Page 391 - Handbook of Materials Failure Analysis

2 Stainless Steels (SSs) and their Welding Characteristics 389

Austenite is the predominant phase in austenitic SSs, while in DSS the chemical
composition is tailored to produce a structure consisting of ferrite and austenite.
Unfortunately, during welding of SSs many discontinuities may be produced. These
discontinuities act as stress raisers that can lead to a decrease in the life of the weld.
Moreover, many of the involved structures include straight runs of tubes, near bends,
and welded tubes. High-strength creep-resistant steels are usually subjected to
normalizing and tempering heat treatment prior to service and therefore, have micro-
structures of tempered martensite. Creep strength of these steels is improved by the
martensitic lath structure, precipitation strengthening effects of M23C6 carbide and
MX (M¼Nb, V, Cr and X¼C, N) carbo-nitrides, and the solid-solution strengthen-
ing effect of Mo and W in the matrix. The precipitation strengthening effect of
MX carbo-nitrides is especially important, because its coarsening rate is small
and the fine particle size is maintained for long-term service [7].

2 STAINLESS STEELS (SSs) AND THEIR WELDING
CHARACTERISTICS
SSs are the most important family of corrosion-resisting alloys. These alloys acquire
their SS properties from the presence of the chromium element. The corrosion resis-
tance of these alloys allows them to find numerous applications in chemical plants,
high-temperature applications, food processing equipment, and others.
The effect of chromium lays in the formation of a tight, impervious layer of chro-
mium oxide, which protects the metal form the environment. The essential level of
chromium in SS is more than 11%. At this level, the chromium oxide layer is con-
tinuous and provides full protection. The other alloying additions in SSs, such as
nickel, provide additional desirable properties such as low-temperature toughness,
improved ductility, and strength.

2.1 CLASSIFICATION OF STAINLESS STEELS (SSs)
The structure of SS is determined by the stability of phases, such as ferrite, austenite,
and martensite in microstructure, and hence by alloy composition leading to these
phase stabilities. In this respect, it is worth mentioning that Cr, Mo, Si, Ti, Nb, V,
and Al are ferrite stabilizers, while Ni, Mn, C, N, Co, and Cu are austenite stabilizers.
Based on the effect of these alloying additions, SSs are composed of four basic fam-
ilies of alloys: ferritic, martensitic, austenitic, and DSSs. In addition, there is a family
of alloys relying on precipitation hardening to improve mechanical properties. The
duplex alloys are designed to have nominally 50% ferrite and 50% austenite. They
provide superior toughness, strength, and corrosion resistance than ferrite alloys and
better resistance to SCC than the austenitic alloys, and thus they find increasing
applications in oil, natural gas, and chemical industries. However, their use is limited
to 250 (for welded structures) and to 300 °C for other structures in order to prevent
precipitation of different undesirable phases.