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Microstructure and mechanical properties of a 5 wt.% Cr cold work tool steel: Influence of heat treatment procedure.
University West, Department of Engineering Science, Research Enviroment Production Technology West. (PTW)ORCID iD: 0000-0001-6300-5923
2017 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The demand for Advanced High Strength Steel (AHSS) in the automotive industry is increasing day by day. It is mainly motivated by the fact that AHSS can be used as thin sheets while having high strengths. It enables weight reduction of the automobiles which consequently increases the fuel efficiency and has proven to be less harmful to the environment. It is also expected that AHSS will have even higher strength in the near future. Cold work tools steels with 5 wt.% Cr are commonly used to process AHSS. Therefore, the tool steel must meet the challenges in the future, i.e. have even higher hardness, compressive strength and toughness. One way of increasing the mechanical properties of the tool steel is by improving the heat treatment parameters. However, it is not possible without a deeper understanding of the heat treatment process. Therefore, this work presents investigations related to phase transformations occurring in a 5 wt.% Cr cold work tool steel during heat treatment. Furthermore, the influence of austenitisation and tempering temperatures on the microstructure and mechanical properties were investigated. The studies revealed that a higher austenitisation temperature can be used to achieve a higher hardness, good compressive strength and adequate toughnessof the steel. However, too high austenitisation temperature may result inexcessive coarsening of prior austenite grains which reduced the impact toughness. It was also found that retained austenite can transform during tempering by two different mechanisms. Firstly, when tempering at 525°C, carbides precipitate in retained austenite lowering its stability and permitting a transformation to marten site on cooling. Secondly, when tempering at 600°Cfor extended holding time retained austenite isothermally transforms to ferrite and carbides. This occurs by precipitation of carbides in retained austenite followed by a final transformation to ferrite and carbides.These results were used to understand the standard tempering procedure of the 5 wt.% Cr cold work tool steel. Furthermore, alternative heat treatment procedures are discussed based on the important findings presented in this thesis. 

Place, publisher, year, edition, pages
Trollhättan: University West , 2017. , 86 p.
Series
Licentiate Thesis: University West, 18
Keyword [en]
Cold work tool steel, Heat treatment, Microstructural characterisation, Mechanical properties, Retained austenite
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology; ENGINEERING, Manufacturing and materials engineering
Identifiers
URN: urn:nbn:se:hv:diva-10915ISBN: 978-91-87531-56-9 (print)ISBN: 978-91-87531-55-2 (electronic)OAI: oai:DiVA.org:hv-10915DiVA: diva2:1090375
Presentation
2017-05-23, C118, Högskolan väst, Trollhättan, 11:19 (English)
Supervisors
Available from: 2017-05-16 Created: 2017-04-24 Last updated: 2017-04-21
List of papers
1. Retained austenite transformation during heat treatment of a 5wt.% Cr cold work tool steel
Open this publication in new window or tab >>Retained austenite transformation during heat treatment of a 5wt.% Cr cold work tool steel
(English)Manuscript (preprint) (Other academic)
Keyword
Tempering, metals, austenitization. heat treatment
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology
Identifiers
urn:nbn:se:hv:diva-10914 (URN)
Note

Ingår i lic avhandling

Available from: 2017-04-21 Created: 2017-04-21 Last updated: 2017-04-24Bibliographically approved
2. Effects of Austenitisation Temperature and Multiple Tempering on the Microstructure and Impact Toughness of a 5 wt. % Cr Cold Work Tool Steel
Open this publication in new window or tab >>Effects of Austenitisation Temperature and Multiple Tempering on the Microstructure and Impact Toughness of a 5 wt. % Cr Cold Work Tool Steel
2016 (English)In: 10th TOOL Conference, Tool, conference proceedings, 10th TOOL Conference , 2016, 1-10 p.Conference paper, Published paper (Other academic)
Abstract [en]

The microstructure and properties of a 5 wt.% Cr cold work tool steel were studied after austenitisation at 1020°C, 1050°C or 1075°C followed by single, double and triple tempering treatments at 525°C. The microstructures were investigated with scanning electron microscopy and X-ray diffraction and phase transformations were studied by dilatometry. Furthermore, hardness and Charpy un-notched and V-notched impact toughness testing was performed and results were correlated to observed microstructures. With higher austenitisation temperature, the martensite and bainite start temperatures were lowered resulting in microstructures containing a higher volume fraction of retained austenite. Retained austenite transformed into martensite on cooling from the tempering temperature. Specimens that were austenitised at 1050°C or 1075°C and tempered twice contained fresh martensite. Applying a third tempering was therefore required to guarantee a fully tempered microstructure. The second tempering resulted in an increase of the un-notched impact energy while the third tempering did not have a pronounced effect. A triple tempering procedure could be preferable when austenitising at high temperatures to avoid undesirable fresh martensite in the tool microstructure.

Place, publisher, year, edition, pages
10th TOOL Conference, 2016
Keyword
Cold work tool steel, microstructure, martensite, retained austenite, toughness, X-ray diffraction, scanning electron microscopy, dilatometry
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology; ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-10563 (URN)
Conference
10th TOOL Conference, Bratislava, Slovaki, October 4-7, 2016
Available from: 2017-01-12 Created: 2017-01-12 Last updated: 2017-04-24
3. Effect of Austenitization and Tempering on the Microstructure and Mechanical Properties of a 5 wt% Cr Cold Work Tool Steel
Open this publication in new window or tab >>Effect of Austenitization and Tempering on the Microstructure and Mechanical Properties of a 5 wt% Cr Cold Work Tool Steel
Show others...
2016 (English)In: Steel Research International, ISSN 1611-3683, E-ISSN 1869-344X, Vol. 12, no 1 December, 1609-1618 p.Article in journal (Refereed) Published
Abstract [en]

The effects of austenitization and tempering temperatures for a 5 wt% Cr cold work tool steel are studied with an aim of understanding the influence on microstructure and mechanical properties. Microstructures are characterized with scanning electron microscopy and light optical microscopy. Retained austenite contents and martensite start temperatures are measured by X-ray diffraction and dilatometry, respectively. Hardness, impact toughness, and compressive yield strength are also determined. When the austenitization temperature is increased from 1020 or 1050 to 1075 °C, followed by tempering at 525 °C, significant hardness is gained while there is no increase in compressive yield strength. Higher austenitization temperatures also produce larger amounts of retained austenite. At the same time, the impact toughness is reduced due to coarsening of the martensitic microstructure. When the steel is tempered at 200 °C, a higher impact toughness and a higher volume fraction of retained austenite are observed. Retained austenite is not found after tempering at temperatures of 525 °C or above. It is concluded that the best combination of mechanical properties is achieved by austenitization at 1020 or 1050 °C followed by tempering at 525 °C.

Keyword
Cold work, tool steel, austenitization, tempering, retained austenite, impact toughness
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology
Identifiers
urn:nbn:se:hv:diva-9689 (URN)10.1002/srin.201600012 (DOI)000392632900005 ()2-s2.0-84971330355 (Scopus ID)
Available from: 2016-12-16 Created: 2016-08-10 Last updated: 2017-04-24Bibliographically approved

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