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:
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 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.
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:
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