409 Stainless Steel

409 Stainless Steel – Composition, Properties, and Applications


409 stainless steel, also known as UNS 40900 and EN 1.4512, is a low carbon, low chromium ferrite stainless steel greatly valued for its durability and corrosion resistance. This ferritic stainless steel is highly versatile and provides a cost-effective substitute for more expensive parts requiring corrosion resistance.

It has become one of the most widely used steels in automotive exhaust systems due to its ability to offer adequate corrosion resistance, excellent high temperature resistance to oxidation, good weldability and forming characteristics that too at moderate price. Additionally, its chromium base enables excellent weldability for fabrication where needed.

Composition of 409 Stainless steel

ElementContent %
Iron, Fe(as remainder)
Manganese, Mn<1.0%
Sulfur, S<0.03%
Phosphorous, P<0.045%
Carbon, C<0.08%
Chromium, Cr10.5-11.75%
Nickel, Ni <0.5%
Silicon, Si<1.0%


Mechanical Properties

Mechanical Properties409 Stainless Steel
Tensile Strength, Ultimate448 MPa
Tensile Strength, Yield238 MPa
Elongation at Break33%
Modulus of Elasticity200 GPa

Thermal Properties

Thermal Properties409 Stainless Steel
Heat Capacity 0.460 J/g-°C
Thermal Conductivity  24.9 W/m-K
CTE Linear409 Stainless Steel
@Temperature 20.0 – 100 °C11.7 µm/m-°C
@Temperature 20.0 – 260 °C11.9 µm/m-°C
@Temperature 20.0 – 480 °C12.4 µm/m-°C
@Temperature 20.0 – 650 °C12.9 µm/m-°C
@Temperature 20.0 – 815 °C13.5 µm/m-°C

Electrical Properties

Electrical Properties409 Stainless Steel
Temperature 20 °C0.0000600 ohm-cm

Microstructure of 409 Stainless Steel

As shown in figure 1 & 2, The as-received AISI 409 plate material, in its fully annealed form, contains a mix of elongated ferrite grains and visible small, cuboidal titanium carbide (TiC) precipitates (highlighted in figure 1). 

With the use of ultrahigh resolution field emission scanning electron microscopy technology, it was identified that within the cores of these TiC particles lay tiny titanium nitride (TiN) particles – rendering them visibly darker – and that most were located within the ferrite grains, with only a limited number sitting on the grain boundaries.

409 Stainless Steel Microstructure

Can 409 steel be welded?

Dissimilar metal joints are used in a wide range of engineering applications ranging from nuclear power plants, coal fired boilers, and automobile manufacturing. 

Pressure welding is the preferred joining method for dissimilar metals due to its ability to reduce weight and costs without sacrificing structural strength and safety. 

In addition, the welds themselves must have enough tensile strength and ductility in order to survive any stresses placed on them over time. 

When welding grade 409 steels, pre-heating to temperatures between 150 – 260°C is a must. While either grade 430 or 409 filler rods/electrodes can be used, AS 1554.6 strongly recommends the use of 309 grade electrodes/filler rods. Welders should take caution to avoid excess heat when welding grade 409 steels as grain growth may occur elsewise. 

For welded products requiring better ductility, post-weld annealing is recommended at 760 – 815°C however this step isn’t applicable for thin welded products. Additionally, for exhaust tubing of automobiles, no filler rod is required as welding can be done without them.

Problems Encountered with High Temperature Working of 409 Stainless Steel

WidmanStatten Structure

When making components out of ferritic stainless steels, it is important to consider their weldability, as temperature and cooling can affect the steel’s properties. Above 1140°C, 17% Cr steels become fully ferritic and grain coarsening increases. 

This can cause a Widmanstatten structure to form at the ferrite grain boundaries, made up of austenite particles that transform to martensite upon cooling to room temperature. Unfortunately this process diminishes the structural integrity of any welds made in the steel and affects its overall weldability. 

To counteract this, engineers must use other methods such as advanced welding techniques and careful temperature management when working with ferritic stainless steel.

Sensitization of Welded Structure

Sensitization in ferritic stainless steels can occur at temperatures above 920 C, caused by the combination of carbon and chromium in solution, forming unstable precipitates that insolubilize the chromium near grain boundaries. This can lead to intergranular corrosion, resulting in a decrease in material reliability and premature failure.

To avoid this from occurring, alloying elements such as niobium or titanium are added to react preferentially with the carbon or nitrogen in solution and restore chromium solubility.

A proper understanding of sensitization is therefore essential for manufacturers when dealing with stainless steels so as to ensure minimal potential for delamination between grains.

More detailed sensitization mechanism in Stainless steel is explained in article, “Does stainless steel rust?”.

Heat treatment

Grade 409 steel is unique in its ability to be annealed as well as preserve its hardness when treated. Annealing of grade 409 steels utilises temperatures that range from 790 to 900°C and is then followed by cooling with furnace-cooling. 

This type of thermal treatment does not harden the material, allowing for greater longevity and improved formability. Grade 409 steels are sought after for their resistance to corrosion and their distinctive properties, making them firm favourites among many industrial manufacturers.

Does 409 stainless steel rust? 

Grade 409 stainless steels are known for their robustness, with superior resistance to exhaust gas and atmospheric corrosion. Despite this, the corrosion resistance of these steels is still lower than grade 430 steels containing 17% chromium. 

Nevertheless, thanks to its composition and hard-wearing properties, it is often used in industrial settings such as automotive exhaust systems where durability outweighs aesthetic needs. While this means it may not be suitable for decorative purposes due to its susceptibility to mild surface corrosion, grade 409 stainless steels offer dependable protection in extreme environmental conditions.

