If 304 and 321 are similar alloys of Stainless Steel, why does Penflex use 321 for its hose instead of 304 or 304L?

At first glance, it appears that 304L is very similar to 321.  In fact, looking at the chemical composition, it is clear that the chromium and nickel ranges of the two alloys are very similar.  The difference appears when the issue of “carbide precipitation” in the “heat-affected zone (HAZ)” is discussed.  This is when the heat of welding causes the available chromium in the grain boundaries to combine with the carbon in the steel and this reduces the corrosion resistance in this zone.  One of the ways to combat this phenomenon is to lower the carbon content (thus the "L" in 304L: .030% max vs. .080%)  But an even more effective way is to add titanium to the alloy to “stabilize it”.  The carbon likes to attach to the titanium better than the chromium, so it leaves the chromium alone so it is still available to resist corrosion.  The 321 is an alloy with added titanium (it has to have at least 5 times the carbon percent).  The reduced risk of corrosion in the HAZ is the main advantage of 321.

In dynamic applications, fatigue strength is also important.   Fatigue or endurance limits (strength in bending) of austenitic stainless steels in the annealed condition are about one-half the tensile strength.  Typical tensile and endurance limits for these alloys (annealed) are:

TEMPERATURE FACTORS - The factors are slightly higher for 321 than for 304L at most elevated temperatures, which for some applications can be a defining factor in choosing 321 over 304L.

(The design guide published by the Stainless Steel Producer’s of North America was used for researching this answer.)