Before you choose 309S stainless steel for any application, you should know the properties, chemical composition, alternative grades available, and applications, amongst others.
What is 309s Stainless Steel?
309S or UNS S30908 is an austenitic stainless steel grade.
UNS S30908 is known for superior resistance to heat due to its high nickel and chromium composition.
This makes it a perfect choice for high-temperature applications. In today’s metal fabrication industry, they are identified as:
- ASTM A240
- AMS 5523
- ASME SA240
309s Stainless Steel Vs 309 Stainless Steel
The 309S is characterized by a lower carbon percentage than in 309 stainless steel.
Normally, the low carbon content makes fabricating stainless steel alloy 309S easy without many defects.
At the same time, it is worth mentioning that there is the 309H. It is also called the UNS S30909. Among the 309 stainless steel grades, it has the highest carbon content.
As a result, it has excellent creep resistance.
Elements in 309S Stainless Steel Composition
Like other stainless steel grades, the 309s is an alloy of 7 key elements. Ideally, the main elements are iron, chromium, and nickel as you can see evident in the table below:
|Amount by %
|20 to 24
|12 to 15
It is these elements that are responsible for high-temperature resistance:
Stainless Steel Grades – Source: Tuolian Metal
309s SS – Source: AZO Materials
Stainless Steel Grade 309s Properties
The 309S grade stainless steel has superior mechanical, physical, and chemical properties. It shares most properties with other grades in austenitic stainless steel.
· Chemical Properties
As you can see from the chemical composition in the table above, high chromium (about 23%) and nickel (approximately 14%) are responsible for:
- Excellent corrosion resistance – it can resist corrosion even at working temperature.
- Superior oxidation resistance – Usually, this is up to 1038°C. Of course, the conditions must be non-cyclic. When you subject the stainless steel grade to frequent cycling, the oxidation temperature will reduce to 1010°C.
- Excellent resistance to chemicals including sulphur. However, it is safe to use it within a maximum temperature of 1000°C.
- Notably, the stainless steel 309S is not suitable for highly carburizing applications.
· Physical Properties
Ideally, stainless steel alloy 309S has a density of 8g/cm3. Its melting point is about 1454 °C. Additionally, at 0 to 100 °C, the thermal expansion coefficient is 14.9 µm/m°C while thermal conductivity is 15.6 W/mK.
· Mechanical Properties
Apart from the superior mechanical properties, 309S grade stainless steel has better mechanical properties. A reason the material is a perfect choice for most structural applications.
There are many variables you can use to determine the mechanical strength of the stainless steel grade such as:
- Ultimate tensile strength is about 620 MPa
- Poissons ratio is approximately 0.3
- About 85 Rockwell hardness
- Modulus of elasticity is about 120 GPa
- Shear modulus is an average of 7Gpa
- 45% elongation at break for about 50mm sheet
It is important to note that these values are simply indicative. When importing stainless steel 309S, let the manufacturer provide you with exact data specifications for mechanical properties.
The data should be based on their laboratory test or accredited institution.
309s SS Fabrication and Machining Properties
Stainless steel fabrication and machining will help you transform stainless steel 309S into useful products.
However, before choosing this metal for any fabrication, you need to evaluate the machining properties, welding, and heat treatment capability.
· Machining 309S SS
Generally, you can easily machine stainless steel grade 309S. However, if you want quality finishes, there are simple rules that will enhance the entire process and they include:
- Use sharp cutting tools to prevent possible work hardening.
- Even if you opt for light cuts, they should be deep enough. Again, this will prevent possible work hardening.
- During the cutting process, use coolants or lubricants. It will prevent heat buildup. Remember, most austenitic stainless steels have low thermal conductivity.
Generally, the cutting properties of 309s are similar to the stainless steel 304.
· Welding Stainless Steel 309s Alloy
Whenever you are fabricating stainless steel 309S, avoid oxyacetylene welding. However, you can use resistance and fusion welding methods.
Additionally, you will not experience any problems when hot working, cold working, annealing, or hardening stainless steel grade 309S.
But at the same time, you should take note of:
|SS Fabrication Process
|Hot Working 309S SS
|· Recommended temperature for hot working is 1177°C
· Reheating temperature should be 982°C
· Rapid cooling optimizes mechanical properties.
|Cold Working 309S SS
|· Known for high work hardening rate
· You can draw or stamp stainless steel grade 309S
· Cold working causes internal stress which requires annealing for optimal mechanical properties.
|Annealing 309S SS
|· Suitable annealing temperature is between 1038 and 1121°C
· After heating, use water for quenching
|Hardening 309S SS
|· Cold working improves hardness
Stainless Steel Grade 309S Product Forms
After stainless steel grade 309S manufacturing, they are available in many forms such as:
- Rolls, etc.
Additionally, they may come in different stainless steel finishes such as:
- Mill finish
- Brush finish
- Patterned finish
- Colored finish, or
- Custom finish
309S Stainless Steel Applications
Due to the excellent heat resistance properties, stainless steel grade 309S is a perfect choice for most high-temperature applications. It is for this reason that you will find fabricated 309S parts in:
- Boiler systems – milling industry, petroleum refining, etc.
- Furnace parts – including fluid bed systems, fans, piping, etc.
- Fire equipment – such as an incinerator, kilns, etc.
309s stainless steel is a perfect choice for most high-heat applications. Of course, this is due to the high chromium and nickel levels alongside other alloying elements.
Although it is susceptible to work hardening, annealing will restore the desired properties.