This blog is a continuation of our series covering the wide variety of steels in the Temperform alloy portfolio. So far in this series, we have covered Stainless Steel for Heat Resistance and Austenitic Manganese Steel. Today, we will be covering another fascinating material with an extremely wide variety of applications and properties, corrosion resistant stainless steel.
The history of corrosion resistant stainless-steel dates back to the discovery of stainless steel itself back in 1910. To define stainless steel plainly, there are really only two conditions that must be met. The first condition for a material to be considered stainless steel is that it has to be a ferrous material, and the second condition is that the alloy must contain at least 12% chromium. Outside of those two aforementioned conditions, stainless steel is essentially a free for all, any variety of alloying elements can be added for any reason, and so long as those two conditions are met, it is still considered to be stainless steel. The most common other elements to be added to corrosion resistant stainless steel are Nickel, Silicon, and Molybdenum. There are no requirements as for what microstructural constituents must be included or excluded from corrosion resistant stainless steel, there are a wide variety of different options that fit different specific applications.
There are a few ways in which stainless steel alloys are identified, the designations that are most commonly used by steel foundries are the ones that were created by the Alloy Casting Institute (ACI), which are the designations that are referenced within the applicable ASTM specifications. The ACI is now known as the Steel Founders’ Society of America.
Performance and Applications
This section will be split up into different sub-categories, due to the fact that there are so many different groups of steel that still qualify as “Corrosion resistant stainless steel”, and their properties tend to vary based upon their microstructural characteristics.
Austenitic with Ferrite Corrosion Resistant Stainless Steel
This is by far the largest subcategory of corrosion resistant stainless steel; it contains many of the grades that have the best corrosion resistant properties. These grades contain some amount of ferrite, which can vary from almost none up to nearly 40% ferrite content, that being said, they remain predominantly austenitic. However, with that austenitic structure also typically comes a reduction in the strength and the hardness of the material. These grades are typically low to medium strength steel with high ductility and high toughness. These grades will vary in their level of magnetism, some of them are almost entirely non-magnetic (though never completely), and others are mildly magnetic, it depends on the residual ferrite content. A typical casting that you might see out of austenitic with ferritic stainless steel are volutes for dealing with saltwater or other corrosive materials.
Common specifications for these grades include the following:
These alloys typically do very well in resistance to acid corrosion in an oxidizing environment, the grades that contain molybdenum tend to also do very well in terms of pitting resistance. These alloys are variable in reduction environments, some of them do very well, and some of them just okay. Alloys in this group include CF8/CF8M, CF3/CF3M/CF3MN and others.
Martensitic Corrosion Resistant Stainless Steel
To sum this one up pretty straightforwardly, Martensitic stainless steel mostly exists for applications that demand high strength, but where corrosion resistance is also required. In some cases, this is as simple as ensuring that the steel does not rust, in other cases, they are alloyed up more to increase elevated temperature strength or increase resistance to specific types of corrosion. These grades are all magnetic.
These steels are typically mid to high strength, low to medium ductility, and tend to be low toughness also, though some grades do have halfway decent toughness. These grades also tend to exhibit a much higher Brinell hardness than the austenitic grades. Common steel types in this category are CA6NM, CA15, etc. These types of steels would also be used for valve bodies and volutes, but typically in situations where the pressure is much higher than what you see in the austenite with ferrite grades.
Common specifications for these grades include the following:
Precipitation Hardening Corrosion Resistant Stainless Steel
This category really only contains one material, CB7Cu, which is a copper containing precipitation hardening material. Depending on the heat treatment cycle, this material can vary from medium to high strength, but is always pretty low ductility with very low toughness. The typical specification for this grade is ASTM A-747, which is the only existing specification for precipitation hardening stainless steel. The corrosion resistant properties of precipitation hardening stainless steel are typically better than those of martensitic stainless steel, but not as good as the austenitic stainless-steel grades. Impellers are sometimes made from precipitation hardening stainless steel.
Duplex Corrosion Resistant Stainless Steel
Duplex stainless steel is a very interesting material and is worth mentioning here. Duplex stainless steels contain 40-60% ferrite, with the aim being to get as close to 50% Ferrite and 50% Austenite as possible. Duplex grades typically have better mechanical properties than the grades that are more predominately austenite, yet also have similar corrosion resistant properties. The most common specification for duplex stainless steel is ASTM A995, which covers Duplex Stainless Steel for Pressure Containing Parts. Duplex stainless steel is frequently used in the pump and valve industries to make valve bodies in high pressure applications.
Ferritic Corrosion Resistant Stainless Steel
There are only two common grades of ferritic corrosion resistant stainless steel, CB30 and CC50, which are alloys that are essentially considered to be just iron and chromium. These materials are mid strength, mid to low ductility, and very low toughness. These grades typically fall under ASTM A-743, which covers iron-chromium castings for corrosion resistant service. These grades tend to do fairly well in oxidizing corrosive environments, though not as good as many of the predominantly austenite containing grades. Temperform does not very commonly produce these materials, as they are often substituted for the Martensitic or Austenite with Ferrite grades. Components for heat exchangers are commonly made of ferritic stainless-steel grades.
