Stainless steel is the most common material used in investment casting; it has many types, and austenitic stainless steel is the most widely used. If you’ve ever wondered why choose this material during the procurement process, this article will explain everything you want to know about austenitic stainless steel and answer all your questions.
What is Austenitic Stainless Steel
Austenitic stainless steel is a high-alloy steel containing various alloying elements that exhibits an austenitic microstructure at room temperature, meaning its internal atomic arrangement forms a face-centered cubic (FCC) crystal structure. Knowing its appearance, composition, and type is helpful, as it will help you get a general understanding of austenitic stainless steel.
Appearance
When no surface treatment is applied, the surface of austenitic stainless steel ranges in color from silver-white to matte gray and appears brighter than ordinary carbon steel or cast iron.
Composition
Austenitic stainless steel typically consists of iron combined with various chemical elements. Chromium is the most critical element, usually accounting for 16%–26%, and is essential for corrosion resistance. Next is nickel, which typically accounts for 6%–22% and stabilizes the austenitic structure. 2%-3% molybdenum is also commonly added, which is key to resisting pitting corrosion. In addition, trace elements such as manganese, titanium, silicon, and phosphorus are often included.
Type
Austenitic stainless steel is classified into the following categories based on the content of its chemical elements. Knowing these categories will help you choose the appropriate stainless steel for your component.
- 304（CF8）
304 stainless steel has a chemical composition of 18% chromium and 8% nickel. It is the most cost-effective and widely used austenitic stainless steel. 304 stainless steel castings are suitable for applications in food processing, medical devices, construction, and the pump and valve industries.
- 316(CF8M)
316 stainless steel contains 2%–3% molybdenum compared to 304 stainless steel, which enhances its resistance to pitting corrosion. 316 stainless steel castings are suitable for marine environments, chemical piping, high-salinity food processing, and water purification.
- 316L(CF3M)
316L stainless steel is a grade of 316 stainless steel with less than 0.03% carbon. It addresses the susceptibility of 316 stainless steel to intergranular corrosion, offering superior corrosion resistance. 316L stainless steel castings are suitable for a wide range of industries, including marine engineering, the pump and valve industry, the chemical industry, and pharmaceutical machinery.
- 347
347 stainless steel contains less than 1.1% niobium, which enhances resistance to intergranular corrosion and maintains mechanical strength under high temperatures. It is primarily used in engine exhaust manifolds, heavy industrial boiler components, heat exchangers, and aerospace castings.
- 904L
904L stainless steel contains 20% chromium and 25% nickel, with an additional 2%–3% copper. With the addition of high chromium, nickel, and copper, it has exceptional corrosion resistance. However, due to the high cost of raw materials, it is primarily used in the petrochemical industry, seawater desalination, and environmental protection.
Advantages& Disadvantages
As the most widely used type of stainless steel, austenitic stainless steel offers numerous advantages, including high toughness, corrosion resistance, low-temperature resistance, and non-magnetic properties. These characteristics make it suitable for most applications. However, it still has many drawbacks. By understanding both its strengths and weaknesses, you can use austenitic stainless steel appropriately in the right applications.
Advantages
- High toughness
The high toughness of austenitic stainless steel is primarily due to its face-centered cubic (FCC) crystal structure. This structure allows the material to absorb energy and deform when under external or tensile forces, thereby dissipating impact and extending the service life of your components.
- Corrosion Resistance
Austenitic stainless steel offers exceptional corrosion resistance and can withstand exposure to many corrosive media. This advantage makes your components suitable for a wide range of environments where they must operate in corrosive conditions, such as marine engineering and chemical engineering.
- Low-temperature Resistance
It also has excellent low-temperature resistance; this advantage could ensure your components continue to function properly without fracturing even in extreme environments at ultra-low temperatures, even at -269°C.
- Non- magnetic
Austenitic stainless steel also has an advantage that many other materials lack: it is generally non-magnetic in the fully annealed state. This advantage ensures that it will not interfere with the operation of your equipment, which makes your components suitable for a wide range of applications that require non-magnetic parts.
Disadvantages
- Not wear-resistant
Austenitic stainless steel has very low hardness in the solution treatment, typically only 150–200 HB, and is highly susceptible to wear when it contacts other metals. This disadvantage makes it unsuitable for applications involving purely mechanical high-speed scraping, such as gears.
- Low hardness
Because its hardness is far inferior to that of martensitic stainless steel, it is not suitable for applications requiring high wear resistance and extreme surface hardness, such as high-load drivetrain components. However, it remains widely used in other areas of the automotive and construction industries where components need corrosion resistance.
- High coefficient of thermal expansion
Compared to other stainless steels, austenitic stainless steel has a relatively high coefficient of thermal expansion(18.2 × 10⁻⁶/°C). This will cause significant thermal stress to build up inside the casting as it cools. If your component is designed with abrupt changes in wall thickness, this makes it highly susceptible to thermal cracking and shrinkage cavities at the joints. This characteristic makes it unsuitable for working environments that require rapid cycling between high and low temperatures, such as heat treatment quenching fixtures and high-temperature steam pipelines subject to frequent start-stop cycles.
