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On the Machinability of High Performance Tool Steels
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The continuous development of hot forming tool steels has resulted in steels with improved mechanical properties. A change in alloying composition, primarily a decreased silicon content, makes them tougher and more wear resistant at elevated temperatures. However, it is at the expense of their machinability. The aim of this study is to explain the mechanisms behind this negative side effect.

Hot work tool steels of H13 type with different Si content were characterised mechanically, and evaluated analytically and by dedicated machining tests. Machining tests verified that materials with low Si content displayed reduced machinability due to their stronger tendency to adhere to the cutting edge. Three hypotheses were tested.

The first hypothesis, that the improved toughness of the low Si steels is the reason behind their relatively poor machinability, was rejected after machining tests with one low Si steel heat treated to the same relatively low toughness as conventional hot work tool steels.

The second hypothesis, that a change in oxidation properties, also associated with the change in Si composition, lies behind the reduced machinability was investigated by dedicated tests and evaluations. It was found that the oxide thickness increased with reduced Si content and that there was an enrichment of Cr at the oxide/steel interface. The differences in oxide thickness and the possible differences in oxidation properties may influence the machinability of the materials through their different abilities to adhere to the cutting edge.

The third hypothesis, that a high enough temperature to initiate phase transformation from ferrite to austenite is generated during machining of the tool steels, was also investigated. This may lead to a reduced machinability because higher austenite content is directly related to higher compressive stresses and higher cutting forces. This causes accelerated tool wear. This hypothesis was verified by ThermoCalc calculation of austenite content in the steels, which showed a good agreement with Gleeble compression tests and cutting force measurements.

This thesis confirms that a reduced Si content in conventional H13 steel improves the toughness, reduces the oxidation resistance and lowers the ferrite-to-austenite transformation temperature. The reduction in austenite temperature is probably the most important factor behind the reduced machinability. 

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2012. , 58 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 927
Keyword [en]
hot work tool steels, alloying composition, machinability
National Category
Tribology
Research subject
Materials Science
Identifiers
URN: urn:nbn:se:uu:diva-172427ISBN: 978-91-554-8349-4 (print)OAI: oai:DiVA.org:uu-172427DiVA: diva2:514634
Public defence
2012-06-04, Häggsalen, Ångströmlaboratoriet, Uppsala, 12:00 (Swedish)
Opponent
Supervisors
Available from: 2012-05-08 Created: 2012-04-10 Last updated: 2012-08-01Bibliographically approved
List of papers
1. Machinability of high performance die steels
Open this publication in new window or tab >>Machinability of high performance die steels
2003 (English)In: Transactions of the 22nd International Die Casting Congress and Exposition: Die casting in the heartland : September 15-17, 2003, Indianapolis, Indiana, Rosemont, Ill.: North American Die Casting Association , 2003, , 327-335 p.327-335 p.Conference paper, Published paper (Other academic)
Place, publisher, year, edition, pages
Rosemont, Ill.: North American Die Casting Association, 2003. 327-335 p.
National Category
Materials Engineering
Identifiers
urn:nbn:se:uu:diva-46380 (URN)
Conference
22nd Int. Die Casting Congress and Exposition, Indianapolis, USA, September 15-17, 2003
Available from: 2008-10-17 Created: 2008-10-17 Last updated: 2012-08-01Bibliographically approved
2. Machinability of modern hot work tool steels
Open this publication in new window or tab >>Machinability of modern hot work tool steels
2006 (English)In: Proceedings of the 7th Int. Tooling Conference, Torino, Italy, 2006, 2006Conference paper, Published paper (Refereed)
National Category
Materials Engineering
Identifiers
urn:nbn:se:uu:diva-172423 (URN)
Conference
7th Int. Tooling Conference, Torino, Italy, 2006
Available from: 2012-04-10 Created: 2012-04-10 Last updated: 2012-08-01Bibliographically approved
3. On the relation between composition, oxidation and machinability of tool steels
Open this publication in new window or tab >>On the relation between composition, oxidation and machinability of tool steels
Show others...
2008 (English)Conference paper, Published paper (Refereed)
National Category
Materials Engineering
Identifiers
urn:nbn:se:uu:diva-172424 (URN)
Conference
The 13th Nordic Symposium on Tribology, Tampere, Finland, June 10-13, 2008
Available from: 2012-04-10 Created: 2012-04-10 Last updated: 2016-04-12
4. Properties affecting machinability of hot work tool steels
Open this publication in new window or tab >>Properties affecting machinability of hot work tool steels
2009 (English)In: Tool Steels - Deciding Factor in Worldwide Production: Tool 09; Proceedings of the 8th International Tooling Conference, RWTH Aachen University, Aachen, Germany, June 2 - 4, 2009 / [ed] P. Beiss, Christoph Broeckmann, Mainz, 2009Conference paper, Published paper (Refereed)
Place, publisher, year, edition, pages
Mainz: , 2009
National Category
Materials Engineering
Identifiers
urn:nbn:se:uu:diva-172425 (URN)9783861307013 (ISBN)
Conference
8th Int. Tooling Conference, Aachen, Germany, June 2-4, 2009
Available from: 2012-04-10 Created: 2012-04-10 Last updated: 2016-04-14Bibliographically approved
5. On wear resistance of tool steel
Open this publication in new window or tab >>On wear resistance of tool steel
2012 (English)In: Journal of Manufacturing Processes, ISSN 1526-6125, Vol. 14, no 3, 195-198 p.Article in journal (Refereed) Published
Abstract [en]

Maintaining a reasonably low cutting tool wear when producing forming tools is a general challenge in the development of new forming tool materials. The tool life of a hot forming tool steel (H13) has been significantly improved by reducing its Si-content from 1.0 to 0.06 wt.%. However, this modified H13 (MH13) also displays a reduced cutting tool life due to higher cutting forces and a stronger tendency to form built up layers (BUE) on the cutting edge. This paper explains why.

Gleeble tests of MH13 revealed a significantly higher flow stress in the 820–900 °C temperature interval in MH13 compared to H13. Thermo-Calc simulations showed that when reducing the Si-content from 1.0 to 0.06 wt.% the initial temperature for ferrite-to-austenite transformation (A1) was reduced from 900 °C to 820 °C. Knowing that austenite has totally different mechanical and thermal properties than ferrite, the difference in A1 between the two steels explains the higher cutting forces and higher tendency for BUE-formation. The conclusion is that the difference in machinability between H13 and MH13 is primarily related to their difference in A1.

An attempt was also made to find a new tool material composition that can combine the wear resistance of MH13 and the good machinability of H13. Thermo-Calc simulations were performed with slightly modified alloying content without changing its properties as a good forming tool material, with the aim to increase A1. For instance, reducing the Mn content from 0.5 to 0.05 wt.% proved to increase A1 from 820 to 850 °C.

National Category
Materials Engineering
Research subject
Engineering Science with specialization in Materials Science
Identifiers
urn:nbn:se:uu:diva-172426 (URN)10.1016/j.jmapro.2012.03.002 (DOI)
Available from: 2012-04-10 Created: 2012-04-10 Last updated: 2013-06-19Bibliographically approved

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