Cracks were found in outer shroud segments made of cast 310 stainless steel after extended use. Investigation revealed the cracks were caused by sigma phase, a brittle intermetallic that formed during operation. Initial weld repairs failed as new cracks formed. A successful repair process was developed using a pre-weld heat treatment to dissolve the sigma phase before welding, preventing further cracking.

1. Abstract:
Outer shroud segments fabricated from a cast 310 stainless steel were found to have cracked
following extended service. Detailed investigation revealed cracking to be associated with a sigma
phase, a brittle TCP intermetallic, which had developed over time during engine operation. Initial
attempts to weld repair the cracks proved unsuccessful as cracks were discovered both during and
after the weld repair procedure. A new weld repair procedure incorporating a pre-weld solution
annealing heat treatment to remove the sigma phase before welding was successfully developed,
thus alleviating the cracking concern.

2. Introduction:
The outer shroud segments installed in many power generation turbines are often fabricated from
cast 310 SS. This particular austenitic stainless steel has been selected as it provides a cost-
effective solution with respect to oxidation resistance, strength, and creep properties for this
particular application. Unfortunately, over a period time, exposure to engine operating
temperatures can result in the formation of sigma phase (r) particles, reducing both the toughness
and ductility of the steel.
Sigma phase is a brittle intermetallic, chromium-rich phase that forms in austenitic stainless steels
after prolonged exposure to temperatures in the range from 595 to 930ο
C range and develops more
rapidly near 870ο
C. Sigma phase in iron–chromium alloys has a hardness equivalent to
approximately 68 HRC (940 HV), with alloys containing more than 16.5% Cr being more prone
to sigma phase precipitation. In addition to sigma phase precipitation, higher temperature
exposures can also result in the precipitation of complex chromium carbides at grain boundaries
along with other deleterious phases.
Typical failures attributed to sigma phase formation have been observed in iron–chromium alloys
exposed to temperatures in the critical temperature range for prolonged periods of time and then
subjected to adverse loading conditions. Scenarios of this type include parts being subjected to
cool down when differential thermal contraction forces arise, during weld repair in a service shop,
or because of low temperature impact. When sigma phase is anticipated, special care must be taken
to minimize or avoid impact when applying high stress to the unit under maintenance or during
repair.
The shroud was found to have multiple cracks on the hot gas flow path side. A large majority of
the engines were discovered to have cracked shrouds after routine boroscope inspection. The outer
shroud component had been in service for over 25000 h at normal engine operating temperatures.
The hardness values measured at individual locations (Fig. 1) varied from 191 HV on the cold side
to 348 HV at the hot side of the shroud. The higher hardness values of 315–348 HV were attributed
to sigma phase precipitation, while the lower hardness value of 191 HV taken at the cold area of
the shroud met the specification [3] requirements of maximum allowable hardness of 95 HRB
(*214 HV) and, therefore, it was likely not affected by sigma precipitation.

3. Fig. 1 Overall macro photo showing cracked outer shroud segment in the as-received condition
Weld Repair Process:
Initial attempts to weld repair the cracked outer shrouds consisted of tungsten inert gas (TIG)
welding using 310L or 309L filler wire with no preheating of the component. Before weld repair,
cracks were completely removed by milling to appropriate depth (Fig. 2) using a special tool.
Fig. 2 image showing the process of removal of pre- existing cracks by milling the slot out

4. After a number of trials using different techniques, such as time between each weld run, weld
current, and filler wire material (Fig. 3), the procedure for repair welding of the cracks with no
preheating was found to give unsatisfactory results.
Fig: 3 schematic showing welding process with no preheating
Cracks were commonly observed adjacent to the weld bead as a consequence of differential
thermal contractions encountered during cool down (Fig. 4).
Fig. 4 Macro image documenting failed weld repair process due to formation of new cracks
adjacent the weld bead during cooling down after welding

5. Metallurgical Evaluation:
Cylindrical specimens at colder non-heat-affected and hotter heat-affected areas were removed
from a cracked outer shroud using electrical discharge machine (EDM) for metallographic
examination (Fig. 5).
Fig. 5 Macro image showing the areas where the specimens were taken using EDM
An evaluation of the microstructure was conducted on cross-sectional mounts taken thru both
cylindrical specimens. General microstructure was revealed by electrolytic etching the mounts in
10% oxalic acid solution according to ASTM standard practice [4]. After etching, microstructure
was examined under a laser confocal microscope.
General microstructure thru Specimen 1 taken from a colder non-heat affected area of the shroud
revealed an as-cast dendritic microstructure void of sigma phase consistent with properly
processed 310 stainless steel material (Fig. 6).

