Page 393 - Handbook of Materials Failure Analysis

2 Stainless Steels (SSs) and their Welding Characteristics 391

2.2.3 Oxidation and depletion of chromium
Chromium is highly susceptible to oxidation and depletion due to the high chemical
affinity of chromium to oxygen and carbon. This may lead to oxide inclusion defects
and depletion of chromium from the weld metal and the HAZ, which deteriorates the
corrosion resistance of welded joints. Therefore, there are some restrictions related to
oxygen for welding SSs. Addition of stabilizing gas as oxygen or CO 2 in GMAW
should be restricted to 2% of the shielding gas to avoid loss of alloying elements.
The fluxes of SAW and shielded metal arc welding (SMAW) should have low oxy-
gen levels to reduce oxidation and fluoride additions to facilitate chromium oxide
fluxing. During the root pass welding of pipes, it is essential to protect the unshielded
side by inert gas purging, this helps to avoid the loss of corrosion resistance at the
inner side of the weld [8,10]. High heat inputs should be avoided to prevent chro-
mium depletion in HAZ during slow cooling after welding of austenitic SSs.
After this concise review on some basic considerations of welding austenitic and
DSSs, it might be useful to discuss some of the welding parameters and the potential
welding problems and their remedies.

2.3 REVIEW OF WELDING PARAMETERS RELEVANT
TO STAINLESS STEELS
The combination effect of heat input and heat treatment on the microstructure and
mechanical properties of weld metal and HAZ in SSs and Cr steels has a major role
on material performance. This combined effect shows on the resulted microstructure
and formed phases and carbides as well as their stability. Accordingly, the final
creep-resistant prosperities and SCC resistance are affected. Stringent control of
welding conditions as per process qualification records requirements is an essential
process for controlling the weld quality.
However, daily failures during service are reported and investigated from the
chemical and energy plants [11,12], many of which prove to be originating from
welded joints as they act as critical places where high residual stresses combined
with heterogeneous structures exist so they act as potential sites for failures by
stress-driven or environmental-driven mechanisms[13].
Though welding of austenitic SS types has reached a mature state of art, it has
been shown that two ferrite morphologies precipitate at the austenite matrix of
AISI/SAE 304 SS welded with an electrode type 308L using SMAW, GMAW,
and flux-cored arc welding (FCAW) processes (skeletal and lathy type, the latter
being characteristic of higher ferrite content). It has been also shown that (unlike
FCAW) SMAW and GMAW cause a variation in grain size, as well as the appear-
ance of exogenous phases [14].
DSS have a two-phase microstructure (austenite and ferrite) resulting their higher
strength, higher resistance to intergranular corrosion, and lower cost compared to aus-
tenitic SSs. However, their difficult thermomechanical processing conditions limit their
applications. Also, they are sensitive to heating and cooling conditions during welding
and deleterious phases such as (σ phase and Cr 2 N) may appear during welding of these
steels. The Cr 2 N phase is reported [2] to be the main precipitate found in the HAZ of