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Resources Industrial Heating Articles Stainless Steels Part One: Classification and Selection | Stainless Steels Part One: Classification and Selection |
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By Daniel H. Herring Stainless steel is primarily used for applications where resistance to corrosion or heat, or both, is required. For their cost, the performance enhancement achieved by stainless steels is unmatched. In the past and continuing through today, there have been a number of ways developed to retard or prevent corrosion, the most common of these being painting or coating with metals and non-metals. Stainless steels offer an attractive alternative. A Little History An Englishman named Harry Brearley is considered the first not only to recognize the superior corrosion resistance of an iron-based alloy containing chromium but also put this property to use for making “rustless” cutlery from a 12.8% Cr alloy. Brearley’s discovery, which included a heat treatment to harden the alloy, was the result of a chance observation. While trying to prevent corrosion and fouling in rifle barrels, he alloyed iron with chromium and observed during metallographic work that these steels resisted attack by etchants. He later gave these ferritic iron-chromium alloys the name “stainless” steels. This name was also applied to the austenitic iron-chromium-nickel compositions, which were being developed into commercial products in Germany about the same time. Today, over fifty grades have designation numbers, and there are at least another 100 non-standard compositions, which are marketed under various trade names. Commonality between Grades
Chromium is the common denominator among the various grades of stainless steel; all grades of stainless steel contain a minimum of 11.5% chromium. This is the minimum amount required for a stainless steel to be resistant to rusting in a non-contaminated outdoor environment with 100% relative humidity. Resistance to oxidizing acids is another property that they share. Stainless steels are typically divided into the following major sub-categories:
Austenitic Grades Austenitic grades (Fig. 1) are those alloys that are widely used in corrosive environments, such as the chemical and food industry. The austenitic grades are non-magnetic, have unusually good mechanical properties and do not respond to conventional quench hardening heat treatments. They consist of the iron-chromium-nickel steels, widely known as the 300 series. The 200 series are also members of this group, where some of the nickel has been replaced by manganese. Martensitic Grades
Martensitic grades (Fig. 2) were developed in order to provide a group of stainless alloys that would be corrosion resistant and hardenable by heat-treating. Their crystal structures change on heating and cooling. The martensitic grades are straight chromium steels containing no nickel. They are magnetic and are used in applications where hardness, strength, and wear resistance are important considerations. Steels in this group carry the 400 series designation (as do some of the ferritic stainless steels – See Fig. 3). The chromium content of these steels is generally lower than for the austenitic grades and, in general, the corrosion resistance of the martensitic grades is far lower than the austenitic grades (and somewhat lower than the ferritic grades). Ferritic Grades Ferritic grades have been developed to provide a group of stainless steels to resist corrosion and oxidation while being highly resistant to stress corrosion cracking. The structure is ferritic at all temperatures (hence the name). These steels are magnetic but cannot be hardened or strengthened by heat treatment. They can be cold worked and softened by annealing if required. Nitriding is the only heat treatment that is sometimes applied to ferritic grades. As a group, they are more corrosion resistant than the martensitic grades but generally inferior to the austenitic grades. Like martensitic grades, these are straight chromium steels with no nickel. Automotive applications are commonplace. Precipitation Hardening Grades These grades of stainless steels were developed primarily for use as aerospace materials but now are gaining wider commercial appeal since they are cost effective for a variety of applications and are available in a range of products (bar, rod, wire, forgings, sheet, and strip). As a class, they offer a unique combination of strength, fabricability, ease of heat treatment, and corrosion resistance not found in the other grades. They are classified as the 600 series but are also known by the designations 13-8Mo, 15-5PH, 17-4PH, and 17-7PH. The austenitic precipitation hardenable alloys have, to a large extent, been replaced by the more sophisticated and higher strength superalloys. The martensitic precipitation-hardenable stainless steels are the workhorse of the family. Duplex Grades
Duplex grades are the newest of the stainless steels. These materials are a combination of austenitic and ferritic materials. They have higher strength and superior resistance to stress corrosion cracking. These are usually mill ordered materials with designations such as 2005. Superalloy Grades These include a variety of alloys with very large amounts of nickel, chromium, and molybdenum, as well as other alloying elements designed to enhance performance and resist attack at high temperatures. These alloys are based on nickel, iron-nickel, and cobalt-nickel and exhibit a good combination of mechanical strength and resistance to surface degradation. They are not usually grouped with the stainless steels and have instead their own classification. Considerations When Selecting Stainless Steels The correct choice of the best stainless steel for a given application is dependent on a number of factors. Key considerations include:
Stainless Steels Part Two: Heat Treatment Techniques
Daniel H. Herring - Tel: (630) 834-3017) References
1. Streicher, Michael A., Stainless Steels: Past, Present and Future, Stainless Steels ’77, A Conference sponsored by Climax Molybdenum Company. Published with the permission of Industrial Heating Magazine |
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