6. Fig. 6 Face-centered cubic (FCC), as-cast austenite microstructure obtained from the Specimen 1
taken at colder area of the shroud
General microstructure observed in the hotter heat affected area revealed needle shaped
intermetallic precipitates confirming the presence of sigma phase (r)as shown in Fig. 7. Formation
of sigma phase occurred during exposure in the elevated temperature range during engine
operation resulting in severe loss of ductility and fracture toughness of the steel. Cracking
occurring in the outer shroud was attributed to deteriorated properties caused by sigma phase
formation in the presence of stresses caused by differential thermal contraction occurring during
part cool down.
Fig 7. platelets of hard, brittle intermetallic sigma phase (r) obtainedfrom Specimen 2 taken at
heat-affected area. Etchant: Electrolytic oxalic reagent.

7. Remedies:
The above findings supported the hypothesis that sigma phase precipitation is primarily
responsible for unsuccessful weld repair due to formation of new cracks during and after welding.
To alleviate the cracking, it was proposed to perform a solution annealing heat treatment on the
entire outer shroud component before the weld repair process. The solution annealing process
consisted of heating the altered material up to temperatures of 1010 ο
C or 1150ο
C and holding it
for 1 and 2 h to allow the sigma phase to go into solid solution. Specimens were given a relatively
fast gas fan quench to prevent carbon from coming out of the solution during cooling. The
microstructure of the sample after solution annealing heat treatment at 1010ο
C for 1 h is shown in
Fig. 8. The presence of needle-like intermetallic of sigma phase particles was still evident
indicating incomplete solution annealing treatment.
Fig. 8. Platelets of sigma phase after solution annealing heat treatment at 1010ο
C for 1 h
The solution annealing heat treatment at 1150ο
C for 1 h followed by quenching dissolved the sigma
phase completely (Fig. 9). After ensuring that the solution annealing heat treatment at 1150ο
C for
1 h completely dissolved the sigma phase, a weld repair trial was then performed utilizing the same
welding parameters as before. After completion of the weld repair, the shroud was subjected to
surface polishing in the weld area plus non-destructive testing (NDT) to check for evidence of any
cracks or other weld imperfections. Figure 10 shows the overall macro photo of the weld-repaired
outer shroud segment subjected to the new weld repair process. No evidence of post-weld cracking
was observed following polishing or NDT examination.

8. Fig 9. Completely dissolved sigma phase by solution annealing heat treatment at 1150ο
C for 1 h.
Electrolytic oxalic reagent
Fig 10. Overall macro photo of the outer shroud segment found to be cracked. The shroud was
subjected to heat treatment at 1150ο
C for 1 h, and then selected crack was removed by milling the
slot out and repair welded. After polishing the surface, NDT did not reveal post-welding cracks.

9. Conclusions:
Cracking of the outer shroud component was attributed to the formation of needle-shaped sigma
phase during extended periods of exposure at elevated service temperatures. The intermetallic
sigma phase is hard and brittle, increasing the cracking susceptibility during cool down when the
part experiences stresses resulting from differential thermal contraction. The presence of sigma
phase also led to cracking after initial attempts to weld repair the cracked shrouds with no pre-
weld solution annealing treatment. A pre-weld solution annealing heat treatment at 1150ο
C for 1
h before the weld repair enabled the removal of the sigma phase and led to a successful weld repair
utilizing the same weld parameters as for the weld repair carried out without any pre-weld solution
annealing treatment.
References:
 Hall, E.O., Algie, S.H.: The sigma phase. Metall. Rev 11, 61–88 (1966)
 Hau, J., Seijas, A.: Sigma Phase Embrittlement of Stainless Steel in FCC Service,
Corrosion 2006 – Nace International, paper no. 06578 (2006)
 ASTM A240: Standard Specification for Chromium and Chromium–Nickel Stainless Steel
Plate, Sheet, and Strip for Pressure Vessels and for General Applications (2011)
 ASTM E 407-99: Standard Practice for Microetching Metals and Alloys (1999)