Austenitic stainless steel grades are the number one choice for materials that can endure corrosive environments. Austenitic stainless steel tube is rich in chromium and nickel, which make it ideal for resisting corrosion. It also has remarkable mechanical properties across the various grades available in the market.
Grade 304 stainless steel
Grade 304 is a basic austenitic stainless steel. It contains a high level of nickel and chromium, with the amount of chromium ranging from 18 to 20% by weight and nickel weighing in at between 8 and 10.5%. Grade 304’s other alloying elements include silicon and manganese with iron comprising the remaining chemical composition.
Chromium and nickel give 304 stainless steel its high corrosion resistance. Common applications for 304 stainless steel include:
- Household appliances
- Heat exchangers
- Making structures in the environment that would wear down carbon steel
- Commercial food processing equipment
Grade 316 stainless steel
316 stainless steel has the same material composition as 304 stainless steel. However, in addition to high amounts of chromium and nickel, it also holds silicon, carbon, and manganese, with a huge portion comprising of iron.
Grade 316 stainless steel has a denser chemical composition. It holds a significant amount of molybdenum of about 2-3% in weight compared to the traces found in 304. The higher molybdenum in 316 results in higher corrosion resistance than in 304.
316 stainless steel does well in salty water, making it ideal for marine applications. Other common applications of 316 stainless steel are:
- Marine surroundings, especially those with high chloride concentrations
- Medical devices
- Refinery equipment
- Chemical storage equipment and for chemical processing
Which should you choose - 304 or 316?
There are situations which set apart the two grades of stainless steel, but both are legends on their turf. The circumstances under which Grade 304 is a better choice are:
The application requires superb formability. The higher molybdenum content in 316 can have serious consequences on formability, thus making 304 a better choice.
The application has underlying cost concerns. Given that grade 316 is more corrosion resistant, it has a higher direct cost.
The following situations highlight cases where Grade 316 may be a better choice:
- When the surroundings include a high concentration of corroding elements like in marine application or the dishwasher
- When the material will be submerged in water or have consistent exposure to water
- In applications where formability is less of a concern and strength and durability are required
- In applications where non-magnetic permeability is required.
- The underlying cost, however, is justified directly by the long life cycle. Therefore formability and corrosion resistance should help you choose between grade 304 and grade 316.
Criteria to select the best of grades
With over 50 stainless steel grades to choose from, picking a grade is not always easy. With expert advice, you can be certain which grade of pipes best suits your project.
Here are some things to consider first:
- The timeline of material generation or application
- Transition in-process phase
- Cost-reduction through the change of basic material
- Adaptation of new material due to bad performance
The basic types of pipes obtainable are:
- Basic ferritic
- Austenitic peak temperature steel
- Basic martensitic
Stainless steel class is a broad spectrum and critical elements include:
- Mechanical strength
- Magnetic response
- Corrosion resistance
- Life span cost
The environment in which the stainless steel product will be located is important when determining the choice of grade. The atmospheric conditions, the concentration of chemicals which comprise of chlorides and acids are the fundamental concerns of corrosion resistance.
For less corrosive environments, the most preferred grades of austenitic stainless steel would be grades 303 and 304. If your project is set in surroundings which are more corrosive based on the presence of acid and chlorine or seawater, grade 316 is best suited for such environments.
The key to choosing the best stainless steel for your application is to evaluate the life cycle cost.
This article was written by Mike Maynes
About the author
Mike Haynes is a Senior Inspecting Engineer with over 12 years experience in the steel industry. He has experience in the analysis, design and development of steel using the latest technology.