Stainless steels are steels with a minimum of 10.5% chromium. They gain their resistance to corrosion from a thin, tenacious surface layer of chromium oxide. If the oxide layer is physically damaged there is rapid regeneration of the layer, thus preserving the corrosion resistance. However, a chemical environment that can disrupt this layer can initiate corrosion. Consequently, stainless steel is highly resistant to atmospheric corrosion, but not immune to corrosion in all environments.
Stainless steels fall into five general classes: austenitic, ferritic, martensitic, duplex, and precipitation hardened. The distinction between each class is based primarily on the predominant phase present in the stainless steel as determined by the major alloying elements.
The major alloying elements in stainless steels are chromium and nickel. Chromium primarily provides corrosion resistance and additional strength. Nickel provides strength and some corrosion resistance. Minor alloying elements include manganese, carbon, and molybdenum. Manganese is present in steels in small quantities, but at higher concentrations it stabilizes austenite and partially replaces nickel in the 200-series steels. Carbon is largely an impurity in austenitic steel, but it is a strengthening element in ferritic and martensitic steel, much as it is for carbon and low-alloy steel. Molybdenum provides additional strength and resistance to chloride pitting. Other elements, such as titanium or niobium, serve other purposes specific to the application for which the alloy was developed.
400-series steels were the first versions of stainless steel. They include the ferritic and martensitic grades that contain only chromium as a major alloying element, making them less expensive than austenitic grades. They are magnetic, generally more resistant to chloride attack than 300-series alloys, and some grades may be strengthened by heat treating.
Type 410 contains about 12% chromium. The ability to strengthen this alloy by heat treating to form martensite makes it a martensitic grade. Its low chromium content provides modest corrosion resistance. Given enough time, exposure to weather will cause it to rust. Type 430 is a ferritic grade that contains about 17% chromium. It cannot be strengthened by heat treating. 400-series stainless steels are generally more resistant to chloride attack than 300-series.
With sufficient quantities of nickel, stainless steel remains austenite at room temperature, creating the austenitic steels. They are nonmagnetic and cannot be heat treated for through hardening like carbon steels because the phase transformation to martensite does not occur in these alloys. The primary reason for their use is their superior resistance to corrosion in the atmosphere and aggressive chemical environments compared to 400-series.
300-series alloys contain chromium and nickel, and are the most popular austenitic grades. Types 301 and 304 are the most common alloys in use and are for general use. They contain 18% chromium, 9% to 10% nickel, and up to 0.15% carbon (301) or 0.08% carbon (304) as an impurity. Other 300-series alloys are modified versions of these alloys to achieve specific properties. Type 316 contains 2%-3% molybdenum to improve the resistance to corrosion in chloride-containing environments. Types 304L, 316L and other L-grades contain reduced carbon, less than 0.03%, to avoid microstructure changes during welding and other thermal processes which can damage the corrosion resistance. This detrimental change is known as sensitization. Types 321 and 347 contain small amounts of titanium and niobium, respectively, to prevent sensitization. They are capable of service at elevated temperatures, while the L-grades are intended to resist sensitization during fabrication. The image below shows the austenite grains in a 304 alloy.
Austenite grains in a 304 alloy. The particles on the grain boundaries are chromium carbides.200-series steels, also austenitic, have manganese substituted for some of the nickel as a cost-saving measure. Grade 201 contains about 17% chromium, 6.5% manganese, and 4% nickel. It has corrosion resistance similar to 301.
Precipitation hardening (PH) steels are strengthened by heat treating to form precipitates, as well as by martensite formation. They can be strengthened to higher hardness than 400-series grades by an aging method similar to that of aluminum alloys. 17-4 PH and 17-7 PH steels contain 17% chromium and 4% or 7% nickel, respectively. Minor alloying elements can include copper, titanium and niobium, and others.
