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Effect of austenitising temperature and cooling rate on microstructures of hot-work tool steels
University West, Department of Engineering Science, Division of Manufacturing Processes. (PTW)ORCID iD: 0000-0001-5249-4207
2015 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The average size of hot-work tools has gradually increased over the past years.This affects the effective temperature cycle tools experience during hardening,as large dimensions prevent uniform and rapid cooling, and thereby the resulting microstructures and properties. In order to avoid the formation of coarse structures or cracking during heat treatment it has become common practise to lower the austenitising temperature below that recommended by the steel manufacturer.In this work, therefore, the effects of austenitising at temperatures lower thancommonly recommended are investigated. Three 5% Cr hot-work tool steelsalloyed with Mo and V were heat treated, resulting microstructures andtempering carbides were studied and transformation characteristics determined for different austenitising temperatures and different cooling rates. The temperatures and cooling rates have been chosen to be representative for heat treatments of different sizes of tools. Bainite rather than martensite formed during slow cooling regardless of austenitising temperature. A lowered austenitising temperature produced largeramounts of both bainite and retained austenite while a higher caused graingrowth. Carbon partitioning during the bainitic transformation resulted in anincrease of the carbon content in the retained austenite of at least 0.3 wt.%. The austenitising temperature influences also the type and amount of tempering carbides that precipitate, which affects the properties of the steel. Higher austenitising temperatures favour the precipitation of MC carbides during tempering. The Mo rich M2C type carbides were proven to be more prone to coarsening during service at 560°C-600°C, while V rich MC carbides preserve their fine distribution. A best practice heat treatment needs to balance the increase of grain size with increasing austenitising temperatures, with the possibility to form more tempering carbides. Higher austenitising temperatures also give less retained austenite, which can affect dimensional stability and toughness negatively after tempering

Place, publisher, year, edition, pages
Trollhättan: University West , 2015. , 85 p.
Series
Licentiate Thesis: University West, 4
Keyword [en]
Tool steel, Heat Treatment, Austenitising Temperature, Large Tools, Tempering Carbides, Bainitic Microstructures
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology
Identifiers
URN: urn:nbn:se:hv:diva-8614ISBN: 978-91-87531-16-3 (print)ISBN: 978-91-87531-15-6 (print)OAI: oai:DiVA.org:hv-8614DiVA: diva2:868510
Opponent
Supervisors
Available from: 2015-11-11 Created: 2015-11-02 Last updated: 2015-11-11Bibliographically approved
List of papers
1. Effect of Austenitising Temperature and Cooling Rate on Microstructure in a Hot-Work Tool Steel
Open this publication in new window or tab >>Effect of Austenitising Temperature and Cooling Rate on Microstructure in a Hot-Work Tool Steel
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2014 (English)In: Proceedings of the 6th International Swedish Production Symposium / [ed] Stahre, Johan, Johansson, Björn & Björkman, Mats, 2014, 1-7 p.Conference paper (Refereed)
Abstract [en]

The effects on microstructucture of austenitising temperature and cooling rate during hardening were studied for a hot-work tool steel. Transformation temperatures were determined by dilatometry, scanning electron microscopy was used to characterise the microstructure and both retained austenite contents and their lattice parameters were measured by neutron diffraction. For lower cooling rates, lower austenitising temperatures produce larger amounts of both retained austenite and bainite. Retained austenite in bainitic structures is higher in carbon than in martensitic structures. Consequently, lowering the austenitising temperature will affect microstructure and properties.

Keyword
Austenitising temperature, bainitic reaction, retained austenite, lattice parameter
National Category
Production Engineering, Human Work Science and Ergonomics
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-7009 (URN)978-91-980974-1-2 (ISBN)
Conference
The 6th Swedish Production Symposium
Available from: 2014-11-20 Created: 2014-11-19 Last updated: 2015-11-11Bibliographically approved
2. Influence of lowered austenitisation temperature during hardening on tempering resistance of modified H13 tool steel (Uddeholm Dievar)
Open this publication in new window or tab >>Influence of lowered austenitisation temperature during hardening on tempering resistance of modified H13 tool steel (Uddeholm Dievar)
2013 (English)In: International Heat Treatment and Surface Engineering, ISSN 1749-5156 1749-5148, Vol. 7, no 3, 129-132 p.Article in journal (Refereed) Published
Abstract [en]

The surface of large tools will be exposed to the hardening temperature for longer times than the core. This might in occasions, result in grain growth. In order to prevent this, it has become practice to lower the hardening temperature. This paper presents the effect of this practice on the precipitation of tempering carbides and the tempering resistance of Uddeholm Dievar. Composition of equilibrium austenite and the undissolved carbides at two different hardening temperatures were estimated by Thermo Calc simulations and the calculations predict that the balance between the amounts of molybdenum and vanadium in the austenite is shifted towards more molybdenum at the lower austenitising temperature. Since molybdenum stabilises M2C precipitates, it was predicted also that the tempering carbides would be almost only M2C in the sample with the lower austenitising temperature, whereas for the higher austenitising temperature, the subsequent tempering would yield a mixture of the much more stable MC together with M2C. Samples were hardened at the simulated temperatures and tempered. The existing carbides were investigated with help of SEM and TEM. The result shows that a lowered austenitisation temperature decreases the tempering resistance. However, the transmission electron microscopy reveals that both samples have the same mixture of tempering carbides, as the samples do not reach thermodynamical equilibrium during the holding time at the hardening temperature. The lower austenitising temperature gives less tempering carbides as less alloying elements are dissolved.

Keyword
Hot-work tool steel, Tempering resistance, Tempering carbides, Hardening temperature, Grain coarsening
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering; Production Technology
Identifiers
urn:nbn:se:hv:diva-8611 (URN)10.1179/1749514813Z.00000000071 (DOI)
Available from: 2015-11-02 Created: 2015-11-02 Last updated: 2015-11-11Bibliographically approved
3. On the evolution of tempering carbides in a modified H!# and a modified H11 when hardening at 1000°
Open this publication in new window or tab >>On the evolution of tempering carbides in a modified H!# and a modified H11 when hardening at 1000°
2013 (English)Conference paper, (Refereed)
Abstract [en]

Hot- work tool steels require high austenitising temperature during hardening in order to yield the high tempering resistance that vanadium- rich carbides supply. Such grades, when offering high cleanness, are also used for plastic injection molding. The hardening temperature can then be lower, yielding a lower content of vanadium in the martensitic matrix and precipitating instead molybdenum-rich carbides, M2C- type, during tempering. M2C- type carbides are metastable and have high carbide/ matrix interface energy, which implies a greater driving force for coarsening than that in the MC- type. In this paper the carbide evolution in two hot- work grades hardened at 1000˚C, is studied after two and threetemperings. Type, size and distribution of tempering carbides were investigated with the help of TEM. Undissolved carbides were documented by SEM investigation and the microstructures classified by LOM. Hardness levels and Charpy V test results are also reported here.

Keyword
hardening, tempering carbides, hot-work tool steel, lowered austenitising temperature, carbide coarsening
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology
Identifiers
urn:nbn:se:hv:diva-8612 (URN)
Conference
4th Heat Treatment Symposium,24 - 25 October 2013, Istanbul
Note

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Available from: 2015-11-02 Created: 2015-11-02 Last updated: 2015-11-11Bibliographically approved

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