Prediction of microsegregation and pitting corrosion ... Â· 3 Vilpas, Martti, Prediction of microsegregation and pitting corrosion resistance of austenitic stainless steel welds

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  • VTT PUBLICATIONS 390

    Prediction ofmicrosegregation and pitting

    corrosion resistance ofaustenitic stainless steel

    welds by modelling

    Martti VilpasVTT Manufacturing Technology

    Dissertation for the degree of Doctor of Science in Technology to bepresented with due permission for public examination and debate in

    Auditorium K216 at Helsinki University of Technology (Espoo, Finland) onthe 22th of June, 1999 at 12 oclock noon.

    TECHNICAL RESEARCH CENTRE OF FINLANDESPOO 1999

  • ISBN 9513853837 (soft back ed.)ISSN 12350621 (soft back ed.)

    9513853845 (URL: http://www.inf.vtt.fi/pdf/)14550849 (URL: http://www.inf.vtt.fi/pdf/)

    Copyright Valtion teknillinen tutkimuskeskus (VTT) 1999

    JULKAISIJA UTGIVARE PUBLISHER

    Valtion teknillinen tutkimuskeskus (VTT), Vuorimiehentie 5, PL 2000, 02044 VTTpuh. vaihde (09) 4561, faksi (09) 456 4374

    Statens tekniska forskningscentral (VTT), Bergsmansvgen 5, PB 2000, 02044 VTTtel. vxel (09) 4561, fax (09) 456 4374

    Technical Research Centre of Finland (VTT), Vuorimiehentie 5, P.O.Box 2000,FIN02044 VTT, Finlandphone international + 358 9 4561, fax + 358 9 456 4374

    VTT Valmistustekniikka, Voimalaitosten materiaalitekniikka,Kemistintie 3, PL 1704, 02044 VTTpuh. vaihde (09) 4561, faksi (09) 456 7002

    VTT Tillverkningsteknik, Material och strukturell integritet,Kemistvgen 3, PB 1704, 02044 VTTtel. vxel (09) 4561, fax (09) 456 7002

    VTT Manufacturing Technology, Materials and Structural Integrity,Kemistintie 3, P.O.Box 1704, FIN02044 VTT, Finlandphone international + 358 9 4561, fax + 358 9 456 7002

    Technical editing Maini Manninen

    Libella Painopalvelu Oy, Espoo 1999

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    Vil pas, Martti, Prediction of microsegregation and pitting corrosion resistance of austeniticstainless steel welds by modelling. Espoo 1999. Technical Research Centre of Finland, VTTPublications 390. 139 p. + app. 27 p.

    Keywords austenitic stainless steel, welding, surface remelting, solidification, microsegre-gation, corrosion resistance, modelling, prediction, pitting corrosion

    Abstract

    The present study focuses on the ability of several computer models to accuratelypredict the solidification, microsegregation and pitting corrosion resistance ofaustenitic stainless steel weld metals. Emphasis was given to modelling the effectof welding speed on solute redistribution and ultimately to the prediction of weldpitting corrosion resistance. Calculations were experimentally verified by applyingautogenous GTA- and laser processes over the welding speed range of 0.1 to 5m/min for several austenitic stainless steel grades.

    Analytical and computer aided models were applied and linked together formodelling the solidification behaviour of welds. The combined use ofmacroscopic and microscopic modelling is a unique feature of this work. Thisprocedure made it possible to demonstrate the effect of weld pool shape and theresulting solidification parameters on microsegregation and pitting corrosionresistance. Microscopic models were also used separately to study the role ofwelding speed and solidification mode in the development of microsegregationand pitting corrosion resistance.

    These investigations demonstrate that the macroscopic model can be implementedto predict solidification parameters that agree well with experimentally measuredvalues. The linked macro-micro modelling was also able to accurately predictsegregation profiles and CPT-temperatures obtained from experiments. Themacro-micro simulations clearly showed the major roles of weld composition andwelding speed in determining segregation and pitting corrosion resistance whilethe effect of weld shape variations remained negligible.

    The microscopic dendrite tip and interdendritic models were applied to welds withgood agreement with measured segregation profiles. Simulations predicted thatweld inhomogeneity can be substantially decreased with increasing welding speedresulting in a corresponding improvement in the weld pitting corrosion resistance.In the case of primary austenitic solidification, the dendrite cores were predicted tobe the weakest link with respect to weld pitting corrosion resistance. In primaryferritic solidification, the second phase austenite in the vicinity of / interfaceswas predicted to show lowest pitting corrosion resistance.

