Atlas of Microstructures
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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. Teodor-Heuss-Allee Teodor-Heuss -Allee 25 60486 Frankfurt am Main Germany 2) Air Products and Chemicals, Inc. 7201 Hamilton Boulevard Allentown, PA PA 18195-1501 USA
Te results, conclusions and recommendations given in this report refer to the specimens and data submitted as well as to the system conditions mentioned. Terefore, the results cannot be applied to other conditions. DECHEMA e.V. e.V. accepts no liability liability.. It is only permitted to reproduce this report unabridgedly. unabridgedly. Publication of the report or of single experimental results are subject to approval by DECHEMA e.V.
Atlas of Microstructures Microstructures
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Atlas of Microstructures Microstructures
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Atlas of Microstructures Microstructures
All rights reserved, reserved, including translations translations ISBN 978-1-57698-046-0 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. Tis document was prepared under the sponsorship of the Materials echnology echnology Institute Inc. (MI) and is approved for release. All data and information contained contained in this document are believed to to be reliable; however however,, no warranty of any kind, express 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. MI, the author author,, or any person acting on its behalf, assume no liability and expressly disclaim liability, 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 echnology Institute www.mti-global.org
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able of Contents Introduction ......................................... ..................................................................................... .................................................................................. ...................................... xx Description of est est Specimens Specimens ........................................... ....................................................................................... ..................................................... ......... xx Creep rupture test specimens..................................... ................................................................................. ................................................................... ....................... xx -
Alloy 35/45 Exposure Conditions Conditions ................ ................................. .................................. .................... ... able 1 Chemistry of Alloy 35/45 Creep Rupture Rupture Stress Samples ............... .................. ... able 2 Alloy HPMA Exposure Exposure Conditions Conditions ............................................... .............................. .................... ... able 3 Chemistry of Alloy HPMA Creep Rupture Stress Stress Samples................ able 4 Alloy HP Modified Exposure Conditions Conditions ............... ................................ .......................... ......... able 5 Chemistry of Alloy HP Modified Modified Creep Rupture Stress Stress Samples....... Samples....... able 6
Service-exposed specimens ........................................ .................................................................................... ................................................................... ....................... xx - History of Service Exposed Samples - Exposure Conditions of ................ ................................. .................................. .................................. ................. able 7 - Chemistry of Service Exposed Samples ............... ................................ ............................... .............. able 8
Sample Preparation Preparation and and Microstructural Characterization Characterization ....................................... ...................................................... ............... xx Metallographic Preparation ......................................... ..................................................................................... ............................................................ ................ xx Investigation echniques .......................................... ..................................................................................... ................................................................... ........................ xx - Differential Interference Contrast echnique echnique (LM-DIC) - Scanning Electron Microscopy (SEM-BSE) - Electron Probe Microanalysis (EPMA) - Interference Layer Metallography (LM-ZnSe) - Image Analysis by the False Color echnique (LM-FC) - Development of the Microstructure by Etching (LM-etched) iv
