Materials for engineering, 3rd Edition - (Malestrom)

Metals and alloys 111
25
Austenite
Nickel equivalent (%)
20
15
10
Martensite
Austenite
+
martensite
Austenite
+
martensite
+
δ-ferrite
Austenite
+
δ-ferrite
5
Martensite
+
δ-ferrite
δ-ferrite
0
0 5 10 15 20 25 30 35 40
Chromium equivalent (%)
3.27 Schaeffler diagram showing the structures of various stainless
steels.
Similarly, the austenite formers give:
Ni equivalent = Ni + Co + 0.5Mn + 0.3Cu + 25N + 30C
The three most important groups are as follows:
Ferritic stainless steels are generally of lower cost than the austenitic steels
and are used when good cold-formability is required. About half of the
stainless steels produced are rolled to sheet, which is subsequently colddrawn
into articles such as cooking utensils, sinks and automotive trim.
Martensitic stainless steels contain more C than the ferritic and can be
heated to form austenite then cooled with excellent hardenability to form
martensite. They can then be tempered to yield strengths in the range 550
to 1860 MPa and can be used for cutting implements.
Austenitic stainless steels are non-magnetic, because of their fcc structure,
and show no ductile/brittle fracture transition. Annealed type 304 stainless
steel has a yield strength of about 140 MPa and an ultimate tensile strength
of about 585 MPa, so work-hardening is an obvious means of strengthening.
Solid solution strengthening can be employed in these steels and N is
most effective in this regard, the 200 series of Cr–Mn–Ni–N steels being
an example.
The steels are readily welded, but care must be taken to prevent the
precipitation of chromium carbide in the grain boundaries. This may occur
in the ‘heat-affected zone’ adjacent to the weld, leaving the material adjacent
to the grain boundary depleted in chromium and, thus, no longer corrosionresistant.
Intergranular corrosion may, therefore, occur is these regions – a