There are hundreds of different grades of stainless steel on the market. Each of these unique formulations of stainless steel offer some degree of corrosion resistance above and beyond that of plain steel.
The existence of these stainless steel variants can cause some confusion—especially when the names & formulations of two stainless steel alloys are almost the same. This is the case with grade 304 and 304L stainless steel.
Composition Table Grade 304 SS Chemical Content by % Grade 304L SS Chemical Content by %
Carbon 0.08 Max 0.03 Max
Chromium 18.00-20.00 18.00-20.00
Iron Makes up the Balance Makes up the Balance
Manganese 2.00 Max 2.00 Max
Nickel 8.00-12.00 8.00-12.00
Nitrogen 0.10 Max 0.10 Max
Phosphorus 0.045 Max 0.045 Max
Silicon 0.75 Max 0.75 Max
Sulfur 0.030 Max 0.030 Max
These two alloys are remarkably similar—but there is one key difference. In grade 304 stainless, the maximum carbon content is set at 0.08%, whereas grade 304L stainless steel has a maximum carbon content of 0.03%. The “L” in 304L can be interpreted as meaning extra-low carbon.
This difference of 0.05% carbon content produces a slight, but marked, difference in the performances of the two alloys.
The Mechanical Difference
Grade 304L has a slight, but noticeable, reduction in key mechanical performance characteristics compared to the “standard” grade 304 stainless steel alloy.
For example, the ultimate tensile strength (UTS) of 304L is roughly 85 ksi (~586 MPa), less than the UTS of standard grade 304 stainless, which is 90 ksi (~620 MPa). The difference in yield strength is slightly greater, with 304 SS having a 0.2% yield strength of 42 ksi (~289 MPa) and 304L having a 0.2% yield strength of 35 ksi (~241 MPa).
This means that if you had two steel wire baskets and both baskets had the exact same design, wire thickness, and construction, the basket made from 304L would be structurally weaker than the standard 304 basket.
Why Would You Want to Use 304L, Then?
So, if 304L is weaker than standard 304 stainless steel, why would anyone want to use it?
The answer is that the 304L alloy’s lower carbon content helps minimize/eliminate carbide precipitation during the welding process. This allows 304L stainless steel to be used in the “as-welded” state, even in severe corrosive environments.
If you were to use standard 304 stainless in the same way, it would degrade much faster at the weld joints.
Basically, using 304L eliminates the need to anneal weld joints prior to using the completed metal form—saving time and effort.
In practice, both 304 and 304L can be used for many of the same applications. The differences are often minor enough that one isn’t considered massively more useful over the other. When stronger corrosion resistance is needed, other alloys, such as grade 316 stainless steel, are usually considered as an alternative.
Post time: Nov-24-2021