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Low Temperature Austenite Decomposition in Carbon Steels
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Martensitic steels have become very important engineering materials in modern society. Crucial parts of everyday products are made of martensitic steels, from surgical needles and razor blades to car components and large-scale excavators. Martensite, which results from a rapid diffusionless phase transformation, has a complex nature that is challenging to characterize and to classify. Moreover the possibilities for modeling of this phase transformation have been limited, since its thermodynamics and kinetics are only reasonably well understood. However, the recent development of characterization capabilities and computational techniques, such as CALPHAD, and its applicability to ferrous martensite has not been fully explored yet.

In the present work, a thermodynamic method for predicting the martensite start temperature (Ms) of commercial steels is developed. It is based mainly on information on Ms from binary Fe-X systems obtained from experiments using very rapid cooling, and Ms values for lath and plate martensite are treated separately. Comparison with the experimental Ms of several sets of commercial steels indicates that the predictive ability is comparable to models based on experimental information of Ms from commercial steels.

A major part of the present work is dedicated to the effect of carbon content on the morphological transition from lath- to plate martensite in steels. A range of metallographic techniques were employed: (1) Optical microscopy to study the apparent morphology; (2) Transmission electron microscopy to study high-carbon plate martensite; (3) Electron backscattered diffraction to study the variant pairing tendency of martensite. The results indicate that a good understanding of the martensitic microstructure can be achieved by combining qualitative metallography with quantitative analysis, such as variant pairing analysis. This type of characterization methodology could easily be extended to any alloying system and may thus facilitate martensite characterization in general.

Finally, a minor part addresses inverse bainite, which may form in high-carbon alloys. Its coupling to regular bainite is discussed on the basis of symmetry in the Fe-C phase diagram.  

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. , xii, 77 p.
Keyword [en]
Carbon steels, Electron backscattered diffraction, Martensite, Microscopy, Microstructure, Thermodynamic modeling
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-100993ISBN: 978-91-7501-449-4 (print)OAI: oai:DiVA.org:kth-100993DiVA: diva2:546034
Public defence
2012-09-27, F3, Lindstedsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Projects
Hero-m
Note

QC 20120824

Available from: 2012-08-24 Created: 2012-08-22 Last updated: 2012-08-24Bibliographically approved
List of papers
1. Investigation of Lath and Plate Martensite in a Carbon Steel
Open this publication in new window or tab >>Investigation of Lath and Plate Martensite in a Carbon Steel
2011 (English)In: International Conference on Solid-Solid Phase Transformations in Inorganic Materials, 2011, Vol. 172-174, 61-66 p.Conference paper, Published paper (Refereed)
Abstract [en]

Martensite in carbon steels forms in different morphologies, often referred to as lath andplate martensite. The alloy composition has a strong effect on the morphology, for instance in car-bon steels there is a morphological change of the martensite microstructure from lath martensite atlow carbon contents to plate martensite at high carbon contents. In the present work a decarburizedhigh-carbon steel, enabling the isolation of carbons' influence alone, has been studied in order to in-vestigate the changes in morphology and hardness. From the results it is concluded that there is acontinuous change of hardness with increased carbon content. The increasing hardness slows down atabout 0.6 wt%C before decreasing at higher carbon contents. This is in accordance with the change inmorphology since it was found that lath martensite dominates below 0.6 wt%C and the first units ofgrain boundary martensite and plate martensite appear above 0.6 wt%C. At high carbon contents thedominating morphology is plate martensite, but retained austenite is also present.

Series
Diffusion and Defect Data Pt.B: Solid State Phenomena, ISSN 1012-0394 ; 172-174
Keyword
Lath martensite, Plate martensite, Microstructure, Carbon Steels
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-66779 (URN)10.4028/www.scientific.net/SSP.172-174.61 (DOI)000303359700009 ()2-s2.0-79960913685 (Scopus ID)
Conference
International Conference on Solid-Solid Phase Transformations in Inorganic Materials, PTM 2010. Avignon. 6 June 2010 - 11 June 2010
Note

QC 20120127

Available from: 2012-01-27 Created: 2012-01-27 Last updated: 2017-03-24Bibliographically approved
2. Thermodynamically Based Prediction of the Martensite Start Temperature for Commercial Steels
Open this publication in new window or tab >>Thermodynamically Based Prediction of the Martensite Start Temperature for Commercial Steels
2012 (English)In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 43A, no 10, 3870-3879 p.Article in journal (Refereed) Published
Abstract [en]

A thermodynamic method for predicting the martensite start temperature of commercial steels is developed. It is based mainly on information on M (s) from binary Fe-X systems obtained from experiments with very rapid cooling, and M (s) values for lath and plate martensite are treated separately. Comparison with the experimental M (s) of several sets of commercial steels indicates that the predictive ability is comparable to models based on experimental information of M (s) from commercial steels.

