Formation of Bainite in Steels Jiaqing Yin Doctoral Thesis in Department of Materials Science and Engineering School of Industrial Engineering and Management KTH Royal Institute of Technology Stockholm, Sweden, 2017 Jiaqing Yin, Formation of Bainite in Steels KTH Royal Institute of Technology School of Industrial Engineering and Management Department of Materials Science and Engineering SE-100 44 Stockholm, Sweden ISBN 978-91-7729-391-0 Akademisk avhandling som med tillstånd av Kungliga Tekniska Högskolan i Stockholm framlägges till offentlig granskning för avläggande av Teknologie Doktorsexamen den 14 juni 2017 kl 10.00 i sal B2, Brinellvägen 23, Kungliga Tekniska Högskolan, Stockholm © Jiaqing Yin (尹嘉清), 2017 This thesis is available in electronic version at kth.diva-portal.org Printed by Universitetsservice US-AB, Stockholm, Sweden Abstract A systematic survey of morphology of bainite as well as proeutectoid ferrite was carried out in order to validate some old thoughts of a sharp transition between the diffusional and diffusionless transformation at the bainite start temperature in steels. Based on investigation of morphology of grain boundary nucleated particles of ferrite as function of temperature, it is confirmed that there is no morphological evidence supporting a sharp change neither between Widmanstätten ferrite and the ferritic component of upper bainite, nor between upper and lower bainite. Instead, following the decreasing holding temperature, the change of morphology is gradual. Among the three products, both Widmanstätten ferrite and upper bainite start with precipitation of ferrite plates at a grain boundary while lower bainite starts with intragranular nucleation. In case of grain boundary nucleation, the precipitates usually form a group of parallel plates with same crystallographic orientation to the parent austenite grain. This process is followed by a second stage of decomposition of the austenitic interspace, which remained in between the primary ferrite plates. At high temperature, when precipitation of cementite at the broad interfaces of primary ferrite is not possible, the austenitic interspace would either retain as thin slabs or transform into pearlite through a nodule originated from a grain boundary. With decreasing temperature, cementite precipitation starts to be possible, which initiates simultaneous growth of ferrite. Such a process is thus a eutectoid reaction. Generally, there are two modes of eutectoid reactions operating in the second stage of bainite transformation, i.e. a degenerate mode and a cooperative mode, which would lead to final morphology of typical upper and lower bainite, respectively, in definition of carbides morphology. It was further demonstrated that both upper and lower bainite according to this definition can form simultaneously in a wide temperature range, indicating that both structures have formed without a sharp change of growth mechanism. It thus seems that a sharp temperature between the upper and lower bainite structures exists only when the definition is based on their nucleation sites, i.e. grain boundary nucleation for upper bainite and intragranular nucleation for lower bainite. Since it is confirmed that the second stage of bainite transformation has characters of eutectoid transformation, the process must occur under diffusion of carbon. On the other hand, if the first stage is a diffusionless process it should have a high growth rate, high enough to prevent carbon diffusion. This is not supported by lengthening rate obtained in current study as well as data from literature for Fe-C alloys. Finally, it is shown that the so-called subunits play no role in the lengthening process of bainite. Keywords: Fe-C alloys, Bainitic transformation, Proeutectoid ferrite, Upper bainite, Lower bainite, Morphology, Steels. i Sammanfattning En systematisk genomgång av morfologin hos bainit lika väl som proeutektoid ferrit har utförts för att pröva gamla tankar om en skarp övergång mellan diffusionsbetingad och diffusionslös omvandling vid bainitens starttemperatur i stål. Baserat på en undersökning av morfologin som funktion av temperaturen hos ferritiska partiklar som kärnbildats på korngränser, blev det bekräftat att det inte finns något morfologiskt bevis som stödjer en skarp ändring varken mellan Widmanstätten ferrit och den ferritiska komponenten av övre bainit eller mellan övre och undre bainit. I stället är den morfologiska förändringen vid lägre omvandlingstemperatur gradvis. Bland de tre omvandlingsprodukterna startar både Widmanstättenferrit och övre bainit med utskiljning av ferritplattor vid någon korngräns medan lägre bainit startar genom kärnbildning inuti austenitkornen. I fallet med kärnbildning vid en korngräns bildas vanligen en grupp av parallella plattor med samma