Atlas of Microstructures
AF234/7226/MSchü
Prepared by:
Ellen Berghof-Hasselbächer 1)
Peter Gawenda 1)
Monika Schorr 1)
Michael Schütze 1)
John J. Hoffman 2)
1) Karl-Winnacker-Institut der DECHEMA e.V.
Theodor-Heuss-Allee 25
60486 Frankfurt am Main
Germany
2) Air Products and Chemicals, Inc.
7201 Hamilton Boulevard
Allentown, PA 18195-1501
USA
The results, conclusions and recommendations given in this report refer to the specimens and data submitted as well as to the system
conditions mentioned. Therefore, the results cannot be applied to other conditions. DECHEMA e.V. accepts no liability.
It is only permitted to reproduce this report unabridgedly. Publication of the report or of single experimental results are subject to
approval by DECHEMA e.V.
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Atlas of Microstructures
Atlas of Microstructures
iii
Table of Contents
All rights reserved, including translations
Introduction ........................................................................................................................................................................................... 1
Description of Test Specimens................................................................................................................................................... 2
ISBN 978-1-57698-046-0
Creep rupture test specimens............................................................................................................................................................... 2
No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means,
electronic, mechanical, photocopying, recording or otherwise, without prior written permission of the publisher. This
document was prepared under the sponsorship of the Materials Technology Institute Inc. (MTI) and is approved for release.
All data and information contained in this document are believed to be reliable; however, no warranty of any kind, express
or implied, is made with respect to the data, analyses, or author of this document; and the use of any part of the document is
at the user's sole risk. MTI, the author, or any person acting on its behalf, assume no liability and expressly disclaim liability,
including without limitation liability for negligence, resulting from the use or publication of the information contained in this
document to warrant that such use or publication will be free from privately owned rights.
Published by Materials Technology Institute
www.mti-global.org
-
Alloy 35/45 Exposure Conditions............................................................................................ Table 1............... 3
Chemistry of Alloy 35Cr/45Ni Stress Rupture Samples........................................................... Table 2............... 3
Alloy HPMA Exposure Conditions......................................................................................... Table 3............... 4
Chemistry of Alloy HPMA Stress Rupture Samples................................................................. Table 4............... 5
Alloy HP Modified Exposure Conditions................................................................................ Table 5............... 6
Chemistry of Alloy HP Modified Stress Rupture Samples........................................................ Table 6............... 7
Metallographic Preparation......................................................................................................................................................... 8
Investigation Techniques. ...................................................................................................................................................................... 8
- Differential Interference Contrast Technique (LM-DIC).................................................................................... 8
- Scanning Electron Microscopy (SEM-BSE)........................................................................................................ 8
- Electron Probe Microanalysis (EPMA)............................................................................................................... 8
- Interference Layer Metallography (LM-ZnSe)..................................................................................................... 8
- Image Analysis by the False Color Technique (LM-FC)...................................................................................... 9
- Development of the Microstructure by Etching (LM-etched)............................................................................. 9
- Results of image analysis of Alloy 35Cr45 Ni Creep Rupture Stress Samples............................Table 9............. 10
Results of the Microstructural Analysis............................................................................................................................ 11
Alloy 35Cr45Ni........................................................................................................................................................................................ 11
- As-Cast.............................................................................................................................................................. 11
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Creep Rupture Samples
- 1800°F/982°C
- 1800°F/1,509 h/3.3 ksi (982°C/1509 h/22.8 MPa)........................................................................................11
- 1800°F/7,217 h/2.9 ksi (982°C/7217 h/20.0 MPa)........................................................................................11
- 1900°F/1038°C
- 1922°F/367 h/3.4 ksi (1050°C/367 h/23.4 MPa)...........................................................................................12
- 1900°F/2,023 h/2.09 ksi (1038°C/2023 h/14.4 MPa)....................................................................................12
- 1900°F/6,331 h/1.6 ksi (1038°C/6331 h/11.0 MPa)......................................................................................12
- 1900°F/10,247 h/1.54 ksi (1038°C/10,247 h/10.6 MPa)...............................................................................13
- 2000°F/1093°C
- 2012°F/335 h/2.0 ksi (1100°C/335 h/13.8 MPa)...........................................................................................13
- 2000°F/1,297 h/1.8 ksi (1093°C/1297 h/12.4 MPa)......................................................................................13
- 2012°F/3,545 h/1.15 ksi (1100°C/3545 h/7.9 MPa)......................................................................................13
- 2050°F/1121°C
- 2057°F/277 h/1.60 ksi (1125°C/277 h/11.1 MPa)...........................................................................................13
- 2050°F/2,606 h/1.00 ksi (1121°C/2606 h/6.9 MPa)........................................................................................13
- 2100°F/248 h/1.50 ksi (1149°C/248 h/10.3 MPa)...........................................................................................14
- Conclusions.......................................................................................................................................................14
- Results of image analysis of Alloy HPMA Creep Rupture Stress Samples................................Table 10............. 15
- 1700°F/927°C
