Role of alloy additions on strengthening in 17-4 PH stainless steel

150 Pages·2017·17.87 MB·English

by  Arpana S. Murthy

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Doctoral Dissertations Student Theses and Dissertations
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Arpana S. Murthy
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Murthy, Arpana S., "Role of alloy additions on strengthening in 17-4 PH stainless steel" (2012). Doctoral
Dissertations. 2253.
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ROLE OF ALLOY ADDITIONS ON STRENGTHENING IN 17-4 PH STAINLESS
STEEL
by
ARPANA SUDERSHAN MURTHY
A DISSERTATION
Presented to the Faculty of the Graduate School of the
MISSOURI UNIVERSITY OF SCIENCE AND TECHNOLOGY
In Partial Fulfillment of the Requirements for the Degree
DOCTOR OF PHILOSOPHY
in
MATERIALS SCIENCE AND ENGINEERING
2012
Approved by
Von L. Richards, Advisor
David C.Van Aken
Simon L. Lekakh
Julia E. Medvedeva
Matthew J. O'Keefe
© 2012
Arpana Sudershan Murthy
All Rights Reserved
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PUBLICATION DISSERTATION OPTION
This dissertation has been prepared in the form of five manuscripts for
publication. Pages 20-44 have been published in International Journal of Metal Casting.
Pages 45-59 have been published in Proceedings of Materials Processing Fundamentals
Symposia of TMS, 2011. Pages 60-73 have been published in Scripta Materialia, 2012.
Pages 74-98 and pages 99-125 have been prepared in the style utilized by Metallurgical
Transactions A. An introduction and a literature review sections have been added in the
beginning, while a summary section has been added to conclude.
IV
ABSTRACT
Alloy modifications by addition of niobium (Nb ), vanadium (V), nitrogen (N) and
cobalt (Co) to cast 17-4 PH steel were investigated to determine the effect on mechanical
properties. Additions of Nb, V, and N increased the yield strength from 1120 MPa to
1310 MPa while decreased the room temperature charpy V notch (CVN) toughness from
20 J to four Joules.
The addition of Co to cast 17-4 PH steel enhanced the yield strength and CVN
toughness from 1140 MPa to 1290 MPa and from 3.7 J to 5.5 J, respectively. In the base
17-4 PH steel, an increase in block width from 2.27 ± 0.10 1-1m in the solution treated
condition to 3.06 ± 0.17 1-!ffi upon aging at 755 K was measured using orientation image
microscopy. Cobalt inhibited recrystallization and block boundary migration during aging
resulting in a finer martensitic block structure.
The influence of Co on copper (Cu) precipitation in steels was studied using atom
probe tomography. A narrower precipitate size distribution was observed in the steels
with Co addition. The concentration profile across the matrix I precipitate interface
indicated rejection of Co atoms from the copper precipitates. This behavior was observed
to be energetically favorable using first principle calculations. The activation energies for
Cu precipitation increased from 205 kJ/ mol in the non-cobalt containing alloy, to 243 kJ/
mol, and 272 kJ/ mol in alloys with 3 wt. %Co, and 7 wt. % Co, respectively. The role of
Co on Cu precipitation in cast 17-4 PH steel is proposed as follows: (i) Co is rejected out
of the Cu precipitate and sets up a barrier to the growth of the Cu precipitate; (ii) results
in Cu precipitates of smaller size and narrower distribution; (iii) the coarsening of Cu
precipitates is inhibited; and (iv) the activation energy for Cu precipitation increases.
v
ACKNOWLEDGEMENTS
I take this opportunity to thank my advisor Dr. Von L. Richards for his support,
encouragement and patience throughout my PhD program. The discussions with him
played an important role in the development of my thoughts and reasoning ability. He
played an important role in shaping me as a researcher. I sincerely thank Dr. David Van
Aken for keeping me motivated throughout this work. His valuable advice at various
stages played an important role in the success of this work. I heartily thank Dr. Simon
Lekakh for training me, especially in my initial years into the program. I thank Dr. Julia
Medvedeva and Dr. Nadezdah Medvedeva for their discussions on ab initio modeling. I
thank Dr. Matthew J. O'Keefe for his time and cooperation during this program.
I sincerely thank the funding agencies: Casting for Defense Readiness (CIDR),
Steel Founder's Society of America, Benet Laboratories, Wolf Professorship of Metal
Casting and the MSE department.
I acknowledge the support from all my colleagues, especially Ryan Howell,
William Peach, Ken Doering, Anthony Wilshire and Jared Teague. I thank Michael
Myers for teaching me quartz encapsulation skills. I heartily thank Dr. Dieter Isheim
from Northwestern University for training me on the Local Electrode Atom Probe
analysis. I acknowledge the assistance received from Clarissa Wisner, Eric Bohannan,
Nathan, Jimmy, Craig, Jack, Rebecca, Denise, Joyce and Amy. I thank my roommates
and friends who brought joy to my life in these years.
I thank my parents for their love, support, patience and belief in me. I thank my
husband for always being there for me, for cheering me up, and for inspiring me to fulfill
this task. I dedicate this dissertation to my late grandmother Mrs. Kamala Puttu.
VI
TABLE OF CONTENTS
Page
PUBLICATION DISSERTATION OPTION ................................................................... iii
ABSTRACT ....................................................................................................................... iv
ACKNOWLEDGMENTS .................................................................................................. v
LIST OF ILLUSTRATIONS .............................................................................................. x
LIST OF TABLES ........................................................................................................... xiii
SECTION
1. INTRODUCTION .................................................................................................... 1
1.1. RESEARCH OBJECTIVE .............................................................................. 1
1.2. PRECIPITATION HARDENED (PH) STAIN LESS STEEL ......................... 2
1.2.1. History ..................................................................................................... 2
1.2.2. Classification ........................................................................................... 3
1.3. 17-4 PH STEEL ............................................................................................... 4
2. LITERATURE REVIEW ......................................................................................... 5
2.1. PHASE CONSTITUTION ............................................................................... 5
2.1.1. Martensite ............................................................................................... 5
2.1.2. Delta Ferrite ............................................................................................ 7
2.1.3. Retained Austenite .................................................................................. 8
2.2. COPPER PRECIPITATION .......................................................................... 10
2.2.1. Size and Structure ................................................................................. 11
2.2.2. Precipitation Kinetics ............................................................................ 12
2.2.2.1. Activation energy .......................................................................... 12
2.2.2.2. A vrami exponent 'n' ..................................................................... 12
