Welding of dissimilar steels

1. Dissertation Report
On
BEHAVIOURAL ANALYSIS OF P22 AND P91 STEELS
AFTER TIG WELDING AND FURTHER PROVIDE A
SUSTAINABLE PWHT PROCESSES
SUBMITTED BY
PARMOD KUMAR
(75117134)
UNDER THE SUPERVISION OF
PROF. ANKIT DUA
(Assistant Professor, Department of Mechanical Engineering)
AND
DR. BIKRAM JIT SINGH
(Professor, Department of Mechanical Engineering)
Department of Mechanical Engineering
MM University, Sadopur

2. CONTENTS
• Introduction of materials
• Application of P91 and P22 steel welding
• Literature Review
• Research Gaps
• Problem Formulation
• Methodology Adopted
• Experimental plan
• Present work findings
• Results and Discussion
• Conclusion
• Future scope
• References

3. INTRODUCTION
Why We Need to Weld P91 & P22 Steels…
• A primary part of supercritical steam boiler Cr-Mo ferritic steels
are used for water evaporators and steam headers, in high
temperature part of super heaters and reheaters Cr-Ni austenitic
steels are employed .
• Thus, the joining of dissimilar steels is unavoidable in boiler
system of power plants
• In steam headers when high temperature steam flows from high
temperature to low temperature with passage it contains bend
and T joints of P91 and P22 which need to be welded and TIG
welding is the most frequently used procedures for commercial
product of power plant and Petrochemical industry.

4. Applications of P91 & P22 Steel Welding
 Used in tubes, shell and thick
section tube plates of steam
generators of sodium cooled
fast reactors.
 Fossil-fired thermal and
nuclear power generating
industries.
 Steam tubes,superheaters,
reheaters.

5. INTRODUCTION TO MATERIALS
P91 Steel –
It was developed at Oak Ridge National Laboratories, in the United
States, during the 1970s.
The steel was approved under ASTM standard A213 as tubing
(T91) in 1983 and further approved for piping applications under
ASTM A335 in 1984 .
High temperature(above 600) material for steam generator
applications in fossil-fired thermal and nuclear power generating
Industries.
Cr Fe P Mo S C Mn Si
9.48 Bal 0.01 0.88 0.07 0.08 0.47 0.35

6. Mechanical Properties:
 Low thermal expansion coefficient (about 7)
 High resistance to stress corrosion cracking in water– steam
systems.
 Better mechanical properties at elevated temperatures.
 High creep strength and ductility over long exposures at high
temperatures
 Good weld ability and micro structural stability over long exposures
at elevated temperatures
 Suitable for hot and cold operations (600 0C) and can be welded by
all recent processes
Grade Tensile Strength Yield Strength Elongation Hardness
P91 > 585 MPa >415 MPa >%20 Max 265 VHN

7. • P22/T22 is especially suited for steam boiler, boiler parts, boiler
drum, and pressure vessel for engineering and similar purposes. It can
be used in permanent operation with wall temperatures up to about
590 °C.
Chemical composition and mechanical properties:
Grade Tensile
Strength
Yield
Strength
Elongation Hardness
P22 415 MPa 205 MPa %30 Max 85HRB
C Mn P S Si Cr Mo
0.11 0.56 0.01 0.007 0.32 2.46 0.88
P22 Material

8. Application of P91 and P22 Steels
 Dee piping Ltd. Palwal
( Faridabad)
 ISGEC Heavy Engineering
(Yamunanagar)
 Cheema Boilers Ltd.
(Mohali)
 BHEL India Ltd.
(Haridwar)

9. LITERATURE REVIEW
Year Author Information Obtained
2002 I A Shibli Five Recent failure in Western bourdon
boilers (U.K. ) were before service period.
There was no poor welding technique used.
Deficiency of PWHT was seen and required
to improve more
2011 Nattaphon
Tammasophon,
Weerasak Homhrajai
Gobboon
Lothongkum
Post-weld heat treatment provided more
homogeneous microstructures ,reduces
hardness, which could lead to a decrease in
weld cracking and most.
This PWHT provides the minimum
hardness of the weld zone between P91 steel
and P22.

