Cable Tray: Material Properties
Design Advice for Proper Material Selection
Ventilated cable tray systems are commonly fabricated from a corrosion-resistant metal or from a metal with a corrosion-resistant finish. The selection of the proper material is essentially an economic consideration.
When pure, aluminum is soft and ductile. However, most commercial uses require greater strength than pure aluminum affords. This strength is achieved by the addition of other alloying elements. These alloys have been classified into seven categories according to their chemical composition for each series of alloys of 1000 through 7000 by the Aluminum Association. In addition to alloying the pure aluminum by doping with elements such as copper, manganese, silicon and zinc, further strengthening is possible by the process of heat-treating.
The initial strength of alloys in this group is enhanced by the addition of such alloying elements as copper, magnesium, zinc and silicon, and are designated as 2000, 6000, and 7000 series. Since these alloys, singly or in various combinations, show increasing solid solubility in aluminum with increasing temperature, it is possible to subject them to thermal treatments which will produce pronounced strengthening.
The initial strength of alloys in this group depends upon the hardening effect of elements such as manganese, silicon, iron and magnesium, singly or in various combinations. The non-heat treatable alloys are designated the 1000, 3000, 4000 and 5000 series. As these alloys are work-hardenable, further strengthening is made possible by various degrees of cold working, denoted by the “H” series of tempers. Alloys containing appreciable amounts of magnesium when supplied in strain-hardened tempers are usually given a final elevated-temperature stabilization as an additional assurance.
In determining the proper aluminum alloy for structural applications such as a ventilated cable tray system, the design engineer should recognize the advantages inherent in using alloys that are heat-treatable and of being able to fabricate the structure from materials possessing known minimum values of yield strength.
Basic structural members of aluminum cable tray systems can be made from 6063-T6 aluminum extrusions, a material which economically meets the requirements of the majority of installations. The 6063-T6 alloy has adequate strength and good corrosion resistance. It is light weight, maintenance free, and because of the non-magnetic properties of aluminum, keeps electrical losses to a minimum.
Steel cable trays are used principally in environments which are relatively free from corrosive attack. They are available with various types of corrosion-resistant finishes; usually hot-dip or mill galvanized. The main advantage of utilizing steel in cable tray fabrication is the high strength to low cost ratio, however, the disadvantages of using steel are the increased structural weight, poor corrosion-resistance, and low electrical conductivity.
The idea that all steels are the same, except for chemical composition is false. Carbon steels may be produced with chemical compositions (carbon, manganese, phosphorus, sulphur and silicon) within the specified limits of a given grade and still have characteristics that are widely dissimilar. Each grade and quality variation has a useful place depending upon the end use and the methods of fabrication.
Basic components of steel cable trays are normally fabricated from either hot or cold rolled carbon steel strips of commercial quality. Steels in this category are ASTM A-569 and A-366. Pregalvanized steel conforms to ASTM A-526.
Today, hundreds of different alloy combinations exist for the endless variety of applications which utilize stainless and heat resisting steels.
The primary elements added to obtain the various properties required in the stainless steels include chromium, nickel, manganese, silicon, molybdenum and the stabilizing elements, titanium columbium and tantalum.
Stainless steel contains at least 11 percent chromium, along with an assortment of other elements to develop the specific material properties required for an application. The material properties of an alloy depend on the distribution of each type of element present, giving each alloy specific weight, strength, toughness, hardness, corrosion and fatigue attributes.
We offer cable trays and accessories in both the 304 and 316 alloy series. These austenitic alloys provide a remarkable strength to corrosion-resitance ratio while remaining non-magnetic and ductile in relativley low climate conditions. 304 stainless is the most common form of stainless steel and typically contains 18% chromium and 8% nickel, while the percentage of carbon is kept relatively low. Very similarly, 316 stainless steel also typically contains 18% chromium and 8% nickel, however, 316 stainless contains increased levels of molybdenum creating a stronger resistance to chloride corrosion, making it commonly known as marine grade stainless steel.
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