- 1700°F/183 h/8.00 ksi (927°C/183 h/55.2 MPa)...........................................................................................16
- 1700°F/2,338 h/6.0 ksi (927°C/2338 h/41.3 MPa)........................................................................................16
- 1700°F/6,072 h/5.30 ksi (927°C/6072 h/36.6 MPa)......................................................................................16
- 1750°F/8,359 h/4.45 ksi (954°C/8359 h/30.7 MPa)......................................................................................16
- 1800°F/982°C
- 1800°F/177 h/5.57 ksi (982°C/177 h/41.2 MPa)...........................................................................................17
- 1800°F/2,436 h/3.51 ksi (982°C/2436 h/24.2 MPa)......................................................................................17
- 1800°F/6,478 h/3.90 ksi (982°C/6478 h/26.9 MPa)......................................................................................17
- 1832°F/12,289 h/3.0 ksi (1000°C/12,289 h/20.7 MPa).................................................................................17
- 1920°F/1050°C
- 1922°F/137 h/3.48 ksi (1050°C/137 h/24.0 MPa).........................................................................................17
- 1950°F/2,558 h/2.80 ksi (1066°C/2558 h/19.1 MPa)....................................................................................17
- 1922°F/4,864 h/2.32 ksi (1050°C/4864 h/16.0 MPa)....................................................................................17
- 1922°F/11,778 h/2.08 ksi (1050°C/11,778 h/14.3 MPa)...............................................................................18
- 2012°F/1100°C
- 2012°F/105 h/2.39 ksi (1100°C/105 h/16.5 MPa)...........................................................................................18
- 2012°F/2,714 h/1.80 ksi (1100°C/2714 h/12.4 MPa)......................................................................................18
- 2012°F/5,580 h/1.45 ksi (1100°C/5580 h/10.0 MPa)......................................................................................18
- 2012°F/8,990 h/1.35 ksi (1100°C/8990 h/9.3 MPa)........................................................................................18
- Conclusions.......................................................................................................................................................18
Alloy HPMA.............................................................................................................................................................................................. 16
- As-Cast.............................................................................................................................................................. 16
Creep Rupture Samples
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Alloy HP Modified. ................................................................................................................................................................................ 19
- As-Cast.............................................................................................................................................................. 19
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vii
Creep Rupture Samples
- 1675°F/913°C
- 1675°F/659 h/6.50 ksi (913°C/659 h/44.6 MPa)...........................................................................................19
- Results of image analysis of Alloy HP Modified Creep Rupture Stress Samples.......................Table 11............. 20
- 1700°F/2,338 h/6.0 ksi (927°C/2338 h/41.3 MPa)........................................................................................16
- 1700°F/6,072 h/5.30 ksi (927°C/6072 h/36.6 MPa)......................................................................................16
- 1750°F/8,359 h/4.45 ksi (954°C/8359 h/30.7 MPa)......................................................................................16
- 1800°F/982°C
- 1800°F/177 h/5.57 ksi (982°C/177 h/41.2 MPa)...........................................................................................17
- 1800°F/2,436 h/3.51 ksi (982°C/2436 h/24.2 MPa)......................................................................................17
- 1800°F/6,478 h/3.90 ksi (982°C/6478 h/26.9 MPa)......................................................................................17
- 1832°F/12,289 h/3.0 ksi (1000°C/12,289 h/20.7 MPa).................................................................................17
- 1920°F/1050°C
- 1922°F/137 h/3.48 ksi (1050°C/137 h/24.0 MPa).........................................................................................17
- 1950°F/2,558 h/2.80 ksi (1066°C/2558 h/19.1 MPa)....................................................................................17
- 1922°F/4,864 h/2.32 ksi (1050°C/4864 h/16.0 MPa)....................................................................................17
- 1922°F/11,778 h/2.08 ksi (1050°C/11,778 h/14.3 MPa)...............................................................................18
- 2012°F/1100°C
- 2012°F/105 h/2.39 ksi (1100°C/105 h/16.5 MPa)...........................................................................................18
- 2012°F/2,714 h/1.80 ksi (1100°C/2714 h/12.4 MPa)......................................................................................18
- 2012°F/5,580 h/1.45 ksi (1100°C/5580 h/10.0 MPa)......................................................................................18
- 2012°F/8,990 h/1.35 ksi (1100°C/8990 h/9.3 MPa)........................................................................................18
- Conclusions.......................................................................................................................................................18
Alloy HP Modified................................................................................................................................................ 19
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As-Cast............................................................................................................................................................19
Creep Rupture Samples....................................................................................................................................19
- 1675°F/913°C................................................................................................................................................19
1675°F/659h/6.50 ksi (913°C/659 h/44/6 MPa)............................................................................................19
Results of image analysis of Alloy HP Modified Creep Rupture Stress Samples.........................Table 11............. 20
- 1700-1750°F/913-955°C................................................................................................................................. 21
1700°F/59 h/8.5 ksi (927°C/59 h/58.8 MPa)..................................................................................................21
1750°F/794 h/5.10 ksi (955°C/794 h/35.2 MPa)............................................................................................21
- 1778°F/970°C.................................................................................................................................................. 21