10. Year Author Information Obtained
2011 N. Arun Kumar
P.Duraisamy
S.Veeramanikandan
Two combinations of materials namely
T91+T22 and T91+SS had been welded using
both GTAW and GMAW process.
It has been found that of the two welded
joints, the joints fabricated by GTAW process
exhibited higher strength value and
enhancement in strength value is
approximately 21% compared to GMAW
joints.
2013 Vani Shankar,
K.Mariappan,R.
Sandhya, M. D.
Mathew
P91 is a heat treatable steel and hence its
microstructure is very sensitive to
temperature.
The high dislocation density initially present
in the martensitic structure undergoes a
rearrangement into a lower energy
configuration.

11. Year Author Information Obtained
2013 J.Baral,
J.S. Swaminathan
R.N.Ghosh
Type IV cracking encountered in the critical
heat affected zones of welded super-heater
tubes.
Considerable amount of creep data are
available on P91 steel.
Creep and stress rupture at several stresses in
temperature range of 600 0C 650 0C.
2013 D.P. Rao Palaparti,
E. Isaac Samuel, B.
K. Choudhary and
M. D. Mathew
Creep-rupture properties of T91 steam
generator (SG) tube steel at 923 K in
normalized and tempered condition in the
stress range 55-150 MPa.
At all stress conditions, the creep deformation
was characterized by a decrease in creep rate.

12. RESEARCH GAPS
Lot of work was done on the creep and fatigue P92and P21, but
creep and fatigue failure were to be found as failure of joints after a
long interval of time.
Most of the Literature includes the similar metal welding and
the strength of the joints .
From the literature studied it was found that five most recent failure in boiler
joint were due to lack of PWHT of P91 and P22 joints.
Their were no poor technique in TIG welding between P22 and P91.
Very little interest is shown by researchers for Post weld heat
treatments of these kinds of dissimilar metal welded joints.

13. PROBLEM FORMULATION
 After TIG welding between P91 and P22 high hardness values of the
heat affected zone (HAZ) of dissimilar weld joints obtained.
 The improper post weld heat treatment (PWHT) can result in a
considerable difference in hardness between P91 steel, HAZ of P91 steel
and weld metal leading to prior crack and failure during high temperature
operation.
 Few researchers are studying hardness and microstructure of these
welded steel.
 Very few researchers dealt with effect of PWHT on these joints’s
hardness and microstructure and to find out proper conditions of PWHT
to avoid these failures so it becomes part of our study and research.

14. METHODOLOGYADOPTED

15. PRESENT WORK FINDINGS

16. EXPERIMENTAL PLAN
Steps Description Tools / Techniques Used
Step-1 Welding of dissimilar materials
(P22 & P91 Materials) with filler metal ER90S-B9
- TIG Welding
Step-2 Visualizing Metallic Structures of Welded Materials
(Zone wise)
- SEM/EDS
Step-3 Assessing Hardness Trend among various Zones
(after TIG welding)
- VHN Hardness Tester
- Analysis of Outcomes
through One-Way ANOVA
Step-4 Post Welding Heat Treatment (PWHT)
(Standard HT Process but Holding Time varies from 8 Mins.
to 135 Mins. As observed from concerned Literature)
- Sample preparation
- Muffle Furnace
- Analysis of Results through
Two-Way ANOVA
Step-5 Prediction of Zone wise Hardness at Average Holding Time
(with 68.3 Mins.)
- Multi Vary Chart
Step-6 Verification of Metallic Structures at 68.3 Mins.
(By taking at least 15 samples at this Holding Time)
- Sample Preparation for
Heat Treatment
- SEM/EDS
Step-7 Demonstration of Hardness Variation among zones at
Holding Time of 68.3 Mins.
- One Way ANOVA
Step-8 Hardness Comparison with in the respective Zones
(Before & After HT)
- Data Distribution Curve

17. SPECIMENS AFTER TIG WELDING

18. Four longitudinal strips were being cut by in following manner having
equal weight and cross section with pipe cutter.
SAMPLES

19. Different Regions (or Zones) Created after TIG

20. 2. VISUALIZING METALLOGRAPHY
(AFTER TIG WELDING)

21. P22 HAZ WELDPOOL P91 HAZ P91
P22

22. The microstructure of P91 consists of Tempered Martensite.
P22 consists of Polygonal Ferrite embedded in Bainitic Ferrite
matrix.
P91 HAZ consists of martensite and retained austenite.
P22 HAZ consists of Ferritic Bainitic, microstructure with
finer grains as compared to base metal.
Welding zone consist of bigger grain size structure of
Martensite and Retained Austenite phases due to higher
temperature conductivity of ER90SB9.
INFERENCES FROM OPTICAL MICROSCOPY

23. P22 HAZ WELDPOOL P91 HAZ

24. INFERENCES FROM SEM
.
P22 HAZ consists of ferritic
bainitic, microstructure with finer
grains as compared to base metal.
P91 HAZ consists of coarse
prior austenite grains with fine
prior delta ferrite grains

25. SEM of Weld Zone shows that (C23C6)
carbides at grain and sub grain boundaries
and fine MX precipitate within sub grains
were present in weld joint.

