Selecting the best alloy for a tubing application

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Finding the Optimal Metal Tubing for Medical and Critical Applications
Selecting the best alloy for a tubing application can be a daunting task considering the wide variety of stainless steel and metal alloys available. The chromium in stainless steel forms a coherent passive oxide film and oxygen barrier, which imparts corrosion resistance. All stainless steels contain at least 10.5 percent chromium, although most grades contain at least 12 percent to maintain passivity. Understanding the different alloy series can aid in material selection. Austenitic, ferritic, martensitic, precipitation hardening (PH) and duplex are the main series of stainless steel alloys.

The austenitic 300 series of stainless steel alloys contain nickel additions as well as chromium, which improves ductility and retains an austenitic crystal structure in the alloy even at room temperature. The 304 and 316 grades are the most widely used. 304 stainless steels nominally contain 18 percent chromium and 8 percent nickel. 316 stainless steels contain approximately 16 percent chromium, 10 percent nickel and 2 percent molybdenum. The molybdenum additions make the chromium oxide film denser, which enhances passivity and improves pitting and crevice corrosion resistance in chloride and marine environments.

Ferritic and martensitic 400 series stainless steels do not contain the nickel additions, so an austenitic structure is not retained at room temperature. The absence of nickel reduces material cost. Austenitic and ferritic stainless steels have low carbon levels that are not hardenable by heat treating. Austenitic and ferritic stainless steels can be work hardened through mechanical processing to partial or full hard tempers. Austenitic stainless steels have superior corrosion resistance compared to ferritic and martensitic grades, especially in wet environments. Ferritic stainless steels are easy to form, with sufficient corrosion resistance for dry applications.

Martensitic grades have higher carbon levels (1.2 percent in 410), so these alloys can be quenched and tempered to high hardness, strength and toughness levels. PH steels can provide corrosion resistance similar to austenitic steels but with higher tensile strength. Duplex stainless steel grades contain both ferrite and austenite. They have superior chloride stress corrosion cracking (SCC) and pitting resistance with about twice the strength of austenitic grades.

Mainstream stainless steel alloys may not have the required properties for special applications. Highly alloyed superaustenitic, superferritic and superduplex stainless steels are available with enhanced properties for extremely demanding applications. For medical implants, titanium tubing has superior biocompatibility. In certain high temperature, extreme corrosion and wear applications, cobalt- or nickel-based alloys might be the only viable options. Copper-nickel or Monel® tubing is useful in desalination and marine applications. Aluminum tubing might be the best option where high thermal conductivity or lightweight components are required.

Performance, Properties and Material Relationships
Identifying the environmental, thermal and mechanical performance requirements is the first step in selection, in order to determine what material properties are needed for reliable service throughout the tubing component’s lifetime. From the material property constraints, a group of candidate tubing materials can be formed.

Environmental Performance: Corrosion and Oxidation Resistance 
Austenitic 304 and 316 stainless steels are considered biocompatible-, surgical- or medical-grade stainless steels. Austenitic stainless steels are suitable for implants, cannula, stents, catheters or medical devices with long-term exposure to blood or human tissue. Martensitic grades are useful for surgical punches, scalpels and medical instruments that must retain sharp edges. These grades cannot match the corrosion resistance of implant grade austenitic stainless steels.

In pharmaceutical, power generation and chemical process applications, austenitic stainless steels like 304 are typically the first choice. Certain critical petroleum, chemical process and marine applications with corrosive chloride gas media require the improved pitting and crevice corrosion resistance of 316 or even 317 grade molybdenum-modified stainless steels. SCC can occur if the tubing component is highly stressed while exposed to a corrosive media. Duplex, superaustenitic and high nickel (30 percent) stainless steel might be warranted for their SCC resistance in these applications.

High-temperature oxidation and scaling in power generation, refineries, furnaces, jet engines and chemical process plants is another performance challenge. Several stainless grades with high nickel and chromium additions have been developed for improved oxidation and scaling resistance at elevated temperatures, such as 309, 310, 321, 314, 330 and 446. Inconel® and Nimonic® superalloys are valuable for tubing within a jet engine, turbocharger or power generation gas turbine.  read more.....

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Custom Tubing Designed for Your Needs
Making a difference in your design… from simple tubing to complex designs.
At Eagle, there’s nothing more important than producing a customized solution which exceeds our clients’ expectations and satisfies the unique specifications of their application. Our expertise with stainless steel custom tubing gives us the ability to help you design the product that will fit your project.

Complex designs need more than just stainless steel custom tubing. Secondary operations such as cutting to precision lengths, CNC and EDM machining, laser welding, bending, and precision grinding can transform a simple tube into just the part you need for your design.

We welcome the opportunity to share our expertise in developing product concepts and helping you solve your engineering problems. With decades of experience in the metals industry, you can count on Eagle as a resource for designing and fabricating your metal products.

Eagle Stainless Tube & Fabrication Inc.
Eagle Stainless Tube & Fabrication Inc.supplies stainless steel tubing to the medical, aerospace and high tech industries. Stainless steel, nickel alloys, aluminum, and titanium.

  • Cut to length
  • Precision machining
  • Laser cutting & welding bending
  • Electro-polishing and passivation.
  • Re-draw to specialty & millimeter sizes in both round, square and rectangular.
  • Grinding to precision tolerances
  • Assemblies

 

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ISO 13485