Page 164 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Section 5.6 Stainless Steels

HSLA 550
DP 600
15% DP 600
HSLA 550
1%
HSLA 250
HSLA
12%

HSLA 350
HSLA
13% DP soo

DP 600
_
Mild Steel
59% Hsu 350
(al (U)
FIGURE 5.6 Advanced high~strength steel applications in the Ford 500. (a) Use of advanced high-strength
steels by weight percent; (b) structural components and alloys used. Source: Courtesy of the American Iron
and Steel Institute-Great Designs in Steel 2005.

5.6 Stainless Steels

Stainless steels are characterized primarily by their corrosion resistance, high strength
and ductility, and high chromium content. They are called stainless because, in the
presence of oxygen (air), they develop a thin, hard, adherent film of chromium oxide
that protects the metal from corrosion (passivation, see Section 3.8). This protective
film builds up again in the event that the surface is scratched. For passivation to
occur, the minimum chromium content should be 10 to 12% by weight. In addition
to chromium, other alloying elements in stainless steels typically are nickel, molyb-
denum, copper, titanium, silicon, manganese, columbium, aluminum, nitrogen, and
sulfur.
The letter L is used to identify low-carbon stainless steels. The higher the carbon
content, the lower is the corrosion resistance of stainless steels. The reason is that the
carbon combines with the chromium in the steel and forms chromium carbide; the re-
duced availability of chromium lowers the passivity of the steel. Also, the chromium
carbide introduces a second phase and thereby promotes galvanic corrosion.
Developed in the early 1900s, stainless steels are produced in electric furnaces
or the basic-oxygen process and by techniques similar to those used in other types of
steelmaking. The level of purity is controlled by various refining techniques.
Stainless steels are available in a wide variety of shapes, and typical applications in-
clude cutlery, kitchen equipment, health care and surgical equipment, and applica-
tions in the chemical, food-processing, and petroleum industries. A more recent
development is the use of thin, high-strength stainless steels for bus bodies and
chassis. Although not commercially successful, the DeLaurean automobile had a
stainless-steel body.
Stainless steels generally are divided into five types (see also Table 5.5).

Austenitic (200 and 300 series). These steels generally are composed of chromium,
nickel, and manganese in iron. They are nonmagnetic and have excellent corrosion
resistance, but they are susceptible to stress-corrosion cracking. Austenitic stainless