Keyword
Critical Driving-Force, Ms Temperature, Transformation, Kinetics, Iron, Nucleation, Alloys
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-100992 (URN)10.1007/s11661-012-1171-z (DOI)000308187100046 ()2-s2.0-84867234353 (Scopus ID)
Funder
Vinnova
Note

QC 20121029

Available from: 2012-08-22 Created: 2012-08-22 Last updated: 2017-12-07Bibliographically approved
3. A Transmission Electron Microscopy Study of Plate Martensite Formation in High-carbon Low Alloy Steels
Open this publication in new window or tab >>A Transmission Electron Microscopy Study of Plate Martensite Formation in High-carbon Low Alloy Steels
2013 (English)In: Journal of Materials Science & Technology, ISSN 1005-0302, Vol. 29, no 4, 373-379 p.Article in journal (Refereed) Published
Abstract [en]

The martensitic microstructures in two high-carbon low alloy steels have been investigated by classical and automated crystallographic analysis under a transmission electron microscope. It is found that the martensitic substructure changes from consisting mostly of transformation twins for 1.20 mass% carbon (C) steel to both transformation twins and planar defects on {101}(M) for 1.67 mass% C steel. In the 1.67 mass% C steel it is further found that small martensite units have a rather homogeneous substructure, while large martensite units are more inhomogeneous. In addition, the martensite units in both steels are frequently found to be of zigzag patterns and have distinct crystallographic relationships with neighboring martensite units, e.g. kink or wedge couplings. Based on the present findings the development of martensite in high-carbon low alloy steels is discussed and a schematic of the martensite formation is presented. Moreover, whether the schematic view can be applied to plate martensite formation in general, is discussed.

Keyword
Transmission electron microscopy, High-carbon low alloy steel, Plate martensite
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-101107 (URN)10.1016/j.jmst.2013.01.016 (DOI)000317800500011 ()2-s2.0-84875631089 (Scopus ID)
Funder
Vinnova
Note

QC 20130524. Updated from manuscript to article in journal.

Available from: 2012-08-23 Created: 2012-08-23 Last updated: 2017-12-07Bibliographically approved
4. Effect of Carbon Content on the Orientation Relationship between Austenite and bct-Martensite in Fe-C Alloys resolved by Electron Backscattered Diffraction
Open this publication in new window or tab >>Effect of Carbon Content on the Orientation Relationship between Austenite and bct-Martensite in Fe-C Alloys resolved by Electron Backscattered Diffraction
(English)Manuscript (preprint) (Other academic)
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-101162 (URN)
Note

QS 2012

Available from: 2012-08-24 Created: 2012-08-24 Last updated: 2012-08-24Bibliographically approved
5. Effect of carbon content on variant pairing of martensite in Fe-C alloys
Open this publication in new window or tab >>Effect of carbon content on variant pairing of martensite in Fe-C alloys
Show others...
2012 (English)In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 60, no 20, 7265-7274 p.Article in journal (Refereed) Published
Abstract [en]

The effect of carbon content on the variant pairing tendency of martensite formed in Fe-C alloys is investigated by means of electron backscattered diffraction analysis. The method used is based on experimentally determined orientation relationships between austenite and martensite. The results show that the carbon content has a strong effect on the martensite variant pairing tendency. This observed change in variant pairing tendency is discussed in relation to the well-known morphological transition from lath to plate martensite in Fe-C alloys and the formation of packets and plate groups. The results indicate that quantitative analysis of variant pairing, as demonstrated here, may facilitate martensite characterization in Fe-C alloys as well as in other alloy systems.

Keyword
Carbon content, Crystallography, Electron backscattered diffraction, Fe-C alloy, Martensite
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-101163 (URN)10.1016/j.actamat.2012.09.046 (DOI)000312679600037 ()2-s2.0-84868214668 (Scopus ID)
Funder
Vinnova
Note

QC 20121214. Updated from manuscript to article in journal.

Available from: 2012-08-24 Created: 2012-08-24 Last updated: 2017-12-07Bibliographically approved
6. On the Symmetry Among the Diffusional Transformation Products of Austenite
Open this publication in new window or tab >>On the Symmetry Among the Diffusional Transformation Products of Austenite
Show others...
2011 (English)In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 42A, no 6, 1558-1574 p.Article in journal (Refereed) Published
Abstract [en]

Information on the diffusional transformation products of austenite in high-carbon steels is reviewed and supplemented with new microscopic studies. A comparison with transformation products in low-carbon steels indicates that there is a symmetry with pearlite in the middle, where ferrite and cementite are equal partners, and with acicular ferrite or cementite on each side. They both form with a surface relief, and at lower temperatures, each one is the leading phase in a eutectoid microstructure, bainite and inverse bainite, respectively. However, there is an asymmetry because at low temperatures bainite appears in high-carbon steels but inverse bainite never appears in low-carbon steels. At a constant high carbon content, there is another kind of symmetry, which is related to temperature. At intermediate temperatures the eutectoid reaction results in spherical nodules in which the cementite constituent originates from Widmanstatten plates. It turns spiky at both higher and lower temperatures with the leading phase in the spikes being cementite at higher temperatures and ferrite at lower temperatures. In the first kind of symmetry, there is an abrupt change among the three reaction products; in the second kind of symmetry, there is a gradual change. Accepting that all the eutectoid microstructures form by diffusion of carbon, one may explain the existence of both symmetries by the variation of the ratio of the supersaturations of ferrite and cementite with carbon content and with temperature.

Keyword
HYPEREUTECTOID STEELS; LOWER BAINITE; DECOMPOSITION; MORPHOLOGY; ALLOYS; CARBON
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-33964 (URN)10.1007/s11661-010-0539-1 (DOI)000290176100021 ()2-s2.0-79958786497 (Scopus ID)
Note
QC 20110531Available from: 2011-05-31 Created: 2011-05-23 Last updated: 2017-12-11Bibliographically approved

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