In fact, all three are 304 stainless steel according to chrome-nickel content.
The ingredients are both 18% chromium (Cr) and 8% nickel (Ni), but the main difference is in the amount of carbon contained.
Table of chemical composition (%) of 304, 304L, 304H
| Item | C | Mn | P | S | Si | Cr | Ni | N |
| 304 | 0.08 | 2 | 0.045 | 0.03 | 0.75 | 18-20 | 8-10.5 | 0.1 |
| 304L | 0.03 | 2 | 0.045 | 0.03 | 0.75 | 18-20 | 8-12 | 0.1 |
| 304H | 0.04-0.1 | 2 | 0.045 | 0.03 | 0.75 | 18-20 | 8-10.5 |
Note: The unspecified range is less than or equal to.
304L is an ultra-low carbon stainless steel with carbon content reduced to less than 0.03%, which can avoid intergranular corrosion, and in theory the effect of stress corrosion resistance is stronger than 304, but in practice, the effect is not obvious.
The purpose of reducing carbon is the same as adding titanium, but 321 with titanium is more expensive to smelt, the steel is thicker and more expensive.
The H in 304H refers to high temperature.
High carbon content is the protection of high-temperature strength, GB150 requires austenitic steel used in 525 °C or more, carbon content is not less than 0.04%, carbide is the strengthening phase, especially high-temperature strength is better than pure austenitic.
Of the three, the highest carbon content is 304H, the lowest carbon content is 304L, and the carbon content of 304 stainless steel is between the two.
The higher the carbon content, the worse the corrosion resistance of stainless steel and the more likely it is to rust.
The difference in carbon content also leads to a difference in price, or in other words, different requirements for different uses.
Mechanical properties table of 304, 304L, 304H
| Item | Tensile strength | Yield strength | 50mm elongation | Hardness | Cold bending | |
| MPa | MPa | Brinell | Rockwell | |||
| 304 | >515 | ≥205 | ≥40 | ≥201 | ≥92 | no requirement |
| 304L | >485 | ≥170 | ≥40 | ≥201 | ≥92 | no requirement |
| 304H | ≥515 | ≥205 | ≥40 | ≥201 | ≥92 | no requirement |
Note: Note: The yield strength is 0.2% yield strength
Also, it can be judged from these two lists, the idea that 304L can be used instead of 304 is wrong.
Whether or not it can be used is related to the place of use, and according to the rules and regulations, we do not have the right to do “material substitution”, only the original design department can have the right to do the work, this point must be remembered.
