The Post Weld Heat Treatment Response in the Heat Affected Zone of 2.25Cr-1Mo Steel THESIS

144 Pages·2013·7.07 MB·English

The Post Weld Heat Treatment Response in the Heat Affected Zone of
2.25Cr-1Mo Steel
THESIS

Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in
the Graduate School of The Ohio State University

By
David K. Hodgson
Graduate Program in Welding Engineering
The Ohio State University
2013

Master‟s Thesis Committee:
Professor John Lippold – Advisor
Professor Sudarsanam Babu
Copyright by
David K. Hodgson
2013
Abstract
F22 (2.25Cr-1Mo) steel is used in a variety of applications in the oil and gas industry.
One of the primary uses is for subsea well equipment where exposure to elevated
temperatures and sour service conditions is common. In order to be acceptable for sour
service conditions, the National Association of Corrosion Engineers (NACE MR0175)
stipulates maximum permissible hardness of 22 HRC (250 VHN). In welded
components made of steel such as F22, this is particularly problematic due to the
formation of hard martensite in the heat affected zone (HAZ). To meet these
requirements a post weld heat treatment (PWHT) is used. In practice, it has been
observed that materials with similar compositions and similar welding parameters will
temper differently during PWHT.
To investigate the cause of this discrepancy, 16 heats of different composition within the
specification limits for F22 were investigated. As a basis of comparison, each heat was
subjected to an austenization treatment followed by a water quench with the goal of
producing a fully hardened martensitic microstructure. When comparing the hardness of
these samples with previous research it was found that the heats with higher carbon
content were softer than expected. Upon further investigation with optical microscopy, it
was found that second phase particles (which are presumed to be alloy carbides) are still
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present in the microstructure. The presence of these carbides (along with observed
autotempering) occupies carbon and contributes to a softer microstructure than as
predicted from the carbon content alone.
To study the temper response, autogenous spot welds were made using parameters
designed to replicate the HAZ observed in a typical weld used for cladding with a
corrosion resistant layer. After welding, the welds were tempered for a range of
Hollomon-Jaffe parameters which was inclusive of the post weld heat treatment (PWHT)
used in practice. The average hardness in the HAZ was used to calculate the average
tempering rates for each heat.
In the as-welded condition, two heats of the same composition displayed radically
different behavior in the HAZ, with one being hardest in the coarse grained heat affected
zone (CGHAZ) and the other being hardest in the fine grained heat affected zone
(FGHAZ). In examination of these regions using scanning electron microscopy, there
was a qualitative difference in the size and distribution of precipitates, but resolution
using this technique was not sufficient to fully characterize the microstructure. Upon
tempering, several different heats exhibited secondary hardening behavior as a result of
specific heat treatments. When compared to the HAZ, the tempering response in the
fusion zone typically displayed similar tempering response with the exception of the
overall hardness being higher and the secondary hardening reactions were greater in the
fusion zone.
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To explore the impact of the welding thermal cycle on the variety of carbides found to be
present in F22, thermodynamic and kinetic simulations were conducted for the
temperature range found throughout the HAZ. These simulations indicated that for
carbides of identical size, those typically found later in the precipitation sequence were
more stable and dissolved slower.
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To my wife Rachel,
your love and support provide constant encouragement to be my best
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Acknowledgments
I would like to thank my advisor, Dr. John Lippold for providing a tremendous resource
in both technical knowledge and professional guidance throughout the research and
graduate school process.
I‟d also like to thank Dr. Suresh Babu for participating on my examination committee,
providing assistance in learning and troubleshooting simulation tools as well as providing
a great foundation and introduction for my continued learning of steel metallurgy.
Additionally, his efforts as Director of the National Science Foundation supported Center
for Integrated Materials Joining for Energy Applications (CIMJSEA) allowed for a great
opportunity to interact with industry members. Also, thanks to all of the other faculty
and members of the welding engineering department at the Ohio State University for
providing a great environment to learn.
Special thanks go to Dean Hannam, Feng Lu and John Bartos at Cameron International
for supporting this research and supplying valuable industrial experience and knowledge,
which proved to be indispensable throughout this project.
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Thank you to Paul Mason and Kaisheng Wu of ThermoCalc for their assistance in
developing the appropriate code and methodology to solve modeling challenges used in
this research.
Thanks to David Tung and Tapasvi Lolla for taking time out of busy schedules to help
with SEM work, to Jeff Rodelas, Adam Hope and Ben Sutton and the other members of
the Welding and Joining Metallurgy Group for always being willing to help and
providing advice when welding research was foreign to me. Also, thank you to all the
great friends I‟ve made during my time in Columbus for support.
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Vita

August 1984 ...................................................Born - Cleveland, Ohio
December 2006 ..............................................B.S. Mechanical Engineering, University of
Toledo, Toledo, Ohio
March 2011 to Present ...................................Welding and Joining Metallurgy Research
Group, The Ohio State University,
Columbus, Ohio

Fields of Study

Major Field: Welding Engineering

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Table of Contents
Abstract ............................................................................................................................... ii
Acknowledgments.............................................................................................................. vi
Vita ................................................................................................................................... viii
List of Tables ................................................................................................................... xiii
List of Figures .................................................................................................................. xiv
Chapter 1: Introduction ....................................................................................................... 1
Chapter 2: Background ....................................................................................................... 9
2.1 Heat Treatment in 2.25Cr-1Mo Steels ...................................................................... 9
2.1.1 Processing of F22 ............................................................................................... 9
2.2 Hardenability ........................................................................................................... 12
2.2.1 Effect of Alloying Additions On Hardenability ............................................... 14
2.2.2 Effect of Prior Austenite Grain Size ................................................................. 17
2.3 Martensite ................................................................................................................ 18
2.3.1 Strengthening Mechanisms in Martensite ........................................................ 18
2.3.2 As Quenched Hardness ..................................................................................... 22
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