What does CDS mean and the difference between T45, 4130 and CDS tube?

CDS means simply ‘Cold Drawn Seamless’ but it is a term that can commonly be used for a long list of tubes. Alloys Steels like T45 and 4130 are CDS tubes, along with aluminium’s, stainless and titanium’s, but it is more common in the market place when people say CDS tube, that they are referring to a more commercial and economically priced variant of a CDS tube. For this reason, Aerocom Metals tend to offer products like our own brand AEROCOM33, or commercial tubes like E355 or E235 when asked for just a CDS tube.

The difference between products like T45 tube or 4130 tube vs. a CDS is down to its mechanical properties. The major chemical element of these steels is Iron, and in T45 tube for example, it is accountable for approximately 97% of the alloy make-up. Other alloying elements are then added like Carbon, Silicon, Manganese, Molybdenum etc, and along with thermal treatments, this gives all alloy designations there uniqueness. In the case of 4130 tube and T45 tube, this rich mix of alloying elements and thermal treatments gives them their high strength, vs. the more economically priced CDS tubes like E235.

Is T45 tube or 4130 tube lighter than CDS?

No. It is a common misconception that the high strength alloys like 4130 and T45 are indeed lighter than a common CDS, but in fact, within 0.00 of a kilo, they are identical weights. With the majority of alloy tubes being 95 to 98% iron, the finishing alloying elements tend not to make enough difference for a standard set of scales to notice.

Where the theory does stem from however is the extra strength the high strength alloys hold. Due to a product like T45 tube and 4130 tube having higher mechanical properties, a user can sometimes select a thinner wall thickness or smaller diameter, and still maintain the structural strength required. An example could run as follows, for 30 meters of tube;

Economically priced CDS tube E235 in 1.500“od x 12swg = 69 kilos

High Strength Alloy T45 in 1.500“od x 16swg = 44 kilos

What does the BS5 T100 & BS6 S100 conditions mean?

These are British Standard Institute (BSI) publications, specific to seamless steel tubes and wrought steel. As they are a BSI publication, they will predominantly only be applicable to a British standard material.

Examples of these are;

• BS4 T45
• BS2 S515
• BS2 S514

The title of BS5 T100 is:
Procedure for inspection, testing and acceptance of seamless steel tubes and tubestock for aerospace use.

The document is 36 pages long sets out the basic principles for the manufacture and upkeep of tubestocks, but is non specific to any particular grade. In the case of a product like BS4 T45, the extract from the specification pictured clearly shows reference to the BS2 T100 (BS5 T100 is now the updated version where the BS (number) indicates the revision. BS4 T45 was made obsolete in December 1979 where BS2 T100 was the latest revision at that time). What this means is products like BS4 T45 tube must be manufactured in accordance with the latest revision at time of manufacture, of the T100 specification, and as the T100 is written into the BS4 T45 tube specification, all T45 tube will be manufactured in accordance to BS4 T45 to BS5 T100.

The title of the BS6 S100 is:
Procedure for inspection, testing and acceptance of wrought steels (other than plate, sheet, strip and tube) for aerospace use.

This document is 74 pages long and sets out the basic principles for the inspection and testing tolerances, and how these procedures should be carried out for wrought steel, in British grade materials.

To keep in line with references mentioned previously, we will look at BS4 T45 tube again. There is no reference to the BS6 S100 within the specification, although the BS5 T100 does make reference to the BS6 S100 as a normative reference to ‘terms and definitions’, and other testing conformities.

In essence therefore, purchasing grades like BS4 T45 tube would automatically be in accordance with the BS5 T100 and BS6 S100
conditions, but acceptable variations are as follows;

• BS4 T45
• BS4 T45 to BS5 T100
• BS4 T45 to BS5 T100 with BS6 S100 conditions

If in doubt, ask the supplier for the manufacturers certification, this is something Aerocom Metals and all AS9120 qualified distributers should do.

Why is 4130 tube and T45 tube termed as an aerospace product?

The originating BS T45 tube specification was written in 1948 and the military specification Mil-T-6736 4130 tube specification was written in 1950. Both standards at the time were written for aerospace and military application where evolutions in steels for high strength and precision demands were prominent.

The BS T45 tube specification is British (BS = British Standard), and as such, core demands come from the UK. Originally, usages for this material came from aerostructures such as fuselage detail on Spitfires and Hurricanes, and was a driving factor behind its development as an aerospace product. Advancement over subsequent years saw the materials advantages used elsewhere in other markets, and although the specification was made obsolete in 1979, it is still produced and used extensively in the UK today.

The 4130 tube specification had a similar demand in the infancy of its development. Demands could have been driven up due to World War escapades, and it would be common to have seen American developed grades work their way into European drawings, however there was a shortage of chromium and molybdenum during this time on European shores, which helped with the development of T45 in the UK. In modern day terms, 4130 is available in a huge array of sizes due to the demands from the American markets, and as such, is popular for this reason.

What does Aerocom Metals recommend, T45 or 4130?

The age old debate, and we couldn’t possibly answer that question. For the 60 years of experience Aerocom Metals has had in these industries, it is a question that arises time and time again.

In whole, it is the same answer, your decision. Listening to our customers, we will hear an equal number of people converge on the benefits of 4130 and T45, some love one and hate another, some will use either or.

What are random lengths and why are they made this way?

Random lengths or r/ls as commonly written, are a result of the way the tubing has been drawn. All cold drawn tubes will usually start as a large hot rolled section known as a hollow. This hollow will be chemically accurate only for the product to be drawn, and is usually a dark scaled finish. Dependent on the starting hollow used and the finished size to be obtained, there is usually an average of two to three cold drawing processes to bring the material down to the required tolerance and dimensions, before going off for thermal treatments to conform to the mechanical properties of the specification being made.

During these two to three drawing processes, the material lengthens, like when pasta is rolled through a hand cranked machine. As the hollow stock is loose on dimensional tolerance, and lengths may deviate slightly, by the end of the two to three cold draw processes, we get what is called random lengths, or lengths that are not all identical.

If identical lengths are required for production manufacturers, it is important to let your sales representative know, as specific hollows can sometimes be selected.