2.3. ALLOY DEVELOPMENT ........................................................................... 14
2.3.1. Niobium, Vanadium, and Nitrogen Additions ...................................... 14
2.3.2. Co Addition ........................................................................................... 15
2.3.3. Heat Treatment. ..................................................................................... 16
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2.3.4. Other Approaches ................................................................................. 17
2.4. INSPIRATION FOR THIS WORK ............................................................... 19
PAPER
I. Microstructure and Properties ofNb, V and N Modified CB7Cu-1 (17-4 PH)
Steel ......................................................................................................................... 20
ABSTRACT ................................................................................................................ 20
1. INTRODUCTION AND THERMODYNAMIC CALCULATIONS .................... 21
2. EXPERIMENTAL PROCEDURE ......................................................................... 24
3. RESULTS ............................................................................................................... 29
3.1. Homogenization treatment ............................................................................. 29
3.2. Austenite conditioning treatment ................................................................... 32
3.3. Age-hardening kinetics .................................................................................. 34
3.4. Mechanical properties and fractography ........................................................ 34
4. DISCUSSION ......................................................................................................... 39
5. CONCLUSIONS ..................................................................................................... 41
6. FUTURE WORK .................................................................................................... 42
REFERENCES ........................................................................................................... 42
II. On the homogenization of cobalt modified 17-4 PH stainless steel. ..................... 45
ABSTRACT ................................................................................................................ 45
1. INTRODUCTION .................................................................................................. 46
2. EXPERIMENT PROCEDURE .............................................................................. 46
3. RESULTS ............................................................................................................... 49
4. DISCUSSION ......................................................................................................... 54
5. CONCLUSIONS ..................................................................................................... 57
ACKNOWLEDGEMENT .......................................................................................... 57
REFERENCES ........................................................................................................... 58
III. Copper precipitation in cobalt alloyed precipitation hardened stainless steel ...... 60
ABSTRACT ................................................................................................................ 60
1. INTRODUCTION .................................................................................................. 61
2. ATOM PROBE TOMOGRAPHY .......................................................................... 62
3. FIRST PRINCIPLES STUDY AND DISCUSSION .............................................. 67
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4. SUMMARY ............................................................................................................ 71
ACKNOWLEDGEMENTS ....................................................................................... 72
REFERENCES ........................................................................................................... 72
IV. Cobalt addition to 17-4 PH steel. Part 1: Matrix microstructure and mechanical
properties ............................................................................................................. 7 4
ABSTRACT ................................................................................................................ 74
1. INTRODUCTION .................................................................................................. 75
2. PROCEDURE ......................................................................................................... 80
3. RESULTS ............................................................................................................... 83
4. DISCUSSION ......................................................................................................... 92
5. SUMMARY ............................................................................................................ 95
ACKNOWLEDGEMENTS ........................................................................................ 96
REFERENCES .......................................................................................................... 96
V. Cobalt addition to 17-4 PH steel. Part II: Kinetics of Copper precipitation .......... 99
ABSTRACT ............................................................................................................... 99
1. INTRODUCTION ................................................................................................ 100
2. PROCEDURE ....................................................................................................... 102
2.1. Experiment. .................................................................................................. 102
2.2. Modeling ...................................................................................................... 104
3. RESULTS ............................................................................................................. 108
3 .1. Modeling ...................................................................................................... 108
3.2. Copper precipitation ..................................................................................... 110
4. DISCUSSION ....................................................................................................... 119
5. CONCLUSION ..................................................................................................... 123
ACKNOWLEDGEMENT ........................................................................................ 123
REFERENCES ........................................................................................................ 123
SECTION
3. CONCLUSIONS ................................................................................................... 126
3.1. NIOBIUM, VANADIUM, NITROGEN ADDITION ................................. 126
3.2. COBALT ADDITION ................................................................................. 126
3.3. RETAINED AUSTENITE ........................................................................... 128
IX
BIBLOGRAPHY ............................................................................................................ 129
VITA ............................................................................................................................. 135