Results of the Microstructural Analyses ................................................................................... xx Alloy 35/45 ............................................................................................................................... xx - Results of image analysis of Alloy 35/45 ............................................ able 9 - As Cast .............................................................................................. Fig. 1-1a-j - Creep Rupture Samples .................................................................... Fig. 1-2a to 1-2j .................................................................... Fig. 1-3a to 1-3r .................................................................... Fig. 1-4a to 1-4r .................................................................... Fig. 1-5a to 1-5r .................................................................... Fig. 1-6a to 1-6z .................................................................... Fig. 1-7a to 1-7r .................................................................... Fig. 1-8a to 1-8r .................................................................... Fig. 1-9a to 1-9z .................................................................... Fig. 1-10a to 1-10r .................................................................... Fig. 1-11a to 1-11r .................................................................... Fig. 1-12a to 1-12z .................................................................... Fig. 1-13a to 1-13v - Conclusions ......................................................................................................................... xx Alloy HPMA ............................................................................................................................. xx - Results of image analysis of Alloy HPMA ............................. able 10 - As Cast .............................................................................................. Fig: 2-1a to 2-1j - Creep Rupture Samples .................................................................... Fig. 2-2a to 2-2r .................................................................... Fig. 2-3a to 2-3r .................................................................... Fig. 2-4a to 2-4r v
- Creep Rupture Samples (continued) ...................................................... Fig. 2-5a to 2-5r .................................................................... Fig. 2-6a to 2-6r .................................................................... Fig. 2-7a to 2-7z .................................................................... Fig. 2-8a to 2-8r .................................................................... Fig. 2-9a to 2-9r .................................................................... Fig. 2-10a to 2-10r .................................................................... Fig. 2-11a to 2-11r .................................................................... Fig. 2-12a to 2-12r .................................................................... Fig. 2-13a to 2-13r .................................................................... Fig. 2-14a to 2-14r .................................................................... Fig. 2-15a to 2-15r .................................................................... Fig. 2-16a to 2-16r .................................................................... Fig. 2-17a to 2-17s - Conclusions ......................................................................................................................... xx Alloy HP Modified ..................................................................................................................... xx - Results of image analysis of Alloy HP Modified ................................. able 11 - As Cast ............................................................................................. Fig. 3-1a to 3-1j - Creep Rupture Samples .................................................................... Fig. 3-2a to 3-2r .................................................................... Fig. 3-2a to 3-2r .................................................................... Fig. 3-3a to 3-3r .................................................................... Fig. 3-4a to 3-4r .................................................................... Fig. 3-5a to 3-5r .................................................................... Fig. 3-6a to 3-6r .................................................................... Fig. 3-7a to 3-7r vi
- Creep Rupture Samples (continued) ...................................................... Fig. 3-8a to 3-8r .................................................................... Fig. 3-9a to 3-9r .................................................................... Fig. 3-10a to 3-10r .................................................................... Fig. 3-11a to 3-11r .................................................................... Fig. 3-12a to 3-12r .................................................................... Fig. 3-13a to 3-13r - Conclusions ......................................................................................................................... xx Service Exposed Samples ............................................................................................................. xx - Results of image analysis of Service Exposed Samples ........................ able 12 - Alloy HPMA ................................................................................... Fig. 2-SE-Fa to 2-SE-Fr .................................................................... Fig. 2-SE-Ga to 2-SE-Gr - Alloy HP Modified .......................................................................... Fig. 3-SE-Ba to 3-SE-Br .................................................................... Fig. 3-SE-Aa to 3-SE-Ar .................................................................... Fig. 3-SE-Ca to 3-SE-Cr .................................................................... Fig. 3-SE-Da to 3-SE-Dr .................................................................... Fig. 3-SE-Ha to 3-SE-Hr .................................................................... Fig. 3-SE-Ia to 3-SE-Ir .................................................................... Fig. 3-SE-Ja to 3-SE-Jr .................................................................... Fig. 3-SE-Ea to 3-SE-Ez - Conclusions ......................................................................................................................... xx
Summary ................................................................................................................................ xx Composition of the Precipitates .................................................................................................... xx Average Compositions of the Precipitates in atomic-%......................... able 13 vii
Precipitation Kinetics Diagrams .............................................................................................. xx - Alloy 35/45 ..................................................................................... Fig. 1-PKD-a to 1-PKD-d .................................................................... Fig. 1-PKD-e - Alloy HPMA .................................................................................. Fig. 2-PKD-a to 2-PKD-c .................................................................... Fig. 2-PKD-d - Alloy HP Modified ................................................................... Fig. 3-PKD-a to 3-PKD-d .................................................................... Fig. 3-PKD-e
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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. o increase tube life, greater carbon concentrations were required to promote precipitation hardening upon elevated temperature exposure. Te increased carbon concentrations, however, generally resulted in excessive work hardening during conventional processing of wrought materials. Te 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. Tus, refinement of centrifugal casting processes was the gateway to further alloy development and more aggressive furnace operations. Te 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. Te 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. Te HK and HP alloys rely on precipitation of M23C6 and/or M7C3 carbides (where M is primarily chromium) for elevated temperature creep strength. Te precipitated chromium carbides in the HK and HP alloys tended to coalesce as exposure temperatures approached 1800 oF (982oC). Te 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
Atlas of Microstructures
extended aging time and temperature have been well documented by Battelle Columbus Laboratories1. Te 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. Te HP-modified alloy had the same nominal chemistry of the HP alloy along with the addition of typically 1 wt% niobium. Te niobium addition results in precipitation of M 23C6, M7C3, and MC type primary carbides upon solidification. In the M 23C6 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. itanium 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. Te 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. Te 35Cr/45Ni alloy has similar creep strength to the HPMA alloy but with notably improved carburization resistance. Te 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
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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 echnology Institute (MI) sponsored the Atlas of Microstructures project. Te specimens analyzed in the Atlas of Microstructures were foundry stress rupture specimens generously donated by Metalek International, Duraloy echnologies, 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 Metalek International. Te thorough, detailed microstructural analyses were completed at the Karl-Winnacker Institut der DECHEMA in Frankfurt, Germany. Te 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. Tis MI 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
Te specimens that had been taken from creep rupture tests can be summarized with regards to their exposure conditions and their chemistry in ables 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. Te test temperatures varied between 1675 and 2100°F (913 and 1149°C), the maximum exposure times reached 12,289 h. Te stress range was between 1.0 and 8.5 ksi (6.9 and 58.8 MPa). Te scatter of the chemical compositions was in the allowed range for these materials.
