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Using Regression Analysis to Optimize the Combination of Thermal Conductivity and Hardness in Compacted Graphite Iron
Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting.
2010 (English)In: Key Engineering Materials, ISSN 1013-9826, Vol. 457, 337-342 p.Article in journal (Refereed) Published
Abstract [en]

In cast iron there is a contradictory relationship between thermal conductivity and strength. In many applications it is desirable to optimize the material properties to obtain both sufficiently high thermal conductivity and sufficiently high strength. The aim of this paper is to investigate how various microstructure parameters and alloying elements affect thermal conductivity and hardness in compacted graphite irons. It was found that the fraction of ferrite, the fraction of cementite, nodularity and content of carbon and silicon are parameters that influence the thermal conductivity and hardness the most. Based on these five key parameters linear regression equations were created for calculation of thermal conductivity and hardness. Ferrite and carbon have a positive influence on the thermal conductivity, while silicon, cementite and nodularity have a deleterious effect. All parameters except ferrite have a positive influence on the hardness. This is because the thermal conductivity is dependent on the movement of free electrons, and therefore unfavourable growth directions and grain boundaries which impede the electron movement will reduce the thermal conductivity. Ferrite has quite high thermal conductivity, while cementite has poor thermal conductivity, due to an unfavourable crystal structure. Nodular shaped graphite has a lower thermal conductivity than compacted graphite which explains the deleterious influence of nodularity. The soft ferrite phase will reduce the hardness value, while increasing the fraction of harder graphite nodules and harder cementite phase will increase the hardness. To investigate how these five parameters affect the combination of hardness and thermal conductivity, values for hardness and thermal conductivity were calculated for all combinations of key parameters in given intervals, using two linear regression equations. From these it is possible to predict the combination of parameters which gives a particular combination of hardness and thermal conductivity in compacted graphite iron.

Place, publisher, year, edition, pages
2010. Vol. 457, 337-342 p.
Keyword [en]
Compacted Graphite Iron (CGI), Regression Analysis, Thermal Conductivity (TC), Vickers Hardness
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:hj:diva-13472DOI: 10.4028/www.scientific.net/KEM.457.337OAI: oai:DiVA.org:hj-13472DiVA: diva2:355952
Available from: 2010-10-08 Created: 2010-10-08 Last updated: 2011-12-14Bibliographically approved
In thesis
1. On Thermal Conductivity and Strength in Compacted Graphite Irons: Influence of Temperature and Microstructure
Open this publication in new window or tab >>On Thermal Conductivity and Strength in Compacted Graphite Irons: Influence of Temperature and Microstructure
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Thermal conductivity, hardness and strength are all highly important material properties, affecting the performance and life expectancy of components operating at elevated temperatures. The main purpose of this work has been to increase the knowledge and understanding concerning how mechanical and physical properties are affected by temperature and microstructure in compacted graphite irons. This was accomplished by investigating the whole chain from how a certain microstructure can be achieved, how that microstructure affects mechanical and physical properties, how those properties were related and how they can be estimated.

 

By investigating how specific alloying elements affected the microstructure it was possible to confirm the pearlite promoting effects of copper and tin, the carbide stabilizing effects of chromium and molybdenum and magnesium’s ability to alter the compactness of the graphite particles. An ausferritic metal matrix could be attained by performing an austempering heat treatment or by increased solidification rate on samples highly alloyed with molybdenum. Increasing the content of ferrite improved the thermal conductivity, while increased content of free carbide, ausferrite or nodularity reduced the thermal conductivity. Ferrite containing high amount of dissolved silicon had a negative influence on thermal conductivity values but also a strengthening effect. Thermal conductivity values in CGI generally showed a maximum at about 300 °C but a large content of ferrite resulted in quite temperature stable values below 300 °C. Tensile strength parameters such as ultimate tensile strength and yield strength were temperature stable for temperatures up to 300 °C before the values rapidly decreased at higher temperatures. The values of Young’s modulus continuously decreased with increasing temperature and seemed to increase slightly with increasing content of free carbide and nodularity. Increasing nodularity also increased the values of the strength parameters. Contradictory linear relationships between strength or hardness and thermal conductivity were found highlighting the problem in optimizing both these properties. Linear regression models based on five key parameters were created to describe thermal conductivity values and hardness values, where the model describing thermal conductivity included temperature. Stress values for plastic deformations were possible to approximate with good accuracy by using constituent equations such as the Hollomon and Ludwigson equations.

Place, publisher, year, edition, pages
Göteborg: Chalmers Reproservice, 2010. 142 p.
Series
Doktorsavhandlingar vid Chalmers tekniska högskola, ISSN 0346-718X ; 3115
Keyword
Compacted Graphite Iron, Thermal Conductivity, Mechanical Properties, Hardness, Microstructure, Plastic Deformation, Hollomon Equation, Elevated Temperature
National Category
Materials Engineering
Identifiers
urn:nbn:se:hj:diva-13473 (URN)978-91-7385-434-4 (ISBN)
Public defence
2010-10-29, Gjuterisalen E1405, Gjuterigatan 5, Jönköping, 10:00 (English)
Opponent
Supervisors
Available from: 2010-11-23 Created: 2010-10-08 Last updated: 2010-11-23Bibliographically approved

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