Stainless steel, known primarily for its corrosion resistance, is used in a wide variety of applications. The diverse range of grades allows it to accommodate various applications across many different industries. However, having so many grades requires the know-how to select the appropriate one for the job.
If the application requires good formability, avoid the martensitic group of stainless steels. Try an austenitic grade such as 304 or a ferritic grade such as 430. Martensitic stainless steels like 410 tend to be brittle and are not readily formable. Austenitic stainless steels are usually the best choice when it comes to formable stainless steels.
Welding stainless steel is very different than welding carbon steel, and can lead to problems such as intergranular corrosion, hot cracking and stress corrosion cracking. The most weldable stainless steels are typically in the austenitic group. When welding austenitic stainless steels, grades such as 304L or 347 should be used. Grade 304L has lower carbon while 347 has niobium stabilizers added to it which help to deter intergranular corrosion. Ferritic stainless steels such as grade 430 or grade 439 are also readily weldable, as are Duplex stainless steels. Martensitic stainless are generally not suitable for welding, however, some martensitic stainless steel grades with lower amounts of carbon can be welded. With precipitation hardened stainless steels, care should be taken to ensure that the original mechanical properties are not compromised during the welding process.
If machining is required, special considerations must be accounted for when working with stainless steel. Most grades of stainless steel can be machined, however, stainless steel is very susceptible to work hardening. The machining process must be optimized to work at a rate that helps alleviate this issue, and the tools used for machining must also be kept in good working condition. Similar to carbon steels, sulfur can be added to increase machinability; grade 303 is an example of this. It is very similar to grade 304 except that sulfur has been added to it for machining purposes. Grade 416 is example of a ferritic stainless steel with added sulfur.
Stainless steel is usually chosen for its corrosion resistant properties, but it is important to know that different grades provide different amounts of corrosion resistance. Austenitic stainless steels generally provide the most corrosion resistance because of their high amounts of chromium. This makes grade 304 an excellent choice when corrosion resistance is important. Grade 316 is similar to grade 304, but it has molybdenum as part of its chemical makeup, further increasing its corrosion resistance. Ferritic stainless steels and martensitic are generally more affordable than austenitic stainless steel because they have less nickel and sometimes less chromium than austenitic stainless steels, which can result in a loss of corrosion resistance. Duplex stainless steels can be used to avoid the stress corrosion cracking associated with austenitic stainless steels.
If the stainless steel is going to be subjected to heat treatment, it is important to know how the various grades of stainless steel can be affected. For the most part, austenitic stainless steels and ferritic stainless steels are non-hardenable when heat treated. The heat treatable stainless steels are typically martensitic or precipitation hardened. Examples of these are grade 440C and 17-4 PH, respectively.
Very high strengths can be achieved with martensitic stainless steels, like grade 440C; and precipitation hardened stainless steels, like grades 17-4 PH and 15-5 PH. Austenitic stainless steels, such as grade 316, can provide high strengths as well, though not as high as the martensitic grades. Austenitic stainless steels also have more nickel than other stainless steels, so a grade like 316 will have greater toughness and ductility than ferritic and martensitic stainless steels. Duplex stainless steels can provide ferritic stainless steel properties while still maintaining a ductility and a toughness close to austenitic stainless steels.
Sometimes the best way to find out what grade of stainless steel should be used is to see what has been used in the past. Here are some examples of where certain grades of stainless steel are used.
Ferritic Stainless Steels:
Austenitic Stainless Steels:
Martensitic Stainless Steels:
Precipitation Hardened Stainless Steels:
Duplex stainless steels:
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