continued on page 8
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Atlas of Microstructures
Table 1: Alloy 35/45 Exposure Conditions Sample_ID Dechema Sample MI
A234_003 A234_004 A234_005 A234_006 A234_007 A234_008 A234_009 A234_010 A234_011 A234_012 A234_013 A234_014 A234_015
est-emp °F
est-emp °C
Stress Ksi
Stress MPa
Expo. ime hrs
as cast 1800 1800 1922 1900 1900 1900 2012 2000 2012 2057 2050 2100
as cast 982 982 1050 1038 1038 1038 1100 1093 1100 1125 1121 1149
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
35/45-1 (c) 35/45-2 35/45-3 35/45-4 35/45-5 35/45-6 35/45-7 35/45-8 35/45-9 35/45-10 35/45-11 35/45-12 35/45-13
Table 2: Chemistry of Alloy 35Cr/45Ni Stress Rupture Samples Sample_ID Sample Dechema MI Cr Ni Fe
A234_003 A234_004 A234_005 A234_006 A234_007 A234_008 A234_009 A234_010 A234_011 A234_012 A234_013 A234_014 A234_015
35/45-1 (c) 35/45-2 35/45-3 35/45-4 35/45-5 35/45-6 35/45-7 35/45-8 35/45-9 35/45-10 35/45-11 35/45-12 35/45-13
Atlas of Microstructures
35.85 35.12 34.84 32.30 32.39 34.34 34.41 32.30 32.65 34.84 32.30 32.59 34.84
44.59 43.69 44.43 43.40 44.77 45.42 45.37 43.40 44.61 44.43 43.40 44.34 44.43
bal bal bal bal bal bal bal bal bal bal bal bal bal
Composition (wt%) Mn Si C
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
1.55 1.83 1.86 1.52 1.85 1.30 1.78 1.52 1.71 1.86 1.52 1.82 1.86
0.38 0.46 0.43 0.41 0.42 0.37 0.42 0.41 0.42 0.43 0.41 0.42 0.43
Nb
N
Other
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
0.08 0 0.06 0 0.04 0 0.065 NR NR NR 0.065 NR 0.04 0 0.065 NR 0.04 0
i, Zr & W i, Zr & W i, Zr & W i i i i i, Zr & W i i, Zr & W
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Table 3: Alloy HPMA Exposure Conditions Sample_ID Dechema Sample MI
A234_018 A234_019 A234_020 A234_021 A234_022 A234_023 A234_024 A234_025 A234_026 A234_027 A234_028 A234_029 A234_030 A234_031 A234_032 A234_033 A234_034
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HPMA-1 (c) HPMA-2 HPMA-3 HPMA-4 HPMA-5 HPMA-6 HPMA-7 HPMA-8 HPMA-9 HPMA-10 HPMA-11 HPMA-12 HPMA-13 HPMA-14 HPMA-15 HPMA-16 HPMA-17
est-emp. °F
est-emp. °C
Stress Ksi
Stress MPa
Expo. ime hrs
as cast 1700 1700 1700 1750 1800 1800 1800 1832 1922 1950 1922 1922 2012 2012 2012 2012
as cast 927 927 927 954 982 982 982 1000 1050 1066 1050 1050 1100 1100 1100 1100
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
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
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 MI Cr Ni Fe
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
Atlas of Microstructures
25.01 24.92 24.29 25.01 25.01 25.85 24.38 24.49 24.49 24.36 25.11 25.01 24.49 24.36 24.49 24.49 25.01
33.79 34,54 32.51 33.79 33.79 35.25 35.13 33.53 33.53 33.18 36.13 33.79 33.53 33.18 33.53 33.53 33.79
bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal
Composition (wt%) Mn Si C
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
1.08 0.80 1.37 1.08 1.08 1.12 0.85 1.14 1.14 1.73 1.33 1.08 1.14 1.73 1.14 1.14 1.08
0.45 0.50 0.42 0.45 0.45 0.47 0.46 0.44 0.44 0.46 0.48 0.45 0.44 0.46 0.44 0.44 0.45
Nb
N
Other
0.65 0.49 0.40 0.65 0.65 0.95 1.08 0.51 0.51 0.83
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
i, Zr & W i W i, Zr & W i, Zr & W
0.65 0.51 0.83 0.51 0.51 0.65
i & W i, Zr & W i, Zr & W i & Zr i & W i, Zr & W i, Zr & W i & Zr i, Zr & W i, Zr & W i, Zr & W
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Table 5: Alloy HP Modified Exposure Conditions Sample_ID Dechema Sample MI est-emp. °F
A234_036 A234_038 A234_039 A234_040 A234_041 A234_042 A234_043 A234_044 A234_045 A234_046 A234_047 A234_048 A234_049
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HPNb-1 (b) HPNb-2 HPNb-3 HPNb-4 HPNb-5 HPNb-6 HPNb-7 HPNb-8 HPNb-9 HPNb-10 HPNb-11 HPNb-12 HPNb-13
as cast 1675 1700 1750 1778 1778 1800 1800 1800 1800 1850 1900 1900
est-emp. °C
Stress Ksi
Stress MPa
Expo. ime hrs
as cast 913 927 955 970 970 982 982 982 982 1010 1038 1038
6.50 8.50 5.10 5.15 3.77 5.40 5.42 5.52 2.80 3.60 2.30 1.90
44.6 58.8 35.2 25.5 26.0 37.4 37.4 37.4 19.3 24.8 15.8 13.1
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 MI Cr Ni Fe
Composition (wt%) Mn Si C
A234_036 A234_038 A234_039 A234_040 A234_041 A234_042 A234_043 A234_044 A234_045 A234_046 A234_047 A234_048 A234_049
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
HPNb-1 (b) HPNb-2 HPNb-3 HPNb-4 HPNb-5 HPNb-6 HPNb-7 HPNb-8 HPNb-9 HPNb-10 HPNb-11 HPNb-12 HPNb-13
Atlas of Microstructures
25.30 25.34 25.26 25.34 25.11 25.11 24.88 24.86 24.86 25.26 25.62 25.26 25.26
34.93 34.88 33.56 34.88 32.85 32.85 33.33 33.79 33.30 33.56 34.75 33.56 33.56
bal bal bal bal bal bal bal bal bal bal bal bal bal
1.40 1.67 1.23 1.67 1.79 1.79 1.29 1.37 1.35 1.23 1.71 1.23 1.23
0.44 0.43 0.42 0.43 0.45 0.45 0.41 0.41 0.42 0.42 0.48 0.42 0.42
Nb
N
1.06 0.75 0.74 0.75 0.83 0.83 1.18 1.22 1.2 0 0.74 0.82 0.74 0.74
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
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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 SiO 2 suspension of size 0.02 µm. Te 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 echnique (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.