1778°F/286 h/5.15 ksi (970°C/286 h/35.5 MPa)............................................................................................21
1778°F/1,185 h/3.77 ksi (970°C/1185 h/26.0 MPa).......................................................................................21
- 1800°F/982°C.................................................................................................................................................. 21
1800°F/191 h/5.42 ksi (982°C/191 h/37.4 MPa)............................................................................................21
1800°F/4,467 h/5.42 ksi (982°C/4467 h/37.4 MPa).......................................................................................21
1800°F/7,833 h/5.42 ksi (982°C/7833 h/19.3 MPa).......................................................................................22
1800°F/10,637 h/2.80 ksi (982°C/10637 h/19.3 MPa)...................................................................................22
- 1850-1900°F/1010-1038°C............................................................................................................................. 22
1850°F/707 h/3.60 ksi (1010°C/707 h/24.8 MPa)..........................................................................................22
1900°F/2,555 h/2.30 ksi (1038°C/2555 h/15.8 MPa).....................................................................................22
1900°F/5,373 h/1.90 ksi (1038°C/5373 h/13.1 MPa).....................................................................................22
- Conclusions.....................................................................................................................................................22
Service-Exposed Samples............................................................................................................................. 24
- Exposure Conditions of Service-Exposed Samples...................Table 7.............................................................. 24
- Chemistry of Service-Exposed Samples....................................Table 8.............................................................. 24
Atlas of Microstructures
ix
- Samples SE-A & SE-B....................................................................................................................................... 25
- Sample SE-C..................................................................................................................................................... 25
- Sample SE-D..................................................................................................................................................... 25
- Sample SE-E..................................................................................................................................................... 25
- Sample SE-F...................................................................................................................................................... 26
- Sample SE-G..................................................................................................................................................... 26
- Sample SE-H..................................................................................................................................................... 26
- Samples SE-I & SE-J......................................................................................................................................... 27
- Precipitation Kinetics Diagrams.......................................................................................................................... 31
- Composition of the precipitates in atomic-% (Mean values of several measurements)Table 13................................................... 32
Alloy 35Cr45Ni.................................................................................................................................................... 33
Alloy HPMA......................................................................................................................................................... 33
Alloy HP Modified................................................................................................................................................ 34
Acknowledgement.................................................................................................................34
Literature..............................................................................................................................34
Addendum............................................................................................................................35
Service-Exposed Samples - Results of the Microstructural Analysis............................................................28
- Results of image analysis of Service-Exposed Samples............Table 12.............................................................. 28
- Alloy HPMA..................................................................................................................................................... 29
-1673°F/94,900 h/2.04 ksi (912°C/94900 h/140 MPa)......................................................................................29
1800°F/65,664 h/1.79 ksi (982°C/65664 h/12.3 MPa)......................................................................................29
- Alloy HP Modified............................................................................................................................................. 29
1607°F/108,306 h/2.12 ksi (875°C/108306 h/14.6 MPa)..................................................................................29
1630°F/108,306 h/2.12 ksi (888°C/108306 h/14.6 MPa)......................................................................................................30
1650°F/74,000 h/0.72 ksi (899°C/74000 h/4.96 MPa)......................................................................................30
1650°F/148920 h/1.16 ksi (899°C/148920 h/8.0 MPa).....................................................................................30
1661°F/131,000 h/1.98 ksi (905°C/131000 h/13.6 MPa)..................................................................................30
1661°F/131,000 h/1.98 ksi (905°C/131000 h/13.6 MPa)..................................................................................30
1661°F/143,000 h/1.98 ksi (905°C/143000 h/13.6 MPa)..................................................................................30
2350°F/35,000 h/1.3 ksi (1288°C/35000 h/8.96 MPa)......................................................................................31
- Conclusions....................................................................................................................................................... 31
Summary..............................................................................................................................31
- Composition of the Precipitates............................................................................................................................ 31
Atlas of Microstructures
Atlas of Microstructures
xi
Introduction
Up to and through the 1940’s, furnace tubes were typically fabricated
from wrought chromium steels and/or austenitic stainless steel alloys.