26. P22 HAZ
WELD POOL
P91 HAZ
The peaks in graphs showing the
major elements i.e. C,V,Fe,Mo,Si
and Cr. In this analysis the
percentage of carbon increases
along with p91 HAZ where as
hardly any carbon on P22 side
lead to decarburized zone.
EDS analysis of weld zone shows
chromium increases in this zone
and presence of molybdenum
indicates the formation of (c23c6)
with Mx. Hence the hardness
increases in this zone

27. 3. ANALYZING HARDNESS TREND
AFTER TIG WELDING
(Among different Zones)

28. Measurement of Micro Hardness (in VHN)

29. P22 Base Metal P22 HAZ Weld Joint P91 HAZ P91 Base Metal
202 289 312 290 231
201 296 298 292 233
204 282 319 273 236
200 338 350 333 234
200 276 288 272 235
202 325 342 330 232
203 336 342 334 231
205 328 342 331 231
201 325 336 320 232
203 290 313 290 231
203 296 301 290 232
203 280 319 272 233
203 339 352 334 234
201 278 284 271 236
202 325 344 330 233
200 336 341 333 231
204 329 342 331 232
204 326 334 322 230
201 289 314 291 230
204 295 304 292 232
201 280 321 273 230
204 339 350 333 234
201 278 286 272 235
200 328 347 331 235
203 337 341 334 231
204 328 345 331 231
200 325 338 320 230
202 289 312 290 231
204 296 304 292 233
202 284 320 272 232
200 340 351 332 233
203 276 284 272 235
204 327 345 332 234
205 337 343 333 230
204 329 343 332 232
204 325 335 320 232
Hardness in
VHN
at
Different
Regions
(Generated
after
TIG
Welding)

30. One-way ANOVA: P22, P22 HAZ, Weld Pool, P91 HAZ, P91
Method
Null hypothesis All means are equal
Alternative hypothesis At least one mean is different
Significance level α = 0.05
Equal variances were assumed for the analysis.
Factor Information
Factor Levels Values
Factor 5 P22, P22 HAZ, Weld Pool, P91 HAZ, P91
Analysis of Variance
Source DF Adj SS Adj MS F-Value P-Value
Factor 4 544499 136125 439.38 0.000
Error 175 54217 310
Total 179 598717
Model Summary
S R-sq R-sq(adj) R-sq(pred)
17.6015 90.94% 90.74% 90.42%
Means
Factor N Mean StDev 95% CI
P22 36 202.417 1.592 (196.627, 208.206)
P22 HAZ 36 310.42 24.60 ( 304.63, 316.21)
Weld Pool 36 337.14 16.34 ( 331.35, 342.93)
P91 HAZ 36 331.92 25.90 ( 326.13, 337.71)
P91 36 232.444 1.827 (226.655, 238.234)
Pooled StDev = 17.6015
P (calculated as 0.00) is less than α -level
we have selected, there are significant
differences in hardness.

31. H
A
R
D
N
E
S
S

32.  The interval plot illustrates both the location and variation in the
data.
 The plot shows the mean hardness for each zone and displays
bars around the mean
 Blue dot Symbols representing the hardness means.
 Confidence intervals (default) or standard error bars, depending
on our selection.
INTERFERENCE FROM INTERVAL PLOT

33. H
A
R
D
N
E
S
S

34. H
A
R
D
N
E
S
S

35.  Shape. The box represents the middle 50% of the data. The line
through the box represents the median. The lines (whiskers) extending
from the box represent the upper and lower 25% of the data
(excluding outliers). Outliers are represented by asterisks (*) which
was found 288 VHN.
 Zone 3 has the largest hardness values, the largest mean, and the
largest median.
 Zone 1 has the smallest values, the smallest mean, and the smallest
median.
 The middle half of the data for zone 4 is very spread out, as indicated
by the large box.
 There are no outliers (asterisks) in the data for any level
INTERFERENCE FROM BOX PLOT

