Page 78 - Mechanical Behavior of Materials

Section 3.3 Irons and Steels 79

The 400-series stainless steels have carbon in various percentages and small amounts of metallic
alloying elements in addition to the chromium. If the chromium content is less than about 15%, as
in types 403, 410, and 422, the steel in most cases can be heat treated by quenching and tempering
to have a martensitic structure, so that it is called a martensitic stainless steel. Uses include tools
and blades in steam turbines. However, if the chromium content is higher, typically 17 to 25%,
the result is a ferritic stainless steel that can be strengthened only by cold work, and then only
modestly. These are used where high strength is not as essential as high corrosion resistance, as in
architectural use.
The 300-series stainless steels, such as types 304, 310, 316, and 347, contain around 10 to 20%
nickel in addition to 17 to 25% chromium. Nickel further enhances corrosion resistance and results
in the FCC crystal structure being stable even at low temperatures. These are termed austenitic
stainless steels. They either are used in the annealed condition or are strengthened by cold work,
and they have excellent ductility and toughness. Uses include nuts and bolts, pressure vessels and
piping, and medical bone screws and plates.
Another group is the precipitation-hardening stainless steels. These are strengthened as the
name implies, and they are used in various high-stress applications where resistance to corrosion
and high temperature are required, as in heat-exchanger tubes and turbine blades. An example is
17-4 PH stainless steel (UNS S17400), which contains 17% chromium and 4% nickel—hence its
name—and also 4% copper and smaller amounts of other elements.

3.3.6 Tool Steels and Other Special Steels

Tool steels are specially alloyed and processed to have high hardness and wear resistance for use in
cutting tools and special components of machinery. Most contain several percent chromium, some
have quite high carbon contents in the 1 to 2% range, and some contain fairly high percentages of
molybdenum and/or tungsten. Strengthening generally involves quenching and tempering or related
heat treatments. The AISI designations are in the form of a letter followed by a one- or two-digit
number. For example, tool steels M1, M2, etc., contain 5 to 10% molybdenum and smaller amounts
of tungsten and vanadium; and tool steels T1, T2, etc., contain substantial amounts of tungsten,
typically 18%.
The tool steel H11, containing 0.4% carbon, 5% chromium, and modest amounts of other
elements, is used in various high-stress applications. It can be fully strengthened in thick sections
up to 150 mm and retains moderate ductility and toughness even at very high yield strengths around
2100 MPa and above. This is achieved by the ausforming process, which involves deforming the
steel at a high temperature within the range where the austenite (FCC) crystal structure exists.
An extremely high dislocation density and a very fine precipitate are introduced, which combine
to provide additional strengthening that is added to the usual martensite strengthening due to
quenching and tempering. Ausformed H11 is one of the strongest steels that has reasonable ductility
and toughness.
Various additional specialized high-strength steels have names that are nonstandard trade
names. Examples include 300 M, which is AISI 4340 modified with 1.6% silicon and some
vanadium, and D-6a steel used in aerospace applications. Maraging steels contain 18% nickel
and other alloying elements, and they have high strength and toughness due to a combination of
a martensitic structure and precipitation hardening.