Stainless Steel Grade Selection Guidelines

Topics Covered

Background

Selection for Corrosion Resistance

Pitting Resistance Equivalent

Stress Corrosion Cracking

Selection for Mechanical and Physical Properties

Selection for Fabrication

Selection Criteria

Background

When considering the choice of a stainless steel for a particular application, the first consideration needs to be on the basis of which of the fundamental "competitive advantage" properties needs to be exploited, as tabulated in Table 1. These basic properties for selection can be initially looked at from the point of view of the five basic alloy groups - austenitic, duplex, ferritic, martensitic and precipitation hardening.

Selection for Corrosion Resistance

The selection of the most cost-effective grade for a particular corrosive environment can be a complex task. Often the most revealing guide to material selection is the simple consideration of what has been used before (here or in a similar environment), what was the service life and how and when did it corrode. For resistance to environments such as strong acids, where uniform general corrosion is the controlling mechanism, there are published tables of recommended grades, and iso-corrosion curves that indicate the rate at which the steel can be expected to corrode. These are usually constructed so that several grades can be compared, and the applicable one selected for the expected environment. Although this approach is useful, some care needs to be taken as there are often minor differences between apparently similar environments that can make a large difference to the corrosion rates in practice. Traces of chloride for instance can be harmful.

Table 1. Basic "Competitive Advantage" Properties of Stainless Steels

Required Property

Alloy Groups and Grades Likely to be Selected

Corrosion resistance

Selection depends upon environment. See later discussion

Heat resistance

Austenitic grades, particularly those high in chromium, often also with high silicon, nitrogen and rare earth elements (e.g. grade 310 and S30815). High chromium ferritic grades can also be useful (e.g. 446).

Cryogenic (low temp.) resistance

Austenitic grades have excellent toughness at very low temperatures.

Magnetic response

Austenitic grades have low magnetic permeability; higher nickel grades (e.g. 316 or 310) are guaranteed non-magnetic even if cold worked.

High Strength

Martensitic and precipitation hardening grades.

Local corrosion is very frequently the mechanism by which stainless steels are likely to corrode. The related mechanisms of pitting and crevice corrosion are very largely controlled by the presence of chlorides in the environment, exacerbated by elevated temperature.

Pitting Resistance Equivalent

The resistance of a particular grade of stainless steel to pitting and crevice corrosion is indicated by its Pitting Resistance Equivalent number or PRE, as shown in table 2. The PRE can be calculated from the composition as:

PRE = %Cr + 3.3 %Mo + 16 %N

Clearly grades high in the alloying elements chromium and especially molybdenum and nitrogen are more resistant. This is the reason for the use of grade 316 (2%Mo) as the standard for marine fittings, and also explains the selection of duplex grade 2205 (S31803) with 3%Mo and a deliberate addition of 0.15%N for resistance to higher chlorides at higher temperatures. More severe chloride-containing environments can be resisted by the "super austenitic" grades (e.g. N08904 and S31254) with up to 6%Mo and by the "super duplex" grades (e.g. S32750 and S32520) with very high chromium, molybdenum and nitrogen additions. The use of these grades can extend the useful resistance in high chloride environments up to close to boiling point.

Table 2. Pitting Resistance Equivalent Number or PRE for Various Grades

Grade

Class

PRE

3CR12

ferritic

11

430

ferritic

17

303

austenitic

18*

304/L

austenitic

18

316/L

austenitic

24

2205

duplex

34

904L

austenitic

34

S31254

austenitic

43

S32750

duplex

43

S32520

duplex

43

*The calculated PRE for 303 is wrong, due to sulphur in the composition.

Stress Corrosion Cracking

A particular problem for the common austenitic grades (e.g. 304 and 316) is stress corrosion cracking (SCC). Like pitting corrosion this occurs in chloride environments, but it is possible for SCC to take place with only traces of chlorides, so long as the temperature is over about 60°C, and so long as a tensile stress is present in the steel, which is very common. The ferritic grades are virtually immune from this form of attack, and the duplex grades are highly resistant. If SCC is likely to be a problem it would be prudent to specify a grade from these branches of the stainless family tree.

Selection for Mechanical and Physical Properties

High strength martensitic (e.g. 431) and precipitation hardening (e.g. 630 / 17-4PH) grades are often the material of choice for shafts and valve spindles - here the high strength is as fundamental to the selection process as is the corrosion resistance. These grades have strengths up to twice that of grades 304 and 316.

More commonly, however, the grade is selected for required corrosion resistance (or resistance to high or low temperature or because of required magnetic response), and then the structure or component is designed around the mechanical and physical properties of the grade selected. These secondary aspects should be considered as early as possible in the selection process. The selection of a high strength duplex grade such as 2205 may not only solve the corrosion problem but also contribute to the cost effectiveness of the product because of its high strength. The selection of a ferritic grade such as 3CR12 may result in adequate corrosion resistance for a non-decorative application, and its low coefficient of thermal expansion could be desirable because of less distortion from temperature changes. The thermal expansion rates of the ferritic grades are similar to that of mild steel and only 2/3 that of austenitic grades such as 304.

Selection for Fabrication

Again it is usually the case that grades are selected for corrosion resistance and then consideration is given to how the product can be fabricated. Fabrication should be considered as early as possible in the grade selection process, as it greatly influences the economics of the product. Table 3 lists some common grades and compares their relative fabrication characteristics. These comparisons are on arbitrary 1 to 10 scales, with 10 indicating excellent fabrication by the particular method.

Table 3. Common Grades and Their Relative Fabrication Characteristics

Grade

Formability

Machinability

Weldability

303

1

8 *

1

304

8

5*

8

316

8

5*

8

416

1

10

1

430

4

6

2

2205

5

4

5

3CR12

5

6

6

* Improved Machinability versions of these grades offer higher machinabilities in some products.

It is important to realise that there may be a trade-off between desirable properties. An example is the grade 303. This has excellent machinability, but the high sulphur content which increases the cutting speed so dramatically also substantially reduces the grade's weldability, formability and corrosion resistance. With this grade the calculated PRE is wrong, as it does not factor in the negative effect of the sulphur. This grade must not be used in any marine or other chloride environments.

Selection Criteria

Before selecting a grade of stainless steel it is essential to consider the required properties such as corrosion resistance, but it is also important to consider the secondary properties such as the physical and mechanical properties and the ease of fabrication of any candidate grades. The correct choice will be rewarded not just by long, trouble-free life, but also by cost-effective fabrication and installation.

 

Source: Atlas Steels Australia

 

For more information on this source please visit Atlas Steels Australia

 

Date Added: Jan 2, 2002 | Updated: Jun 11, 2013
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