Stainless Grades and Properties
There are a couple of hundred different stainless steels standardized in the specifications. However, the most widely used ones are austenitic Types 304 and 316 and ferritic Types 409 and 430 stainless steels. The many grades differ mainly in their chemical composition. The composition of alloying elements in a stainless steel determines its corrosion resistance as well as its mechanical and physical properties.
Below is a list of the most important stainless steel grades and the most important molybdenum-containing stainless steel grades.
| EN | AISI | Cr | Mo | Ni | N | PREN |
|---|---|---|---|---|---|---|
| Ferritic Stainless Steels | ||||||
| 1.4512 | 409 | 11.5 | 11.5 | |||
| 1.4016 | 430 | 16.5 | 16.5 | |||
| 1.4113 | 434 | 16.5 | 1 | 19.8 | ||
| 1.4526 | 436 | 17.5 | 1.25 | 21.6 | ||
| 1.4521 | 444 | 17.7 | 2.1 | 24.6 | ||
| (UNS S44660) | 27 | 3.7 | 2 | 39.2 | ||
| Austenitic Stainless Steels | ||||||
| 1.4301 | 304 | 18.1 | 8.3 | 18.1 | ||
| 1.4401 | 316 | 17.2 | 2.1 | 10.2 | 24.1 | |
| 1.4438 | 317L | 18.2 | 3.1 | 13.7 | 28.4 | |
| 1.4439 | 317LMN | 17.8 | 4.1 | 12.7 | 0.14 | 33.6 |
| 1.4539 | 904L | 20 | 4.3 | 25 | 34.2 | |
| (6%Mo) | 20 | 6.1 | 18-24 | 0.2 | 43.3 | |
| Duplex Stainless Steels | ||||||
| 1.4362 | 2304 | 23 | 0.3 | 4.8 | 0.1 | 25.6 |
| 1.4462 | 2205 | 22 | 3.1 | 5.7 | 0.17 | 35.0 |
| 1.4410 | 2507 | 25 | 4 | 7 | 0.27 | 42.5 |
Table 1: Common ferritic, austenitic and duplex stainless steels with and without molybdenum.
PREN is the Pitting Resistance Equivalent; this number is a measure of the relative pitting corrosion resistance of a stainless steel grade in a chloride containing environment. The higher the PREN the more corrosion resistant a stainless steel is. The formula for PREN is:
PREN = %Cr + 3.3*%Mo + 16*%N
This formula expresses that molybdenum is 3.3 times more effective against pitting corrosion than chromium. However, chromium is always necessary to give a base corrosion resistance. Molybdenum cannot replace a base amount of chromium in stainless steels. It boosts the corrosion resistance of a stainless steel.
The figure shows that ferritic, austenitic and duplex stainless steels are available on different levels of corrosion resistance. For example, Type 444 ferritic, Type 316 austenitic and 2304 duplex stainless steels all have a similar level of pitting corrosion resistance in chloride environments.
Stainless steels are selected based on the corrosiveness of the environment and the requirements for mechanical properties.