Fully Austenitic Corrosion Resistant Stainless Steel
This group of materials is similar to the austenite with ferrite group, but they contain pretty much no ferrite, perhaps some trace amounts at most. For the most part, these grades are low strength, high ductility, and mid to high toughness. When compared to the austenite with ferrite grades, these alloys contain more nickel and more carbon, which further stabilizes the austenite at room temperature. These alloys typically perform better at higher temperatures than the grades that contain both austenite and ferrite, and they handle most types of acids very well. Pitting resistance in the fully austenitic grades containing Molybdenum is also very good, depending on how much Molybdenum is in the material. These materials are essentially the last stop in stainless steel in terms of the quantity of alloying elements before the materials become non-ferrous and you start to get into nickel-based alloys. Temperform frequently makes discharge and exhaust manifolds out of fully austenitic corrosion resistant stainless steel.
Castability
The castability of corrosion resistant stainless steel is fairly straightforward with the one large exception being the duplex grades (We will address some of those challenges later). In general, there are several consistent challenges, the largest one is preventing the formation of oxide films in the castings. Oxide films form at metal fronts as the casting fills, so when a casting is filled from multiple different in-gates, each metal front forms an oxide film, and when those two metal fronts meet, the oxide films are either trapped between the metal fronts or broken apart and swept up into the metal flow. This oxide film phenomenon is not entirely unique to stainless steel; however, it is the most prominent in stainless steel. In most steels, these oxide films continue to form at the metal fronts, but they are swept up into the metal flow and broken apart when the metal fronts converge. In stainless steel, the chromium and the manganese content strengthens these oxide films, which allows them to remain intact, which in turn can trap them in the castings. These oxide film defects are sometimes misidentified as cold laps or cold shot, as they have a very similar visual appearance to those defects.
Another challenge is the decreased fluidity of stainless steel when compared to low alloy steel, manganese steel, or cast iron. While cast iron behaves almost just like water in the fluid state, stainless steel has a consistency that I can best describe as being similar to that of maple syrup (maybe not quite as sticky, and definitely not as sugary). The poor fluidity of stainless-steel means that foundries must be a bit pickier when it comes to the complexity of the shapes that they cast and the thickness of the casting walls, particularly in a no-bake sand molding process like the one that Temperform utilizes.
Duplex Stainless Steel Challenges
In addition to the aforementioned challenges with all stainless steels, Duplex contains a few additional challenges, which can be extremely significant. The first large challenge with Duplex is in regard to the very low carbon content. Most grades of Duplex Stainless Steel contain a maximum carbon content of 0.04% or less (there are exceptions), which is very hard to achieve. Most foundries melt multiple different types of steel, many of which contain carbon as the primary alloying element. The presence of carbon in the furnace, even if it is not added directly to the charge (which is difficult in and of itself to deal with), will often mean that the metal bath will pick up carbon content from the furnace and/or ladle linings. To get around this issue with the carbon content, many foundries utilize a piece of equipment and subsequent process called “Argon Oxygen Decarburization” or ‘AOD’ for short. In the argon oxygen decarburization process, the metal is tapped out of the main melting furnace (usually either an electric arc furnace or an induction furnace) and is transferred to the AOD, which is a large, almost egg-shaped piece of equipment that spins on an axis. From there, a mixture of Argon, Oxygen, and sometimes Nitrogen are injected into the metal bath to intentionally form Carbon Monoxide gas, which is then removed from the metal bath. The formation of Carbon Monoxide and subsequent removal from the metal bath lowers the carbon content in the metal and allows foundries to achieve a much lower carbon content.
Another significant challenge with Duplex stainless steel is the presence of sigma iron in the as-cast state. The issue with sigma iron is that it makes the castings very brittle, which can lead to processing issues in the foundry. If you have ever been in a foundry, I am sure that you know it can be a very rough environment, and in order to successfully process Duplex Stainless Steel, you need to be VERY gentle with the castings to avoid cracking them. If you were to even drop one on the floor, that could be enough to cause a significant problem.
The last large challenge with Duplex stainless steel is the issues associated with the Nitrogen content in the material. The content of Nitrogen in the material leaves it much more prone to forming evolved gases than materials without it. This leads to a lot of care needing to be taken in the gating of the material to minimize turbulence during the fill, which can allow for the nitrogen in the metal to readily interact with the Nitrogen in the atmosphere, or the Nitrogen present in the molding materials and cause defects.
Heat Treatment
For the most part, the heat treatment of corrosion resistant stainless steel is fairly simple, it is almost always a solution anneal followed by a water quench. The temperature of the anneal varies by material, but it is almost always done in excess of 1900F. In some cases, the solution anneal is done in excess of 2100F. The purpose of the solution anneal is to dissolve the grain boundary chromium carbides, and also to eliminate the sigma iron. This is, in effect, a type of homogenization that prevents the precipitation of chromium carbides, it ensures that there is a uniform distribution of chromium throughout the solid solution.
Conclusion
Stainless steel is an extremely versatile material with a broad range of applications, though corrosion resistance is where it truly shines. Stainless steel is a material that is very simple to cast, however it takes a large amount of understanding to truly master and harness its potential. Temperform is among the best when it comes to the production of Stainless Steel, our experience is difficult to match, and our dedication to our customers is unrivaled. If you are looking for Stainless Steel castings, reach out to one of our steel experts today by clicking here.