Duplex stainless steels allow savings in material costs in corrosive applications such as chemical processing, including chloride- and sulfur-bearing environments. They consist of a mixture of austenite and ferrite in roughly equal proportions. Duplex stainless steels are subdivided into lean, standard, super, or hyper duplex based on the quantity of alloying elements. Duplex stainless steels contain more chromium and less nickel than 300-series and typically include nitrogen as an additional austenite stabilizer and molybdenum for corrosion resistance. 2205 (22% chromium, 5% nickel, and 3% molybdenum) is a common standard duplex stainless steel, and 2507 (25% Cr, 7% Ni plus 4% Mo) is a common super-duplex steel. The micrograph below shows a duplex stainless steel.
Micrograph of duplex stainless steel with ferrite (dark-colored phase) and austenite (light-colored phase). (Courtesy of Charles Young, P.E.)The broad array of stainless steels available provides a vast portfolio of capabilities. However, each alloy has distinct advantages and disadvantages. When selecting a grade of stainless steels available it is important to consider how components will be fabricated and joined together, the specific environment to which it will be exposed, in addition to the considerations common to other alloys such as mechanical requirements and cost.
More information about stainless steels is in ASM Specialty Handbook: Stainless Steels by J.R. Davis and ASM Metals Handbook Volume 1: Properties and Selection of Irons, Steels, and High-Performance Alloys. This article was written with the assistance of Charles Young, P.E.
The material we know as stainless steel (also commonly referred to as "Inox" or "Rostfrei") is such a common feature of 21st century living that there can be few of us who have not seen or handled articles made from it. But how many of us really know what stainless steel is?
Like all types of steel, stainless steel is not a single metal but an alloy that is a material made from two or more separate elements alloyed or "melted" together. What all steels have in common is that their major "ingredient" (alloying element) is the metal iron, to which a small amount of carbon has been added. Stainless steel was invented early in the 20th century when it was discovered that a certain amount of the metal chromium (usually a minimum of 11 per cent) added to ordinary steel gave it a bright shiny gloss and made it highly resistant to tarnishing and rusting. This rust-resisting property which we call “corrosion resistance” is what sets stainless steel apart from most other forms of steel.
For more information about the corrosion-resistance of stainless steel, see Corrosion Properties.
It is important to appreciate that stainless steel is a solid material and not a special coating applied to ordinary steel to give it “stainless” properties. Conventional steels and, indeed, several other metals, are often coated or "plated" with white metals such as chromium, nickel or zinc to protect their surfaces or to provide other surface characteristics. While such coatings have their own benefits and are still widely used, the danger exists that the coating can be penetrated or damaged in some way, such that its protective effect is undermined.
The appearance of stainless steel can, however, vary and will depend on the way it is made and finished.
For more information about the ways in which stainless steel can be fabricated and finished, see Fabrication and Surface Treatment
It is, of course, the rust-resisting characteristic which gives stainless steel its name. However, soon after its discovery, it was realised that the material had many more valuable properties which make it suitable for a vast range of diverse uses. In fact, the number of uses to which stainless steel can be put are almost limitless, a point which can be illustrated with just a few examples:
In the home:
For more information on the uses of stainless steel in the home and in the office, see Home & Office Uses
In the town:
For more information on the uses of stainless steel in the building industry, see Architecture, Building and Construction
In industry:
For more information on the uses of stainless steel in industry, see Industrial Applications
While the original form of stainless steel, (iron with around 12% Chromium) is still in widespread use, engineers now have a wide choice of different types (grades). In all, there are more than 100 different grades but these are usually sub-classified into distinct metallurgical “families” such as the austenitic, ferritic, martensitic and duplex families.
For more information on the different types of stainless steel, see Categories and grades.
The proportions of iron to chromium may be varied and other elements such as nickel, molybdenum, manganese and nitrogen may be added to widen the range of capabilities. Each particular grade of stainless steel has its own unique mechanical and physical properties and will usually be produced in accordance with an established national or international specification or standard.
For more information on the various national and international standards which define the various grades, see Standards.
Whether you are an engineer, metallurgist, designer, architect, or simply someone who wishes to know more about this remarkable material called stainless steel, this site aims to provide you with the essential information you are likely to need. In many cases we provide links to other sites where complementary documents and/or publications can be found.
Welcome to the world of stainless steel!
For more crimped wire mesh bbq mesh factories, perforated copper sheet metal, carbon steel crimped wire meshinformation, please contact us. We will provide professional answers.