Difference Between 304 304L and 321

304 stainless steel is low carbon chromium nickel stainless and heat resisting steel somewhat superior to Type 302 in corrosion resistance.

321 stainless steel is known as stabilized grades of stainless steel, is Chromium nickel steel containing titanium. Recommended for parts fabricated by welding which cannot be subsequently annealed. Also recommended for parts to be used at temperatures between 800°F and 1850°F (427 to 816°C), have good properties resistance to intergranular corrosion. The titanium element in 321 stainless steel makes it more resistant to chromium carbide formation.

321 stainless steel is basically from 304 stainless steel. They different by a very very small addition of Titanium. The real difference is their carbon content. The higher the carbon content the greater the yield strength. 321 stainless steel has advantages in high temperature environment due to its excellent mechanical properties. Compared with 304 alloy, 321 stainless steel has better ductility and resistance to stress fracture. In addition, 304L can also be used for anti-sensitization and intergranular corrosion.

The real problem with most headers/upipes is a difference in the coefficient of thermal expansion (CTE) As you block gets hot it expands, as it cools it contracts. What you want is a material that expands and contracts at the same rate as your cast iron block. This allows the seals (gaskets/flanges) to undergo less stress. Most leaks (besides improper installation) are caused by this unmatched CTE. That is why stock exhaust manifold is cast iron, which really meany it has a 2% or more carbon content.

321=(17-19Cr, 9-12Ni + Titanium)

As for the dual designation theory, that is incorrect. L stands for low carbon.

304 L grade Low Carbon, typically 0.035% Max

304 grade Medium Carbon, typically 0.08% Max

Carbide precipitation

The weld areas with temperatures 930°F – 1470°F are often called carbide precipitation zone – in which Chromium (Cr) combines with Carbon (C) and precipitates chromium carbides at the grain boundaries significantly reducing corrosion resistance of steel in this zone. One of the ways to combat this phenomenon is to lower the carbon content in steel to decrease the carbide precipitation – 304L SS is an example of such steel; the “L” in 304L is for “Lower carbon” (.030% max vs. .080% max for 304 steel). Even more effective way against carbide precipitation is addition of Titanium (Ti) to the alloy to “stabilize it”. The carbon is more attracted to the Titanium (Ti) and therefore it leaves the chromium alone. To be a true “stabilized” grade the 321 steel has to have Titanium (Ti) content at least 5 times of Carbon’s (C). Reduced risk of corrosion in the HAZ is the main advantage of 321.

Fatigue strength

In dynamic applications, fatigue strength is also important to consider. And in this respect 321 Stainless Steel has a slight advantage over 304 Stainless Steel. 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 presented in the table below:

Alloy	Typical Tensile Strength	Typical Endurance Limit
304L	68 ksi	34 ksi
304	70 ksi	35 ksi
321	76 ksi	38 ksi

Temperature Factors

Tempearture factors could be another factor to consider in some aplications. As we can see in the table below the temperature redaction factors are slightly higher for 321 than for 304L at most elevated temperatures:

Temperature °F	304L Factor	321 Factor
70	1.00	1.00
150	0.95	0.97
200	0.91	0.95
250	0.88	0.93
300	0.85	0.91
350	0.81	0.89
400	0.78	0.87
450	0.77	0.85
500	0.77	0.83
600	0.76	0.80
700	0.74	0.76
800	0.73	0.68
900	0.68	0.59
1000	0.63	0.65
1100	0.58	0.59
1200	0.53	0.53

 

  Source: https://tubingchina.com/Difference-Between-304-321.html