Is 409 stainless steel good for exhaust?

The complicated work atmosphere in automotive exhaust systems generally requires tougher material performance requirements. At higher temperatures, exhaust system components like the exhaust manifold, front pipe, and catalytic converter frequently operate at temperatures above 600 C, requiring the use of materials with exceptional heat resistance.

Furthermore, at the cold end of the exhaust system, the destructive moisture of exhaust gas could cause pitting puncture in the exhaust system.

Stainless steels were utilized in vehicle exhaust systems to meet the demands for high quality assurance and lightweight construction. The ferritic stainless steels are particularly well-known for their superior properties like low coefficient of thermal expansion and good thermal conductivity, but also because they are more affordable than austenitic stainless steels.

The popularity of type 409 in this proposal is due to its good corrosion resistance (the atmosphere in an exhaust system may contain harsh species such as Cl-, SO4 2-, SO3-, CH3COOH, CO3 2-, and HCO3 – due to the presence of condensed water and exhaust gas), low cost, and capability to resist running temperatures of up to 600 °C.

Alternate Grades to 409

3CR123CR12 offers easier weldability and good corrosion resistance, but availability of heavy sections is limited when compared to 409
304Grade 304 is a great choice as it provides high heat resistance and excellent corrosion resistance, although its cost is considerably higher than 409.
321321 has superior heat resistance compared to 304 or 409 for optimal use in exhaust systems.
Aluminized steelAluminized steel is significantly less expensive than grade 409, although it provides poorer exhaust gas resistance.

Common uses of 409 Stainless Steel

Grade 409 stainless steel is most commonly used for its applications in the automotive industry. It is primarily seen as a material for exhaust tubing and catalytic converter systems, providing excellent strength and durability in this harsh environment. 

Beyond these features, it can also be used in mufflers to reduce noise and vibrations generated by engines. Although 409 stainless steels are typically utilized for automotive purposes, they can also be found in other areas such as furniture, transportation and food processing facilities. 

Additionally, its corrosion resistance capabilities make it an invaluable material for medical parts and equipment as well. In summary, grade 409 stainless steels lend themselves to a variety of uses due to their strength and corrosion resistance capabilities.

Common Questions Related to 409 Stainless Steel

How long will 409 steel last in its workspace?

10 to 15 years. Most modern exhaust systems in the factory are produced in 409 stainless steel – Stainless Steel. They are robust and will last 4-6 years in mild conditions.

What is the main use of 409 steel?  

Stainless alloy 409 provides chromium, titanium stabilized, ferrite stainless steel mainly used in automotive filtration equipment.  It is primarily seen as a material for exhaust tubing and catalytic converter systems, providing excellent strength and durability in this harsh environment.

Is 409 Stainless steel magnetic?

409 is magnetic, just like other stainless steels of the 400 grades i.e., 410, 420, and 440a. This form of stainless steel is magnetic largely because it has a high concentration of ferrite.

Which stainless steel is better, 304 or 409?

Stainless steel can hold 85% iron, meaning the magnet is stuck to its surface causing rust. 304 stainless steel provides excellent quality exhausts parts and is considerably less oxidized with less iron than 304 Stainless steels. 

Magnets can’t stick together with this object. The use of the austenitic grades is limited worldwide because of the high cost due to the relatively large amount of nickel present.

What is the difference between SS 409 and SS 409L?

Grade 409L is a more stable version of 409. This is made up of proper chromium, a low carbon and nitrogen content, with the small addition of titanium. This results in improved performance in areas like corrosion resistance, weldability, and formability. 

The 409L grade of stainless steel has experienced a significant rise in industrial applications since its inception. Automotive Exhaust systems have particularly benefited from the adoption of this material due to said properties. 

These features make it well-suited for parts exposed to moderate heat range and corrosive environments. Many vehicle manufacturers have made the switch from less cost-effective grades of steel such as 409, paving the way for 409L as the new industry standard. 

As reliable performance and sustainability become more important factors in automotive production, it’s almost certain that 409L will remain a mainstay in exhaust systems far into the future.

Which grade of stainless-steel pipe is suitable for the transportation of IC engine exhaust gases?

409 is a great option for engine exhaust systems, as well as many other parts due to its high temperature oxidation resistance, medium strength, better forming properties and low cost of production.

The higher grade of 304 stainless steel has increased corrosion resistance over 409, therefore making it a more reliable option when looking for durable parts. However, the 304 grade will discolor to a golden yellow when exposed to heat, so it’s good to know about this should you choose 304 for your project.

Because of its improved corrosion resistance, the 304 grade tends to be slightly more expensive than 409 but if you’re willing to accept some colour change on heating then it might be worth the additional cost.

What is the difference between SS 409 and SS 409M?

 SS 409

It is a straight Chromium Ferro-magnetic stainless steel with a low strength and hardness. It is used to replace carbon steels and low alloy steels in applications requiring high-temperature corrosion resistance, general vicinity, and weldability. Titanium is used to increase weldability.

SS 409M

A modified form of SS 409 with less carbon (0.03%) is known as Type SS 409 M. Wherever greater strength, resistance to wear, ductility, and slide ability are required, it takes the place of carbon steels and low alloy steels. 

It has good oxidation and scaling resistance at high temperatures, even in sulfur-containing atmospheres. 

It is used in sewage treatment facilities, transport (wagons), petrochemical, sugar, agricultural, fisheries, bulk solids handling areas/bunkers, mining & quarrying, and civil construction.