Stainless Steel Tubing Alloys
Stainless Steel Tubing Alloy Information
Within the family of stainless steel tubing, there are a variety of alloys offered by Vita Needle. Alloys are defined by the composition of various elements that comprise the metal, measured as % of the weight. Iron is the primary alloy for stainless steel tubing alloys, and is augmented by a variety of other elements so that the alloy provides certain performance characteristics desired by the end user. For example, 304 stainless steel tubing products contain around 18% Chromium and 8% Nickel (bounded by allowable minimum and maximum values) which along with other alloying elements provides the bright, tough and durable product familiar to all users whether in tube form or kitchen utensils. From a base austenitic alloy like 304, other elements can be added and the content percentages of each alloy are varied to produce other alloys with enhanced tube performance characteristics, such as improved corrosion resistance, better performance at high temperatures, greater tensile strength values etc. The tradeoff with the alloys that have improved performance is that they often cost more because the base metal is less widely available and these other alloys have a greater percentage of their composition from expensive and rarer elements, like titanium and molybdenum.
Not surprisingly because of its relative cost effectiveness and widely accepted use, the majority of stainless steel tubing and wire sold by Vita Needle Company falls in the alloy 300 series stainless steel family. The key element in all stainless steel is chromium, which when alloyed with the remaining elements, permits formation of a microscopically thin, passive layer on the surface of the metal which resists oxidation (rusting). This invisible layer is consistently self repairing in a normal environment---in other words, if the tubing surface is damaged, the layer reforms to protect the underlying tubing from oxidation at the atomic level. The condition of this layer on the surface of the tubing can be enhanced by a process known as passivation, which removes free iron and contaminants from the surface of tubing that might otherwise limit or reduce the formation of the passive layer. Certain alloys, like 316, have increased percentages of select elements that improve the passive layer’s resilience. However, under exposure to aggressive oxidizing environments, the self-repairing nature of the passive layer can be reduced, with the level of degradation depending in part on the alloy selection.
Key benefits of using stainless steel in tubing applications include:
- Cost effectiveness relative to other tubing alloys. Stainless steel tubing is widely approved and accepted material in applications ranging from industrial to medical devices.
- Relative ease to clean tubing and maintain acceptable hygiene for medical, dental and food applications.
- Appearance of stainless steel tubing is pleasing to the eye and consistent with other medical instruments of similar construction.
- Tubing made from stainless steel can be fabricated by numerous methods, from traditional machining/grinding methods to conductive machining methods like electrochemical grinding (ECG) or electrical discharge machining (EDM).
- Good toughness and strength to weight characteristics for hard temper tube during cold work applied during tubing manufacture.
Below is a listing of most commonly available stainless steel tubing alloy selections.
Stainless Steel Tubing Alloy Selections
304 / 304 L: The most common 300 stainless series alloy in tube form, 304 has good corrosion resistance, cold working characteristics and relative cost effectiveness that make this alloy our most popular selection. The "L" grade version has a lower carbon content for reduced carbide precipitation during secondary welding operations.
316 / 316 L / 316 LVM: The addition of molybdenum to the base metal improves corrosion resistance for use in medical / food service and other demanding tubing applications. "L" grade is a low carbon version for reduced carbide precipitation during secondary welding operations. 316 LVM (LVM stands for L grade vacuum melt) is melted in a vacuum furnace before forming to reduce impurities introduced from air and create a uniform chemistry.
17-7: The addition of aluminum in this alloy provides improved tube strength and hardness while maintaining good corrosion resistance. Can be heat treated (CH-900 heat treatment) to a higher tensile strength than can be achieved by cold work alone. Far better strength to weight characteristics than same size 304 hard temper tube.
302: Similar corrosion resistance to stainless 304, but higher strength due to carbon content. Typically not available in tubing, sold in wire/solid bar form only. Generally, slightly less expensive than 304 wire of the same size and temper.
303: An ideal stainless steel alloy for machining parts due to the addition of elements which reduce tool wear / expense. Available for machined parts only, not offered as tubing.
321: Increased strength and corrosion resistance at higher temperatures, are characteristics of this alloy due to the addition of titanium as a stabilizing ingredient to the base metal. The titanium bonds with much of the carbon in the alloy, preventing the carbon from bonding with the chromium as chromium carbides. The result is that more chromium is “free” in the alloy to resist corrosion.
347: Characteristics are similar to 321 but with improved corrosion resistance at higher temperatures. The addition of columbium elements allows tubing to withstand high temperatures for longer periods without suffering from chromium carbide precipitation.
Finally, Vita supplies other specialty stainless alloys on request. Although immediate inventory for delivery may be limited for specialty alloys, our fabrication capabilities allow us to manufacture off-list tubing alloys in reasonably short lead times.