Manufacturing Process
Austenitic stainless steel offers exceptional corrosion resistance, but its high work hardening, structural complexity, and the need for high surface finish often result in high processing costs. Using the investment casting process can significantly reduce these troubles.
Near- Net Shape(NNS)
Austenitic stainless steel, particularly 316 or the low-carbon 316L, exhibits work-hardening properties, that is, the more it is machined, the harder it becomes, which significantly increases manufacturing costs. However, investment casting is renowned for its near-net-shape technology, which allows components to be cast directly to match the final product, eliminating the need for extensive post-processing and greatly reducing machining time and costs.
Integral Casting
Investment casting is a one-piece casting process that allows for complex designs to be created directly within the mold cavity, enabling intricate internal channels and thin walls to be cast as an integral piece. This eliminates the need for complex post-casting machining, enabling the production of austenitic stainless steel castings.
Surface Finish
Since austenitic stainless steel is widely used in the medical, food, and marine industries, all of which require castings with exceptionally high surface quality. Under such requirements, investment casting is an ideal process. This is because it employs metal molds with high surface precision, which achieve excellent investment casting surface finish roughness of Ra 3.2μm to Ra 6.3μm. Subsequent processes, such as Barrel finishing and passivation, are used to further enhance surface finish.
Property
The properties of austenitic stainless steel can be divided into mechanical and chemical properties. Understanding these properties will help you know why austenitic stainless steel is the best choice for you.
Mechanical Properties
- Non-magnetic
To prevent interference during equipment operation, components are typically made of non-magnetic materials; austenitic stainless steel is one such material. This is because its internal austenitic structure is non-magnetic.
However, if the component undergoes machining or vigorous grinding, part of the internal austenitic structure will transform into martensite, resulting in slight magnetic properties. But the component can be restored to its non-magnetic state through re-heat treatment.
This non-magnetic property ensures that the components don’t interfere with other components during use. For example, it ensures the safety and image clarity of medical devices, as well as zero interference with the signals of precision instruments.
- Low Temperature Resistance
In many martensitic stainless steels or carbon steels, the inherent toughness of the material drops precipitously under low-temperature impact, making them highly prone to cracking. This is due to their “cold brittleness.”
However, austenitic stainless steel, due to its face-centered cubic (FCC) crystal structure, does not exhibit this characteristic. It maintains its extremely high toughness even in extreme environments ranging from -196°C to -269°C, particularly the 316L grade.
It is suitable for applications in industrial refrigeration, gas liquefaction, and deep-space exploration, such as high-pressure cryogenic pump housings, structural components for liquid helium cooling systems, and liquid oxygen engines.
- High Tensile Strength
If a component has low tensile strength, engineers must increase its wall thickness during the design process, which significantly drives up costs. Austenitic stainless steel, however, offers high tensile strength, typically ranging from 500 to 700 MPa at room temperatures. When cold hardening, this strength can even surge to over 1,000 MPa.
This capability allows for the direct use of thin-walled designs, thereby reducing material costs. High tensile strength also reduces the risk of valve bodies and pump housings cracking under impact, thereby extending the service life of the components.
Chemical Properties
- Comprehensive corrosion resistance
When austenitic stainless steel is exposed to air or an oxygen-containing environment, the high chromium content in the steel reacts chemically with oxygen, forming a dense, thin layer of chromium oxide on the surface of the component, which is known as a passivation film. This film protects the part from corrosion by external corrosive agents. Furthermore, as long as oxygen is present, the steel can absorb oxygen to repair itself if the surface is damaged.
- Pitting resistance
Although the presence of chloride ions in seawater can corrode the passivation layer, adding 2%–3% molybdenum to austenitic stainless steel can significantly enhance the stability of the passivation layer in chloride-ion environments. This is why 316 stainless steel, a type of austenitic stainless steel, is commonly used for castings such as stainless steel pump castings, valve castings, and impeller castings in seawater engineering projects.
- Intergranular corrosion resistance
When austenitic stainless steel is held at temperatures between 427°C and 816°C, the carbon within the material may react with chromium to form chromium carbide, a process known as sensitization. Once sensitization occurs, intergranular corrosion will develop at the edges of the crystals due to a lack of chromium. However, adding titanium or niobium to austenitic stainless steel, such as 316L and 347, could cause these elements to combine with carbon first, reducing the possibility of intergranular corrosion.
- Strong acids and alkali resistance
Due to the synergistic effect of high chromium and high nickel content, austenitic stainless steel is capable of withstanding corrosion from strong acids and alkalis. 904L stainless steel is particularly outstanding in this regard, and its copper content results in an extremely low corrosion rate during exposure to strong acids and alkalis. It exhibits near-immunity to strong alkali solutions such as sodium hydroxide and potassium hydroxide at both room and high temperatures, making it commonly used in valves and pumps for alkali transport in chemical plants.
Application
Austenitic stainless steel has an extremely wide range of applications. Understanding these application areas can help you save time and quickly select the parts best suited for your industry.
