications.
No Sigma-Phase Formation
With a nominal nickel of 35% and a moderate level of chromium, the nickel content is sufficient to prevent the formation of harmful sigma phase that completely embrittles many stainless steel alloys, even 310. While sigma-phase embrittlement does not adversely affect material at operating temperatures, sigma phase can render stainless steels quite brittle at room temperature. Sigma phase does not precipitate in RA330.
Resistance to Surface Heat-Treat Atmospheres
Carburization resistance in an alloy is conferred by the protective oxide and the nickel content.[3,4] The primary oxide scale is chromia. Silicon gives a potent boost to the carburization resistance. It should be understood that the operating phrase is resistance, not elimination. There is still iron in the alloy that will absorb the carbon and carburize.
The nickel lowers the solubility of carbon in the alloy so that a very high-nickel grade does not carburize to the same level as a lower-nickel alloy. Nickel provides similar resistance to nitriding for the same reasons. Figure 1 shows a carburized section of RA330 round bar from a wire bar basket after significant use in carburizing.
Thermal Fatigue Resistance
Thermal fatigue is cracking, which occurs from the repeated subjection of metal pieces to heating and cooling cycles. Heat-resistant alloys have high coefficients of thermal expansion but low coefficients of thermal conductivity. Stated simply, this means that the metal does not heat up uniformly during heating. The surface is at temperature a lot faster than the center.
Likewise, the surface is at quenchant temperature faster than the center during cooling. That in turn means the center is strained because the metal near the surface is growing during heating but restrained by the cooler center, which is not expanding at the same rate. Also the surface is contracting faster than the center during cooling, thus exerting a compressive stress on the center.
At some point this repetition of tensile and compressive strain will cause cracks to develop in the interior of the material. The higher the nickel content, the more pronounced the effect. However, the higher-nickel alloys generally have more creep strength than stainless steels. The most effective way to avoid thermal fatigue cracking is to design thinner sections. Figure 2 shows the effect of thickness.
While stainless steels would not exhibit as much stress because of higher coefficient of thermal expansions, they do not have the resistance to surface heat treating, as previously described, and therefore cannot be used in applications involving these processes. RA330 is the lowest-cost nickel alloy and is the best compromise between creep strength and the ability to resist carburizing and nitriding.
In summary, RA330 exhibits good properties for most heat-treating applications. The combination of cost multiples of the higher-cost nickel alloys and reasonable performance in terms of no sigma phase, resistance to surface treatments and acceptable thermal fatigue resistance leads many to use RA330 for heat-treating applications because it optimizes the competing factors of cost versus benefits.
Resistance-Welded Wire Bar Baskets
This is the most common application for RA330. A typical wire bar basket is shown in Figure 3. These are the workhorse for fixturing in many heat-treating facilities. They are used for multiple processes, including austenitizing and quenching, tempering, carburizing, nitriding and more. In facilities that do not surface heat treat, RA330 baskets have been observed to last 10 years and longer. Since the material is wrought, it can be straightened and rewelded even after long exposures to temperature. That means that routine maintenance and straightening can keep baskets in service for years.
In carburizing processes, RA330 will carburize from the surface inward. When used primarily in carburizing processes, RA330 baskets typically have a useful life of approximately a year, depending on carbon potential. While a cast basket will better resist distortion, it has been reported that their useful life is no longer due to thermal fatigue cracking. It is also reported that many shops prefer RA330 baskets because of the ductility they retain, which is required to withstand the beating that they take from forklifts and normal handling. Cast products are notoriously brittle and cannot tolerate abuse.
Many heat-treating companies have gone to cast trays because of the higher creep strength. In doing so, ductility is sacrificed. In addition, castings are usually heavier sections in order to be of acceptable quality. Every heat-treating furnace has a weight capacity. As a result, the heavier the fixture, the fewer parts per load can be run. Therefore, cast fixtures can eat into the profitability of heat treaters because too much furnace capacity is wasted on heating fixtures instead of parts.
As a result, serpentine grids made of RA330 (Fig. 4) – as well as other alloys for more stringent operations – have been developed and are increasingly being used. The concept is that there is a series of flat and bent members being held together by passing threaded round bar through predrilled holes in the plate sections. A nut is affixed to the threaded bar end on each side and welded. These are the ONLY welds in the fixture.
The design includes allowances for a gap between the end of the nut and the plate so that each individual plate member can move freely during heat-up and cool-down. The bars and nuts are also made from RA330. Once again, because these materials are wrought, the assembly can be broken down and the individual plate sections can be straightened, and the entire tray can then be reassembled and used.
With the ability to resist both carburization and nitriding, RA330 has been a choice for many retort shells, especially in furnace systems where multiple processes are performed in one piece of equipment. RA330 can be used in retort applications up to 1800°F for these applications. One furnace manufacturer had a working life of more than 10 years in a large laboratory, pilot plant furnace that was subject to multiple atmospheres and working temperatures up to 1700°F (927°C) using RA330.
RA330 has been widely used for muffles to temperatures as high as 2100°F. While some muffle manufacturers use a higher-nickel alloy, many more use RA330 for the muffles and brace them with either a SiC hearth plate or specially designed support members. Proper support at the higher temperatures has extended working life enough that the more expensive nickel alloy 601 is not always the economical choice.
RA330 is often used for furnace fans for moderate temperature exposures because creep strength is sufficient up to 1800°F. Proper design engineering is required to ensure adequate strength for the intended operation.
Sheaths and Probes
RA330 is used for both thermowells and carbon-potential probes, where reasonable life and the ability to withstand the atmospheres are important considerations.
General Furnace Repair
RA330 often is used for repair patches in furnaces and other thermal equipment. There are even occasions where a more robust alloy might have been used by the OEM, but the repair has to be performed immediately. In such cases, material availability is the most important consideration, and this alloy can be used until the next planned shutdown where the more robust material can be fitted in. Of course, in other cases, the RA330 could be the correct material as well as the patch material.
- RA330 Data Book. http://content.rolledalloys.com/technical-resources/databooks/RA330_DB_US_EN.pdf
- James Kelly, Heat Resistant Alloys, Art Bookbindery, Canada, 2013
- Binary Alloy Phase Diagrams, Thaddeus B. Massalski, Editor, 1986, American Society for Metals, Metals Park, Ohio
- Stanislaw Mrowec, Teodor Werber, Gas Corrosion of Metals. Translation published by the Foreign Scientific Publications Department of the National Center for Scientific, Technical, and Economic Information, Warsaw, Poland, 1978