Having rather low carbon concentrations, these alloys had poor creep
strength and limited service lives. To increase tube life, greater carbon
concentrations were required to promote precipitation hardening upon
elevated temperature exposure. The increased carbon concentrations,
however, generally resulted in excessive work hardening during conventional processing of wrought materials. The use of centrifugal casting,
pouring molten metal in a horizontal spinning mold, allowed founders
to develop high carbon alloys as the molten metal solidified into near
final shape without the need for subsequent metal working operations.
Thus, refinement of centrifugal casting processes was the gateway to
further alloy development and more aggressive furnace operations.
The first widely used centrifugal cast alloy for steam-methane
reformer tube applications was HK40 in the 1950’s timeframe. HK40 is
essentially the cast equivalent to wrought 310 stainless steel nominally
containing 25 wt% chromium, 20 wt% nickel, with iron as the balance.
However, HK40 nominally contains 0.40 wt% carbon while wrought
310 stainless steel contains only 0.08 wt% carbon. The increased carbon
content and precipitation of primary carbides resulted in HK40 having
greatly improved high temperature strength as compared to wrought 310
stainless steel. In the 1960’s, the cast HP alloys (nominally 25 wt% chromium, 35 wt% nickel, 0.50 wt% carbon, with iron as the balance) were
developed to provide greater creep strength as compared to HK40.
The HK and HP alloys rely on precipitation of M23C6 and/or M7C3
carbides (where M is primarily chromium) for elevated temperature
creep strength. The precipitated chromium carbides in the HK and HP
alloys tended to coalesce as exposure temperatures approached 1800oF
(982oC). The carbide coalescence decreased the creep strength and,
therefore, limited the strength of these alloys at elevated temperatures.
Microstructural changes that occur in the HK and HP alloys with
xii
Atlas of Microstructures
Atlas of Microstructures
extended aging time and temperature have been well documented by
Battelle Columbus Laboratories1. The Battelle data has proven to be a
valuable resource in estimating reformer tube exposure conditions associated with reformer tube failure analyses and remaining life assessments.
User demand for higher temperature/stronger alloys fueled continued
alloy development resulting in the introduction of the HP-modified alloy
in the 1970’s. The HP-modified alloy had the same nominal chemistry
of the HP alloy along with the addition of typically 1 wt% niobium. The
niobium addition results in precipitation of M23C6, M7C3, and MC type
primary carbides upon solidification. In the M23C6 and M7C3 carbides,
niobium substitutes for some of the chromium with the complex
niobium-chromium carbides being more stable at elevated temperatures
as compared to chromium carbides. In the HP-modified alloy, niobium
is the primary carbide forming element in the MC type carbides.
In the 1980’s, the demand for more severe design conditions and/or
design lives in excess of 100,000 hours led to the introduction of the
HP-micro-alloyed material. HP-micro-alloyed (or commonly designated as HPMA) material was based on the HP-modified chemistry
with “micro” additions of alloying elements. In general, micro-alloying
refers to intentional alloying additions at a concentration of 0.10 wt% or
less. Titanium is the most common micro-alloying addition with other
micro-alloying additions including zirconium, tantalum, or tungsten.
Rare earth elements such as lanthanum, cerium, and/or yttrium may
also be added. The micro-alloying additions provide a fine dispersion
of MC type carbides that are stable at temperatures well in excess of
2,000oF (1093oC).
