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.
Heat treatment 1 By
P.SENTHAMARAI KANNAN,
ASSISTANT PROFESSOR ,
DEPARTMENT OF MECHANICAL ENGINEERING,
KAMARAJ COLLEGE OF ENGINEERING AND TECHNOLOGY,
VIRUDHUNAGAR, TAMILNADU.
INDIA.
Conventional heat treatment of low carbon steelAyush Chaurasia
Heat treatment of Low Carbon Steel via heat treatment processes of annealing, quenching and normalising and observing the structural changes affecting the hardness property of material.
Chapter 3: Metal Works, Casting & Heat Treatmentsyar 2604
This topic explains the processes of metal works and casting. It also describes the types and purpose of heat treatment for steels and the effects of heat treatment on mechanical properties of steels.
Heat treatment defects &and its remediesNIAJ AHMED
Heat Treatment involves various heating and cooling procedures performed to effect structural changes in a material, which turn affect its mechanical properties
Heat treatment 1 By
P.SENTHAMARAI KANNAN,
ASSISTANT PROFESSOR ,
DEPARTMENT OF MECHANICAL ENGINEERING,
KAMARAJ COLLEGE OF ENGINEERING AND TECHNOLOGY,
VIRUDHUNAGAR, TAMILNADU.
INDIA.
Conventional heat treatment of low carbon steelAyush Chaurasia
Heat treatment of Low Carbon Steel via heat treatment processes of annealing, quenching and normalising and observing the structural changes affecting the hardness property of material.
Chapter 3: Metal Works, Casting & Heat Treatmentsyar 2604
This topic explains the processes of metal works and casting. It also describes the types and purpose of heat treatment for steels and the effects of heat treatment on mechanical properties of steels.
Heat treatment defects &and its remediesNIAJ AHMED
Heat Treatment involves various heating and cooling procedures performed to effect structural changes in a material, which turn affect its mechanical properties
A low-carbon steel wire of AISI 1022 is used to easily fabricate into self-drilling tapping screws,
which are widely used for construction works. The majority of carbonitriding activity is performed to improve
the wear resistance without affecting the soft, tough interior of the screws in self-drilling operation. In this
study, Taguchi technique is used to obtain optimum carbonitriding conditions to improve the mechanical
properties of AISI 1022 self-drilling tapping screws. The carbonitriding qualities of self-drilling tapping screws
are affected by various factors, such as quenching temperature, carbonitriding time, atmosphere composition
(carbon potential and ammonia level), tempering temperature and tempering time. The quality characteristics of
carbonitrided tapping screws, such as case hardness and core hardness, are investigated, and so are their
process capabilities. It is experimentally revealed that the factors of carbonitriding time and tempering
temperature are significant for case hardness. The optimum mean case hardness is 649.2HV. For the case
hardness, the optimum process-capability ratio increases by about 200% compared to the original result. The
new carbonitriding parameter settings evidently improve the performance measures over their values at the
original settings. The strength of the carbonitrided AISI 1022 self-drilling tapping screws is effectively improved.
Surface hybrid nanocomposites via friction stir processingmohammed noor
Friction stir Processing (FSP) is a new innovative technology developed based on the principle of Friction Stir Welding (FSW) technique.
In FSP, the ceramic particulates are reinforced into the base metal by adding it into the groove and Friction Stir Processing (FSP) is performed.
In this study, the aluminum alloy 6061 is chosen as the base metal, alumina and graphite Nano powder as reinforcement.
The process parameters such traverse speed of 64 mm/min and the tool rotational speed of 1060 rpm and tilt angle of 2deg were selected, The Friction Powder Processing was carried out on vertical milling machine.
New parameters such as powder type and number of passes were involved and we also study the effect of heat treatment.
The influence of FSP was checked using some tests such as the microstructure analysis that was carried out using optical microscope (OM) and the mechanical characteristics were analyzed using tensile test and hardness test.
The micrograph results revealed that powder particulates were evenly distributed in the stir zone and reduction in grain size also observed; the reason for the grain size reduction was stirring action of the FPP tool’s pin.
The tensile strength results showed a significant improvement in strength by a percent of
50% compared to base metal but when T6 heat treatment is applied, the tensile strength decreased.
Investigation on the Rate of Solidification and Mould Heating in the Casting ...IOSR Journals
Abstract: The quality of casting in the foundry can be measured by the rate at which solidification of the
molten metal takes place, which is consequent upon the rate the mould, is able to dissipate the heat of
solidification to the surroundings. The faster or slower the heat removal process during solidification the
structure of the grains formed by the casting is either finer of coarser. An experimental investigation was
carried out to compare the rate of solidification of commercially pure aluminium in metallic moulds. The rate at
which solidification occurred was compared with the rate at which the mould absorbed and dissipates heat. The
experiments conducted recorded the temperature fields at different casting location and that of the moulds
respectively. The results showed that there is a direct relation of the rate of heat absorption by the mould and
the rate of solidification in metallic moulds.
Keywords – Aluminium, casting, heat, mould, solidification, temperature.
International Journal of Engineering Research and Applications (IJERA) is an open access online peer reviewed international journal that publishes research and review articles in the fields of Computer Science, Neural Networks, Electrical Engineering, Software Engineering, Information Technology, Mechanical Engineering, Chemical Engineering, Plastic Engineering, Food Technology, Textile Engineering, Nano Technology & science, Power Electronics, Electronics & Communication Engineering, Computational mathematics, Image processing, Civil Engineering, Structural Engineering, Environmental Engineering, VLSI Testing & Low Power VLSI Design etc.
Effect of Hardness and Wear Resistance on En 353 Steel by Heat Treatment IJMER
En 353 steel is an easily available and cheap material that is acceptable for heavy duty
applications. Heat treatment on En 353 steel is improved the ductility, toughness, strength, hardness and
relive internal stress in the material. Spectrographic method is used to analyze the composition of the
alloy material. The experimental results of hardness and dry wear testing on pin-on-disc are done to get
idea about heat treated En 353 steel. It is found that the hardness and wear resistance of the En 353 steel
is improved after the heat treatment and the microstructure is changed from ferrite to martensite.
Wear Properties of Thixoformed Al-5.7Si-2Cu-0.3Mg Aluminium AlloyDr. Manal Abdullatif
Earlier work has shown that Al-5.7Si-2Cu-0.3Mg aluminium alloy is suitable for
thixoforming process. Here, the dry sliding wear behaviour of the alloy, in the as-cast and
thixoformed conditions were investigated. The cooling slope technique was used to produce the alloy
with globular microstructure for the thixoforming process. Both the thixoformed and cast samples
were subjected to T6 heat treatments prior to the wear tests. The tests were carried out using a
pin-on-disc tribometer, against a hardened M2 tool steel disc of 62 HRC at different loads, under dry
sliding conditions at fixed sliding speed and sliding distance of 1 m.s–1 and 5 km respectively. The
microstructural response, worn surfaces was thoroughly and carefully examined using various
methods such as scanning electron microscopy, energy dispersive spectroscopy, and differential
scanning calorimetry. The density of the heat treated thixoformed alloys showed significant increase
in the hardness property, among others, due to its reduced porosity. Their wear test results also
observed that the weight loss of materials increase with an increase in the input load and the sliding
distance for all samples. However, the as-cast alloy displayed higher wear rate compared with the
thixoformed alloys. In general, the wear mechanisms showed a mixture of abrasive, oxidative and
delamination wear (mild wear) at low applied loads and mainly an adhesive (severe wear) at high
applied loads.
investigation of Cracking of the outer shroud component of a turbine blade
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)