Page 395 - Handbook of Materials Failure Analysis

2 Stainless Steels (SSs) and their Welding Characteristics 393

react with chromium; (ii) use of stabilized SS grades (e.g., 321 and 347), which con-
tain alloying additions with higher affinity to carbon than chromium, such as tita-
nium and niobium; and (iii) carrying out a solution treatment for the welded joint
at a temperature range of 1050-1100 °C, this treatment helps to dissolve any chro-
mium carbide in the joint and restores the corrosion resistance [8,17].

2.4.2 Stress-corrosion cracking
This damage takes place as a result of tensile stresses combined with corrosive con-
ditions [18]. Environments with halogen ions such as chlorides are commonly attrib-
uted to these failures. The welding residual stresses play major role in these failures.
As a consequence, stress relieving may constitute a remedy for this problem.

2.4.3 Cracking
There are several causes of cracking in SSs welds. Hydrogen cracking is common in
ferritic and martensitic grades. Stabilized austenitic SSs are prone to HAZ cracking
after welding and reheat cracking in-service. The first is due to the formation of low
melting point compounds at grain boundaries, while the second mechanism is due to
precipitation hardening inside the grains. Both mechanisms are promoted by the sta-
bilizing element [8].

2.4.4 Solidification cracking
Typical solidification cracking appears at the centerline of weld metal. This problem
is caused by two factors: the presence of sulfur and phosphorus impurities, and the
rejection of these elements to grain boundaries, where it forms a low melting phase,
which may tear out due to tensile stresses of shrinkage late during solidification. The
mode of solidification also affects this phenomenon, so that crack-free weld can be
obtained when the metal solidifies as ferrite-austenite rather than autenite-ferrite,
since ferrite has high solubility of sulfur and thus, little rejection to grain boundaries
occurs[19].

2.5 POST-WELD HEAT TREATMENT
With regard to boiler steels, the final heat treatment for T/P91, T/P92, and T/P911
consists of normalizing and tempering, an austenitising temperature of about 1060 °C
is adopted for hardening. A fully martensitic structure is obtained upon cooling to
room temperature over a wide range of cooling rates. Welding of these steels is sim-
ilar to welding hardenable alloy steels, since complete transformation to martensite
during air cooling after welding occurs, therefore, the specification of preheat and
post-weld heat treating is a major requirement for these steels. A detailed investiga-
tion on this steel is given in Ref. [20]. P91 can be welded satisfactorily by many pro-
cesses including manual metal arc, submerged arc, and gas tungsten arc welding.
Postweld heat treatment (PWHT) is necessary for tempering the martensite formed
during welding, and many investigations have highlighted the need for optimizing
the PWHT temperature and time as well as filler material composition [21,22].