In the 1990’s, the 35Cr/45Ni alloy family grew in popularity. The
35Cr/45Ni alloy has similar creep strength to the HPMA alloy but with
notably improved carburization resistance. The excellent carburization
resistance made the 35Cr/45Ni alloy well suited for ethylene pyrolysis
furnace tubes.
As outlined above, alloy development has continued in centrifugally
cast heat resistant alloys allowing users to design for and operate at more
severe conditions. Unlike the HK40 and HP50 alloys, there is virtually no published data on the microstructural changes that occur in the
HP-modified, HPMA, or 35Cr/45Ni alloys upon long term aging. In
an effort to address industry need, the Materials Technology Institute
(MTI) sponsored the Atlas of Microstructures project. The specimens
analyzed in the Atlas of Microstructures were foundry stress rupture
specimens generously donated by MetalTek International, Duraloy
Technologies, Kubota Metal Corp., and Manoir Industries. In addition,
service exposed samples, having longer aging times than the foundry
stress rupture specimens, were donated by operating companies including Air Products & Chemicals, Syncrude Canada, Eastman Chemical,
and MetalTek International. The thorough, detailed microstructural
analyses were completed at the Karl-Winnacker Institut der DECHEMA
in Frankfurt, Germany.
The Atlas of Microstructures documents microstructural changes
with increased aging time and temperature, identification and chemical
composition of precipitated phases, as well as diagrams characterizing
the kinetics of phase transformation for the HP-modified, HPMA, and
35Cr/45Ni alloy classes. This MTI Atlas of Microstructures bridges the
gap from the Battelle project from the early 1970’s to the most important alloys used in the petrochemical industry today.
Description of the
Test Specimens
Creep rupture test specimens
The specimens that had been taken from creep rupture tests can
be summarized with regards to their exposure conditions and their
chemistry in Tables 1-6. For each of the three materials investigated an
as-delivered (as-cast) specimen as reference sample was available. For
alloy 35Cr/45Ni and alloy HP Modified twelve creep specimens had
been supplied for the investigations. For alloy HPMA sixteen creep specimens existed. The test temperatures varied between 1675 and 2100°F
(913 and 1149°C), the maximum exposure times reached 12,289 h.
The stress range was between 1.0 and 8.5 ksi (6.9 and 58.8 MPa). The
scatter of the chemical compositions was in the allowed range for these
materials.
continued on page 8
Atlas of Microstructures
Table 1: Alloy 35/45 Exposure Conditions
Sample_ID Dechema
Sample MTI
A234_003
35/45-1 (c)
A234_004
35/45-2
A234_005
35/45-3
A234_006
35/45-4
A234_007
35/45-5
A234_008
35/45-6
A234_009
35/45-7
A234_010
35/45-8
A234_011
35/45-9
A234_012
35/45-10
A234_013
35/45-11
A234_014
35/45-12
A234_015
35/45-13
Test-Temp °F
as cast
1800
1800
1922
1900
1900
1900
2012
2000
2012
2057
2050
2100
Table 2: Chemistry of Alloy 35Cr/45Ni Stress Rupture Samples
Sample_ID
Sample
Dechema
MTI
Cr
Ni
Fe
A234_003 35/45-1 (c)
35.85
