Dairy, Food and Environmental Sanitation, Vol. 20, No. 7, Pages 506-517 Copyright© International Association for Food Protection, 6200 Aurora Ave., Suite 200W, Des Moines, IA 50322 Stainless Steels: An Introduction to Their Metallurgy and Corrosion Resistance Roger A. Covert and Arthur H. Tuthill* and why they sometimes do not. In most cases, selection of the proper stainless steel leads to satisfactory performance. COMPOSITION, NOMEN- CLATURE AND GENERAL PROPERTIES Most metals are mixtures of a primary metallic element and one or more intentionally added other ele- This article has been peer-reviewed by two professionals. ments. These mixtures of elements are called alloys. Stainless steels are alloys, as are brasses (copper + zinc), bronzes (copper + tin), the many alu- INTRODUCTION better understanding of stainless minum alloys, and many other me- Worldwide, in industry, in busi- steels, especially to the non-metal- tallic materials. In general, solid ness and in the home, metals called lurgist. metals and alloys consist of randomly stainless steels are used daily. It is Industries are concerned with oriented grains that have a well-de- important to understand what these integrity of equipment and product fined crystalline structure, or lattice, materials are and why they behave purity. To achieve these, stainless within the grains. In stainless steels, the way they do. This is especially steels are often the economical and the crystalline structures within the true because the word “stainless” is practical materials of choice for pro- grains have been given names such as itself somewhat of a misnomer; these cess equipment. However, before ferrite, austenite, martensite, or a materials can stain and can corrode intelligent decisions can be made mixture of two or more of these. under certain conditions. People need regarding the proper selection from Many of the properties of stainless to know why these metals are the various types of stainless steel, it steels depend upon which crystalline usually bright and shiny and is necessary to have an understanding lattice occurs. Examples of these why they sometimes depart from of what stainless steels are. It is crystal structures are given in Fig. 1, this expected appearance. In this important to know what different where the black dots represent atoms paper, we hope to explain some grades of stainless steel are available, and the lines are present to help the of these phenomena and provide a why they perform satisfactorily structure to be seen. 506 Dairy, Food and Environmentol Sanitation - JULY 2000 Figure 1. Crystal structures of stainless steels (Fig. 1b) becomes stable at room temperature if nickel, manganese, nitrogen, or carbon is added, singly or in combination, to iron or iron/ chromium alloys. The resulting ma- terials are called austenitic stainless steels. In general, they are easier to shape and bend, more weldable, and less brittle than ferritic alloys. Martensite is a stable structure at ambient temperature and more similar to ferrite than to austenite. It also has a body-centred structure (Fig. 1c), but one axis of the cube has been elongated to form a tetragonal structure, that is, a crystal having all three axes at right angles Figure 2. The influence of chromium on the atmospheric corrosion of low carbon steel and with two equal sides and one unequal. It is produced by heat treat- ing or cold working cubic crystals of ferrite or austenite. Martensite is the hardest, and strongest of the three crystalline forms, but it is also the least workable. In fact, these alloys are seldom intentionally deformed. As mentioned previously, alloys are combinations of two or more elements, at least one of which is always a metal. All of the many and varied stainless steels are alloys. They are always iron-chromium alloys, but they often contain other elements, such as molybdenum or nickel. The better known varieties of stainless steel are wrought (hot- rolled or hot-forged after casting into an ingot). There are also cast counterparts that have properties similar to those of most of the wrought grades but that are altered slightly in composition in order to improve casting properties. To define the different materials, the publication Metals and Alloys in the Ferrite is the basic crystal struc- grains of other substances. They can Unified Numbering System (1) lists ture of iron or low-alloy steel at am- be quite complicated and often play over 250 types within the broad bient temperatures. To understand it, an important role in the mechanical definition of stainless steels. These envision a cube with an atom at each properties and corrosion behaviour are iron base alloys containing more of the eight corners and in the geo- of metals. than 11% chromium. Various grades metric centre of the cube. This body- Austenite is the crystal form of also contain nickel, molybdenum, centred cubic structure (Fig. 1a) is unalloyed iron in the grains at higher manganese, nitrogen