Stainless Steel Grade Selection Guidelines

While choosing stainless steel for a specific application, it must be noted that the fundamental properties listed in Table 1 need to be considered. These properties vary for all five basic types of alloys, such as precipitation hardening, martensitic, Ferritic, duplex and austenitic.

Selection for Corrosion Resistance

Selecting an inexpensive steel grade for a particular type of corrosive environment is difficult. One of the key factors in material selection is the consideration of the service life of the metal, corrosion resistance properties and environment in which the material can be used. For selecting grades that are resistant to strong acids, several tables of recommended grades have been published, in addition to iso-corrosion curves, which indicate the corrosion rate of each grade. These tables are usually developed in order to compare different grades and select the suitable one. However, care must be taken, as there are minor differences between similar environments, which can, in turn, create a large difference in terms of corrosion rates.

The table below provides the fundamental properties of stainless steels:

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

Stainless steels corrode often through local corrosion. The presence of chloride in the operating environment controls the mechanisms of crevice and pitting corrosion.

Pitting Resistance Equivalent

The pitting and crevice corrosion resistance of a particular stainless steel grade is denoted by a Pitting Resistance Equivalent number (PRE), as displayed in table 2. PREs can be calculated using the following equation:

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

It was estimated that grades having high alloying elements such as molybdenum, nitrogen and chromium are more resistant to crevice and pitting corrosion. Due to this reason, grade 316, with 2% molybdenum, is used in marine fittings. Super austenitic steel grades, having up to 6% molybdenum and high chromium and nitrogen additions, can be used in rigorous chloride-containing environments.

The PRE number for various stainless steel grades are tabulated below:

Table 2. Pitting resistance equivalent number or PRE for various grades

*The calculated PRE for grade 303 is inaccurate, due to the presence of sulphur in the composition.

Stress Corrosion Cracking

Stress corrosion cracking (SCC) is one of the most common problems of austenitic steel grades. SCC occurs in the presence of traces of chloride. The duplex steel grades are highly resistant to SCC, and Ferritic grades are moderately resistant to this attack.

Selection for Mechanical and Physical Properties

Precipitation hardening and martensitic steels with high strength are the best choice for valve spindles and shafts. These grades were estimated to have more strength than that of grades 304 and 316. High strength, like corrosion resistance, is a very important criterion for selecting stainless steel grades.

The grade is more often selected based on the required corrosion resistance, and the structure is developed with respect to the physical and mechanical properties of the selected grade. The secondary properties of these grades should also be considered as early as possible during the selection process. The selection of high strength duplex grades, such as grade 2205, solves the problem of corrosion and provides cost effectiveness due to its high strength.

On the other hand, the selection of Ferritic grades, such as grade 3CR12, can provide sufficient corrosion resistance for a non-decorative application. In addition, a low thermal expansion coefficient could be desirable because of less distortion from temperature changes. The Ferritic grades have thermal expansion rates similar to that of mild stee,l and two-third of austenitic grades such as grade 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.

Fabrication affects the cost-effectiveness of the product; hence this factor must be considered very seriously. Some of the common grades and their fabrication characteristics are listed in Table 3.

The following table shows the important fabrication characteristics of certain steel grades. The rate of comparisons in the table is on arbitrary scales with 10 representing excellent fabrication using a specific method.

Table 3. Common grades and their relative fabrication characteristics

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

It should be noted that the desirable properties of some products might be compromised during the selection process. For instance, grade 303 has excellent machinability and high sulphur content, which not only increases its cutting speed but also reduces corrosion resistance, formability and weldability. Therefore, this grade is not recommended for use in chloride or marine environments.

Selection Criteria

It is necessary to consider key properties, such as corrosion resistance, prior to the selection of stainless steel grades. However, secondary properties, such as mechanical and physical properties, and the ease of manufacture of any grade should also be taken into account. The appropriate choice is not only based on a prolonged, error-free service life, but also on inexpensive installation and fabrication.