7 THINGS TO CONSIDER WHEN CHOOSING A STAINLESS STEEL GRADE

 

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.

 

Here are 7 things to consider when choosing a stainless steel grade:

Does it need to have good formability?

Does it need to be welded?

Does it need to be machined?

How much and what type of corrosion resistance is desired?

Does it need to be heat treated?

What are the strength requirements?

What are the typical applications?

Does the stainless steel need to have good formability?

 

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.

 

Does the stainless steel need to be welded?

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.

 

Does the stainless steel need to be machined?

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.

 

How much and what type of corrosion resistance is desired?

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.

 

Does the stainless steel need to be heat treated?

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.

 

What are the strength requirements of the stainless steel?

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.

 

Typical Applications

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:

Grade 409: Automotive exhaust systems and heat exchangers

Grade 416: Axles, shafts, and fasteners

Grade 430: Food industry and appliances

Grade 439: Automotive exhaust systems components

 

Austenitic Stainless Steels:

Grade 303: Fasteners, fittings, gears

Grade 304: General purpose austenitic stainless steel

Grade 304L: Grade 304 applications that require welding

Grade 309: Applications involving elevated temperatures

Grade 316: Chemical applications

Grade 316L: Grade 316 applications that require welding

 

Martensitic Stainless Steels:

Grade 410: Generable purpose martensitic stainless steel

Grade 440C: Bearings, knives, and other wear resistant applications

 

Precipitation Hardened Stainless Steels:

17-4 PH: Aerospace, nuclear, and chemical applications

15-5 PH: Valves, fittings, and fasteners

 

Duplex stainless steels:

2205: Heat exchangers and pressure vessels

2507: Pressure vessels and desalination plants