What Exactly Is the Difference Between Stainless and Ordinary Steel?
All steel is iron based, but stainless steel contains chromium at 10% or more by weight. Steel is susceptible to rust, an active iron oxide film that forms when steel is exposed to air and moisture. This iron oxide film is active and accelerates corrosion by forming more iron oxide. Stainless steel is rust resistant because of passivity. Passivity is when chromium forms a passive film of chromium oxide to repair the surface layer and prevent further corrosion.
Stainless steel’s non-corrosive characteristic gives it a long lifespan. With the addition of nickel, the material survives even longer and in several types of environments. By adding molybdenum and nitrogen, stainless gains improved pitting and crevice corrosion resistance. Stainless steel has a high strength-to-weight ratio even at high temperatures. Adding alloys to stainless steel helps it resist heat and scaling. Stainless steel is easy to clean because the surface is non-porous. It is well suited for medical and food-grade applications because it prevents the absorption of bacteria and germs. Stainless steel does not stain and can be polished to create an aesthetic appearance.
How Do you Classify Different Types of Stainless Steel?
Stainless steels are classified according to their metallurgical structure: austenitic, ferritic, martensitic, precipitation hardening, and duplex. Austenitic stainless steels contain chromium and nickel. They are hardened only through cold working and are non-magnetic although some may become magnetic by cold working. Austenitic stainless steels are identified as T300 series stainless steels (typical alloys T304, T316). Alloys containing chromium, nickel, and manganese are identified as T200 series (T201, T202). Ferritic stainless steels are straight chromium. They are magnetic, have good ductility, and resist corrosion and oxidation. Ferritic stainless steels cannot be hardened by heat treatment and are identified as T400 series (T409, T430). Martensitic stainless steels are also straight chromium but can be hardened by heat treatment. They are magnetic and resist corrosion in mild environments. They have fairly good ductility and some can be heat treated to tensile strengths in excess of 200,000 psi. Martensitic stainless steels are identified as T400 series (T410, T416). Precipitation-hardening stainless steels are chromium-nickel types identified as alloys T17-4 or T17-7 PH. They can be hardened by solution treating and aging. Duplex stainless steels have an annealed structure which is typically equal parts of austenite and ferrite. The duplex grades are highly resistant to corrosion and pitting. They have about twice the yield strength of conventional grades. T2205 is the standard alloy for duplex grade.
What Typical Applications Are Suitable For Each Series of Stainless?
Applications for stainless steel can vary but some common examples include cookware, cutlery, sinks, aerospace components, medical device instruments, oil and gas lines, beverage applications, or laboratory instruments. Stainless steel is a good choice for applications where corrosion and heat resistance are important in material selection. More specifically, austenitic stainless steels are used for food equipment, chemical equipment, and architectural applications. Ferritic stainless steels are used for appliance trim and cooking utensils. Martensitic stainless steels are used for fasteners and pump shafts. Precipitation hardening stainless steels are used for valves, gears, and petrochemical equipment. And duplex stainless steels are used across applications similar to austenitic and ferritic stainless steels. Examples include marine environments and pollution control equipment.
What Forms Does Stainless Steel Come In?
Stainless steel is also available in bar, wire, tubing, pipe, and plate form.
Is Stainless Steel Magnetic?
All stainless steel with the exception of Austenitic steels (300 series) are magnetic.
What Does the L After a Stainless Steel Type Mean?
The L indicates low carbon grades. The “L” grade material helps reduce the possibility of carbide precipitation forming during processes such as welding. Carbide precipitation may then lead to intergranular corrosion attack where the material becomes more sensitive to possible corrosion. Typically the carbon is kept to .03% or under; however, depending upon the specification or standard referenced it may vary.
What is the Difference Between Tubing and Pipe?
In some instances the terms may be used interchangeably; however, there are some differences is in how the material is ordered and toleranced. Tubing is typically ordered to outside diameter and wall thickness; however, it may also be ordered as OD & ID or ID and Wall Thickness. Although tubing has three dimensions (O.D., I.D. and wall thickness) only two may be specified with tolerances and the third is theoretical. Pipe is typically ordered using the Nominal Pipe Size (NPS) standard and by specifying a nominal diameter (pipe size) and schedule number (wall thickness). Also tubing is usually ordered and held to tighter and more stringent tolerances and specifications than pipe.
What is the “Annealed” Condition?
Stainless Steel is usually sold in the “annealed” condition. It means that the material is in the “soft” or annealed condition. The 300 series of stainless can not be hardened by heat treatment (like carbon steels) but can be hardened by cold working. This cold work can be eliminated by heat treatment (annealing) that will restore the original soft condition.
What is Cold Working? (Cold Forming)
Any mechanical operation that creates permanent formation, such as bending, rolling, drawing performed at room temperature that increases the hardness strength of stainless steel.
How Do You Harden Stainless Steel?
Some types of stainless steel can be hardened through heat treatment while other types need to be cold worked. Examples of cold working include swaging or drawing.
What Kinds of Techniques Are Used to Fabricate Stainless Steel?
Stainless steel can be fabricated using techniques such as lathes, milling machines, welding torches, laser machining centers, stamping presses, centerless grinding, swaging, forming machines, electrical discharge machining, and more.
Why Is Fabricated Stainless Steel Treated With Oxidizing Chemicals?
Iron particles or other substances can become embedded in the stainless surface during fabrication or polishing operations. These particles must be removed or else they will cause discoloration, rusting, or even pitting. Oxidizing chemicals are used to form a passive layer along the surface of stainless steel to enhance resistance to corrosion. This process is the final preparation for stainless steel parts. It consists of immersing the parts in a nitric acid solution, rinsing with clear running water, and drying.
If the Part Is Too Large to Submerge in a Chemical Bath, How Can you Tell If All Areas of the Part Have Been Treated?
If immersion of the stainless steel piece is impractical due to size, the acid solution may be applied with a suitable swab and removed by rinsing with water. There are several tests which can be performed on the oversized part to determine passivity such as a high humidity test, salt spray test, copper sulfate test or free iron test.
Why Choose Stainless Steel vs. Aluminum?
Both stainless steel and aluminum have benefits and weaknesses. Stainless steel is often chosen for applications that require corrosion resistance, heat resistance, and high strength to weight ratio. Aluminum is chosen over stainless steel when the application requires less weight. The same part made from aluminum versus stainless steel is one-third the weight. This is why aluminum is well suited for applications such as airplanes.