Stainless Steel: A metal for all seasons

A metal for all seasons By George Genevro The exhaust manifold, waste gate, hot side of the turbo, and the exhaust elbow are made of various grades of stainless steel. Exhaust system and turbosupercharger for the Orenda engine. The...


The American Iron and Steel Institute (AISI) then developed the three-digit numbering system that was used for many years. It is still used, along with the recently developed Unified Numbering System, in the stock lists of suppliers and in technical literature. For stainless steels, the UNS system consists of five numbers preceded by the letter S. In cases where a letter suffix is used in the American Iron and Steel Institute system, in the UNS system it is replaced by a number, usually the fifth digit. For example, two common alloys such as 316 and 316L, (a low carbon version of 316 that contains a maximum of .03 percent carbon) are designated as UNS S31600 and UNS S31603 respectively. The UNS system is now used to designate all non-ferrous and ferrous metals.

CHART 1
STAINLESS STEELS
CHROMIUM TYPES Martensitic

Ferritic

CHROMIUM NICKEL TYPES


17-4CuMo, 17-10P,
and others

Semi-Austenitic





Strengthened by aging

Iron-chromium-nickel alloys were assigned 300 series numbers and the iron-chromium alloys were given 400 series numbers. These two series account for all but a few of the stainless steel alloys available. The relatively few alloys in the 200 series are usually modifications of 300 series materials. For example, 203S (UNS S20300) is a free-machining version of 303 with less nickel, more manganese, and more copper. In the three-digit AISI system minor alloy changes are indicated by a letter following the number. For example, AISI-304L is a low carbon version of 304. When selenium is added to improve machinability the chemical symbol Se is added as a suffix. Another variation in the AISI system is that carbon content in the hardenable 440 stainless steels is indicated by the letters A, B, and C.

The two major stainless-steel categories - the iron-chromium nickel 300 series and the iron-chromium 400 series - can be further subdivided by a study of their microstructures since this is the key to their various properties. As shown in the chart on page 18, the terms martensitic, ferritic, austenitic, and precipitation hardening are used to show which alloys fall into a particular microstructure category.

Martensitic - About 10 of the 400 series stainless steels are hardenable by heat treatment and are therefore useful in applications where high strength, hardness, and abrasion resistance are necessary. The chromium content varies between 11.5 and 18 percent and the carbon content is about .18 percent in all of the 400 series stainless steels except the 440A, 440B, and 440C alloys. The 440A, B, and C stainless steels have carbon contents varying from .60 to 1.2 percent, with the highest carbon content yielding the greatest hardness and least ductility when fully hardened.

Ferritic - The six stainless steels in the 400 series classed as ferritic cannot be hardened by heat treatment. How-
ever, they can be hardened slightly by cold-working processes such as rolling and stamping. The chromium content ranges from about 14 to 27 percent, giving better corrosion resistance than martensitic steels. Other features include relative ease of fabrication, good mechanical properties, and relatively low cost.

Austenitic - About 65 percent of all stainless steels fall into the austenitic category. These steels are very corrosion resistant, do not become brittle at high temperatures, and are highly ductile, and therefore relatively easy to form. The chromium content varies from 16 to 28 percent and the nickel content ranges from 6 to 22 percent, providing a wide range of metallurgical characteristics. One feature is the ability to resist temperatures up to 2,000 F when the chromium content is at or above 25 percent. Austenitic stainless steels cannot be hardened by heat treatment but can be strengthened by cold-working. They are non-magnetic.

Precipitation hardening - Several of the chromium-nickel steels are either semi-austenitic or martensitic. They are also known as precipitation-hardening types and are denoted by a number/letter system (17-4 PH, for example). These materials are solution-annealed at high temperatures and after cooling remain relatively soft at ambient temperatures. They may be fabricated or machined in this condition and then hardened by means of a relatively low temperature heat treatment (about 900 to 1,100 F) that precipitates out elements that had been held in solid solution by the annealing process. The precipitation hardening process reduces both ductility and machinability and increases hardness and tensile strength. New PH type alloys, including some produced by the Vacuum Arc Remelt process, are being introduced as the family of stainless steels expands.

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