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           020-31190575 & 18925189619 (In Mandarin, Cantonese & English)


          <早知道> 關于不銹鋼最常見的問題


          關于不銹鋼有很多困擾人們的問題, 比如:不銹鋼會生銹嗎? 不銹鋼沒有磁性嗎? 既然不銹鋼那么好, 為什么不用來造航母? 等等, 下面是部分問題的答案。



          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.


          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.


          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 heat exchangers, food & beverage equipment, chemical equipment, and architectural applications. Ferritic stainless steels are used for automotive applications, 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.


          Because the predominant element in steel—iron—is magnetic, most steels do have magnetic properties. This is true of many stainless steels, too. All the ferritic, martensitic, and precipitation hardening grades are magnetic.
          The high nickel and chromium contents of the T300 series of austenitic stainless steels renders the alloys initially non-magnetic. For instance, a common austenitic steel, 304 stainless contains 18% chromium and 8% nickel. However, these alloys can also become magnetic during the cold-working processes used to harden them.


          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.


          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.




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