44.59
bal
A234_004
35/45-2
35.12
43.69
bal
A234_005
35/45-3
34.84
44.43
bal
A234_006
35/45-4
32.30
43.40
bal
A234_007
35/45-5
32.39
44.77
bal
A234_008
35/45-6
34.34
45.42
bal
A234_009
35/45-7
34.41
45.37
bal
A234_010
35/45-8
32.30
43.40
bal
A234_011
35/45-9
32.65
44.61
bal
A234_012
35/45-10
34.84
44.43
bal
A234_013
35/45-11
32.30
43.40
bal
A234_014
35/45-12
32.59
44.34
bal
A234_015
35/45-13
34.84
44.43
bal
Atlas of Microstructures
Test-Temp °C
as cast
982
982
1050
1038
1038
1038
1100
1093
1100
1125
1121
1149
Mn
0.60
1.32
1.46
1.02
1.40
1.04
1.31
1.02
1.37
1.46
1.02
1.43
1.46
Stress Ksi
Stress MPa
Expo. Time hrs
3.30
2.90
3.40
2.09
1.60
1.54
2.00
1.80
1.15
1.60
1.00
1.50
22.8
20.0
23.4
14.4
11.0
10.6
13.8
12.4
7.9
11.1
6.9
10.3
1,509
7,217
367
2,023
6,331
10,247
335
1,297
3,545
277
2,606
248
Composition (wt%)
Si
C
1.55
0.38
1.83
0.46
1.86
0.43
1.52
0.41
1.85
0.42
1.30
0.37
1.78
0.42
1.52
0.41
1.71
0.42
1.86
0.43
1.52
0.41
1.82
0.42
1.86
0.43
Nb
0.91
0.78
0.83
1.21
1.25
1.22
1.20
1.21
1.14
0.83
1.21
1.25
0.83
N
0.080
0.060
0.040
0.065
NR
NR
NR
0.065
NR
0.040
0.065
NR
0.040
Other
Ti, Zr & W
Ti, Zr & W
Ti, Zr & W
Ti
Ti
Ti
Ti
Ti, Zr & W
Ti
Ti, Zr & W
Table 3: Alloy HPMA Exposure Conditions
Sample_ID Dechema
Sample MTI
A234_018
HPMA-1 (c)
A234_019
HPMA-2
A234_020
HPMA-3
A234_021
HPMA-4
A234_022
HPMA-5
A234_023
HPMA-6
A234_024
HPMA-7
A234_025
HPMA-8
A234_026
HPMA-9
A234_027
HPMA-10
A234_028
HPMA-11
A234_029
HPMA-12
A234_030
HPMA-13
A234_031
HPMA-14
A234_032
HPMA-15
A234_033
HPMA-16
A234_034
HPMA-17
Test-Temp. °F
as cast
1700
1700
1700
1750
1800
1800
1800
1832
1922
1950
1922
1922
2012
2012
2012
2012
Test-Temp. °C
as cast
927
927
927
954
982
982
982
1000
1050
1066
1050
1050
1100
1100
1100
1100
Stress Ksi
8.00
6.00
5.30
4.45
5.97
3.51
3.90
3.00
3.48
2.80
2.32
2.08
2.39
1.80
1.45
1.35
Stress MPa
55.2
41.3
36.6
34.1
41.2
24.2
26.9
20.7
24.0
19.1
16.0
14.3
16.5
12.4
10.0
9.3
Expo. Time hrs
183
2,338
6,072
8,359
177
2,436
6,478
12,289
137
2,558
4,864
11,778
105
2,714
5,580
8,990
Atlas of Microstructures
Table 4: Chemistry of Alloy HPMA Stress Rupture Samples
Sample_ID
Sample
Dechema
MTI
Cr
Ni
Fe
A234_018 HPMA-1 (c)
25.01
33.79
bal
A234_019
HPMA-2
24.92
34,54
bal
A234_020
HPMA-3
24.29
32.51
bal
A234_021
HPMA-4
25.01
33.79
bal
A234_022
HPMA-5
25.01
33.79
bal
A234_023
HPMA-6
25.85
35.25
bal
A234_024
HPMA-7
24.38
35.13
bal
A234_025
HPMA-8
24.49
33.53
bal
A234_026
HPMA-9
24.49
33.53
bal
A234_027
HPMA-10
24.36
33.18
bal
A234_028
HPMA-11
25.11
36.13
bal
A234_029
HPMA-12
25.01
33.79
bal
A234_030
HPMA-13
24.49
33.53
bal
A234_031
HPMA-14
24.36
33.18
bal
A234_032
HPMA-15
24.49
33.53
bal
A234_033
HPMA-16
24.49
33.53
bal
A234_034
HPMA-17
25.01
33.79
bal
Atlas of Microstructures
Mn
0.65
0.78
1.31
0.65
0.65
0.89
0.79
0.56
0.56
1.37
0.91
0.65
0.56
1.37
0.56
0.56
0.65
Composition (wt%)
Si
C
1.08
0.45
0.80
0.50
1.37
0.42
1.08
0.45
1.08
0.45
1.12
0.47
0.85
0.46
1.14
0.44
1.14
0.44
1.73
0.46
1.33
0.48
1.08
0.45
1.14
0.44
1.73
0.46
1.14
0.44
1.14
0.44
1.08
0.45
Nb
0.65
0.49
0.40
0.65
0.65
0.95
1.08
0.51
0.51
0.83
0.65
0.51
0.83
0.51
0.51
0.65
N
0.06
NR
NR
0.06
0.06
NR
NR
0.05
0.05
NR
NR
