Welding of Stainless Steel -Sndo Kou
-
Upload
govind-raj -
Category
Documents
-
view
270 -
download
2
Transcript of Welding of Stainless Steel -Sndo Kou
-
8/13/2019 Welding of Stainless Steel -Sndo Kou
1/17
Welding of Stainless Steel
Submitted by A R Dhamodaran
-
8/13/2019 Welding of Stainless Steel -Sndo Kou
2/17
-
8/13/2019 Welding of Stainless Steel -Sndo Kou
3/17
Weld Decay
Definition
The HAZ of ASS with
more than .05% Carbon
content is susceptibleto Intergranular
Corrosion known as
Weld Decay
C
-
8/13/2019 Welding of Stainless Steel -Sndo Kou
4/17
Cause for Weld Decay
Precipitation of Cr carbide at the grain
boundaries (known as Sensitization)
Cr carbide is Cr enriched M23
C6 ,
where M
represents Cr and some amount of Fe
Within the Sensitization temperature range,
C rapidly diffuses freely to the grainboundaries and combine with Cr to form Cr
carbides
-
8/13/2019 Welding of Stainless Steel -Sndo Kou
5/17
TEM micrograph of Cr carbide particles along
a grain boundary of a 316 stainless steel that
has been sensitized at 750C for 1 hour
63-71% wt Cr along grain boundaries
-
8/13/2019 Welding of Stainless Steel -Sndo Kou
6/17
Interference
Above slide shows, because of Cr carbide
precipitation at the grain boundary, the areasadjacent to the grain boundary are depleted
of Cr and this is explained in below figure,
These areas become anodic to the
rest of the grain and hence are
preferentially attacked in corrosive
media, resulting in intergranularcorrosion
-
8/13/2019 Welding of Stainless Steel -Sndo Kou
7/17
Isothermal precipitation curve for
Cr23C6in 304 stainless steel
Sensitization Temperature600-850 C
-
8/13/2019 Welding of Stainless Steel -Sndo Kou
8/17
Solvus curves for Cr23C6 and TiC in
304 stainless steel.
Sensitization temperature for Tic
850-1100 C
Sensitization temperature for Cr23C6
600-850C
-
8/13/2019 Welding of Stainless Steel -Sndo Kou
9/17
Sensitization in austenitic stainless steel
Phase Diagram Thermal Cycle Precipitation Curve
Microstructure
-
8/13/2019 Welding of Stainless Steel -Sndo Kou
10/17
The sensitizedtemperature
reached is held for
longer time
Thus more Cr23
C6
gets precipitated
Peak temperature
is too low to
obtain Cr carbideprecipitation i.e.,
T
-
8/13/2019 Welding of Stainless Steel -Sndo Kou
11/17
Thermal cycles and weld decay in 304
stainless steel weld
-
8/13/2019 Welding of Stainless Steel -Sndo Kou
12/17
Effect of carbon content on isothermal precipitation of
Cr23C6in 304 stainless steel
-
8/13/2019 Welding of Stainless Steel -Sndo Kou
13/17
Effect of Heat Input
-
8/13/2019 Welding of Stainless Steel -Sndo Kou
14/17
Remedies
Post Weld Heat Treatment Reduction of Carbon Content
Addition of Strong Carbide Formers
-
8/13/2019 Welding of Stainless Steel -Sndo Kou
15/17
Post Weld Heat Treatment
The weldment can be heat treated at10001100C followed by quenching.
The high-temperature heat treatment
dissolves the Cr carbide that has precipitatedduring welding, and quenching prevents its re-formation.
This treatment is not always possible, howeverbecause of the size and/or the quenching-induced distortion of the weldment.
-
8/13/2019 Welding of Stainless Steel -Sndo Kou
16/17
Reduction of Carbon Content
Low-carbon grades such as 304L and 316L
stainless steels can be used.
These stainless steels are designed to have
less than 0.035% C to reduce susceptibility to
weld decay
-
8/13/2019 Welding of Stainless Steel -Sndo Kou
17/17
Addition of Strong Carbide Formers
Elements such as titanium (Ti) and niobium
(Nb) have a higher affinity for C than Cr and
thus form carbides more easily than Cr.
Types 321 and 347 stainless steels are
essentially identical to 304 stainless steel
except for the addition of Ti and Nb,
respectively.