37. 4. POST WELDING HEAT TREATMENT
(PWHT)

38. During water flow in pipes if un-heat treated steel will used it results in
great affinity of hydrogen that can cause cracking.
Also the residual stresses during welding will not be relieved that can
cause to failure of joint. Homogeneous microstructure and hardness
also obtained by PWHT.
The hardness variation in the base metal,HAZ and welded joint need to
reduce than the obtained value otherwise crack formation may occur
neat the HAZ of P91 and welded joint.
PWHT is required to change the phase transformation from martensite
to ferrite structure in case of P91 after PWHT it is achievable.
WHY PWHT….?
FOR P91 AND P22 TIG WELDMENT

39. Execution of Heat Treatment Process at MMU Labs

40. PWHT AT DIFFERENT HOLDING TIMES
S.No. Sample No. Heating Temperature Holding Time
1 Sample No.1 750 0C 8 minute
2 Sample No.2 750 0C 15 minute
3 Sample No.3 750 0C 30 minute
4 Sample No.4 750 0C 45 minute
5 Sample No.5 750 0C 60 minute
6 Sample No.6 750 0C 75 minute
7 Sample No.7 750 0C 90 minute
8 Sample No.8 750 0C 105 minute
9 Sample No.9 750 0C 120 minute
10 Sample No.10 750 0C 135 minute

41. EXPERIMENTAL RESULTS OF HARDNESS
(W.R.T. VARIOUS ZONES)
AFTER PWHT
AT
DIFFERENT HOLDING TIMES
Time (minutes) P22 P22 HAZ Weld Pool P91 HAZ P91
8 155 149 180 195 165
15 151 145 185 190 161
30 187 184 214 207 200
45 184 184 206 186 193
60 127 139 175 181 136
75 137 152 181 173 167
90 139 154 169 189 186
105 138 159 171 182 189
120 143 159 183 191 179
135 177 184 171 183 181

42. Two-way ANOVA: Hardness (VHN) versus Region, Holding Time (in mins)
Source DF SS MS F P
Region 4 8463.7 2115.92 17.68 0.000
Holding Time (in mins) 9 8456.5 939.61 7.85 0.000
Error 36 4308.7 119.69
Total 49 21228.9
S = 10.94 R-Sq = 79.70% R-Sq(adj) = 72.37%
Individual 95% CIs For Mean Based on
Pooled StDev
Region Mean --------+---------+---------+---------+-
P22 153.8 (-----*-----)
P22 HAZ 160.9 (-----*-----)
P91 175.7 (----*-----)
P91 HAZ 187.7 (----*-----)
weld pool 183.5 (-----*-----)
--------+---------+---------+---------+-
156 168 180 192
Holding Individual 95% CIs For Mean Based on
Time(in Pooled StDev
mins) Mean ---------+---------+---------+---------+
8 168.8 (----*----)
15 166.4 (----*----)
30 198.4 (----*----)
45 190.6 (----*----)
60 151.6 (----*----)
75 162.0 (----*----)
90 167.4 (----*----)
105 167.8 (----*----)
120 171.0 (----*---)
135 179.2 (----*----)

44. Least hardness is obtained 60 minutes holding time for P22 base metal and highest
hardness is obtained at 30 minutes holding time on the zone of weld pool.

45. The scatter plot for the hardness and holding time shows a strong positive and linear
relationship between the two variables.

46. 5. PREDICTION OF
HARDNESS AT
AVERAGE HOLDING TIME
(For Various Zones)

47. Hardness Prediction in Respective Regions
at Average Holding Time (68 Mins)

48. Regions Predicted Hardness (in VHN) at Average
Holding Time of 68.3 Mins
P22 Material 154
P22 HAZ Zone 161
Weld Pool 184
P91 HAZ Zone 188
P91 Material 176
It is obvious from above findings that At Holding time of
68. 3 Mins (during PWHT):
- The individual hardness value of each region has been dropped,
Sufficiently
- and even the Range of hardness variation of each region has been
reduced, appropriately.

