It appears that grade 304/304L SS tubing is nearly similar to type 321 SS at first glance. It is clear that the chromium (Cr) and nickel (Ni) ranges of these alloys are very similar when comparing the chemical composition of the 321 SS and 304/304L SS. When the issue of carbide precipitation in the heat-affected zone or temperature and fatigue strength are considered, the difference is seen clearly.
Stainless Steel 321 Tubes have many versatile advantages in higher temperature setting for their excellent mechanical properties. 321 stainless steel has greater ductility and resistance to stress fracture compared to the 304 alloy. Moreover, it is also possible to use 304L for anti-sensitization and intergranular corrosion.
The welding areas are sometimes referred to as the carbide precipitation zone with temperatures of 930 degrees F to 1470 degrees F, in which chromium (Cr) combines with carbon (C) and precipitates chromium carbides at the grain boundaries, significantly reducing steel corrosion resistance in this region. The reduction of the carbon content in steel to minimise carbide precipitation is one of the ways to tackle this phenomenon. 304L Stainless Steel tubing is an example of such steel where the “L” in 304L stands for “Lower carbon”. Stainless Steel 304H Tubes are an example of higher carbon content.
An even more powerful way to minimise carbide precipitation is by stabilising it by applying Titanium (Ti) to the alloy. The carbon is more attracted to titanium and leaves the chromium alone as a result. The 321 steel must have a titanium (Ti) content of at least 5 times its carbon (C) content in order to be a true ‘stabilized’ grade. The key benefit of Stainless Steel 321H Tubes is a decreased chance of corrosion in the HAZ.
It is also important to consider fatigue strength in dynamic applications. In the annealed state, the toughness or fatigue limits of austenitic stainless steels are around half the tensile strength.
Another element to consider in some applications may be temperature variables. The temperature reduction factors of 304L are greater than 321 at elevated temperatures ranging from 70 degrees F to 1200 degrees F, as we can see in the table below.
Another element to consider in some applications may be temperature variables. The temperature reduction factors are significantly greater for Stainless Steel 321 Tubes than for 304L at most elevated temperatures ranging from 70 degrees F. The real issue with most headers / upipes is a difference in the thermal expansion coefficient (CTE) as the block gets hot, it expands as it cools down. A material that expands and contracts at the same pace as your cast iron block is what you want. This causes less tension to be endured by the seals. This unmatched CTE causes most leaks (besides improper installation).