0.06
0.05
NR
0.05
0.05
0.06
Other
Ti, Zr & W
Ti
W
Ti, Zr & W
Ti, Zr & W
Ti & W
Ti, Zr & W
Ti, Zr & W
Ti & Zr
Ti & W
Ti, Zr & W
Ti, Zr & W
Ti & Zr
Ti, Zr & W
Ti, Zr & W
Ti, Zr & W
Table 5: Alloy HP Modified Exposure Conditions
Sample_ID Dechema
Sample MTI
Test-Temp. °F
A234_036
HPNb-1 (b)
as cast
A234_038
HPNb-2
1675
A234_039
HPNb-3
1700
A234_040
HPNb-4
1750
A234_041
HPNb-5
1778
A234_042
HPNb-6
1778
A234_043
HPNb-7
1800
A234_044
HPNb-8
1800
A234_045
HPNb-9
1800
A234_046
HPNb-10
1800
A234_047
HPNb-11
1850
A234_048
HPNb-12
1900
A234_049
HPNb-13
1900
Test-Temp. °C
as cast
913
927
955
970
970
982
982
982
982
1010
1038
1038
Stress Ksi
6.50
8.50
5.10
5.15
3.77
5.40
5.42
5.52
2.80
3.60
2.30
1.90
Stress MPa
44.6
58.8
35.2
25.5
26.0
37.4
37.4
37.4
19.3
24.8
15.8
13.1
Expo. Time hrs
659
59
794
286
1,185
191
4,467
7,833
10,637
707
2,555
5,373
Atlas of Microstructures
Table 6: Chemistry of Alloy HP Modified Stress Rupture Samples
Sample_ID
Sample
Dechema
MTI
Cr
Ni
Fe
A234_036 HPNb-1 (b)
25.30
34.93
bal
A234_038
HPNb-2
25.34
34.88
bal
A234_039
HPNb-3
25.26
33.56
bal
A234_040
HPNb-4
25.34
34.88
bal
A234_041
HPNb-5
25.11
32.85
bal
A234_042
HPNb-6
25.11
32.85
bal
A234_043
HPNb-7
24.88
33.33
bal
A234_044
HPNb-8
24.86
33.79
bal
A234_045
HPNb-9
24.86
33.30
bal
A234_046
HPNb-10
25.26
33.56
bal
A234_047
HPNb-11
25.62
34.75
bal
A234_048
HPNb-12
25.26
33.56
bal
A234_049
HPNb-13
25.26
33.56
bal
Atlas of Microstructures
Mn
1.29
0.74
0.72
0.74
1.34
1.34
0.84
0.90
0.83
0.72
0.94
0.72
0.72
Composition (wt%)
Si
C
1.40
0.44
1.67
0.43
1.23
0.42
1.67
0.43
1.79
0.45
1.79
0.45
1.29
0.41
1.37
0.41
1.35
0.42
1.23
0.42
1.71
0.48
1.23
0.42
1.23
0.42
Nb
1.06
0.75
0.74
0.75
0.83
0.83
1.18
1.22
1.20
0.74
0.82
0.74
0.74
N
NR
0.06
0.06
0.06
NR
NR
NR
NR
NR
0.06
0.06
0.06
0.06
Other
W & Zr
W & Zr
W & Zr
W
W
W
W & Zr
W
W & Zr
W & Zr
Metallographic
Preparation
From the specimens available sections were taken for metallographic
preparation with the surfaces to be investigated oriented parallel to the
longitudinal direction of the tensile creep rupture specimens or perpendicular to the longitudinal axis of the tube specimens. Cutting was
performed by using a precision sectioning machine with direct water
cooling of the specimen. Some of the specimens had been delivered
in the embedded state from which smaller sections were taken by saw
cutting which were embedded again. Hot embedding in a conductive
epoxy resin was used with a diameter of the moulds of 25 mm and after
this the specimens were ground on SiC papers of the grades 180, 220,
320, 500, 1000 down to grit 2400 (Struers Standard 43-GB-1984, DIN
69176, Part 1,2,4) with a pressure of 70-80 N and water as a lubricant.
Fine polishing was performed with a two step diamond polish of 3 μm
and 1 μm followed by the finalizing polish with SiO2 suspension of
size 0.02 μm. The area on the specimen surface to be investigated was
marked by four Vickers hardness imprints (HV1) with a distance of
500 μm. Usually these areas were marked in the center of the specimen
(in some cases, however, in addition regions close to the surface were
investigated after marking).
