Choosing materials for any piece of equipment or process system is usually not an easy task, unless you’re simply copying something that’s already been successful.
Often engineers will have a checklist to help them narrow down the options, eliminating groups of materials that are not suitable for various reasons.
Where a stainless steel is desired, austenitic 300-series stainless steels will generally tick most of the boxes.
These 300-series alloys, which contain between 8% and 35% nickel, are known for their corrosion resistance, high strength, and excellent formability.
They are commonly used in a wide variety of applications, including the automotive and aerospace industries, as well as in kitchen appliances, food processing equipment, and medical instruments.
There are several different grades of 300-series stainless steel, including 304, 309, 310, 316, 321, and 347. Each grade has its own unique combination of properties and is suited for different applications.
Codes and standards
300-series alloys are featured in every stainless steel application standard, specifying the grades of material that should be used in various applications.
Codes and standards that pertain to the use of 300-series stainless steel include ISO, EN, ASME, ASTM and AWS standards. These standards cover a wide range of applications, including pressure vessel construction, piping, welding, and more.
Corrosion resistance
The 300-series has ‘good’ to ‘excellent’ corrosion resistance properties, making it suitable for many applications chemical processing, food processing, the medical industries and the oil and gas industries.
There are many different 300-series alloys available for general and more specific applications, ranging from the 18/8 versions (304/L or 1.4301/7) to highly-alloyed variants with a chromium content of up to 30% and containing other corrosion-resisting elements, such as molybdenum and nitrogen.
For example, 316 stainless steel, which contains 16% chromium, 10% nickel, and 2% molybdenum, is more corrosion-resistant than 304 stainless steel and is often used in marine and coastal applications.
In general, 300-series stainless steel alloys are resistant to corrosion from a wide variety of sources, including:
- Acids: These alloys are resistant to corrosion from many types of acids, including sulfuric, hydrochloric, and phosphoric acids.
- Alkalis: 300-series stainless steel alloys are resistant to corrosion from alkalis, such as sodium hydroxide and potassium hydroxide.
- Salts: These alloys are resistant to corrosion from many types of salts, including sodium chloride (table salt) and magnesium chloride.
300-series stainless steel can be easily sterilized, making it ideal for use in the production of medical equipment and in the food and beverage industry.
Ductility
The 300-series alloys have outstanding ductility, which means they are easily formable. That is important in deep drawing, stretch forming and producing complex shapes where austenitic stainless steels are frequently the best choice.
At room temperature, most 300-series stainless steel alloys have a ductility of around 50%. This means that they can be stretched up to 50% of their original length before breaking.
Duplex grades are also suitable for many forming operations but as ductility decreases with increasing strength, the duplex grades can be more difficult to form.
The ductility of 300-series stainless steel alloys makes them an excellent choice for a wide range of applications that require the material to be stretched or deformed, such as in the automotive and aerospace industries.
Weldability
Of all the families of stainless steels, the 300-series are the most commonly and most easily welded, maintaining their mechanical properties and corrosion resistance.
They are by far the most common family of stainless used in fabrications.
The 300-series austenitic stainless steels are readily welded with or without filler wire and are suitable for welding by all the standard methods, including shielded metal arc welding (SMAW), flux-cored arc welding (FCAW), and gas tungsten arc welding (GTAW).
One factor that can affect the weldability of 300-series stainless steel alloys is the carbon content. Alloys with a higher carbon content may be more prone to weld decay, which is a form of corrosion that can occur in the heat-affected zone (HAZ) of the weld. To reduce the risk of weld decay, it is important to use a low-carbon alloy or to properly preheat and post-weld heat treat the material.
Another factor that can affect the weldability of 300-series stainless steel alloys is the presence of certain alloying elements, such as molybdenum and nitrogen. These elements can improve the corrosion resistance of the steel, but they can also make the material more challenging to weld. It’s important to use proper welding techniques and equipment to ensure good weld quality when working with these alloys.
Ambient temperature strength
300-series alloys have moderate strength, so specify dual-certified grades 304/304L and 316/316L where necessary. These dual-grade alloys often contain a small nitrogen addition, however, and the nitrogen-alloyed versions (e.g. 304N) are slightly stronger.
If high strength is needed, consider a slightly cold-worked 300-series alloy or a duplex alloy. If no welding is involved and relatively low corrosion resistance is needed, consider a greater cold-worked 300-series alloy, or a martensitic or
precipitation-hardening stainless steel.
Low-temperature properties
The wrought austenitic alloys remain tough (that is, do not become brittle) down to very low temperatures, some even down to near absolute zero. This makes them an excellent choice for cryogenic applications such as in the medical industry and the transportation and storage of liquified gases. All the other families have a much higher ductile-to-brittle transition temperature.
Higher temperature properties
The strength of the austenitic structure shines compared to other families, especially in the creep strength temperature range. A major consideration, though, is how the properties will transform with time.
Many stainless steel alloys become brittle in certain temperature ranges, but the higher-nickel austenitic alloys are most often the best choice.
Recyclability
Stainless steel is 100% recyclable. Choosing 300-series stainless steels for an application ensures that it will be produced in a responsible way, with an extremely low life-cycle inventory, and have low, or close to zero, maintenance costs during its long life. Stainless steel can be reused indefinitely because it is easy to recycle without any loss of quality or material integrity.
The 300-series are more likely to be recycled because they have a higher inherent value at end-of-life.
Availability
The 300-series are available in most product forms including coils, sheets, plates, bars, sections, angles, tubes, pipes, fittings, flanges, forgings and castings.
300-series stainless steel can be supplied in a variety of finishes, such as hot-rolled, cold-rolled, polished and annealed.
Austenitic stainless steels are the most widely-produced family of stainless steels and are readily available in many forms via our BSSA members.
Economics
There are many applications where the 300-series alloys have demonstrated that they offer a low overall cost, due to their corrosion resistance, durability, and long service life.
300-series stainless steel alloys require less maintenance and are easier to clean than other types of metal. The long service life means that they require fewer repairs or replacements, further reducing maintenance costs.
While stainless steel has a higher melting point than carbon steel, which means that it requires more energy to produce, the longer service life and low maintenance requirements of stainless steel can help to offset this initial cost and may result in lower overall energy costs over the lifetime of the material.
Combining these benefits with the others discussed earlier, such as exceptional recyclability, corrosion resistance and favourable behaviour at different temperatures, 300-series stainless steel offers a range of economic benefits.
Learn more
Overall, 300-series austenitic stainless steel is a versatile and widely-used group of alloys that offer excellent corrosion resistance, good formability, and high strength.
If you are in the process of specifying materials for a project, hopefully this article has given you some reasons to consider selecting 300-series stainless steel.
If you found this article useful, you may also wish to read my article discussing the importance of stainless steel in the UK economy.
Should you require additional information regarding stainless steels and their applications, please contact the British Stainless Steel Association. We also offer training and education in the use of stainless steels.
Connect with Rob Cooper on LinkedIn and find out more about the BSSA by visiting their website: bssa.org.uk
Learn about the new Stainless Steel in the Food and Beverage Industry event, jointly hosted by the BSSA, the Nickel Institute and the European Hygienic Engineering and Design Group, by reading Eric Partington’s article in the January 2023 edition of the magazine: Stainless Steel in the Food and Beverage Industry
