At first glance, it appears that type 304/304L Stainless Steel is very similar to type 321 Stainless Steel. Comparing the chemical composition of of 321 Stainless Steel and 304/304L Stainless Steel, it is clear that the chromium (Cr) and nickel (Ni) ranges of these alloys are very similar. The difference appears when the Stainless Steel using of “carbide precipitation” in the “heat-affected zone (HAZ)” is discued Stainless Steel or fatigue strength and temperature have to be considered.
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 stainleStainless Steel 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 | Typical Endurance Limit |
| 304L | 68 ksi | 34 ksi |
| 304 | 70 ksi | 35 ksi |
| 321 | 76 ksi | 38 ksi |
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 Stainless Steel is an example of such stainless steel; the “L” in 304L stainless steel is for “Lower carbon” (.030% max vs. .080% max for 304 stainless 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 Stainless 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.
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 Stainless Steel is an example of such stainless steel; the “L” in 304L stainless steel is for “Lower carbon” (.030% max vs. .080% max for 304 stainless 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 Stainless 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.
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 Stainless Steel than for 304L Stainless Steel at most elevated temperatures:
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 Stainless Steel than for 304L Stainless Steel at most elevated temperatures:
| TEMP ° F | 304L Stainless Steel FACTOR | 321 Stainless Steel 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: Zhejiang Yaang Pipe Industry Co., Limited (www.yaang.com)