    Solidification parameters used in the modelling were verified by cooling rate anddendrite arm spacing measurements as well as by analytical calculations.Experimental investigations using electron probe microanalyses (EPMA, CMA),electron microscopy (SEM, FEG-STEM), microstructural investigations andpitting corrosion tests were used in assessing the calculated microsegregation andCPT-temperatures and showed a reasonably good compatibility with the results ofmodelling.

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    Preface

    The majority of the work presented in this thesis was accomplished at VTTManufacturing Technology as a part of a research project Optimisation of weldproperties by solidification modelling during the years 1995 - 1997. The projectwas financed by the Technology Development Center of Finland (Tekes),Outokumpu Polarit Oy, Ahlstrom Machinery Oy and VTT ManufacturingTechnology. A part of the study was implemented during a technical visit to Japanthat was contributed by Japan-Scandinavia Sasakawa Foundation and VTT. Iwould like to thank all these parties for their economical and technical support.

    Professor Hannu Hnninen at Helsinki University of Technology and ProfessorRisto Karppi at VTT Manufacturing Technology, the supervisor and instructor ofthis thesis, respectively, deserve my sincere gratitude for their valuablediscussions, good advise and encouragement.

    I would like to express my sincere thanks also to the pre-examiners of this thesis,Professor Veli Kujanp at Lappeenranta University of Technology and GeneralManager Niilo Suutala at Outokumpu Polarit Oy for their constructive criticismand valuable comments.

    I am deeply indebted to Research Director Heikki Kleemola, Research ManagerRauno Rintamaa and Group Manager Heli Talja at VTT ManufacturingTechnology for providing me opportunities and resources that were needed tomake this effort possible.

    Professor Kazutoshi Nishimoto at Osaka University deserves my special thanksfor his technical advises and for the possibility to implement a part of the study asa visiting researcher at Osaka University in spring and summer 1997. During thetechnical visit to Japan a part of the modelling work and experiments wereimplemented also at Nippon Steel Co. For this possibility and fruitful co-operationI would like to express my sincere thanks to Dr. Nobutaka Yurioka, Dr. MasaoFuji, Dr. Toshihiko Koseki and Dr. Hiroshige Inoue.

    The macroscopic simulations were carried out by means of a computerised modeldeveloped by Pennsylvania State University. For the possibility to use the model,I am thankful to professor Tarasankar DebRoy. Concerning technical support inmodelling, I am very much indebted to my colleague Mr. Hannu Martikainen fromVTT Manufacturing Technology. I am also very grateful to Dr. Jyrki Miettinen atHelsinki University of Technology for collaboration in part of microscopicmodelling.

    The experimental investigations including delivery of test materials, pittingcorrosion tests, and part of the metallography were conducted in closecollaboration with Mr. Antero Kyrlinen and his co-workers at Outokumpu

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    Polarit Oy to whom I express my special thanks. Concerning line scan analysesand part of the microscopy of the test welds I would like to thank also Mr. OlliNousiainen at University of Oulu. Mr. Tapio Saukkonen at Helsinki University ofTechnology is acknowledged for his contribution in a part of scanning electronmicroscopy studies. Concerning the nitrogen analyses I am grateful to Mrs.Connie Westman from the Swedish Institute of Metals Research in Stockholm.

    The experiments at VTT Manufacturing Technology were carried out jointly withMr. Esko Kallinen (welding), Mr. Jouni Alhainen (weld cooling measurements),Mrs. Anja Norring (metallography) and Mr. Pertti Nenonen (electron microscopy)to whom I am very grateful for fruitful collaboration. I would like to thank alsoMrs. sa vall for help in editing of the manuscript and Mr. Tuomo Hokkanenfor preparing the drawings. Dr. Gary Marquis deserves sincere thanks for revisingthe English text. To these and all other colleagues working for this effort I wouldlike to express my most sincere gratitude.

    Finally, I would like to express my warmest thanks to my family, dear wife Pirjo,and two sons Mika and Juha for love, patience, encouragement and understandingwhich were all needed to finalise this effort.

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    Contents

    Abstract ....................................................................................................................3

    Preface......................................................................................................................4

    List of symbols and acronyms..................................................................................8

    Original features.....................................................................................................10

    1. Introduction ........................................................................................................111.1 Problem description.....................................................................................111.2 Weld pool behaviour ...................................................................................13

    1.2.1 Heat and fluid flow and weld geometry..............................................131.2.2 Weld pool solidification .....................................................................15

    1.3 Solidification and cooling of