Steels_ Metallurgy and Applications, Third Edition

Stainless steels 301

..... , ,. .,,, .=, ,,,, , .,..

25 Austenite

.. 20

\ \ / ,u,,oo,to

15- ~. Austenite~ / .+.

+ ~ / 840rrite

Z --

5 -- Nartensite

0 I I ,I , , I ,,, I , ,,I ,,

0 5 10 15 20 25 30 35 40

Chromium equivalent (%)

Figure 4.7 Schaeffler diagram-modified (After Schneider 3)

In a similar manner, the austenite-forming elements are expressed in terms of a
nickel equivalent:

Ni equivalent = (Ni) + (Co) + (0.5Mn) + (0.3Cu) + (25N) + (30(2)

all concentrations being expressed as weight percentages.

Commercial grades of stainless steels

From the foregoing remarks, it can be appreciated that stainless steels embrace

a wide range of microstructures which are controlled by means of a complex

relationship with composition. Although chromium may be the principal alloying

element in a stainless steel, the level may give little indication of structure, and

steels containing 17% Cr can be martensitic, ferritic or austenitic, depending on

heat treatment and the presence of other elements. In discussing the characteristics

of stainless steels, it is therefore more convenient to categorize these materials

in terms of microstructure rather than composition.

Although most industrialized countries have developed their own national

standards for stainless steels, these steels are referred to almost universally

by means of the American Iron and Steel Institute (AISI) numbering system.

Thus the martensitic stainless steels are classified as the 400 series but, rather

confusingly, the 400 series also includes the fetritic grades of stainless steel. The

more important grades of austenitic stainless steel are classified in the 300 series.

The nominal compositions of the common stainless grades, according to the AISI

numbering system, are shown in Table 4.2. In the UK, the familiar specification