and other repeated regularly in three dimensions temperature (>800°C). It is different alloying elements. As can be seen in throughout the grain until it meets a from ferrite. As in ferrite, there is an Fig. 2 (6), chromium's primary grain of different orientation. At these atom at each corner of a cube, but effect is to impart corrosion contacts are areas termed grain instead of one in the geometric resistance. The diagram shows the boundaries. Grain boundaries consist centre, there is one in the centre of influence of chromium on corrosion of many things, including the each of the six faces of the cube. when it is added to iron or steel. As interface, defects, impurities and This face-centred cubic array can be seen, when it reaches JULY 2000 - Dairy, food and Environmental Sanitation 507 steels), valve steels and iron-base Figure 3. Effect of adding nickel to Fe-Cr alloys “superalloys”, the J to cast steels (except tool steels), and N to nickel and nickel alloys. The UNS system also provides for classification of many of the newer, more complex alloys that would not fit into the old system, and it covers many types of alloys in addition to the stainless steels. It should be noted that in other countries different nomenclatures and systems may be used. For example, in Europe the EN system of numbering alloys is in common usage. With this method, S30400 (304) and S31600 (316) are replaced by the numbers 1.4301 and 1.4401, respectively. Most stainless steels have similar designations, some of which are given in Tables 2 and 3. Figure 4 (6) shows how compo- sition variations have led to many related stainless steels that have evolved from the basic S30400 (304) composition. By altering the com- position, as indicated by the arrows 11-14%, corrosion is practically Institute, applied to the cast grades. and text in the figure, various com- negligible in the atmosphere. These early systems divided positions are produced to meet par- Nickel in stainless steel promotes stainless steel alloys into groups ticular needs. In many cases, this is austenite stability and reduces the according to crystal structure. done by adding or omiting small temperature at which austenite can However, many of the newer alloys amounts of other constituents without exist. Figure 3 illustrates this did not fit into the earlier categories, making major changes in the primary effect. Above the diagonal line in and it became necessary to have a alloy content. the diagram, austenite is stable at more complete system. Therefore, the indicated temperature; below these older nomenclatures are now the line, either ferrite or martensite being replaced by the, Unified COMMON STAINLESS STEEL is the stable crystal structure. Numbering System (UNS)(1) ALLOY SYSTEMS Manganese is similar to nickel developed by the Society for when it is added to or substituted Automotive Engineers (SAE) and Austenitic alloys - iron-chrom- for nickel and also increases the American Society for Testing ium-nickel and iron-chromium- strength. Molybdenum increases and Materials (ASTM). These groups manganese-nickel alloys the resistance to localized have developed a six-character corrosion phenomena, such as notation that assigns a unique Some of these alloys also con- pitting and crevice corrosion. designator to metals and alloys in a tain nitrogen, copper, silicon, and Nitrogen also improves resistance way that consistently defines a other elements for special purposes. to crevice corrosion, as well as material. For example, the UNS They have an austenitic, i.e., face- increasing strength and acting as number S30403 replaces AISI 3041; centred cubic crystal structure an austenite stabilizer. Elements the final two digits, 03, indicate the within the grains. To obtain this such as copper and silicon improve maximum permitted carbon content. structure, the austenite/ferrite corrosion resistance in special In other alloys, the various digits transition temperature is suppressed environments, and silicon also may refer to other parameters; so it by the addition of alloying agents, improves casting properties. cannot always be assumed that the primarily nickel, but also To reduce confusion and sim- latter numbers mean carbon content. manganese and nitrogen, so that plify nomenclature, standard The equivalent cast alloy is J92500, the resulting austenite is stable numbering systems have been which formerly was ACI CF-3. It at ambient temperature (see Fig. 3). developed for the various stainless should also be noted that the letter These alloys are grouped in the steel alloys. For many years in the before the numbers in the Unified 300 and 200 series, respectively, in United States, the three-digit Numbering System pertains to the old AISI system. They are method of the American Iron and different alloy classes. All letters non-magnetic, unless heavily cold- Steel Institute (AISI) was common used will not be defined here, but worked, and hardenable only for wrought stainless steels.