Nitrogen-strengthened stainless steels can provide many benefits, says Jeff Kirchner, COO of High Performance Alloys
Jeff Kirchner: Armco, the inventor of these grades, had several alloys. They were calling them 21-2N, 21-4N/22-4-9, and those became 21-6-9, 18-2Mn, 18-3Mn and 22-13-5. The commonality among these alloys was chromium, manganese and nickel with saturation additions of nitrogen. For marketing purposes in the 1970s, these alloys were branded as the Nitronic series and given alloy designation numbers. A common misconception is that the number relates to strength, when in actuality all the Nitronic grades have yield strengths in the 50-60 ksi range when annealed. The numbering system has more to do with the order in which the alloys were developed than with strength, so it just makes it more literal and associative to the manufacturer.
MM: What are some of the benefits of adding nitrogen to stainless steel?
Kirchner: Adding nitrogen to alloys has several effects on the properties. The most common benefit is an immediate boost in the yield strength, or the point at which a material will deform. The nitrogen also improves the impact resistance of alloys. For an example of the enhanced impact resistance, many of these grades will need to be Charpy tested at sub zero temperatures to conduct a valid V-notch test, which is where it breaks.
MM: Can you briefly touch on the different grades of Nitronic alloys, their characteristics and applications?
Kirchner: Nitronic 30 is a corrosion resistant, or basic, stainless-manganese steel, meaning it is good for abrasion resistance. It is commonly used for liners, chutes, and because of its extra strength and lightweight properties, it is now also being used for body panels in transportation. Nitronic 30 is available primarily in heavy sheet and light plate.
Nitronic 40 is a high-temperature stainless with low magnetic permeability. Common applications include instrument or hydraulic tubing, but it is also found in bar and sheet. 22-13-5 Nitronic 50 is a corrosion-resistant stainless, commonly used for saltwater and marine applications. It has a very low magnetic permeability. 18-8Mn (+Si +N) Nitronic 60 is an oxidation resistant (elevated temperature) as well as a corrosion- and wear-resistant stainless. It is well known for its resistance to wear and galling, but the composition performs well in many environments.
MM: What should material specifiers consider when questioning the applicability of Nitronic alloys?
Kirchner: The obvious advantage to the Nitronic series is an immediate increase in strength. Many times we have recommended Nitronic 60 to replace grade 304L or Nitronic 50 for grade 316L. They are basically quick upgrades with little change in their machining or processing capabilities. Another consideration is that these two alloys excel at the low magnetic permeability, but the biggest difference is the permeability doesn’t change when forming and fabricating. During forming and welding, grades 304L and 316L are known to transform some austenite to ferrite, raising the magnetic permeability. There are definitely some applications where this magnetism potential is not an issue at all, but it can be a concern if you have magnetic fields or sensitive electronics near a rotating assembly.
Nitronic 60 solves a common problem where a stainless or corrosion-resistant grade is needed that must also resist wear and galling. It is unique in that it has these characteristics and can still be formed and fabricated with a moderate price point. There are some great cobalt alloys that can be used for wear and galling and will resist staining from the atmosphere or cleaning, but these cost more as cobalt is normally above $12 per pound.
High Performance Alloys, based in Tipton, Indiana, with sales locations in Oregon and Texas, is a global distributor of wear-, corrosion- and heat-resistant alloys. Kirchner has been with the company for more than 25 years. Call or browse at 800/472-5569 or www.hpalloy.com.
Source: Modern Metals, November 2016