Stainless Steel Grade Shootout: 304 Vs. 409 - What's The Difference?

Stainless Steel Grade Shootout: 304 Vs. 409 - What's The Difference?

Before we get to the two most popular grades of stainless steel used in the auto industry, lets take a dive into the four different types of stainless steel out there:

 

Ferritic Stainless Steel

Based on Chromium with small quantities of Carbon, ferritic stainless steel has a similar microstructure to both carbon and low alloy steels. Compared to other types of stainless steel, it is usually limited to use of relatively thin sections, due to of a lack of toughness in welds. Although, contractors use ferritic stainless steel for a wide range of applications that do not require welding. Additionally, you cannot harden ferritic steel with heat treatment. But you can use it in sea water or other aggressive conditions when you include an addition of Molybdenum. Ferritic stainless steel is also magnetic, but not as formable as austenitic stainless steel for example. On the other hand, steel workers often choose ferritic stainless steel because of its resistance to stress corrosion cracking.

Austenitic Stainless Steel

Austenitic stainless steel is one of the most common types of stainless steel on this list. It has a microstructure that includes an addition of Nitrogen, Nickel, and Manganese. The structure of austenitic stainless steel is the same as what you would find in regular steel. But only in a much higher temperature giving it formability and weldability. Furthermore, you can make austenitic stainless steel corrosion resistant by adding Nitrogen, Chromium, and Molybdenum. While you cannot harden it with heat, austenitic stainless steel has the useful property of retaining a helpful level of toughness and ductility when hardened to high strength. Typical austenitic stainless steel is susceptible to stress corrosion cracking, but austenitic stainless steel with higher nickel content has increased resistance to stress corrosion cracking. Nominally non-magnetic, austenitic stainless steel shows some magnetic response depending on its composition.

Martensitic Stainless Steel

Similar to ferritic stainless steel, martensitic stainless steel bases on Chromium with higher Carbon levels. You can temper and harden martensitic stainless steel much like carbon and low-alloy steels. We use martensitic stainless steel where a moderate level of corrosion resistance and high strength is needed. As it is counted among the magnetic types of stianless steel, it has low formability and weldability. Companies use martensitic stainless steel imostly in long products that require sheet and plate form.

Duplex Stainless Steel


With a microstructure that is half austenitic and half ferritic, duplex stainless steel has a higher strength than these types of stainless steel. It is also resistant to stress corrosion cracking. “Lean” duplex stainless steel is designed to have similar corrosion resistance to regular austenitic stainless steel. But it includes enhanced resistance and strength to stress corrosion cracking. “Super duplex” stainless steel also has enhanced resistance and strength to corrosion in comparison to regular austenitic stainless steel. Furthermore, they are weldable as long as you take care to use the right heat input and welding consumables. Duplex stainless steel is also magnetic with moderate formability.

Precipitation Hardening Strengthens Different Types of Stainless Steel

When you add elements like Niobium, Copper, and Aluminum, precipitation hardening stainless steel can develop very high strength. On the other hand, when you use an appropriate “aging” heat treatment, you create strength by the formation of very fine particles in the matrix of the steel. Finally, when you machine this type of steel, you can create intricate shapes. But those need good tolerance levels before going through the final aging treatment. The corrosion resistance of precipitation hardening steel is also comparable to regular austenitic steel.

 

Now that we've covered the four different types of stainless steel, lets move onto the most popular grades used in the auto industry! 

 

304 Grade 'Austenitic' Stainless Steel

 

304 stainless steel is the most popular exhaust grade selection among auto enthusiasts. Why is that? The high percentage duo of chromium and nickel utilized offers more corrosion resistance than the second most popular selection of 409 grade stainless steel. The purpose of using chromium and nickel deposits in the fabrication of stainless steel is to increase the potential resistance too long term oxidation from harmful chlorides found in salt. As you can imagine for those driving through unforgiving winters, the importance of rust resistance suddenly becomes a necessity! Conversely when running a 304 grade exhaust system in higher temperatures, the steel will actually develop into a golden/bronze finish due to its low ferrous content. There are mixed opinions about the color change in the steel for higher temps, however, the agreed consensus is that 304 grades can be successfully used in both spectrums of the climate, raising its status as the versatile option for exhaust upgrades. The major reason why you find a handful of manufacturers offer 409 grade versus 304 is the cost for fabrication. For those that reside in tropical climates from South California to Miami will typically choose a 409 grade system to avoid the extra cost of a 304 grade. I mean, wouldn't you opt to save a couple extra bucks if you had no plans on driving north? For those that prefer that progressive golden exhaust finish in these same climates will often choose a 304 grade system. So like many options in the aftermarket world, it's a matter of preference! If you're a chemistry fanatic like 10 Second Racing feel free to review the specs of 304 grade stainless steel in the charts below:

 

 

 

 

 

409 Grade 'Ferritic' Stainless Steel 


You likely noticed the steel class change between 304 grade steel! That is because 409 grade steel offers a higher ferrous content than 304 grade and thus, does not alter its original color even at higher temps like its 304 grade cousin. Another cool feature of 409 grade steel is that due to its much higher iron composition, the steel actually becomes magnetic. So the next time a friend needs help figuring out which type they have, take that magnet off the fridge and give it a shot! As discussed earlier, the major factor between 304 and 409 grade steel as it relates to a consumer is the cost for production. The exhaust aftermarket is extremely competitive, so manufacturers have a difficult decision to make when choosing to cater to potential clients in certain regions with variable climates. A handful of exhaust companies offer both 304 and 409 grade options, however the vast majority opt to focus on one grade to make life easier for their fabrication teams. This is why you will often see huge margins in retail prices for different exhaust upgrades. You may have developed an inference that the chromium and nickel deposit percentage in 409 grade steel is lower then 304, so here are the chemical characteristics to justify your logic:

 

 

 

 

 

 

 Concluding Results


Now that we have a better understanding of the difference between 304 & 409 grade stainless steel, which works for you?  Do you live in a region punished by salty roads during winter months? Or do you need a reliable performing exhaust for track day that won't fold under pressure? Both are excellent questions to consider when investing your hard earned money toward an aftermarket system. 10 Second Racing provides a plethora of product information that derives directly from the manufacturers of these options. So rest assured, you will have plenty of specifications to work with to come to a reliable decision! If you are more of a visual learner, Holley Performance created an excellent video about the advantages of both types:


 

 

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