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Stainless grades and properties

Specifications define a couple hundred stainless steel grades, differing from one another mainly in chemical composition. Composition is a fundamental characteristic of stainless steel because it determines the alloy's corrosion resistance, microstructural phase balance, mechanical properties, and physical properties. The table below lists the most important stainless steel grades and the most important molybdenum-containing grades. The most widely used grades are austenitic Types 304 and 316 and ferritic Types 409 and 430.

Common ferritic, austenitic and duplex stainless steels
Ferritic grades
1.4512 409 S40900 11.5       11.5
1.4016 430 S43000 16.5       16.5
1.4113 434 S43400 16.5 1     19.8
1.4526 436 S43600 17.5 1.25     21.6
1.4521 444 S44400 17.7 2.1     24.6
    S44600 27 3.7 2   39.2
Austenitic grades
1.4301 304 S30400 18.1   8.3   18.1
1.4401 316 S31600 17.2 2.1 10.2   24.1
1.4438 317L S31703 18.2 3.1 13.7   28.4
1.4439 317LMN S31726 17.8 4.1 12.7 0.14 33.6
1.4539 904L N08904 20 4.3 25   34.2
  (6%Mo)   20 6.1 18-24 0.2 43.3
Duplex grades
1.4362 2304 S32304 23 0.3 4.8 0.1 25.6
1.4462 2205 S32205 22 3.1 5.7 0.17 35.0
1.4410 2507 S32750 25 4 7 0.27 42.5

The Pitting Resistance Equivalent Number (PREN) is a measure of the relative pitting corrosion resistance of stainless steel in a chloride-containing environment. Higher PREN values indicate greater corrosion resistance. The formula for PREN is:

PREN = %Cr + 3.3*%Mo + 16*%N

This formula suggests that molybdenum is 3.3 times more effective than chromium at improving pitting resistance, which is true within limits. Chromium must always be present in stainless steel to provide basic corrosion resistance. Molybdenum cannot provide this basic resistance, but it significantly enhances a stainless steel's corrosion resistance, as the formula shows.

The table shows that ferritic, austenitic and duplex stainless steels with different levels of pitting resistance are available, and the figure below shows that it is possible to select several different grades with similar pitting resistance. For example Types 444 (ferritic), 316 (austenitic) and 2304 (duplex) have similar resistance to pitting corrosion in chloride environments.

Selection of the appropriate stainless steel grade depends primarily on the corrosiveness of the application environment and the application's mechanical property requirements. When more than one alloy meets these requirements, other factors like physical properties, fabricability, availability and cost are considerations.

Comparative PREN

Comparison of PREN values for different ferritic, austenitic and duplex stainless steels (after ArcelorMittal)