49. .
6. VERIFICATION OF METALLOGRAPHYAT
HOLDING TIME OF 68.3 MINS

50. P22 HAZ WELDPOOL P91 HAZ P91
P22
No significant difference in micro structural characteristics was detected in
specimens after predicted PWHT.
All these microstructures consist of ferrite and ferrite with carbides

51. 7. DEMOSTRATION OF
HARDNESS TREND AT
HOLDING TIME
OF 68.3 MINS.
(Among different Zones)

52. P22 Base Metal P22 HAZ Weld Pool P91 HAZ P91 Base Metal
172 170 172 171 180
172 177 177 176 175
175 170 174 173 172
168 171 178 177 180
162 179 179 178 173
166 175 175 174 178
178 172 175 174 174
180 173 181 180 173
178 177 185 184 176
168 177 186 185 180
169 181 177 176 173
167 171 172 171 169
176 180 174 173 177
185 168 184 183 179
181 175 170 169 175
166 169 186 185 174

53. No evidence of nonconstant variance, missing terms, or outliers exists.

54. One-way ANOVA: P22 Base Metal, P22 HAZ, Weld Pool, P91 HAZ, P91 Base Metal
Method
Null hypothesis All means are equal
Alternative hypothesis At least one mean is different
Significance level α = 0.05
Equal variances were assumed for the analysis.
Factor Information
Factor Levels Values
Factor 5 P22 Base Metal, P22 HAZ, Weld Pool, P91 HAZ, P91 Base Metal
Analysis of Variance
Source DF Adj SS Adj MS F-Value P-Value
Factor 4 271.5 67.88 2.99 0.095
Error 75 1877.3 25.03
Total 79 2148.8
Model Summary
S R-sq R-sq(adj)
5.00300 92.64% 90.98%

55. H
A
R
D
N
E
S
S

56. H
A
R
D
N
E
S
S
Among all these mean value there is a very little difference in hardness at various
zones. So a constant trend of hardness all zones is obtained

57. H
A
R
D
N
E
S
S
In obtained box Plot the values of hardness among all the zones lies within the
mean value which is shown in boxes. And there are no outliers (asterisks) in the
data for any level.

58. 8. COMPARISON OF HARDNESS
WITHIN THE
RESPECTIVE ZONES
(Before & After HT)

59. HARDNESS BEFORE THE PWHT WAS
CONTAINING MORE DRIFT WHICH IS
REDUCED TO PROCESS SHIFTS AND ALL
DRIFTS ARE ELIMINATED BY APPLYING
PWHT.

63. In the limit graph it was compared that if the
hardness is above or below the mean values
and we obtained a significant drift in graph.
which indicates that the hardness reduced is
within significant limits which customers need
to achieve

64. 8. CONCLUSION
 From literature it was conclude that for dissimilar metal welding (P91
and P22steel) TIG welding is most suitable compared to other
techniques of welding. As TIG welding is defect free and provides
suitable strength to the joint.
From experiments it was found that there exists a substantial difference
in harnesses of various zones formed after TIG welding. This has been
further verified by using one way ANOVA. This variation can also be
visually inspected by optical microscopy and SEM.
To overcome this variation in hardness PWHT is the most suitable
process since it provides more homogeneous microstructures among
different zones formed, which could lead to a decrease in weld
cracking.

65. The results obtained after PWHT (taking time as variable) are further
analyzed by two way ANOVA.
 The homogeneity in microstructures of different zones after PWHT is
again rectified by optical microscopy.
The most suitable post weld heat treatment condition for these gas
tungsten arc welding GTAW is at 750 °C for 68.3 minutes holding time.
This condition provides the uniform hardness of the weld zone P91 steel
, HAZ P91,P22 HAZ and P22 base metal.

66. Researchers can further emphasized on properties like tensile stress,
impact strength of the materials which could also lead to failure.
Combination of two different properties and their simultaneous effect
could also be studied.
Variation in these properties could also be reduced by suitable
designing of PWHT through this approach.
Experimentation may be done using large number of sample for better
accuracy and refinement of the proposed materials.
FUTURE SCOPE

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endplate”,Proc. Conf. on "Case Histories in Failure Investigation", Milan,
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service’,Published in the CD version of the Proceedings of the Swansea
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Power Materials.
[18] S. Huysmans and J. Vekeman, “A challenging weld repair of Grade
91 tubing by avoiding PWHT,” (2009),EPRI welds,pp.1-12
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71. APPENDIX

74. There is no any red dot on the diagnostic report which means no any
kind of outliers found in the data.

75. In comparison chart the blue interval represents that mean hardness for each
zone do not differ significantly.