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Comparing Three Equations Used for Modeling the Tensile Flow Behavior of Compacted Graphite Cast Irons at Elevated Temperatures
Jönköping University, School of Engineering, JTH. Research area Materials and Manufacturing - Casting. Jönköping University, School of Engineering, JTH, Mechanical Engineering.
2010 (English)In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 41, no 11, 2805-2815 p.Article in journal (Refereed) Published
Abstract [en]

A comparison between three constituent relationships used to approximate the plastic part of a tensile test curve, was performed on compacted graphite cast iron (CGI) samples at temperatures between room temperature and 600{degree sign}C. The investigated relationships were Hollomon, Ludwigson and Voce equations, where Ludwigson equation adjusts Hollomon equation by adding a correction term. The investigated CGI materials were alloyed with four different amounts of molybdenum and each chemical composition was cast with three different solidification rates. The coefficients in the equations generally were quite temperature independent between room temperature and 300{degree sign}C, but changed much in value as the temperature was further increased. Compared to Hollomon and Ludwigson equation, the coefficients in Voce equation directly showed correlation between proof stress at 0.1 % strain and ultimate tensile strength. Another difference between Voce equation and the other two equations is that Voce is not intended to give an accurate description of the stress values at small plastic strain values. The overall best approximation of the stress values was made by Ludwigson equation, followed by Hollomon and last Voce equation. The downside with Ludwigson equation was that its correction term could either be positive or negative, depending on the total plastic strain and the value of coefficients in Hollomon equation. This makes it harder to use as a general equation to approximate stress values compared to Hollomon and Voce equations.

Place, publisher, year, edition, pages
2010. Vol. 41, no 11, 2805-2815 p.
Keyword [en]
Hollomon, Ludwigson, Voce, Compacted Graphite Iron, Elevated Temperature, Plastic Deformation
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:hj:diva-12566DOI: 10.1007/s11661-010-0354-8OAI: oai:DiVA.org:hj-12566DiVA: diva2:325038
Available from: 2010-06-17 Created: 2010-06-17 Last updated: 2010-11-23Bibliographically 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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