Food & Medical Facilities
Food and medical facilities require components that are completely free of contamination and pose no health risks. Ordinary steel is highly prone to leaching iron ions into liquids when exposed to liquid flow. However, stainless steel is globally recognized as a sanitary-grade metal, with austenitic stainless steel standing out as the best of the bunch. This is because it forms a passivation layer on its surface, exhibiting extreme chemical inertness at room temperature and never leaching any metal ions into liquids.
Additionally, its non-magnetic properties offer significant benefits to the medical industry. For example, in MRI equipment, using components made from completely non-magnetic austenitic stainless steel, such as 316L, could ensure that the images produced in hospitals are exceptionally clear.
Chemical Facilities
In the chemical industry, chemical processing equipment, particularly piping systems, is exposed to corrosive liquids such as strong acids and alkalis daily. Ordinary steel quickly corrodes and fractured when subjected to these corrosive liquids. However, austenitic stainless steel possesses surface passivation properties; the passivation film formed on its surface can withstand corrosion from strong acids and alkalis, and it can even repair itself.
Furthermore, the stable face-centered cubic crystal structure within austenitic stainless steel ensures that components made from it do not suffer from stress corrosion cracking even under high pressure and corrosive conditions. This resistance is particularly pronounced in 316L, which contains an extremely high nickel content.
Pump & Valve Facilities
Pump and valve equipment often operates 24 hours a day in environments characterized by high pressure, high flow rates, and highly corrosive liquids. Ordinary steel is highly susceptible to corrosion and cracking when subjected to such high-stress, highly corrosive conditions.
In contrast, austenitic stainless steel possesses exceptional tensile strength and toughness. Even when subjected to intense erosion, castings made from this material, like valve body castings, can absorb energy through slight elastic deformation, thereby preventing the possibility of sudden component failure.
Furthermore, austenitic stainless steel offers near-net-shape capabilities in the investment casting process, allowing for the production of components with excellent surface finish and complex structure. This eliminates the need for extensive post-processing, significantly reducing manufacturing costs.
Final Thoughts
We believe you already have a good understanding of the composition, types, properties, and applications of austenitic stainless steel. If you are looking for a reliable austenitic stainless steel casting manufacturer, please contact us. Pingheng Machinery has specialized in stainless steel investment casting for 27 years. With our one-stop service and stringent quality control, we can produce stainless steel castings that meet your exact specifications.
FAQ
Q1. Besides austenitic stainless steel, what other types of stainless steel are there?
Based on their internal microscopic metallographic structures, stainless steel can be classified into five major categories. In addition to austenitic stainless steel, they include martensitic stainless steel, ferritic stainless steel, duplex stainless steel, and precipitation-hardening stainless steel.
Q2. How can you verify that the austenitic stainless steel you supply, particularly grades 304 and 316, meets the required chemical composition standards?
We have professional spectrometers to perform identity testing before pouring and after finishing. These tests aim to ensure that the chemical composition meets your requirements.
Q3. Do you have custom capability for 304 and 316 stainless steel castings?
Yes. We have full casting capabilities in customizing the casting parts based on your specific designs, shapes, sizes, surface finishes, thicknesses, and more.
Q4. How do you control the dimensional accuracy of austenitic stainless steel?
Due to the high coefficient of thermal expansion of austenitic stainless steel, it is difficult to control its dimensional accuracy. Therefore, after the wax pattern is removed from the mold, we will place it immediately in a specially designed fixture and subject it to water or air cooling, firmly locking in the dimensions until the wax pattern has completely hardened and set.
Then, in the process of pouring, we do not remove the shell prematurely. Instead, we allow the casting to cool slowly and uniformly within the ceramic mold shell. By leveraging the shell’s thermal insulation properties, we minimize temperature differences between the inner and outer surfaces of the stainless steel components, as well as between thick and thin sections, thereby ensuring more uniform shrinkage.
Q5. How can we resolve the questions that may arise during the investment casting of austenitic stainless steel?
Austenitic stainless steel can present certain challenges during investment casting, such as casting shrinkage and casting porosity, but there are appropriate solutions for each of these issues. Please click“What is Casting Shrinkage and How to Control It?” and “What is Casting Porosity and How to Prevent It?”
Q6. How should austenitic stainless steel be maintained?
Taking pump and valve castings as an example, we begin by acid washing and passivating the austenitic stainless steel to form a passivation film on its surface, which is the key to its corrosion resistance.
Subsequently, high-pressure water is used to remove deposits, salt scale, grease, and biofouling from the surfaces of pump housings and valve bodies, fully exposing the stainless steel surface to the air to prevent crevice corrosion.
It is important to note that during routine cleaning, the use of any cleaning agents containing chlorine or hydrochloric acid is strictly prohibited. Instead, cleaning agents such as nitric acid, phosphoric acid, or alkaline solutions that meet hygiene standards should be used.
Q7. What is the difference between 304 and 316 stainless steel casting?
The difference lies primarily in their chemical composition, which results in varying levels of corrosion resistance. If you want to know more, please click ”304 vs 316 Stainless Steel Investment Casting: How to Choose the Right Casting?”
