PAGE No. 46

Stainless Steel in Telephone Equipment
January 1936

The wide range of iron alloys in the stainless class is the outcome of intensive research and technical progress in this important branch of metallurgy. Users requirements have been studied closely and grades developed to meet individual demands in the engineering, chemical and other industries.

There are several types of alloy. The one introduced early to industry, is a stainless steel containing 12% to 15% chromium, with varying proportions of carbon. This material has fairly good corrosion resisting properties. The metal can be machined without great difficulty, while the higher carbon containing alloys can be hardened and tempered by usual methods of heat treatment.

The next type is the high chromium, high nickel class of alloy. This metal contains 12% to 18% chromium with 8% to 13% nickel. The combined percentage of the two constituents is not usually less than 24% in these types. They are practically non-magnetic, having a permeability of about 1.1, and possess remarkable resistance to corrosion. Chemical engineers use the material widely for the manufacture of plant. Supplies can be produced in every form required for normal industrial requirements, including sheets, rods, wire, tube, castings and sections.

These alloys can be obtained in qualities suitable for deep drawing purposes or in sheets rolled to various degrees of mechanical hardness and possessing a very high finish, if desired.

Structurally, the steels are of the class described in metallurgy as “austenitic” and they are often referred to by manufacturers and in specifications under this name.

A metal in the stainless class containing 25% chromium and 25% nickel possesses long life at high temperatures and is used for the manufacture of equipment for heat treatment. Electrical concerns make use of this alloy for annealing boxes for parts used in magnetic circuits.

Stainless steel which can be hardened and tempered, and possessing superior resistance to corrosion, contains 18% chromium with 2% nickel. This is used for heavy duty constructional parts which are highly stressed in service.

In the telephone industry the materials described are being used to an increasing extent. As with other manufacturers, the comparatively high cost, together with some difficulty in machining, are the limiting factors to more wide-spread use. At the same time, with careful selection of quality and purpose, stainless steel is a very important addition to the existing range of metals.

Here are the main instances where the alloys give greatest service in telephone equipment, as alternatives to other products:-

  1. For tropical equipment where metal having good mechanical strength is necessary, combined with high resistance to corrosion.
  2. In the place of steel parts which cannot be plated or finished owing to precise machining limits.
  3. For details which are subject to mechanical wear or severe use when in service. Cases are often met where electro plate or enamel does not last sufficiently, particularly where surfaces are in rubbing contact.

Stainless steels have excellent mechanical properties. They possess high tensile strength combined with ductility and great shock resistance. Their use is often made worth while for this reason alone.

Little short of a metallurgical treatise could adequately summarise the properties of these chromium alloys, but the following table will give a brief indication of typical instances where the materials are used on telephone and allied parts:-

Description of Stainless Steel Type of Alloy Tensile strength
Tons, sq. in.
Elongation % Form Purpose
Low Carbon Stainless Iron 12/15% chromium 37 30 Bright Hard, Drawn Rod Switchboard Plug centre stems, Small machined parts
Medium Carbon Stainless Steel 12/15% chromium 40 30 Hard Drawn, Precision, Ground Rod Automatic Switch Select or Shafts, Bearing Pins
Austenitic Stainless Steel 18% chromium
8% nickel
50 35 Hard Rolled Polished Sheet Auto-Dial Finger Plates, Totalisator Springs, etc.
Key Cams for Tropics
Austenitic Stainless Steel 18% chromium
8% nickel
50 35 Hard Drawn Wire Coil Springs, fine sizes
Austenitic 12% chromium
12% nickel
35 60 Sheet Pressings on Tropical and Exterior Equipment


BACK Home page BT/GPO Telephones Ericsson Telephones Ericsson Home Page Ericsson PABX All Telephone Systems

Last revised: October 14, 2019