Investigation Techniques
Differential Interference Contrast Technique (LM-DIC)
For documentation of the microstructure in the light microscope
at high magnification (500x and 1000x) the differential interference
contrast (LM-DIC) technique2 was used by which the different phases
due to the different hardness reveal certain topographical structures
which facilitates distinction of these phases.
Scanning Electron Microscopy (SEM-BSE)
After the LM-DIC investigations the same spot was investigated
by the scanning electron microscope using the back-scattered electron
imaging technique (SEM-BSE). In this case the differences in contrast
of the different precipitates resulting from differences in the density can
be used in order to distinguish between the phases.
Electron Probe Microanalysis (EPMA)
As a next step the phases documented by the other two techniques
were analyzed quantitatively for selected specimens with regard to their
chemical composition. In each case several spot measurements were
taken with a beam diameter of 2 μm. The results of these measurements were averaged. From the ratio between the different metals in the
precipitates conclusions were drawn on the respective carbide type by
taking carbon and nitrogen respectively from the difference of the sum
of the determined metal fractions and 100%.
Interference Layer Metallography (LM-ZnSe)
After the EPMA investigations the specimens were coated by a PVD
process in an evaporation equipment (Edwards) at a vacuum of about
10-3 mbar3-7. As a coating material zinc selenide (ZnSe) was used which
allowed the comparison of these marked areas in the colored state with
the images from the other techniques. The evaporation source consists
of a little vessel made of tantalum with ZnSe grains filled into this vessel.
By resistance heating the vessel is heated to a point where the ZnSe
starts to evaporate. The evaporation rate is controlled by controlling the
heating current together with observation of changes of the color of
the section surface. When reaching the desired color (violet as a macroscopic color) of the specimen surface the heating current is switched off
immediately in order to achieve a reproducible thickness of the interference layer. Since the reflection characteristics change periodically with
the layer thickness and manual control of the heating current is not easy
this coating technique requires significant experience in order to achieve
Atlas of Microstructures
a suitable interference layer. In some cases the layer had to be removed
again from the section surface several times and a new coating had to be
applied in order to come to satisfactory results. Due to the small dimensions of the different phases there may be slight deviations between the
different photographs taken by the different techniques.
Image Analysis by the False Color Technique (LM-FC)
By the use of the automated image analysis system Leica QWin in
combination with an automated laboratory microscope Leica DMLA
the different fractions of the precipitations which had been characterized
before by the other techniques were measured. For these measurements
specimens were used which had been contrasted by the interference
layer technique beforehand. An automated program routine was developed by which for each specimen several focused images were taken
at representative spots in the specimen center and partially also at the
specimen edges at a magnification of 1000x. The images were stored
and in a second run the lower and upper threshold values for the RGB
(red, green, blue)-colors of the different phases were determined and
recorded. Since, during the coating process the ZnSe layer can vary
from specimen to specimen, this procedure had to be performed for
all specimens at least once. For specimens where the microstructure in
the center and in the outer region was different this procedure had to
be applied separately for the two regions. The program routine allows
manual interaction with the measurement procedure, i.e. artifacts like
creep pores or cracks can be eliminated in order not to influence the
measurement results. Based on the information in the binary memory,
each of the phases was represented by a defined (false) color. For the
different types of carbides and other phases the following false colors
were selected:
M7C3 - purple
M6C
- red
M23C6 - yellow
M2(C,N) - green
M(C,N), MN - blue, cyan
G-phase - magenta
Atlas of Microstructures
The matrix was not binarized and was used as a background in true
colors behind the false color image.
Development of the Microstructure by Etching (LM-etched)
Finally the interference layers were polished off and sections were
etched for 30 seconds at 50°C in etchant V2A (composition: 100 ml
H2O, 100 ml HCl 1.19, 10 ml HNO3 1.40, 0.3 ml Dr. Vogel’s pickle.
Dr. Vogel’s pickle is a mixture of organic solvents with Thiourea. It
consists of 1-Methoxy- 2-propanol (40-50%), Thiourea (3-5%) and
Nonylphenol-ethoxylate (5-7%).
The interesting areas of the specimen were photographed at 200x
and 500x magnification. By etching the matrix is partially removed
so that edges are formed at the transition from matrix to precipitate.
Due to the local reflection situation of the light which hits the surface
perpendicularly the edges of these phase boundaries appear dark in the
photograph so that all phases have a dark seam and the grain boundaries
become visible.
(For more discussion on the etching technique, see the
Addendum).