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C-Curves for Lengthening of Widmanstätten and Bainitic Ferrite
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.ORCID iD: 0000-0003-2766-976X
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.ORCID iD: 0000-0002-7656-9733
2017 (English)In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 48, no 9, p. 3997-4005Article in journal (Refereed) Published
Abstract [en]

Widmanstätten ferrite and bainitic ferrite are both acicular and their lengthening rate in binary Fe-C alloys and low-alloyed steels under isothermal conditions is studied by searching the literature and through new measurements. As a function of temperature, the lengthening rate can be represented by a common curve for both kinds of acicular ferrite in contrast to the separate C-curves often presented in time-temperature-transformation (TTT) diagrams. The curves for Fe-C alloys with low carbon content show no obvious decrease in rate at low temperatures down to 623 K (350 °C). For alloys with higher carbon content, the expected decrease of rate as a function of temperature below a nose was observed. An attempt to explain the absence of a nose for low carbon contents by an increasing deviation from local equilibrium at high growth rates is presented. This explanation is based on a simple kinetic model, which predicts that the growth rates for Fe-C alloys with less than 0.3 mass pct carbon are high enough at low temperatures to make the carbon pileup, in front of the advancing tip of a ferrite plate, shrink below atomic dimensions, starting at about 600 K (323 °C).

Place, publisher, year, edition, pages
Springer, 2017. Vol. 48, no 9, p. 3997-4005
Keywords [en]
Alloy steel, Bainite, Bainitic transformations, Binary alloys, Ferrite
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-212245DOI: 10.1007/s11661-017-4196-5ISI: 000406486700008Scopus ID: 2-s2.0-85022224686OAI: oai:DiVA.org:kth-212245DiVA, id: diva2:1133964
Funder
VINNOVA
Note

QC 20170817

Available from: 2017-08-17 Created: 2017-08-17 Last updated: 2018-05-23Bibliographically approved
In thesis
1. Modeling Bainite Formation in Steels
Open this publication in new window or tab >>Modeling Bainite Formation in Steels
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This study examines the fundamental aspects of bainite formation in a guided effort to lay a foundation for development of a model capable of predicting bainite formation. In the first part of this study, the tenets of an existing model for growth, developed by Zener and later modified by Hillert are examined. A number of interacting and adjustable parameters are identified namely, diffusivity, driving force, radius of the ferrite plate tip, interfacial carbon content, and thermodynamic barriers. Amongst which, there are a number of assumptions which are no longer justifiable because of the availability of software and databases from which more accurate calculations can be obtained. The approximation of the driving force is one such example. Another is the carbon content in ferrite, which was assumed negligible. The capillarity effect of the curved ferrite interface had initially been assigned by Zener as having a fixed, optimal value at the maximum growth rate. Although this principle was kept it led to quite different results when the earlier approximations were removed. It is shown that the shape of the C-curves is largely dependent on the diffusivity.

The second focus of this thesis is aimed towards developing a model for the start temperatures of bainite, WBs and is achieved twofold. The first procedure was to develop an empirical model. Experimental information on which it is based refers to the start conditions for both bainite and Widmanstätten ferrite. A systematic approach is adapted by which Fe-C is the basis and the effect of alloying elements are evaluated separately from ternary alloys. Regression analysis of data on five ternary systems with Mn, Ni, Si, Cr and Mo gives separate coefficients. A linear empirical equation for the WBs is defined from their sum which was possible because their effects were independent. Carbon had by far the largest effect and Ni the smallest. The equation has good agreement with data but further improvement can be achieved with more reliable experimental data. The second procedure is directed towards a thermodynamic description of the start condition. The critical driving forces corresponding to the critical conditions depicted in the empirical equation are calculated. The results are presented with a dependence on temperature and the same could be translated to a carbon dependence. The critical driving force increases substantially with decreasing temperature and increasing carbon content and the effect of alloying elements is varied.

In the final section, the growth model is further developed. A more generalized expression for the barrier is formulated and together with the capillarity effect, consists of the energy requirement to move the growth interface. This is balanced with the available driving force and is solved with an optimization procedure. The predicted C-curves are compared with experimental results and reasonably good agreement is found.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2018. p. 51
Series
TRITA-ITM-AVL ; 2018:16TSC-MT
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-227863 (URN)978-91-7729-779-6 (ISBN)
Public defence
2018-06-15, B3, Brinellvägen 23, Stockholm, 10:00 (English)
Opponent
Supervisors
Available from: 2018-05-15 Created: 2018-05-14 Last updated: 2018-05-18Bibliographically approved

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