kristallografiska orientering till moderkornet. Denna process flöjs av ett andra stadie med sönderfall av de austenitiska mellanrum som återstod mellan de primära ferritplattorna. Vid hög temperatur där utskiljning av cementit vid de breda ytorna hos de primära ferritplattorna inte är möjlig kommer de austenitiska mellanrummen antingen att bevaras som tunna skivor eller omvandlas till perlit i form av noduler utgående från korngränserna. Vid lägre temperatur börjar cementitutskiljning bli möjlig vilket initierar samtidig tillväxt av ferrit. En sådan process är alltså en eutektoid reaktion. Generellt sett finns det två slags eutektoida reaktioner som kan ske i det andra stadiet av bainitens bildning, det degenererade slaget och det kooperativa slaget, och som skulle leda till den slutliga morfologin av övre och lägre bainit enligt en definition baserad på karbidens morfologi. Det demonstrerades vidare att både övre och lägre bainit enligt denna definition kan bildas samtidigt i ett brett temperaturområde vilket indikerar att dessa två strukturer bildas utan en skarp ändring av tillväxtmekanismen. Det synes alltså som om en skarp temperatur mellan övre och undre bainit exiserar bara med en definition baserad på deras kärnbildningsställen, dvs. korngränskärnbildning för övre bainit och intragranular kärnbildning för lägre bainit. Eftersom det är bekräftat att det andra stadiet av bainitbildningen har karaktär av eutektoid omvandling måste den ske under diffusion. Å andra sidan, om det första stadiet sker med en diffusionslös process så borde denna ha en hög tillväxthastighet, tillräckligt hög för att förhindra diffusion av kol. Detta stöds inte av förlängningshastigheter erhållna i detta arbete eller i litteraturen på Fe-C legeringar. Slutligen har det visats att så kallade ”subunits” inte spelar någon roll i förlängningsprocessen av bainit. ii Preface The present thesis is based on the following appended papers: I. Morphology of Proeutectoid Ferrite Jiaqing Yin, Mats Hillert, Annika Borgenstam Metall. and Mat. Trans. A, 2017, vol. 48, pp. 1425-43. II. Second Stage of Upper Bainite in a 0.3 Mass Pct C Steel Jiaqing Yin, Mats Hillert, Annika Borgenstam Metall. and Mat. Trans. A, 2017, vol. 48, pp. 1444-58. III. C-curves for Lengthening of Widmanstätten and Bainitic Ferrite Jiaqing Yin, Lindsay Leach, Mats Hillert, Annika Borgenstam Submitted manuscript to Metall. and Mat. Trans. A. IV. Morphology of Upper and Lower Bainite in a 0.7 Mass Pct C Steel Jiaqing Yin, Mats Hillert, Annika Borgenstam Submitted manuscript to Metall. and Mat. Trans. A. V. Widening of Laths in Bainite Jiaqing Yin, Mats Hillert, Annika Borgenstam Submitted manuscript to Metall. and Mat. Trans. A. iii Author contributions: I. Most of literature survey, all the experimental work, take part in discussion, writing of the first version of manuscript II. Most of literature survey, all the experimental work, take part in discussion, writing of the first version of manuscript III. Part of literature survey, all the experimental work, take part in discussion, writing of the first version of manuscript IV. Most of literature survey, all the experimental work, take part in discussion, writing of the first version of manuscript V. Most of literature survey, all the experimental work, take part in discussion, writing of the first version of manuscript Parts of this work have been presented in the following international conferences: 1. Jiaqing Yin, Mats Hillert, Annika Borgenstam, “Role of Cementite on the Formation of Baintie in 0.3 Mass% C Steels”, oral presentation at International Conference on Solid-Solid Phase Transformation in Inorganic Materials (PTM), Whistler, Canada, 2015. 2. Jiaqing Yin, Mats Hillert, Annika Borgenstam, “On the Feathery Structure of Bainite”, oral presentation at TMS, Nashville, USA, 2016. iv Contents Chapter 1 .................................................................................... 1 Introduction ................................................................................ 1 1.1 Bainite in steels ................................................................................................... 1 1.2 Bainite controversy ............................................................................................. 1 1.3 Scope of the present work ................................................................................... 2 Chapter 2 .................................................................................... 5 Methodology ............................................................................... 5 2.1 Light optical microscopy ..................................................................................... 5 2.2 Electron microscopy ...........................................................................................