409 Stainless Steel vs 304 Stainless Steel
So you are setting out to build a stainless steel exhaust system. But what material do you choose? 409 and 304 stainless steel are the two most popular choices in the exhaust / muffler industry. So we are going to try and explain both of these metals in a way that everyone can understand.
409 Stainless Steel:
409 stainless steel is widely used in the automotive industry. This material has been used for quite some time on factory exhaust systems and catalytic converters. It is a ferritic metal alloy with 10.5 to 11.5 % chromium content and 0.5% nickel content. This means that the minimum 10.5% chromium content allows the alloy to be classified as stainless steel but does have a higher iron content. This higher iron content means this material will stick with a magnet.
The higher iron content allows for better formablity (bending / stamping), weldability (easier to weld), and heat cycling (less tendency to crack). 409 stainless is has a temperature range of sustaining 1250 degrees Fahrenheit with temperatures able to reach up to 1500 degrees Fahrenheit intermittently. The corrosion resistance of 409 stainless has an increased corrosion resistance compared to mild steel or aluminized components. This particular stainless steel will oxidize and turn a brownish colour. This oxidation actually helps with the corrosion resistance.
304 Stainless Steel:
304 stainless steel is the most widely used stainless steel in commercial and industrial applications. It is an austenitic stainless steel alloy with 18 - 20 % chromium content and 8 - 10 % nickel content. This means with the high chromium and nickel content allow for a much more improved heat and oxidation resistance compared to 409 stainless steel. This higher nickel content also means a magnet will not stick in straight sections.
The bent portions of a tube or sheet metal will tend to have magnet stick. With the low iron content of 304 stainless steel this tends to make the formability (bending / stamping) and weldability to be be more difficult. 304 stainless does have a tendency to stress crack. (make sure the exhaust is hung well) 304 stainless steel is able to reach a sustainable temperature of 1700 degrees. The corrosion resistance of 304 stainless steel is far superior to 409 stainless. The material after being heated generally goes a goldish blue colour. Corrosion can still occur when in high salt applications. Also with the nickel content this allows the 304 stainless to be easily polished.