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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. Te 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. Te 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. Te 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 echnique (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. Te 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. Te 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
Te 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 Tiourea. It consists of 1-Methoxy- 2-propanol (40-50%), Tiourea (3-5%) and Nonylphenol-ethoxylate (5-7%). Te 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).
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Table 9: Results
Sample_ID
of image analysis of Alloy 35Cr45Ni Creep Rupture Stress Samples Sample Temp. Temp °F Stress Stress Expo. Time core °C °F MPa Ksi hrs M(C,N) M7C3 M23C6 M6C 0.8
area fraction in % M2(C,N) G-Phase M(C,N)
outer zone / edge M7C3 M23C6 M6C
M2(C,N)
A234_003
35/45-1 (c)
as cast
as cast
3.9
A234_004
35/45-2
982
1800
22.8
3.30
1,509
0.5
7.9
1.5
0.8
5.9
6.3
A234_005
35/45-3
982
1800
20.0
2.90
7,217
0.3
7.8
1.8
0.3
6.6
12.5
+
A234_006
35/45-4
1050
1922
23.4
3.40
367
0.4
6.9
0.7
0.3
7.4
4.2
+
A234_007
35/45-5
1038
1900
14.4
2.090
2,023
< 0.05
8.3
< 0.03
3.0
< 0.02
8.8
13.1
+
A234_008
35/45-6
1038
1900
11.0
1.60
6,331
< 0.05
10.5
7.5
< 0.05
+
+++
++
A234_009
35/45-7
1038
1900
10.6
1.54
10,247
0.1
15.1
10.7
+
+++
++
A234_010
35/45-8
1100
2012
13.8
2.00
335
1.1
7.6
A234_011
35/45-9
1093
2000
12.4
1.80
1,297
0.4
10.8
0.3
A234_012
35/45-10
1100
2012
7.9
1.15
3,545
0.4
12.0
3.8
A234_013
35/45-11
1125
2057
11.1
1.60
277
1.1
7.2
A234_014
35/45-12
1121
2050
.06.9
1.00
2,606
0.2
12.7
9.2
A234_015
35/45-13
1149
2100
10.3
1.50
248
0.6
8.6
1.6% mass-%) promotes the formation of G-phase by facilitating the conversion of NbC into G-phase. In the G-phase as well as in the M6C particles enrichment of Nb is observed which directs towards the dissolution of former MC 10-15. Te dissolution of MC with increasing exposure time is confirmed by a decrease of their volume fraction. In the cast state the fraction amounts to about 0.8%, while after short exposure times in which no G-phase has formed yet it amounts to 0.6-1.1%. When G-phase formation or after longer times G-phase and M6C formation start the fraction goes down to less than 0.02-0.4%. After several thousand hours above 1900°F (1038°C) the secondary carbides (M23C6) dissolve and form blocky, elongated particles. Besides these coarse carbides of the type M23C6 where the particles sometimes also have grown together only coarse carbides of the type M6C and in the sub-surface zone of the type M2(C,N) are present. Te G-phase is no longer found.
Atlas of Microstructures
Table 10: Results of image analysis of Alloy HPMA Creep Rupture Stress Samples Sample_ ID
Sample
Temp.
Temp °F
Stress
Stress
°C
°F
MPa
Ksi
Expo. Time hrs
area fraction in % M(C,N)
M7C3
0.9
3.4
core M23C6 M6C
M2(C,N) G-Phase
M(C,N)
M7C3 +
outer zone / edge M23C6 M6C
M2(C,N)
A234_018
HPMA-1 (c)
as cast
as cast
A234_019
HPMA-2
927
1700
55.2
8.00
183
0.6
7.0
0.8
+
+++
+
A234_020
HPMA-3
927
1700
41.3
6.00
2,338
View more...
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