The common austenitic grades of stainless steel, such as 304 and 316, are also available “standard” and with controlled low carbon “L” or high carbon “H” contents, with particular applications.
| Grade | UNS | EN-Number | Specified Carbon (%) | Remarks |
|---|---|---|---|---|
| 304 | S30400 | 1.4301 | 0.07 max | |
| 304L | S30403 | 1.4306 | 0.03 max | |
| 304H | S30409 | – | 0.04-0.10 | |
| 316 | S31600 | 1.4401 | 0.07 max | |
| 316L | S31603 | 1.4404 | 0.03 max | |
| 316H | S31609 | – | 0.04-0.10 |
Low carbon “L” grade
Low carbon or “L” grades are used to prevent or delay sensitisation of stainless steel at elevated temperatures and the resulting lower corrosion resistance. The problematic temperature zone is 450-850 °C, encountered during welding or specific application environments. “L” grades are often available in thicker selection sizes, greater than about 5mm in flat products. The low carbon “L” grades are used where high temperature exposure will occur, including welding of medium or heavy sections. The low carbon is one way of delaying or preventing grain boundary carbide precipitation (often referred to as sensitisation) which can result in intergranular corrosion in corrosive service environments.
The “L” grades are most readily available in plate and pipe, but often also in round bar. In the absence of heavy section welding, or of high temperature exposure, the corrosion resistances of the standard and “L” grades are usually identical.
High carbon “H” grade
H” grades are higher carbon versions of standard grades and have increased strength, particularly at elevated temperatures (generally above 500 °C). Long term creep strength is also higher. “H” grades such as 304H and 316H are primarily available in plate and pipe. These grades are susceptible to sensitisation if held in the temperature range of 450-850 °C. Once sensitised, impaired aqueous corrosion resistance and some reduction in ambient temperature ductility and toughness will result (usually irrelevant in high temperature applications).
